You are here » Home » Find a Distributor »

Tom Albin

Find a Distributor

Darcor caster solutions are available wherever you need them.

Make your selection(s) below to find a distributor near you who can help generate results from the ground up.

Industrial Ergonomic Best Practices in Cart Design

Industrial Cart Handle Placement and Caster Selection and Placement are Crucial to Support Industrial Ergonomics and Reduce Workplace Injuries

industrial design concept on the gearwheels, 3D rendering

When designing industrial carts for manual material handling (MMH), a primary goal of an industrial ergonomist is to reduce the risk of musculoskeletal injuries by keeping the forces required to start, stop and maneuver the cart within recommended safe limits. In industrial ergonomics, the hierarchy of controls used to reduce risk gives priority to engineering controls and is commonly described as “designing out the problem”.

There are many facets of industrial cart design to consider to keeping cart operating forces within recommended limits – we’ll focus on two aspects:

  • Handle placement
  • Caster selection and placement

Placement of Handles on Industrial Carts

Cart handles should be placed at about hip height vertically and at about shoulder width horizontally relative to the individual user. This allows an individual to exert his or her maximal push pull force. For the North American population, the vertical height range of a cart handle should be about 94.8 cm (37.3 in) to 117.8 cm (46.4 in) above the floor (3 cm/1.2 in added as an allowance for shoe height). The distance between the handles also affects the maximum force that can be exerted. The fifth percentile shoulder width value for females is about 33.5 cm (13.2 in) and the 95th percentile value for males is about 44.7 cm (17.6 in).

The distance between handles is also important when turning carts. Turning a cart requires exerting torque on the cart and the horizontal distance between the points where an operator grips the cart’s handles determines the amount of torque that he or she can exert.

Industrial Cart Caster Selection and Placement

The type of casters used directly affects the amount of force required to move and maneuver a cart. For example, a larger diameter caster will more readily roll over debris and surface irregularities. Generally, a larger caster requires less force to maintain the motion of a cart once the cart is in motion. A larger diameter caster also reduces the force of exertion for heavier loads.

This relationship between caster diameter and the force required to keep a cart in motion can be expressed in this equation:

push pull force equation ergonomics

Reducing the weight carried by a caster, increasing the diameter of the caster, or doing both will reduce the force required to keep the cart in motion.

The type of caster used affects the force necessary to turn carts. If the casters are fixed and cannot swivel, turning the cart drags the casters over the floor surface, adding frictional force to the effort required to turn the cart, as well as increasing wear to the casters.

Mounting a caster on a swivel allows it to turn and align with the cart’s new heading. However, when a single caster turns, it pivots in place against the floor. Although the work of pivoting the caster is relatively small, it also can wear away the caster. In contrast, a pair of freely-rotating casters mounted on a swivel turns like the differential joint in a car; each caster turns separately, rather than being dragged over the surface and abraded.

Location of swiveling casters on a cart is also an important consideration. A common recommendation is to place the swiveling casters under or as near as possible to the point where the push or pull force is applied to the cart. However, in some instances there is an advantage to having swiveling casters at the front and back of a cart to facilitate placement of the cart within restricted spaces. It is generally useful to include a locking feature on such casters to improve tracking when precise positioning is not required.

Casters offset relative to the swivel point give leverage and mechanical advantage to the operator, which reduces both the turning radius and the turning force. Similarly, placing swiveling casters on the cart at a greater distance from the cart’s center of gravity than the fixed casters reduces the weight that the swivel casters support, reducing the amount of force required to turn.

For more on caster selection considerations, read some of our other blogs on caster technology and caster selection best practices:

Observing to Industrial Cart Design Best Practices Can Reduce MMH-Related Workplace Injuries

In summary, investing in well-designed industrial carts for MMH tasks to keep the operating forces within safe limits throughout the life of the cart pays a return by reducing the risk of musculoskeletal injuries. In addition, reduced cart operating forces can achieve operational efficiencies and productivity gains. Designing industrial carts to achieve these two goals, reduced injury risk and enhanced productivity are laudable goals for all industrial ergonomists.

To learn about reducing workplace injuries and supporting cost justification for crucial ergonomic solutions, download the Economics of Ergonomics Guide now.


This blog was originally published in June 2018. 

About the Author

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Risk Mitigation in Manual Materials Handling (MMH) Push-Pull Tasks

mmh task push pull workplace ergonomicsOver the past four years, Darcor’s Workplace Ergonomics and Caster Technology Blog has emphasized the utility of good cart design, especially with regard to careful consideration of the required operating forces. Failure to act to control risk or making poor decisions in the design of manual cart handling tasks or in the evaluation of risk during push and pull tasks incurs costs from injuries, productivity losses, and operational inefficiencies.

The Cost of Not Addressing Push Pull Risk

The cost of a single lost-time back injury case is estimated to be somewhere between $40,000 and $80,000. Hodgins noted that reducing the force required to push order-picking carts in a warehouse resulted in significant gains in productivity; on average, the warehouse workers were able to complete about 10% more order picking trips per day. Finally, there are often less apparent costs associated with injuries. For example, a study by Freburger et al found that individuals who are experiencing back pain are only fully productive about 90 percent of all workdays each year.

