Throughout history, the invention of the wheel has been helping humankind move objects from place to place, whether it’s moving a block of limestone across a desert or powering a car through a race track. Regardless of the application, wheels were invented by man and have always been utilized to easily get something moving.
Of course, during the natural evolution of the wheel, it has become an increasingly improved tool, which includes the gradual evolution of the caster. Because of this evolution, casters have reached levels of becoming ergonomically effective products. How is ergonomics used to measure the effectiveness of a caster? It’s using push/pull and swiveling forces and Liberty Mutual’s Snook tables 1. Push/pull forces are the rolling resistance when a caster begins moving and continues to move.
Swiveling forces are typically higher than the push/pull starting and rolling force and occur when casters are placed 90 degrees from the direction of travel. There’s a force required to “swivel out” these casters to the direction the user is moving to.
The Liberty Mutual Snook Table 1 takes into account the specified probability for a male and female to move an object with a given push/pull or swiveling force, with a handle height and frequency of pushing. With that information, an ergonomist can determine quantitatively the probability of someone being able to move a cart.
Two examples below shows a possible scenario, where company ABC’s ergonomist has a health and safety mandate to reduce push/pull forces by an amount so that 80% or higher of the female population is able to move the 2,000 lbs cart across a concrete plant floor every 5 minutes.
Example 1
The Requirement: 80% of the female population must be able to initially push a load capacity of 2,000 lbs
every 5 minutes.
A push/pull test is conducted, speeding up 0.2 in/s2:
Urethane Wheel Tested Push/Pull Results:
Using the Liberty Mutual Snook Tables for pushing tasks1:
You can see that only 54 percent of the female population can successfully push the 2,000 lbs load capacity with a 5 minute frequency. Well out of range of the 80% requirement.
If we look at the swiveling force of 92.2 lbs, the population is below 10 percent that can move this caster with this wheel as seen below (the ‘-‘ symbol in the Snook table denotes under 10 percent of the female population can push this load capacity):
So, we can see that this particular caster wheel doesn’t meet this company’s workplace ergonomic
requirements. Example 2 will show another example using Darcor’s SE™ wheels for the same
requirements.
Example 2
The Requirement: 80% of the female population must be able to initially push a load capacity of 2,000 lbs
every 5 minutes.
A push/pull test conducted, speeding up 0.2 in/s2:
Solid Elastomer Wheel Tested Push/Pull Results:
The Liberty Mutual Snook Tables for pushing tasks1:
This example shows that the initial force for Darcor’s SE™ wheel exceeds the requirements for this company, with more than 90 percent of the female population able to move this cart with a 2,000 lbs load capacity (the ‘+’ in the Snook table depicts more than 90% of the population can push this load capacity).
Again, if we consider the swiveling forces, we’re looking at a resistance force of 47.6 lbs. On the Snook table, 49lbs initial force is the closest reference:
We see that we’re just above the 80 percentile requirement and considerably better than the Urethane wheel.
In fact, for those companies who will be going fork lift free in the next couple of years, this is a great tool to assess the ability of its workforce that will be able to push the same load capacities manually.
The examples above show the capacity in which workplace ergonomics can depend on the quality of caster used. Each company that relies on casters can always have a better understanding of how efficient and safe it is for a worker to move a cart.
1 Liberty Mutual Snook Tables,
https://libertymmhtables.libertymutual.com/CM_LMTablesWeb/taskSelection.do?action=
initTaskSelection: 2014 Liberty Mutual Insurance Company.
