Side Pushing': When You Can't Push It or Pull It
An alternative movement works better for maneuvering large material handling carts, this study shows.
MATERIAL handling carts (also known as utility, hand, or service carts) are used in most manufacturing environments to physically transport goods or parts to other areas of the operation for manipulation, storage, or shipment. Many times, safety and ergonomics professionals will advocate pushing a cart from the back when it has to be moved.
But what if the carts in your operations are too long or too high to safely push them from the back? Or what if your operators regularly pull the cart and must awkwardly hyperextend their arms backward to reach it? Is there a better option to safely move these larger carts?
This article summarizes both a recent study and resultant recommendations on alternate methods of safely moving "large" material handing carts. For the purposes of this article, "large" carts are defined as being more than 55 inches in either length or height. This study by the author was conducted to determine whether these carts exceeded maximum push and pull limits and to recommend safer methods of moving large carts from one area of a facility to another. The recommendations, including improvements in cart design, cart modifications, and best-option methods of large cart movement, are offered here in an effort to help other organizations with large material handling carts proactively address similar risks.
Material Handling Cart and Movement Characteristics
Material handling carts are available in numerous designs, sizes, and configurations. Typical carts can vary in structure from steel or aluminum to wire frames or plastic, and can weigh anywhere from under 100 to more than 400 pounds.
Given the number of models available and the varieties of conditions of use, even the most casual observer would expect ease of operation and risk of injury would vary greatly. Some of the factors that affect operation and injury risk are cart design, intended use, floor and wheel characteristics, cart weight, and operator anthropometric data. Common practice in moving large carts is to load the cart and push it from the rear, or to pull it from the front.
For the purposes of this study, carts were selected that exceeded Rodgers' recommended height of 55 inches (Rodgers, 379). The length and height of the carts (60 inches) were purposely chosen because when pushed from the back, they would restrict operators' forward vision, when pulled from the front, they would require awkward operator body positioning and would typically require greater forces to move when compared with smaller, lighter carts.
60 inches long, 36 inches wide, 60 inches tall
Steel, corners 2 inches x 2 inches hollow steel stock
200 pounds empty, 350 pounds full
Ball bearing, rear wheels in fixed position, front wheels swivel
Prior to this study, the facility's generally accepted method of moving these carts was to have the material handler stand in front of the cart (nearest the swivel wheels) and pull the cart forward with one arm extended behind the employee, gripping the side post, walking in front of it, and leading it toward the intended destination. The injury and illness risks included upper extremity, shoulder, and back strains and sprains, soft tissue injuries of the hand that gripped the square posts, and heel injuries due to inadvertent contact with the front wheels.
A Shimpo digital force gauge was used to measure the expected push and pull forces of a typically loaded (boxes of product) large cart. The measured forces necessary for either pushing or pulling the cart were similar, as expected. The data results of 13-17 pounds push/pull forces were then compared to Snook and Ciriello's "Maximum acceptable forces of push for females" and "Maximum acceptable forces of pull for females" (Snook and Ciriello, 1207). All measurement results indicated that the forces to both push and pull the cart were acceptable for 90 percent of the study female population. Based on these measurements, it was confirmed that no redesign of the cart/load was necessary at the time. It had not been confirmed, however, that operators would not experience an injury or illness if they continued pulling it from the front.
Recommendations were first provided on improving the cart as it was designed. The first suggestion was to develop a consistent preventive maintenance (PM) program to ensure that wheels on the carts have well-maintained bearings so the wheels will pivot and roll easily. (Ergonext).
Next, it was recommended that larger-diameter wheels be trailed (coupled with alternate wheel material trials) in a "continuous improvement" effort to reduce the push/pull forces the material handlers experience. It was noted that larger-diameter wheels would generally reduce the initial inertial forces (Rodgers, 378). Similarly, a harder wheel material and/or a narrow tread will typically reduce the inertial forces it takes to move the cart from a standstill versus a softer wheel material with wide tread.