Evaluation of MMH Task Risks

The evaluation of risk of injury associated with manual cart handling tasks is commonly accomplished via use of tools such as the Liberty Mutual Manual Material Handling Tables and the Ohio State tables. Each of these gives an estimate of the percent of people capable of exerting a given level of force. Several researchers have found that individuals exerting forces in excess of the strength capability are more prone to injury; Snook found that using push or pull forces that at least 75 percent of females were capable of exerting lowered the risk of injuries for both men and women.

MMH Cart Design Best Practices

A good practice for the design of carts and for reducing the risk of injury resulting from manual handling of carts is to specify cart designs and components that allow the use of operating forces that at least 75 percent of females are capable of exerting to start the cart moving, sustain movement and to turn the cart. The ISO standard for push pull forces recommends 90 percent capability for cart handling forces in some situations; a higher percent capable for each component force may be advisable for composite MMH tasks. Composite MMH tasks are those that include multiple force exertions; for example, lifting a box and placing it on a cart, starting the cart moving, turning the cart at a corner, etc.

Avoiding MMH Task Risk Re-entry

In order to ensure continuous cart ergonomics performance, periodic maintenance is recommended to ensure that the cart operating forces remain within recommended limits throughout the economic life of the cart. Over time, cart frames may be knocked out of alignment so that they don’t track true, casters may develop flat spots, bearings may wear, etc.

Similarly, use patterns may affect safe operating forces. A cart that was originally intended to carry a load of 250 kg (550 lb) may be put into service to handle a 500 kg (1100 lb) load, or the frequency of use may change from one every 10 minutes to once every 5 minutes, etc.

Wear and tear on the cart or changes in the way in which it is used affect the forces needed to operate the cart, and it is important to be sure that the operating forces stay within safe limits. A good cart handling ergonomics program must periodically reassess carts to be certain that the operating forces are maintained within the recommended limits and that the use conditions are as originally specified. Not to do so means that an operator’s exposure to injury risk is unknown and uncontrolled.

A key aspect of any cart maintenance program is the frequency with which carts need to be inspected. High cart loads, frequent trips or surface debris that could damage casters are examples of factors that can affect the length of time between inspection. Because casters play such an important role in determining push pull forces, they should be selected for durability and to match the usage conditions. Matching the use conditions to the appropriate caster can increase the time between regularly scheduled cart maintenance inspections.

Mitigating MMH Risk through Cart Design and Ongoing Maintenance

To mitigate MMH risk from the start and ongoing, it is crucial to initially design the cart and task so that all operating forces are within safe limits and perform routine verification and maintenance to ensure that the operating forces remain within those recommended limits throughout the economic life of the cart.

The team at Darcor has developed the Guide to Designing Manual Materials Handling Carts for cart designers to consider specific aspects in the design process including environmental considerations, operator factors, and task-related scenarios.

Get the guide today!

Push / Pull Injuries Impact the Whole Body, Not Just the Back

Shoulders and Knees are Also at Risk of Workplace Injuries due to Poorly Designed Carts

People, healthcare and problem concept - unhappy man suffering from neck or shoulder pain at home

When we talk about pushing and pulling as potential causes of manual material handling (MMH) workplace injuries, we tend to focus on low-back injuries. This is not without good reason; back injuries and back pain are both prevalent and expensive. A study of the prevalence of back pain noted that about one in six individuals in the workplace has experienced back pain within the past two weeks. The Spine Research Institute of Ohio State University estimates the total cost of a lost-time back injury, including wage replacement cost for workdays lost to injuries, as ranging between US$40,000 to US$80,000. Back pain and injuries affect productivity as well; one study estimated that back pain annually limits productivity of affected individuals by about 10 percent. However, back injuries are not the only consequence of poorly-designed cart handling MMH tasks; shoulders and knees are also at risk; slips and falls while pushing or pulling carts are also of concern.

Although Less Frequent, Shoulder and Knee Workplace Injuries Can Cost Companies the Same or More than Back Injuries

The US Bureau of Labor Statistics (BLS) estimates the incidence rate of lost time back injuries as 15.2 cases per 10,000 workers per year worked; the prevalence of knee and shoulder lost time cases is lower, 7.6 and 7.2 cases, respectively. There are about twice as many lost-time back injury cases as shoulder and knee injury cases. However, the median lost days per case tell a different story. While the median number of lost work days for a back-injury case is 8 days lost, the median number of lost work days for a shoulder injury is 26 days and the median lost work days for knee injuries is 14 days. The cost of replacing daily wages for a median back injury case at the average US private sector hourly earnings rate of $27 per hour for eight days is roughly US$1,700, but increases to about $5,600 for a shoulder case and about $3,024 for a knee injury case. Taking the relative incidence and median days lost into account for each type of injury, the expected cost of lost wage replacement is roughly equal for backs and knees, but shoulders cost about 60 percent more due to the greater number of days lost. Although knee and shoulder cases are less common, the number of lost work days associated with them are roughly equivalent to the days lost to back injuries (knees), or much higher, as in the case of shoulders. A study of workers compensation costs in Ohio showed approximately equivalent costs between lost time cases involving the lumbar spine (low back) and those involving the shoulder.

