Another consideration is “embedding,” which occurs when debris gets “stuck,” or embedded on the wheel sur face. Like flat spots on a wheel, embedded materials can result in increased rolling resistance, vibration, and noise. The likelihood of debris embedding in a wheel is dependent on the elasticity of the wheel material. A wheel material that does not “bounce back” is more likely to become embedded than a material that quickly reshapes to its intended form. That is, a more elastic material effectively ejects the embedded debris.

Generally, a “softer,” elastic wheel material is better, unless you can be sure of a hard, clean, smooth floor. Often, there is some trade-off between wheel diameter and wheel “softness.”

Sloped Surfaces

You have no doubt experienced what occurs when you push wheeled equipment up or down a slope. On flat sur faces, the resisting forces are restricted to those previously described. When a slope is encountered, the weight of the equipment also comes into play, acting either against or for the operator.

Going down usually requires no push force, because the force created by gravity overcomes the other forces acting to resist movement. In fact, as the slope increases, the operator may have to apply pulling forces so as not to lose control of the free moving equipment. Brakes are recommended for wheeled equipment that has a tendency to “run” when going down sloped sur faces.

In the same way, gravity acts against the operator when equipment is pushed up a slope. The steeper the slope, the more the equipment weight must be borne by the operator. As the slope approaches vertical, the operator is essentially bearing the entire weight of the equipment, plus any friction, physical, or dynamic forces.

Special Environments or Contaminants

Certain operating environments require specialized casters and wheel materials. For example, in flammable environments and medical facilities, static electricity is a significant safety concern, and special equipment selection is required. Clean rooms and environments where chemicals may be present also require special equipment selection, and you are encouraged to consult with experienced manufacturers and vendors in these situations.

Starting, Rolling, Turning, Stopping, and Positioning

To better understand the forces in a typical pushing/pulling task, imagine a task that requires moving a cart some distance, turning the cart around a corner, and then stopping and positioning it at the end of the route. There are four phases in this task:

• starting or Initial Force
• Rolling or Sustained Force
• Turning Force
• Stopping or Positioning Force

Starting

To start the motion, the operator must overcome inertial forces, friction forces, and any other mechanical/physical forces that may be due to such factors as flat spots on the wheel, debris or irregularities on the floor. If a caster is turned, additional resistance must be overcome until it aligns in the direction of travel. Under typical conditions, the force to initiate movement (the starting or initial force) is always higher than the force to sustain movement.

Rolling

Once started, the operator usually does not need to apply much, if any, acceleration. Therefore, the inertial forces either go to zero or become low once moving at a relatively constant velocity. (Remember, any change in velocity means acceleration. So, if the operator tries to speed up, slow down, or turn, iner tial forces will occur.) Once in motion, at a relatively constant velocity, the forces resisting movement are restricted to friction and physical inter ference from wheel or floor irregularities, and momentum tends to keep the equipment in motion.

Turning

Two primary forces combine when the cart is turned: iner tia due to acceleration in a new direction and friction in the swivel housing and between the floor and the wheel. The cart’s momentum, which is related to its mass (weight), wants to carr y the car t in the direction it was traveling, so the operator must overcome that by applying higher forces in the new direction. A well-designed and maintained caster will have low frictional resistance to turning at the bearings in the caster housing, so the real friction concern is related to any pivoting at the wheel/ground inter face. Swivel casters are designed with an offset for this ver y purpose, as discussed previously. Depending on the weight of the cart, the acceleration at which it is turned, and the friction at the casters, the turning forces can be significant. The result is that an operator will need to apply new forces in new directions, often in asymmetric body postures and muscle exertions, which can increase the likelihood of injur y.

Stopping/Positioning

If, at the end of the travel route, the operator can simply release the cart and let it roll to a stop on its own, there is no need to apply any force. However, if it must be stopped or positioned in a specific place, the forces can be significant and multidirectional in the case of positioning. Such multidirectional forces can expose the operator to potentially hazardous postures and muscle exertions. Stopping, in terms of inertial forces, is the same as starting, but additional force is applied to decelerate, rather than accelerate. Positioning is a series of starting, stopping, and turning forces, which are typically the highest force conditions required in a pushing task.

