Maintaining Carts for Optimal Push/Pull Forces

Use of Safety Tables and Maintenance of Carts and Casters to Reduce Injury

Have you heard of the wonderful one-hoss shay,
That was built in such a logical way
It ran a hundred years to a day,
And then, of a sudden, it — ah but stay,
I’ll tell you what happened without delay – Oliver Wendell Holmes

Pushing and pulling heavily-loaded carts by hand is an everyday occurrence in the workplace. There are many parts to a discussion of the best way to systematically manage cart handling, but let’s start first with a discussion of the how the injury risk associated with pushing and pulling carts is assessed. While one might think that the risks are limited to back injuries, there are good reasons to be concerned about shoulder injuries as well. And of course, there are those incidents where a cart ran into or over an unseen individual.

In North America, limiting the risk of injury while handling carts is commonly done by keeping the operating forces at or below a reference force limit. For example, starting a cart moving shouldn’t require a force more than about 220 newtons (or 50 pounds of force).

However, that recommended force changes depending on various conditions:

  • the gender of the person doing the work,
  • the distance that they move the load,
  • whether it’s the force necessary to start the cart moving or just to keep it in motion,
  • how often the loaded cart is moved, etc.

A handy reference tool that takes these various conditions into account when recommending a push or pull force limit can be found in the Snook tables, published by the Liberty Mutual Research Institute for Safety. These tables describe the amount of force workers judged to be the maximum acceptable push or pull forces under a given set of conditions.

Some different tools are used in the European Union to determine acceptable force limits for pushing and pulling loads. One is an international technical standard (International Standards Organization 11228-2: 2007) that gives two different means of calculating acceptable forces for pushing and pulling based on conditions like those used in the Snook tables.

However, of all these tools, only the British Health and Safety Executive’s Risk Assessment of Pushing and Pulling (RAPP) tool calls out the importance of maintaining carts in order to maintain the operating forces at or below the recommended limits.

Unlike the wonderful “one hoss shay” that lasted 100 years to the day, carts wear out. The frames get bent and don’t track quite true. Wheels get cut or go out of round. Bearings wear out. All these change the force required to operate the cart. What was once low risk may no longer be safe. It is not possible to minimize the risk of injury if carts are not well maintained.

A good, systematic approach to managing the safety of pushing and pulling carts must ensure that the operating force is within the recommended limits to begin with and then remains there throughout the cart’s working life. Consequently, a manual cart safety program must do more than an initial test of cart operating forces or documenting a purchasing specification for cart push pull forces. Routinely testing carts to ensure that the operating forces are maintained within safe limits is an essential part of managing cart safety. Not to do so means that an operators’ risk exposure is unknown and uncontrolled.

 

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.


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