Pushing and Pulling Heavy Loads May Be More Likely to Cause Shoulder Pain than Back Pain

An interesting study from the Netherlands found that individuals who reported that their job involved quite often or very often pushing or pulling loads greater than 50 kg (110 pounds) were roughly twice as likely to experience back pain than individuals in a control group whose job did not involve frequently pushing similar loads. However, for those same individuals whose jobs involved quite frequently or very frequently pushing or pulling loads greater than 50 kg were two to six times more likely to report shoulder pain than the individuals in the control group. A prospective study based on direct observation by the same authors reported similar findings.

Impact of Pushing/Pulling and Loading on All Parts of the Body

An operator exerts force on a cart through his or her hands. At the same time, an equivalent force is transmitted through the hands and arms, through the shoulder, down the back, through the legs and feet, to the surface. Each joint in this chain has its own unique ability to resist the forces and moments applied to it. For example, it is well-known that the end-plates between the spinal vertebrae and the spinal disks fracture under compressive loads of about 3,500 Newtons (786 pounds force), and that moments (force times a lever arm) about the spinal vertebrae manifest as compressive loads. Pushing and pulling carts applies compressive and shear loads to each link in the chain: hands to arms, arms to shoulders, shoulders to back, back to legs, legs to floor. The loading changes with posture; for example, Chaffin used a biomechanical model to demonstrate that different postures while lifting a battery from a truck and placing it on the floor created different loads to the shoulder and low back. The operators’ posture when picking the battery up exceeded the shoulder strength capability of about 65 percent of workers; the different posture used by the operator while setting the battery down on the floor exceeded the low back strength of a similar percentage of workers.

Moments about the shoulder joint are minimized when the push or pull force is applied to a cart at the same height as the shoulders and with the arms straight out. However, the standard recommendation is to place cart handles at about waist level, as that is where the maximum push strength can be developed. Pushing with that difference in height between the shoulders and the waist guarantees that pushing will create a torque, or rotational force, on the shoulder joint; similarly, moving carts up or down slopes requires the operator to push or pull up, which also creates a torque around the shoulder joint. Hopefully, the torques created do not exceed the shoulders’ strength capability. Unfortunately, the injury data suggest that is too frequently not the case.

How to Design Carts to Minimize the Risk of Back, Shoulder and Knee Workplace Injuries due to MMH Push/Pull Tasks

Carts should be designed so that the torques and forces applied to the joints by pushing and pulling carts match the capabilities of the individuals who will operate them. Tools such as the Liberty Mutual Manual Materials Handling Tables and the Ohio State University push, pull and turn tables are excellent resources for determining appropriate push, pull and turn forces to reduce the risk of back injuries.

workplace ergonomics guide darcor

A third tool, the Michigan 3-dimensional static strength models, offers guidance for acceptable forces for multiple joints, including the shoulders and the knees. Designers should incorporate the information from these sources in order to design carts with reduced risk of injury to both the shoulders and low back.

To learn more about designing better carts to reduce the push/pull workplace injuries risks, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Workplace Ergonomics: Cumulative Stress & Back Injuries

Cumulative Stress to the Spine Plays a Key Role in Assessing Risk of Occupational Back Injuries

workplace ergonomics cumulative stress back injuriesBack pain and back injuries cost companies in the US more than 100 billion US dollars per year, in terms of both treatment costs and in lower productivity. An effective ergonomics program that identifies and reduces all Manual Materials Handling (MMH) risk factors is a key component of any effort targeted at reducing the cost of injuries.

Recently experts have suggested that, as employers recognize and reduce the risks associated with lifting, attention in MMH work has shifted towards reducing the risk associated with pushing and pulling activities. Key to the analysis of push and pull forces has been the determination of maximum acceptable forces, which have been determined by either psychophysical methods or biomechanical methods.

The basis of these analyses is that, so long as the force used to maneuver a cart is less than or equal to the acceptable force when starting a load moving, maintaining movement, turning, or stopping the load, the risk of injury is reduced and the job design is considered acceptable. To determine those acceptable force limits by psychophysical methods, persons performing the push or pull task subjectively judge whether or not a task is acceptable based on the force exerted, the frequency of exertion, the distance the load is moved, etc. In the biomechanical method, static or dynamic models of MMH forces on body structures such as the spine are used to determine acceptable force levels that do not stress the spinal structure beyond its ultimate strength. Biomechanical ultimate strength can be thought of as the maximum stress that a material such as an intervertebral disc can withstand while being stretched or pulled before breaking.

Push-Pull Risk is Underestimated when Elements are Considered in Isolation

However, current practice for both psychophysical analyses and biomechanical analyses of MMH risk is that each element of a push-pull task is considered separately and in isolation. An MMH task might have three elements: the initial force to start a cart moving, the force necessary to sustain movement, and the force necessary to turn the cart. If the initial force is acceptable and the sustained force is acceptable, and the turning force is acceptable, then the task composed of all three of these three elements is considered acceptable. But this current approach to analyses of push-pull forces doesn’t take the interaction or cumulative effect of all these elements of a cart-pushing task into account. That is, the risk of injury may be underestimated if the cumulative effect of the interaction of forces is not considered.