C. Factors That Affect a Person’s Ability to Push or Pull

So far, this paper has focused on the forces that combine to resist movement. It is the operator – a person – that must generate and apply enough force to overcome the resistance. Additionally, there must be enough traction, or friction at the feet for the person to successfully apply the push/pull force without slipping. We will now focus on the factors that affect a person’s ability to safely and effectively complete a pushing and pulling task.

Ergonomists seek to design work, tools, equipment, etc., to fit as many people as possible in the expected user population. The rule that “one size does not fit all” becomes apparent in every situation.

Perhaps the most frustrating par t about designing equipment for use by people is that we have ver y little control over the size, shape, age, physical strength, etc., of the people who will use the equipment. Certain design features can influence how people will use the equipment, but there is still much variability in who uses it and the way they use it in the real world. There are occasions when a push/pull task might be designed for one specific person. However, in most workplaces, any given task may be completed by a variety of people.

When there is little control over the size and abilities of the people in a process, ergonomists recommend designing or selecting easily adjustable equipment that each user can fine-tune for his or her par ticular needs and abilities. Where adjustable equipment is not feasible, ergonomists recommend designing for the reasonably expected worst-case scenario, with the goal being a design that makes the task safe and efficient for the greatest number of people in the expected user population. For pushing and pulling, it is often the required force that dictates who can and cannot perform the task, so we want to design for the lower strength capabilities in the user population. Therefore, we usually select the 5th, 10th, or 25th percentile female as our lower strength design limit. If she can accomplish the task safely, we expect that larger and stronger people will also be able to do so.

Three primar y analysis and design perspectives can be applied to determine appropriate design limits for manual material handling work: psychophysics, biomechanics, and physiological approaches (Figure 7).

Biomechanics

Biomechanical research and analysis is an approach ergonomists use to establish strength, force, and posture guidelines. Biomechanical methods use posture, gender, anthropometry (body size), and push/pull forces to calculate resultant muscle force requirements and bone and joint compression forces. The calculated values are then compared to accepted limits for working populations. Biomechanical analysis methods are useful when analyzing high exertion tasks, but often do not consider the effects of the dynamics, repetition, or duration of the task or job.

Physiology

When a manual material handling job requires highly repetitive, fast paced, or forceful exer tions, Physical Work Capacity (PWC) and fatigue must be considered. Each person has a unique PWC, which is a measurement of maximum aerobic capacity, or metabolic expenditure capabilities. Your PWC is affected by age (decreases with age), fitness, gender (men typically have a higher PWC than women), maximum heart rate, and the energy demands of the job (repetition, exertion levels, and duration/length of time spent per forming the job). When physiological limits are exceeded, fatigue occurs, and in severe cases, a person’s cardiovascular system may be stressed to the point of hear t failure. This is especially impor tant to consider if the expected user population for a physically demanding job will include older or “out of shape” people, which is a reasonable expectation when designing for the general working population.

The body also produces heat, which must be dissipated at a rate high enough that the body temperature does not rise and cause heat stress, or even death. The rate at which heat can be dissipated depends on a variety of physiological factors, and is affected by clothing, external temperatures, humidity, and air movement.

Psychophysics

The psychophysical approach has proven to be very useful when designing a new push/pull task or when analyzing an existing task. The psychophysical approach to evaluate or design manual handling tasks was pioneered by Snook, Ciriello, and their associates at the Liber ty Mutual Insurance Company Research Center. These studies, conducted since 1967, culminated in an extensive published data set in 1991.

In simple terms, psychophysics is a research method that takes human perceptions into account. Liber ty Mutual successfully applied the method to lifting/lowering tasks, carr ying tasks, and pushing/pulling tasks. For pushing and pulling, they developed a set of guidelines based on these key factors:

• Type of Task
• Type of Force
• Gender of the Person
• Percent of the Industrial Working Population that Should be Able to Safely Per form the Push or Pull
• Distance of the Push or Pull
• Height of the Hands from the Floor When Performing the Push or Pull
• Frequency or Repetition of the Task

In “Guide to Designing a Push/Pull Task,” to follow, data from the Liberty Mutual Studies will be presented that will help you identify the appropriate push/pull forces for your situation.

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