The Ergonomics of Manual Material Handling

I. Introduction

Carts and mobile equipment are used in nearly every industry. Medication, supplies and patients are moved about a hospital on wheeled devices; process equipment is often on wheels to allow for greater flexibility in lean manufacturing facilities; office supplies and mail are delivered by cart, and most office chairs are fitted with casters. Nearly all manufacturing and distribution facilities rely on a variety of wheeled carts and equipment throughout their processes.

Wheeled equipment is often taken for granted and selecting the right designs, including wheels and casters, is often overlooked. Careful forethought in the design of pushing or pulling tasks, on the other hand, will result in measurable bottom-line improvements. Without this care, the resulting costs to your company may be significant. This White Paper provides an overview of the issues involved in manual pushing and pulling, including ergonomics; cart, wheel, and caster design; and important operating environment factors.

II. Ergonomics

scholar


Figure 1. A poor match between people, work, tool, and equipment design has financial costs in at least three areas: productivity, quality, and health and safety.

fig 1

Wojciech Jastrzebowski, a Polish scholar, first used the term ergonomics in 1857. He derived it from the Greek words ergon (work) and nomos (principle or law) to mean the Science of Work. Ergonomics has since evolved into an important bottom-line opportunity that affects all competitive businesses, and extends well beyond the workplace into our daily lives. In business terms: er·go·nom·ics \,ûrg-go-‘näm-iks\ – Ergonomics removes barriers to quality, productivity and safe human performance by fitting equipment, tools, tasks, and environments to people.

A. The Economics of Ergonomics

Health and safety issues are perhaps the most talked about costs and consequences related to ergonomics, yet ergonomics historically grew from the business realm of efficiency and quality improvements. Today, business and social forces have driven the science to encompass a large set of concerns, including productivity, quality, and health and safety (Figure 1). Each of these work factors has an associated cost, and, alone or together, they may carry a large hidden price tag for your company.

B. Ergonomics, Productivity, and Quality

Ergonomics has deep roots in the productivity improvements that characterized much of the technology advancements of the 1900s. Fredric Taylor achieved dramatic productivity improvements in the steel industry by studying the optimal relationships between specific tools and tasks and the people who used the tools to per form the tasks. He was able to maximize the amount of material handled in a day, reducing wasted effort and increasing employee job security and compensation in the process.

By studying micromotions in great detail, Frank and Lillian Gilbreth were able to assign reliable time estimates to each type of task (e.g., reach, grasp, move, release). Their work provided a framework in which to define and monitor productivity as it relates to human task motions.

Any ergonomics intervention must be viewed in light of its effect on productivity, and the best ergonomics solutions will often improve efficiency. Simply put, reducing unnecessary or awkward postures and forces almost necessarily cuts the time and effort it takes to complete a task.

Body motions, visibility, workload, and other important ergonomic parameters will also affect the quality of work and the quality of work product. When a task is matched with the ability of the people who per form it, they make fewer errors and produce less waste.

C. Ergonomics, Health, and Safety

Musculoskeletal Disorders (MSDs) are injuries and disorders of the muscles, nerves, tendons, ligaments, joints, cartilage, and spinal discs. Examples include rotator cuff tendonitis, herniated or ruptured lumbar discs, and carpal tunnel syndrome. MSDs can be directly and indirectly related to aspects of the work or the work environment known as risk factors. Non-work activities and environments that expose people to these risk factors also can cause or contribute to MSDs. When an MSD is associated with work it is usually referred to as a Work Related Musculoskeletal Disorder (WRMSD or WMSD). Other similar terms include cumulative trauma disorder (CTD), repetitive stress injury (RSI), and repetitive motion injury (RMI). MSD risk factors can be defined as actions in the workplace, workplace conditions, or a combination thereof that may cause or aggravate an MSD. Examples include forceful exertion, awkward postures, repetitive exertion, and exposure to environmental factors such as extreme heat, cold, humidity, or vibration. Often, a combination of these risk factors over time can lead to pain, injury, and disability. These risk factors can be reduced through informed purchasing and workplace design, retrofit engineering controls, administrative controls, work practice definitions, or in some cases, personal protective equipment.