Kumar studied cumulative exposure to spinal loading, where cumulative loading was defined as force (Newtons) multiplied by the duration of the exertion in seconds (N.s). He found that back pain was associated with higher cumulative loading of the spine and suggested that cyclic or repetitive loading could lead to fatigue and a reduction in the stress-bearing capacity of the spine, even for stress loads less than the ultimate strength of the spine. In that study, individuals who did not report back pain had higher average compressive forces on the spine than individuals who did report back pain, but the key difference was that the individuals who experienced pain had much higher cumulative exposures.

S-N Curve Concept Applied to Interaction of Force and Repetition

More recently, Gallagher et al have employed the concept of an S-N curve to describe fatigue failure of biological tissues, such as those in the spine. An S-N curve plots the magnitude of repeated applications of stress (S) against the number of repetitions (N) before failure occurs. If this seems familiar to ergonomics practitioners, it is because it might be thought of as a quantitative statement of the well-known rule of thumb in occupational ergonomics, that the interaction of force and repetition increases the risk of musculoskeletal injury. High-force, high repetition tasks are known to have a greater risk for MSD injury than do either high force, low repetition tasks or low-force, high repetition tasks.

Gallagher and his colleagues at Auburn University have developed a tool, Lifting Fatigue Failure Tool (LiFFT), to evaluate the cumulative fatigue of spinal tissues such as the intervertebral discs in response to the application of repetitive compressive loading to the spine while lifting. The LiFFT model is a cumulative risk model; the additive effect of each stress applied to the spine is taken into account with regard to fatigue and failure of the spinal tissue. While each lift may vary and apply differing levels of stress to the spine, the LiFFT model considers the cumulative effect of all elements of the lifting task towards fatigue failure of the spinal tissues.

It is certainly possible to measure the stress to the spine created by pushing and pulling tasks in the same manner as those created by lifting. Among others, Weston et al have modelled compressive and shear loads to the spine during push and pull tasks, including the initial forces to start the load moving, to sustain movement or to turn the load. These task-dependent stress loads could readily be compared to the ultimate strength of the spine and the likelihood of fatigue failure determined, in the same manner as suggested by Gallagher et al for lifting.

Complex MMH Tasks – Magnitude & Repetition Increases Risk of Fatigue Failure

It seems quite plausible that the fatigue failure – cumulative stress approach establishes a common basis for evaluation of injury risk associated with complex MMH tasks, such as those that involve combinations of manually lifting, lowering, carrying, pushing, pulling, turning, and stopping loads. The risk of injury from fatigue failure of the spinal tissues would be determined as a function of the magnitude and repetition of the stresses applied to the spine during performance of those complex MMH tasks.

In conclusion, while evaluating single elements of a push pull task such as initial, sustained or turning forces has proven effective, it likely underestimates the risk of back pain and injury. The cumulative effect of all exertions with regard to fatigue failure of the spine should be considered when assessing the risk of back injuries and back pain during complex MMH tasks.

To learn more about the benefits of industrial ergonomics programs that reduce risks when pushing, pulling and maneuvering carts, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

The Hidden Ergonomic Risks of Manual Material Handling Carts

Risk-management gurus tell us that we need to anticipate and make allowances for those things that we know could happen (but hope that they don’t).

In the management of manual material handling carts, a common hidden and unknown factor is the stability or consistency of operating forces over the life of the cart. Operating forces are those forces necessary to initiate and sustain movement of the cart. Those forces will change as carts and casters wear. This factor can mean risk of injury for the company as, if the operating forces change, the risk of injury to the worker increases substantially.

While we have good processes in place to measure the initial and sustained operating forces and to ensure that those forces are within safe limits, we are perhaps less accomplished in utilizing risk management processes to ensure that the required forces remain within safe limits over the course of time. How can we assure ourselves that a cart’s long-term operating forces aren’t hidden and that they remain within the desired performance envelope six months or a year from now?

Industrial Ergonomists Know How to Safely Track and Measure Cart Fleets

Process for Proactive Cart Maintenance

Ergonomists with experience in managing cart fleets recommend two processes that can help to track cart performance:

1. Operator training

Operators are trained to do a quick, once-over inspection of a cart before they use it. They are empowered to flag and remove carts from service – carts that are obviously damaged or require more force to operate than it should. The operator training also highlights the importance of observing the rated capacity of carts; an over-loaded cart requires more force to maneuver and overloading it may also accelerate the rate of wear on the casters.

An astute operator may well want to give extra attention to a cart that is near its service date or send it to maintenance if it is past due for service. If an operator notes that a cart is routinely loaded to a greater capacity than it is rated for, they may want to request re-evaluation of the recommended limits of the operating forces or request a re-evaluation of the design including handle height and higher quality/more suitable casters. As a colleague recently said, “The right casters make all the difference in the world.”

