Imperial
Most carts have handholds of one kind or another. Handholds are important, because they send a message to the person regarding where and how to apply force to the equipment. Some equipment may not have designated handholds, and the person therefore seeks the most convenient or mechanically advantageous method to apply force. When equipment is to be moved manually, it is advisable to incorporate designated handholds or a surface area that will provide good force application contact points for the person.
As a matter of safety, handholds should not require or encourage the person to have the hands, fingers, or arms protruding to the side of the equipment, because a crushing injury between the equipment, walls, and other equipment is very likely in such instances.
Handhold Height
Figure 8. Handhold height
is important because it defines, in part, the posture a person will assume, and a person’s ability to generate a push/pull force is directly related to posture.
is important because it defines, in part, the posture a person will assume, and a person’s ability to generate a push/pull force is directly related to posture.
Handhold height is important because it defines, in par t, what posture the person will assume. Unfortunately, there is no single handle height that is “correct” for all people. Figure 8 demonstrates the effect of handhold height on posture, showing a small female and a large male reaching to the same height. A height that is appropriate for the small female may cause the large male to bend or stoop. Likewise, a handle height preferred by the tall male will cause the small female to reach up. This is significant, because the force a person is able to generate is directly related to posture.
An adjustable handle system is one way to accommodate people of most sizes, but such adjustability may not be feasible for some applications, and few vendors offer adjustable features at the time of this writing. Another approach is a handhold system that offers continuous vertical handles that can be grasped anywhere along their length or a series of handholds at different heights.
Handhold Width
Operators should be able to contact handholds as near as safely possible to the outer edge of a car t, avoiding crushing injuries, but providing ample leverage for turning and positioning.
Handhold Type
Handle type can significantly influence the amount of force a person can apply through the hands. Ergonomists refer to the hand-equipment inter face as “coupling,” and research shows that poor coupling can lead to as much as a 65% decrease in push-pull force capabilities.
In general, a handle should be shaped so that it does not concentrate pressure on any specific part of the hand (i.e., it should not have sharp edges, pronounced ridges, etc.). The person should be able to grip the handle with a power grip, meaning the fingers and the palm of the hand should be in contact with the handle. The fingers should not overlap, and the handle should be wide enough to accommodate the entire hand.
A handhold that accommodates a grip (i.e., the fingers wrap around it) is required for pulling tasks. However, pushing capabilities are comparable with or without handles, as long as there is a good surface for stable hand-equipment coupling.
Figure 9. When significant force must be applied by the operator, she will assume a posture that maximizes her ability to generate high forces using her large muscle groups, and extending her feet behind her center of gravity, requiring high foot-floor friction forces (top). At lower forces, the operator will stand in a more upright posture (bottom).
The human musculoskeletal system is essentially a series of mechanical levers. Each muscle begins on one bone and attaches, across a joint, to an adjacent bone. The position of the joint – the posture – defines the length and position of the muscle and lever arms. Some postures are more mechanically advantageous than others, and a person is able to produce a greater amount of force in these optimal postures than she is in an awkward posture. Also, certain muscle groups are bigger and more powerful than others, and a person is able to generate the largest forces when these muscle groups are used, especially when they are used in their optimal exertion postures. This is evident when you see a person pushing an object that requires excessive force; she will attempt to align her body with the horizontal force requirement, such as the posture pictured in Figure 9. In such a posture, the person is able to use the large muscle groups in the legs and torso. This person has the added benefit of using part of her own body weight to generate the force.
The best posture for star ting a push is not necessarily the best posture for pushing once the equipment is in motion. Balance, as related to foot placement, becomes a primary factor, and the appropriate posture will become more upright in many movement situations (e.g., Figure 9.)
The posture used while pushing is defined in large part by the height of the handholds and the location of the feet. A person is able to generate the greatest push force when the feet are separated, one foot some distance ahead of the other (e.g., Figure 9.) In this posture, the rear foot, and sometimes the front foot as well, may be behind the body’s center of gravity (or ahead of the body’s center of gravity in the case of pulling). Thus, if the person loses her footing or handhold, a fall can occur. Forces that require this level of exertion should be avoided in pushing tasks, especially if the task is repetitive. Such high forces will also be beyond the safe performance of many workers.
Friction Forces, or "Traction" at the Feet
Friction forces at the foot/floor are one of the most important, yet often the most overlooked, factors in pushing and pulling tasks. Isaac Newton, who stated the previously discussed F=Ma relationship, also observed the physical law that for every force on a body, there is an equal and opposite reaction force. In the case of pushing, whatever force is applied to the equipment by the hands must be reacted to by an equal force at the foot/floor inter face. If, for instance, you apply 30 lbs. of horizontal force to a car t handle, the friction force at your feet must be equal to 30 lbs. If the foot slips easily on the floor, meaning there is a low coefficient of friction (COF) between the shoe and the floor, the amount of force a person can apply to the equipment will be limited to the amount of friction force or traction at the feet. Furthermore, if the person has limited traction at the feet, she is unable to safely optimize her posture by leaning into the equipment (or away in the case of pulling), because her feet will begin to slip, and she may completely lose balance and fall.
