Measuring Push/Pull Forces on Carts
Volume 1, Number 19
Low back pain is often associated with force exerted in maneuvering carts and hand trucks. The force intensity, distance traveled, handle height, and frequency and velocity of exertions are considered risk factors for musculoskeletal problems in these manual handling tasks. Psychophysical analysis, a method which measures the load an individual perceives he can handle to define human capabilities and limitations, is a proven method of quantifying manual handling task specifications. A 1991 study by Snook and Ciriello led to the development of guidelines for designing and evaluating lifting, lowering, pushing, pulling, and carrying tasks. The purpose of these guidelines is to encourage the control of industrial low back pain by reducing the number of instances, the duration of injuries, and the duplication of injuries.
Some elements of these very general guidelines suggest:
- Keep the starting forces to 50 pounds of push force or less. Force is measured with an instrument called a force gauge. With the wheels aligned with the direction of travel, the force gauge is positioned on either the cart or handle level to the operator and pushed forward. The force gauge measures both the initial push force and the force needed to keep the cart moving, which is called sustained or rolling force.
- The rolling force should be less than 40 pounds. If the force has to be sustained for a minute, or if the cart has to be pushed for more than 10 feet, this should drop to 25 pounds or less.
- If the exerted force is sustained without a break for four minutes, the acceptable force drops to about 7.5 pounds.
- Emergency stopping forces of more than 80 pounds should not be needed to bring the cart or truck to a stop within 3 feet.
The above guidelines are based on the assumption that the cart design permits the handler to exert the force around waist height or a little higher and the coefficient of friction of the handler’s shoes with the floor is about 1.0 (or non-slippery).
Some suggestions when required forces exceed the recommended guidelines:
- Floor characteristics. Uneven or sloped surfaces and floors with noticeable seams or cracks result in greater and sometimes unexpected exertions. Where possible, these surfaces should be repaired.
- Cart characteristics. The height of the handles, the distance between handles, and the size and quality of the gripping surface may all influence the amount of strength the handler has in controlling cart movement. "The preferred handle height for straight horizontal pushing is at about elbow height, or about 35 inches above the floor," (Hoozemans et al. 1998). A retrofit using external handles is often possible, as long as the structural integrity of the cart is not compromised. Ideally longer vertically-mounted handles (or bars) should be installed, probably at least 12 to 24 inches in length. This would provide a wide range of users with an ideal elbow-height position from which to manipulate each cart. "The preferred height for carts should be less than 50 inches so that the shorter handler can see over them when pushing them around the plant," (S. Rodgers, Kodak’s Design for People at Work, 2004).
- Wheels. The size, type, and design of the casters or wheels can profoundly affect the ease of handling carts and hand trucks. Larger wheels mean less force is needed to push or pull the cart. Having a proactive wheel maintenance program—wheel cleaning, lubrication, and repair on a scheduled basis—can go a long way in reducing the risk of excessive push/pull forces.
- Footwear. The handler’s footwear is another possible risk factor. A coefficient of friction of about 1.0 is desirable between the handler’s shoes or boots and the floor. If the footwear has a smooth bottom surface and slips on the floor, more effort will needed to move the cart.
- Loads. A general guideline for the size of a load on carts suggests that loads greater than 500 pounds should not be transported on a handcart.
- Suzanne Rodgers, et al., “Kodak’s Ergonomic Design for People at Work”, John Wiley & Sons, Inc., 2004
- Humantech, Inc., “Applied Ergonomics Manual”, Humantech, Inc., 1995
- Chaffin, D.B. and Andersson, G. “Occupational Biomechanics”, Second Edition, John Wiley & Sons, Inc., 1991
- Ergoweb, Inc., “Applied Workplace Ergonomics Training Course”, Ergoweb, Inc., 2001
The information herein is for reference only and State Fund does not warranty its accuracy or fitness for a particular purpose. Any products, references, or links to Web sites are not an endorsement by State Fund or its employees, but serve only as examples to assist you with your workplace design changes. State Fund cannot be held liable or accountable for content on linked Web sites.