2. Scheduled maintenance checks for carts

Facilities with well-developed programs to manage cart handling ergonomics routinely schedule carts for preventative maintenance to ensure that all components, especially casters, are functioning as expected to keep the forces within recommended limits.

There is often something of an art to scheduling these maintenance intervals, as different equipment and different use patterns can affect the rate at which casters and carts wear out. Experienced managers note that a part of maintenance scheduling is learning from experience. For example, an experienced manager knows that carts that receive intense use or are used in adverse environments might be scheduled for maintenance checks more frequently.

Tracking Carts for Proactive Maintenance

One critical component in a cart management programs is tracking and marking the carts with:

  • Unique identifier like a number or barcode,
  • Rated capacity, and
  • Scheduled service dates.

Proactive Industrial Risk Management Translates to a Safer Shop Floor and Increased Operational Efficiency

The obvious benefit to a material handling cart management program which is rooted in proactive ergonomics is the reduction in the risk of injury. Research conducted at the Liberty Mutual Safety Institute noted that keeping operating forces within the capability of at least 75 percent of females significantly reduced the risk of injury.

Less widely known is the potential positive effects on operational efficiency and productivity. However, in a recently published case study, A Closer Look at How Low Quality Casters Will Cost Your Organization in Productivity, Hodgins has noted that, by equipping carts with more ergonomically-suitable casters, the manual exertion forces while handling carts was reduced which resulted in a 10% increase in productivity. It also achieved some notable operating efficiencies; fewer carts and personnel were required to perform the same volume of work.

economics of ergonomics guide cover manual material handlingA proactive approach to cart risk management ensures that operating forces are kept in check throughout the service life of the cart which means reduced risk of injury to valuable workers. In addition, organizations can hope to also achieve improvements in productivity, operational efficiency, and peace of mind.

To gain a thorough understanding of how you can manage the financial risk of workplace overexertion injuries, download the Economics of Ergonomics in the Material Handling Industry Guide.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Current MMH Evaluations may Underestimate Risk from Push and Pull Forces

Measure the word Risk with measuring tape on white background

Current methods of assessing push pull forces in industrial ergonomics are useful, but they need further development as they likely overestimate the safety of push and pull task forces.

Back injuries remain one of the most-costly of all occupational injuries. It is estimated that about 10 percent of adults experience back pain severe enough to limit their productivity 25 days or more per year and that workplace lost-time back injuries cost businesses about $460 per employee per year. The physical stresses created by Manual Materials Handling (MMH) are known to play a causal role in back pain and injury, and studies by Snook and Marras et al have shown that using ergonomic analyses of MMH stresses to design reduced-stress MMH jobs reduces the number of injuries that occur.

However, the methods currently used to evaluate MMH risk are generally applied only to single elements of a composite task that are considered in isolation from other elements, e.g. applying only the guidelines for an initial push to start a cart moving or only to the recommended weight that should be lifted, even when the task includes both. This is problematic, as ergonomic practitioners know that most MMH tasks are not single element tasks, but are composites; combinations of lifting, lowering, carrying, pushing, pulling, stopping, and turning.

Considering multiple task elements in estimating MMH risk is not a new concept. The NIOSH lifting equation uses a composite lifting index to evaluate the risk of tasks that have different lifting parameters, for example, a task that involves combinations of different horizontal distances and vertical heights. Loading or unloading objects onto or off of a pallet is an example of a case where such composite lifting conditions might be encountered within a single task.

Current practice in determining the risk of injury when pushing or pulling carts is to assess the initial force required to start the cart moving and the sustained force required to keep the cart moving once started. The acceptable initial or sustained force is determined based on the frequency of the exertion, the height of the hands and the distance that the cart is pushed or pulled.

As an example, the Liberty Mutual MMH tables indicate that 77 percent of females are capable of exerting an initial 44-pound push force at a height of 35 inches once every minute. If we evaluate the job based only on the initial force required to start the cart moving, then, based on Snook’s safety criterion of 75 percent of females capable, we would expect the job to be within the recommended limits and at lower risk of injury. However, what if the definition of initial force is too narrow?

We know that carts are not always pushed in a straight line, but are commonly turned and maneuvered around obstacles or to position them in a specific location. Let’s take a closer look at the forces employed: we know how to assess the starting force and the sustained force, but how do we assess the turning force?

The International Organization for Standardization’s (ISO) standard ISO 11228-2:2007 Ergonomics – Manual handling – Part 2: Pushing and pulling offers this definition: “Initial forces are used to overcome the object’s inertia, when starting or changing the direction of movement.” Clearly the forces exerted on a cart to maneuver it around a corner or to position it are forces exerted to overcome the object’s inertia or to change the direction of the movement and would be classified by the ISO standard as “initial forces”.

Why is this important? Research indicates that these turning forces may equal or even exceed the initial starting force. Consider the effect in the example above if the cart is turned only once; now there are potentially two initial force exertions of equal magnitude during the same time interval rather than one, that is, a 44-pound force exerted every 30 seconds instead of once per minute. The percent capable drops from 77 percent of females to 67 percent and the task would be at greater risk than originally thought.