Researchers have shown that a person pushing with good traction (high COF, e.g., 0.6 or more) can generate as much as 50% more force than when pushing in a poor traction (low COF, e.g., 0.3 or less) environment.
Angle of Push/Pull Force Application
The force required to move a cart or equipment is in the plane horizontal to movement. That is, for a car t being pushed on a flat surface, the most effective force application will be in the direction parallel to the floor. In an actual pushing situation, however, the person may be unable to apply her force exactly in the horizontal direction. For example, a high or low handle height may make it difficult or impossible to align her body in such a way as to apply a strong horizontal exertion. In other cases, the person will intentionally apply force to the handholds at some angle from the horizontal in order to increase her foot/floor traction. She can increase the vertical reaction force at her feet, and thus her foot/floor traction, by applying an upwardforward force at the handholds. Likewise, if she is pulling, she can increase traction by applying an upward-reward force to the handholds, resulting in an increased vertical reaction force at her feet.
Applying a downward-forward push force does not help foot traction, but it does allow her to utilize body weight to her advantage.
The amount of force a person can apply is also influenced by how far the equipment must be pushed. The amount of force a person can sustain decreases as the distance traveled increases.
Frequency, or Repetition of Task
Repetition, or frequency, is typically related to the job or task cycle. For instance, if a task cycle includes pushing equipment five times every hour, then the repetition rate is 5/hour, or 0.083/minute. As repetition increases, the force a person can exert decreases, especially as the length of time (duration) of the task increases. Repetition increases metabolic demand, and also reduces the amount of time body tissues have to recover between loading.
The duration of a task or job is simply the length of time it is performed. For example, if a worker pushes equipment for 8 hours a day, the duration of that pushing task is 8 hours. In this case, duration is not the duration of a single exertion, but the duration of the push/pull task in a given day.
Clearly, performing a pushing/pulling task for 8 hours a day will be more taxing than doing the same for 1 hour per day. Therefore, a person will be able to push with a higher force in a lower duration job than in a high duration job.
IV. Quick Guide to Designing a Push / Pull Task
If a pushing/pulling job is to be performed 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:
- A. The people
- B. Task design
- C. Operating environment and floor conditions
- D. Cart or equipment design
- E. Caster and wheel design
Liberty Mutual Insurance Company has published a large set of data, commonly referred to as the “Snook Tables,” that can be used to determine the appropriate force levels for straight line pushing tasks. The entire data set, including many combinations of pushing and pulling activities for both males and females, is too extensive to reproduce here. However, a useful subset of the data is available in the Appendix.
If the task requires turning or positioning, special attention must be paid to those additional force demands. Often, a wheel and caster will perform differently when traveling in a straight line than it will when being turned. Further, the design, location and configuration of wheels and casters on the equipment can have a significant effect on the force requirements. Turning and positioning requirements must be reviewed and treated on a case-by-case basis, and wheel and caster designs should be carefully reviewed with your caster supplier. In some cases, an effective task design involves both manual pushing and pulling segments and mechanically assisted segments, as demonstrated in the case study previously discussed.
Also, where force levels cannot be reduced to acceptable levels through design and caster selection, administrative controls such as assigning two people to perform the task may be an option (although, design solutions that minimize potential hazards are always preferable to administrative approaches).
A. The People
Unless you are designing for a specific person, you will usually tr y to design for the widest
range of people you might expect to perform the task. In most workplaces, you have little
control over who will perform any given job. Even if you know the person or people that are
performing it today, that can quickly change. Therefore, in most cases, the following will apply:
Design Force Requirements for the Smaller Female
A small female is likely to be able to generate the least amount of force overall and therefore
represents a reasonable “worst-case.” Companies in the United States often design manual
material handling tasks so that at least 75% of the female population and 99% of the male
population can safely perform them. If you wish to be more conservative in your design, meaning
you will protect a larger portion of the working population, you might design for 90% or more
of the female population to make the job more accessible to a wider population of workers.
Match Footwear With Floor Conditions to Maximize Traction
To avoid slipping, researchers suggest a COF of 0.6 or greater.
B. Task Design
Use the Data in the Appendix to Explore the Effects of Distance Pushed, Repetition, and Duration of Task on Push Force Limits
Depending on task, equipment, and operator factors, you will find that acceptable force levels
for females can range from as low as 13 lbs. to as high as 57 lbs.
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