C. Operating Environment

If a pushing/pulling job is to be per formed manually, your primar y goal is to minimize the forces required by the operator to initiate and sustain rolling, turning, and positioning. Five main topics must be considered in order to design a safe and productive push/pull task:

The Rougher or More Uneven the Rolling Surface, the Larger the Wheel Diameter Should Be.

Even in facilities with ver y smooth floors, the operator often crosses cracks, seams, expansion joints, grates, door thresholds, or other surface irregularities that can cause a small diameter wheel to stop. A larger diameter wheel will roll over such irregularities with relative ease.

The More Potential for Floor Debris, the Larger the Wheel Diameter Should Be

Debris on the rolling surface is much the same as a rough or uneven surface.

Special Conditions: Oil, Grease, Chemicals, Etc.

Floor contaminants can reduce the traction between the shoes and floor, making it difficult and dangerous for the person to apply the necessar y push/pull forces, and may also inter fere with caster maintenance and function. Consult a qualified caster supplier to match wheels and casters to your conditions.

Special Environments: Special Floor Coatings, Dust, High Moisture or Wash Down, Extreme Temperatures, Etc.

In some industries, carts and equipment must be washed regularly, and the casters must therefore be able to withstand this without detriment to their per formance. Consult a qualified caster supplier to match wheels and casters to your conditions.

The Path of Travel Should Be Free of Obstacles, and the Operator Should have Clear Visibility in the Direction of Travel

Implement Effective Floor Inspection and Maintenance Procedures

Floor maintenance and housekeeping can have a dramatic effect on the forces experienced by the operator, the stability of the load, the life of the equipment, etc.

D. Cart or Equipment Design

Optimally, each person should be able to select their own point of contact, either through an adjustable handle system, or a continuous handle system that may be grasped at the height of choice. The following general rules apply:

For Pushing, Handhold Height Should Be Between Elbow and Hip Height

Since elbow and hip heights var y from person to person, there is no single recommended handhold height for pushing. If an adjustable height horizontal handle or continuous vertical handles are supplied, a range of approximately 29 in. to 47 in. will accommodate about 90% of the American working population.

For Pulling, Handhold Height Should Be Between Hip Height and Knee Height, and the Handhold May Need to Be Offset From the Equipment to Ensure Adequate Foot Clearance

Since hip and knee heights var y from person to person, there is no single recommended handhold height for pulling. If an adjustable height horizontal handle or continuous ver tical handles are supplied, a range of approximately 18 in. to 39 in. will accommodate about 90% of the American working population.

The Loaded Car t or Equipment Should Be Stable

An unstable load can fall and injure people, and damage equipment and product. Load Instability can also increase the amount of required force, as the operator attempts to control the load. Fur ther, if the load begins to fall, the person may attempt to catch it, resulting in sudden exposure to high forces, a common cause of injury.

Handholds Should Not Extend Beyond the Sides of Equipment

Extending body parts beyond the side of the equipment exposes them to crushing injuries.

A Handle is Required for Effective Pulling, But Not Always for Pushing

For pulling, the best grip is a power grip (using the palm, fingers and thumb). The fingers should not overlap, and the handle should be wide enough to accommodate the entire hand. For a cylindrical handle, this equates to about a 1.5 in. to 2.0 in. diameter (3.8 cm. to 5.1 cm.), and at least 5 inches in length to accommodate the width of the hand. Pushing can be performed with such a handle, or the person can apply force to a flat sur face, as long as the coupling is good and the hands do not slip or contact edges, sharp protrusions or other pressure points. For most applications, a designated handhold is advisable.

E. Caster and Wheel Selection

Selecting the right caster and wheel design can be the most critical par t of your manual push/pull task design, because reducing the rolling and turning forces reduces the forces that the person must apply. There are numerous casters on the market, and a competent supplier can and should assist you in selecting the right design for your specific application. Impor tant goals include:

Understand the Specific Task, Operating Conditions, Environment, and the People that will be Per forming the Work Before Beginning Caster Selection

Do your homework before talking to vendors.

Match Wheel Material and Diameter with Floor Sur face Conditions

This may involve a trade-off between wheel material characteristics and wheel diameter.

Match Weight of Loaded Equipment with Load Ratings for Specific Casters

A general rule is that each caster should be able to withstand at least 1/3 of the total load weight by itself.