Finally, stopping a cart also involves overcoming the object’s inertia. Consequently, there are four separate forces involved in handling a cart: starting, sustaining, turning, and stopping, but generally only the starting and sustaining forces are considered when evaluating risk, and the stresses created by additional exertions are not. As the thought experiment above shows, this can lead to overestimation of safe levels of exertion, and we haven’t even considered that turning carts involves a combination of single-handed push and pull forces, or the additional stresses if the same individual who moves the cart loads and unloads it manually.

In conclusion, MMH tasks such as manually moving carts are composites of multiple elements: starting movement, sustaining movement, turning, and stopping. Each element applies stress to the individual who exerts forces to perform the task. Risk analysis of MMH such as cart handling must consider the stress created by all the elements of a task, not just starting and sustaining movement. When all MMH elements of a task are considered, the safe forces are likely lower than current assessment techniques suggest.

To learn more about the benefits of industrial ergonomics programs that reduce risks when pushing, pulling and maneuvering carts, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

New Methods for Assessing Risks Needed in Industrial Ergonomics

Most Manual Materials Handling are Compound Tasks – Existing Assessments Underestimating Risks

Problem, Danger, Risk and Liability words on a speedometer 3d rendering

No matter where you work – aerospace, health care, high tech, or other industries – manual materials handling (MMH) is an everyday part of the job. MMH plays a causal role in the development of low back pain and injuries; low back disorders continue to be among the most common and most costly musculoskeletal disorders seen in the workplace. It is estimated that more than 80 percent of all adults will develop back pain at some point in their lives and lost time back cases account for about one-third of all workers’ compensation costs.

Identifying musculoskeletal injury risk factors associated with MMH is an important component of any industrial ergonomics program that targets the reduction of workplace injuries. MMH tasks are often compound tasks or aggregates of several different tasks. For example, an MMH task might consist of loading boxes onto a cart, pushing the cart to a first location, maneuvering the cart around corners, unloading a few boxes, pushing the cart to a second location, and unloading or loading boxes there. Compound MMH patterns of this type are routinely seen in industrial workplaces.

Currently, separate tools are used to individually evaluate each simple component of MMH tasks, such as each instance of lifting, carrying, pushing, and pulling. Applying the tools to a compound task yields several separate risk assessments, e.g. one for the lifting component and another for the pushing component. How should the combination of these individual analyses be evaluated to estimate the risk of injury for the compound task which includes multiple tasks, e.g. both lifting and pushing?

Assumptions during MMH Risk Analysis may result in Overlooking Possible Risks

Suppose that we determine that 75 percent of females are able to exert the push force necessary to move a cart and that 75 percent of females are able to exert the force necessary to lift materials to place them onto the cart. Are we safe in assuming that 75 percent of women are able to perform both the lifting and pushing actions?

Although it seems counterintuitive, I would argue that it likely isn’t a safe assumption, because it all depends on the correlation between lifting strength and pushing strength. The same 75 percent of females would be able to perform both tasks only if lifting and pushing strengths have a perfect correlation value of 1.0. While the different types of strength used to push and lift are correlated, they aren’t perfectly correlated.

A study of US Army recruits noted that female recruits’ lifting and pushing strengths were only weakly correlated, with a correlation value of 0.15. Without going into detail, a mathematical calculation tells us that, rather than 75 percent of females being capable of performing both the required pushing and lifting exertions, only about 59 percent of females are capable of doing both. Relying on separate analyses of the lifting and the pushing exertions can lead to a serious overestimation of the acceptability of the compound task.

One solution to protect 75 percent of females would be to design the task so that about 85 percent of females are capable of exerting the pushing force and 85 percent are capable of exerting the lifting force. Then the math tells us that, when the lift and push are combined in the compound task, about 75 percent of females will be capable of exerting both the required lifting and pushing forces, even though about 85 percent of females are capable of doing one or the other.

Complexity of Compound MMH Task Means Taking a Closer Look at Risk Analysis to Reduce Workplace Injury Risks

Back injuries related to MMH are common and costly. Evaluating the safety of MMH tasks is critical to reducing workplace injuries. However, workplace MMH tasks are often complex with different amounts of pushing, pulling, lifting and carrying. Relying on separate risk analyses of each lift, push, pull and carry task to evaluate the safety of an MMH task may lead to serious over-estimation of acceptable force levels for compound MMH tasks. Research into the development of improved methods of assessing the risk associated with compound MMH tasks is urgently needed.

To learn about reducing workplace injuries and supporting cost justification for crucial ergonomic solutions, download the Economics of Ergonomics Guide now.

Industrial Ergonomics: Best Practices for Cart Management

A recently published paper on reducing back injury risks, “Biomechanically-Determined Guidelines for Occupational Pushing and Pulling”, suggests that “the manual materials handling burden has shifted towards pushing and pulling”; presumably, away from lifting and carrying. Manual material handling using carts is a prime example of an occupational push and pull task. Managing the risk associated with pushing, pulling and maneuvering carts in the workplace is a critical component of an industrial ergonomics program.

As most facilities or organizations have fleets of carts, a systematic approach is useful in managing and reducing the risks associated with cart handling. While such a system will vary to suit the idiosyncrasies of different operations, there are some common components to these ergonomics cart management programs.