From a human factors aspect, it was also recommended that the carts be outfitted with a welded-on, elongated, vertical, padded handle to each cart's front side. Operators would be able to grab the handle at a height appropriate for their size. Another benefit of incorporating a long handle is it would afford operators with an intuitive and appropriate place to grab. Previously, the cart had square corner posts that were difficult for operators with smaller hands to get a full grip on. Not only that, but regardless of the size of the operator's hands, the corners had edges that would be repeatedly pressed into the soft tissues of the hand.
The second set of recommendations focused on the material handlers and the preferred method of moving these carts. First, it should be noted that a widely recognized method of having personnel move a small to medium-sized cart is from the back, where operators can lock their arms and backs and drive forward with the legs. This method inherently reduces back strain because the back is not hyperextended, as it can be when pulling.
Regarding the wheels on the cart, it was noted there were two swivel wheels on only one end. Interestingly, the operators had already determined that the easier method of steering the cart was from the end with the swivel wheels, whether pulling or pushing (Rogers, 378). Having four swivel wheels instead of two would have caused the operator to continually monitor and correct the cart's direction both forward and sideways, requiring considerably more force to maintain.
Having four stationary wheels would not have allowed the operator to effectively steer the cart at all. With a large, fully-loaded cart, the vast majority of operators who attempted pushing from the back could not safely see what was in front of them--regardless of whether the swivel wheels were in the back or the front. The risk of bumping the cart into objects, or more importantly, people, was too great and it was determined that pushing the carts from behind was not a viable option.
It was also known that the currently accepted method of pulling the carts from the front, with an outstretched arm reaching back, was likely causing considerable stresses on the back, shoulders, and upper extremities, not to mention pressure on the soft tissues of the hands from the square posts.
Negating the above two previous options of moving the cart, it was determined that the optimal method of movement would be from the front and side, with the feet initially parallel to the front swivel wheels. The term "Side-Pushing" was coined. In this position, an operator would hold the handle (or front frame) with an arm bent and locked at a 90-degree angle, lock his back in an upright or ever-so-slight forward-tilted position, and drive the cart forward with his legs. It should be noted that in no way should the operator use her back as a force driver to start the cart moving.
Lastly, it was advocated that in addition to training all operators on the proper methods of body positioning and movement of the cart, posters be created that depict an image of an operator standing in the recommended position for moving a cart. This way, the posters would serve as a constant reminder of the preferred method of moving this particular cart, given the product moved and cart design constraints.
The results of these recommendations were an almost immediate reduction in incidents because the carts were not running into either people or property, and a sustainable reduction in personal injury and illness when moving large carts. Interviews with the material handlers indicated "Side-Pushing" allowed them to better see where they were going and allowed them to better steer the cart from the front. It was felt this method helped to minimize arm and shoulder pain and the likelihood of injury.
Recapping, here are suggestions that can be incorporated when pushing or pulling a large cart is not a preferred option because of its height, length, or other factors:
1. Consider alternate methods of moving products to eliminate manual material handling completely. There is no value added by moving product from one area to another.
2. Ensure a properly functioning preventive maintenance program on the carts.
3. Consider modifying the cart design for optimum operator usability.
4. Train all handlers and post reminder examples of the Side-Pushing method of moving large carts.
When you can't safely push it or pull it, Side-Pushing can be a viable method of manually transporting large industrial carts to effectively reduce risks for injuries and illnesses.
1.Cart images provided by:
* Don Buckman, Collins & Aikman, www.collinsaikman.com
* Ancare at www.ancare.com
* Department of Industrial Relations, Cal/OSHA Consultation Service
* National Cart, www.nationalcart.com
* Lyon Workspace, www.lyonworkspace.com
2. Department of Industrial Relations, Cal/OSHA Consultation Service, Education and Training Unit, Easy Ergonomics: A Practical Approach for Improving the Workplace, 1999.
3. Material Handling, Ergonext.com, www.ergonext.com, 2001.
4. Rodgers, S.H. Ergonomic Design for People at Work. New York: John Wiley & Sons, Inc., 1986.
5. Snook, S and Ciriello, V. The design of manual handling tasks: revised tables of maximum acceptable weights and forces. Ergonomics. 34 (9), pp. 1197-1213.
This article originally appeared in the January 2003 issue of Occupational Health & Safety.