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