Cart Ergonomics Best Practices

Best Practice #1 – Measure Cart Operating Forces Throughout the Life of the Cart

The most basic component of a cart management program is the ability to accurately and consistently measure the forces required to move carts during typical usage. It is crucial to then compare those forces to reference standards, such as the Liberty Mutual Tables, to evaluate the acceptability of the required forces. It is important to note that this force measurement should be repeated at intervals for carts in service to ensure that the required operating forces remain at acceptable levels. Experience with these periodic measurements will also help to guide decisions as to whether the maintenance interval may be lengthened or shortened. It can also identify whether a particular type of cart requires more or less frequent reassessment.

Best Practice #2 – Design for Cart Performance at Acquisition

Specifying cart performance at the time of acquisition is a second major component of a good program for managing cart handling ergonomics. Why bring problems in with new carts when acceptable force levels can be made part of the purchase and cart design specification? This includes assessing specific cart design and components of the cart for their ergonomic benefits or detriments. In particular, the cart’s caster wheels can make a significant impact on push/pull forces. So, upfront cart design, engineering improvements and investment in the right casters for the environment and usage can lower push/pull forces and achieve reduced maintenance requirements.

An important part of specifying new carts’ performance is to be aware of the environment in which they are used. For example, are they likely to be used on unpaved surfaces or paved surfaces with cracks or litter? This information is critical to selecting the appropriate casters.

To find out more about how casters can affect your carts in manual material handling tasks, you may want to read some recent articles from Darcor:

Best Practice #3 – Track and Monitor Each Cart in Service

A third component is the ability to track each cart in service, beginning with the characterization of push, pull and maneuvering forces when it is first put into service and subsequent monitoring to ensure that its operating force requirements remain within acceptable limits. Carts whose performance degrades over time require servicing, for example cleaning debris from casters, replacing worn or damaged casters, etc. Although most good quality casters have sealed bearings, others may not and may require lubrication as well.

Effective Industrial Ergonomics Cart Management Program to Reduce Risk of Injury

By implementing industrial ergonomics best practices for cart management, organizations can benefit by lowering risk of back injury and often will experience increased operational efficiency. To learn more about the benefits of industrial ergonomics programs that reduce risks when pushing, pulling and maneuvering carts, download the Guide to Workplace Ergonomics.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

Industrial Ergonomics Regulations around the World

Understanding Manual Material Handling and Cart Ergonomics Standards Everywhere is the Key to Minimizing Risk of Injury

Map of the world connected by thumbtacks. Logistics concept

Injuries related to manual materials handling (MMH) tasks are a major concern in industrial ergonomics. The Canadian Center for Occupational Health and Safety (CCOHS) reports that “three of every four individuals whose jobs include MMH will suffer pain due to back injury at some time. Those injuries account for at about one-third of all lost work and more than one third of all compensations costs”. In addition, there are productivity losses associated with back pain; one study published by the US National Library of Medicine National Institutes of Health found that individuals working with back pain were fully productive on only about 90 percent of all workdays.

Not surprisingly, a good deal of attention has been given to the design of manual materials handling tasks to minimize the risk of injury or pain. That work forms the basis of many regulations, guidelines, and technical standards around the world. While much of that work covers lifting and carrying loads, pushing and pulling loads has also received close attention.

These push and pull guidelines may be quick, “rule of thumb” recommendations, such as those given by the Canadian Centre for Occupational Health and Safety (CCOHS) and the United States Occupational Safety and Health Administration (OSHA); essentially that the maximum cart pushing force should be about 225 newtons (50 pounds-force). However, these guidelines should be used carefully, lest they inadvertently lead to the design of high-risk tasks as a result of inaccurate measurement of forces and/or the variables that affect the specific MMH task.

As CCOHS notes, the recommended maximum force varies depending on several variables such as:

  • cart design,
  • distance the load is moved,
  • frequency with which the load is moved,
  • whether the force is starting or maintaining movement,
  • gender of the person doing the work, etc.

What is a Safe Level of Push/Pull Force?

A colleague once pointed out that there is no general answer to what is the safe force level for pushing or pulling. Whenever he is asked that question, his response is “You have to do an analysis”.

International MMH Standards Examined

There are many technical guidelines that describe how one goes about doing the measurement and analysis of push and pull forces. Of these, the Liberty Mutual Manual Materials Handling Tables (LMMH) are perhaps the most commonly used guideline for determining the acceptability of push or pull forces. The criterion for an acceptable force used in the LMMH is that at least 75 percent of females can exert the required force under the given conditions. In Europe, the European Committee for Standardization or Comité Européen de Normalisation (CEN) standard EN 1005-3 sets limits for operating machinery, recommending that at least 85 percent of the adult population should be able to exert the required force in workplace settings and 99 percent of the adult population capable of exerting the required force in non-work settings.

Some of these standards have unique features well worth additional attention. For example, the International Organization for Standardization ISO 11228-2, which focuses specifically on push and pull forces, offers a very interesting definition of initial forces: “Initial forces are used to overcome the object’s inertia, when starting or changing the direction of movement”.