Locate Swivel Casters Under the Handholds

Often a cart will have two swivel casters and two rigid casters. For such designs, the swivel casters should be located on the same side of the cart as the handholds.

Brakes May Be Needed if Heavy Loads Will Be Moved On Sloped Surfaces

Test Potential Wheels and Casters Under Actual Operating Conditions

Remember, your goal is to match the horizontal force requirements with the force levels you determined using the data in the Appendix. For best results, test in actual operating conditions using a push-pull force gauge to measure initial (star ting), sustained (rolling), and turning forces.

A Mix of Manual and Assisted Pushing and Pulling May Be Needed in Some Situations

Sometimes one or more people can per form parts of an equipment movement task, but other parts of the same task may require powered assistance due to elevated force requirements (e.g., going up or down a slope).

V. Conclusion

This paper focuses on some of the ergonomics issues involved with manual pushing or pulling activities. Ergonomics is an applied science that is used to improve human performance. Companies can expect to improve bottom-line measures in productivity, quality, health and safety, and other product and process areas by applying ergonomics principles. By studying a task or job in detail, and carefully matching equipment and people with those demands, surprisingly heavy loads and equipment can be manually moved, safely and efficiently. In some cases, depending on required task factors such as repetition, distance traveled, force requirements, and handhold locations, a combination of manual and assisted material handling can be used. This is effective where mechanical devices per form “brute force” tasks that may expose people to injury.

Understanding the task requirements, operating environment and conditions, and the people that will per form the work when selecting carts, casters and wheels will pay off.

VI. For Further Information

Andres, R.O., Chaffin, D.B., and Garg, A., (1983), “Volitional Postures During Maximal
Push/Pull Exertions in the Sagittal Plane,” Human Factors, Vol. 25, pp. 541-550.

Ayoub, M.M., and McDaniel, J.W., (1974), “Effect of Operator Stance and Pushing and Pulling
Tasks,” Transactions of American Institute of Industrial Engineers, Vol. 6, pp. 185-195.

Bacchus, C., Kumar, S., Narayan, Y., and (1995), “Symmetric and Asymmetric Two-Handed
Pull-Push Strength of Young Adults,” Human Factors, 37(4), pp. 854-865.

Budnick, P., (2001), “Applied Materials Leverages Ergonomics to Improve Business Bottom-
Line,” published on http://www.ergoweb.com.

Chaffin, D.B., Herrin, G.D., Lee, K.S., and Waiker, A.M., (1991) “Effect of handle Height on
Lower Back Loading in Cart Pushing and Pulling,” Applied Ergonomics, 22(2), pp. 117-123.

Chaffin, D.B., Resnick, M.L., and (1995), “An Ergonomic Evaluation of Handle Height and Load
on Maximal and Submaximal Cart Pushing,” Applied Ergonomics, 26(3), pp. 173-178.

Ciriello, Vincent M., and Snook, Stover H., (1991), “The design of manual handling tasks:
revised tables of maximum acceptable weights and forces.” Ergonomics, 34(9), 1197-1213.

Darcor Ltd., (2000), “Caster and Wheel Comparison Guide.”

Ergoweb Inc., (1995 – 2001), “Liberty Mutual Acceptable Pushing/Pulling Force Tool,” published
in the Ergoweb Job Evaluator Toolbox™ software system, on http://www.ergoweb.com.

Ergoweb Inc., (2001), “Applied Workplace Ergonomics Training Manual,” available at
http://www.ergoweb.com.

Grieve, D.W., Pheasant, S.T., Rubin, T., and Thompson, S.J., (1982), “Vector Representations
of Human Strength in Whole Body Exertion,” Applied Ergonomics, 13(2), pp. 139-144.

Imrhan, Sheik N., (1999), “Push-Pull Force Limits,” The Occupational Ergonomics Handbook,
Edited by Karwowski, W. and Marras, W.S., CRC Press, pp. 407-420.

Kroemer, K.H.E., and Robinson, D.E., (1971), “Horizontal Static Forces Exer ted by Men
Standing in Common Working Postures on Sur faces of Various Tractions,” Aerospace AMRL-TR,
WPAFB, Ohio, pp. 70-114.

Mital, A., Nicholson, A.S., and Ayoub, M.M., (1993), “A Guide to Manual Materials Handling,”
Taylor & Francis, Washington, D.C.

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