The manner in which a risk factor leads to an injury/disorder is usually through the accumulation of exposure to risk factors. An event such as pushing or pulling a car t may stress soft tissues in the arms, shoulders, back, or legs, but the exposure may be too low for traumatic injury, and the tissues recover. Repeated exposure to this stress, on the other hand, may interfere with the normal recovery process and produce disproportionate responses and eventually an MSD-type injury.

Corporate initiatives designed to identify and control workplace ergonomic concerns have proven to be effective in reducing the incidence of MSDs and have been efficient investments producing measurable bottom-line benefits.

III. The Ergonomics of Pushing and Pulling

Figure 2. Given the choice between pushing and pulling, a task should be designed for pushing.

Avoid Pulling
fig 2a

Pushing Is Preferrable
fig 2b

Manual Material Handling (MMH) tasks are physical work activities that involve exertion of considerable force because a particular load is heavy or the cumulative loads during a workday are heavy. Examples of MMH tasks include lifting or lowering, carrying, and pushing or pulling. This paper focuses specifically on pushing and pulling activities while using a car t or equipment with wheels or casters.

Researchers have identified a number of key factors that must be considered when designing manual pushing and pulling tasks. Surprisingly, as the following case study shows, the weight of the load or equipment, though significant, is not as important as most people think. It is the horizontal push force that matters most, and with the right caster selection and job design, thousands of pounds can be moved safely and efficiently.

Pushing is preferable to pulling for several reasons. You may, from your own experience, recall that your feet are often “run over” by the equipment when pulling. If a person pulls while facing in the direction of travel, the arm is stretched behind the body, placing the shoulder and back in a mechanically awkward posture, increasing the likelihood of painful, debilitating, and costly injury. Alternatively, pulling while walking backwards is a recipe for an accident, because the person is unable to view the path of travel. Further, research demonstrates that people can usually exert higher push forces than pull forces. In some situations, pulling may be the only viable means of movement, but such situations should be avoided wherever possible, and minimized when pulling is necessary.

This paper refers to the person pushing or pulling (the operator) as “she.” This is to emphasize an important point when designing a manual handling task: when the application of force is required in a task, it is often best to design for the smaller female members of the population, because if they can do it, presumably so can most other woman and men.

Case Study

Applied Materials Moves 7,000 lb. Equipment with Ease

Ergonomics engineers at Applied Materials, a manufacturer of silicon chip processing equipment, saw an opportunity for improvement on several fronts when they observed workers moving pieces of equipment that weighed up to 7,000 lbs.

When they started the project, four workers were needed each time the equipment was pushed. Each system was typically moved 10-14 times a day, 7 days a week, as it flowed through the lean manufacturing process. Each move required 2 technicians to leave their regular jobs to assist 2 other technicians in moving a system, creating productivity and workflow disruptions, and increasing the risk of error and injuries.

Powered pallet jacks were in use in 10% of the manufacturing lines, but they did not perform as intended, and they lacked safety features the company wanted. The engineers established design goals for the new system based on safety, ergonomics principles, functionality, and low push force requirements. They then began scientifically testing the push/pull forces for prototype systems to find an optimal solution.

Their ergonomics approach proved to be a huge success. The new system involves several dolly designs with ergonomically designed low resistance casters, and a modified electric pallet jack called a “tugger.” Jon Paulsen, Ergonomics Engineering Supervisor, explains: “We tested six dolly and caster designs and learned that not all casters are equal. After four design iterations, we arrived at the new dolly and tugger design based on ergonomics, safety, usability on all system types and configurations, product damage avoidance, and cost. In the end, we were able to reduce the number of technicians needed to push a system by 50%, leaving the others to attend to their designated work without disruption. When pushing the systems in a straight line, we were able to reduce the push force, distributed between two employees, to 60 lbs. and thus avoid using the tugger in many areas of our manufacturing lines. Clean room floor space is very expensive, so we wanted to use as little space as possible. A 60 lb. push force for a 7,000 lb. piece of equipment is an incredible achievement. We are very pleased with the advances in caster technology that allowed us to achieve this push force. Our time studies show that we increased productivity by almost 400% in terms of man-hours. Plus, there haven’t been any injuries related to this task since we instituted the new system over a year ago.”

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