Careful reading of that definition clearly suggests that the forces exerted to turn a cart (change direction) while it is in motion are considered as initial forces. There is research which studies exertion patterns in nursing home environments that supports this definition and has shown that the forces used to turn a cart may be equivalent to those used to start it in motion.

Increased Push/Pull Task Frequency Means Much Higher Force Required

The result is that the frequency of exertion of initial forces may be greater than first thought. Using this definition, and assuming that turning forces are approximately equal to starting forces, turning a cart once after starting it in motion would double the frequency with which the initial force is exerted, likely reducing the maximum acceptable force recommended.

Ongoing Cart Maintenance and Consistent Ergonomic Testing Crucial to Maintain Acceptable Force Requirements

The United Kingdom’s Health and Safety Executive’s Risk Assessment of Pushing and Pulling (RAPP) tool calls out the importance of cart maintenance. An initial assessment of a cart may indicate that the required initial and sustained force requirements are within acceptable limits; however, wear and tear to the cart and its casters may result in increased force requirements. Assurance that cart push and pull forces are maintained within safe limits is a critical part of managing an ergonomics program for manual cart handling.

Considering International Industrial Ergonomic Standards for Push/Pull Tasks Ensures Best Chance of Superior MMH Ergonomics Program

In summary, although specific force recommendations may vary somewhat, there is general agreement among technical guidelines and standards regarding the approach to determining acceptable push and pull force levels. Each approach considers similar variables such as frequency of exertion, distance pushed or pulled, etc. All recommend recognizing strength differences between males and females and designing tasks so that a strong majority of women are capable of safely exerting the required forces to push or pull carts. Accommodating both women and men in this way is protective against injury for both sexes, as has been shown by Snook Tables and research.

Whatever tool is used, the design of carts and manual cart handling tasks can reduce the risk of back injuries and positively affect productivity.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.

How Reliable is Your Measurement of Forces to Maneuver Carts?

Understanding Measurement of Forces to Move Material Handling Carts Safely – Comparing Apples to Apples

measurement of forces to maneuver cartsEffective management of manual cart handling ergonomic programs relies on measuring the forces required to move carts manually. Forces that exceed acceptable levels are more likely to result in injury and may also adversely affect productivity. An accurate, reliable measurement of the force required to manually move carts is essential for the effective use of the Liberty Mutual Tables or any other tools that evaluate the acceptable level of the forces required to move carts.

But, how reliable is your measurement of the forces exerted to maneuver carts in your facilities? A recent Institute of Caster and Wheel Manufacturers (ICWM) webinar underlined the importance of accurate measurement of the forces manually exerted when moving carts, but also noted that there isn’t a consensus at present as to how those forces should be measured in the field.

Why is Accurate Measurement of Push/Pull Forces Important?

Why is this important? Well, without a reliable methodology of measuring the required force, you never know if you’re comparing apples to apples or apples to oranges when you assess the acceptability of the force.

For example, suppose that we measure the force used to start a 1,000 kg (2,200 pound) cart moving from standstill until it reaches a velocity of 0.33 meters per second (1.1 foot per second) when it crosses a line exactly one meter away. The peak initial force measured could be 330 Newtons or it could be 17 Newtons (74 or 4 pounds-force respectively), depending on the time allowed to move the cart from the starting point to the finish line.

Although it’s the same cart and the same distance, the 330 Newton force would almost always be judged to be unacceptable; the 17 Newton force would almost always be acceptable. The acceptability of the force depends on the rate of change of the velocity over time (the acceleration) with which the cart is moved. A standard force measurement protocol that specifies how the forces are to be measured is required for apple to apple comparisons.

The International Standard for manual push pull forces, ISO 11228-2 discusses a protocol for measuring sustained and initial forces in an informative annex (Annex D). However, it’s not completely clear what was intended regarding initial force measurements.

Steps to Accurate Force Measurement

So, what is the best way to interpret that force measurement protocol?

First, follow the ISO standard’s recommendation for measurement of the sustained force. That measurement should be done while the speed of the cart is constant and is greater than or equal to 0.33 meters per second. The cart should move a distance of at least one meter at a constant velocity while the sustained force is measured. So, the sustained force should be measured as the cart travels at a velocity of 0.33 meters per second through a distance of one meter.

Measure the initial force by starting the cart moving from standstill to a velocity of 0.33 meters per second in a time period of 3 seconds. Record the peak force observed. An acceleration of 0.11 meters per second squared is sufficient to accomplish this and that rate of acceleration should not be exceeded.

Finally, take several sets of measurements for both the sustained and initial forces. The ISO standard recommends using the average of at least three consistent measurements. A consistent measurement is defined one which is within 15 percent of the others.

Correct Measurement of Force is Critical for an Ergonomically-Safe Cart Handling Program

Accurate, reproducible force measurements are the foundation on which an effective ergonomic cart-handling program is built. When paired with a method of evaluating the acceptability of the required forces, such as the Liberty Mutual tables, the practitioner has an effective tool to design, implement and manage a cart handling program that efficiently reduces the risk of injury.

Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.

Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.