A. Factors That Affect Pushing and Pulling

Figure 3. When people push wheeled equipment, they generate force and transmit that force through a contact point with the equipment. Friction at their feet must be at least equal to the resisting forces of the equipment, otherwise their feet will slip.

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.

Figure 4. Some key factors that must be considered when designing a safe and productive pushing/pulling task, including human factors, task factors, cart and caster design, and floor and environmental conditions.

B. Rolling Resistance: Forces That Resist Movement

Figure 5. Forces at the
caster and wheel that resist movement include friction in the axle, friction at the swivel axle, and friction and physical interference at the floor-ground interface.

The forces that resist movement, generally referred to as Rolling Resistance, define how much force a person must generate and apply. Several types of forces combine to resist movement (Figure 5):

• Dynamic, or Inertial Forces
• Forces Due to Physical Interference
• Friction Forces

The force required to push/pull wheeled equipment is always greatest at the start, just before movement begins. Ergonomists refer to this force as the initial, or star ting force. Fortunately, the initial forces typically last a short time and drop to the sustained force levels once the acceleration and any mechanical interference at the start of movement is overcome. Once in motion at a relatively constant speed, the force requirement is generally lower. This force is called the sustained, or rolling force. Turning forces occur when the path of travel is changed while the equipment is already in motion, or they can occur when a cart or equipment is being positioned (e.g., small motions while trying to precisely position the equipment).

Dynamics, or Inertia

The initial push force is always higher than the sustained force, in par t because it includes the force required to overcome inertia. Push force is directly related to the acceleration with which the force is applied. The famous 17th Century scientist Isaac Newton determined the relationship among force, acceleration, and the mass (which is directly related to the weight) of an object to be:

Force = Mass * Acceleration
F = Ma

The dynamic forces exist only when the equipment is being accelerated (or decelerated). Acceleration occurs at the start of a push, as the load is accelerated from a stationary position to some movement velocity; when the load is slowed, causing a change in velocity; and when the cart or equipment is turned, causing an acceleration in a new direction.

Friction at the Wheels/Casters

Whenever two surfaces are in contact, friction will resist movement between them. In “perfect” conditions, which exist primarily in theory, a laboratory, or other highly controlled environments, a hard, smooth wheel rolling on a hard, smooth surface would experience the least resistance to rolling. (Other factors, including diameter, tolerance in the round (concentricity), material resilience, and energy loss affect rolling resistance, as well.) In realistic operating environments, however, these perfect conditions rarely, if ever, exist. Using hard wheels under typical conditions will often result in higher rolling resistance, increased noise and vibration.

Friction is defined as either static (star ting) or dynamic (rolling). The static forces are usually higher than the dynamic. Therefore, when considering the force a person needs to apply to a stationary piece of wheeled equipment, the initial force to create motion will almost always be higher than the force needed to sustain motion. This is because acceleration is applied, and the static friction forces must be overcome. The starting force is also affected by physical interference, which is discussed in more detail below.

In a wheel or caster system, there are three locations where friction can act to resist movement, increasing the required push forces:

1. 1. In the axle-wheel interface;
2. 2. In the swivel housing (for swivel casters); and
3. 3. At the ground-wheel interface when a wheel is slid or pivoted on a surface.

By selecting well-designed casters that utilize modern design technology and materials, resistance due to friction can be kept to a minimum. Friction between the wheel and the floor is negligible, unless it occurs from pivoting the wheel on the floor surface, or from sliding the wheel across the floor perpendicular to its rolling direction.

Resistance to Rolling in the Wheel/Axle/Bearings

Typically, wheels and casters are offered with either precision bearings, which are best when sealed and therefore should be maintenance free, or bearings that require maintenance, such as cleaning and lubrication. Some wheels are offered with only a bushing and these should be avoided. Bearing technology has improved to the point that for better casters, the wheel material and diameter are actually more important than the type of bearing. However, sealed precision bearings provide the added advantage that they are maintenance free. Maintenance is often overlooked in caster selection, which can be an expensive mistake. When bearings become dirty or contaminated with debris, or the lubricant breaks down and is not refreshed through maintenance, the rolling resistance can quickly and significantly increase. If precision bearings are not chosen, a strict maintenance or inspection regime should be put in place to ensure that rolling resistance at the bearings is kept to a minimum.

Swiveling, or Turning Resistance

Figure 6a. A swivel offset
helps pivot the wheel. This offset can be varied to suit the application.

Figure 6b. Single wheel casters pivot on the wheel surface, leading to force due to friction and wheel surface damage.Twin wheeled casters (right) reduce friction and wheel damage by rolling when the caster is rotated.

Three types of forces combine to resist turning: friction in the swivel housing and at the wheel-ground contact point; inertial forces due to acceleration applied in the turning direction; and any physical interference that may be present at the wheel-ground interface. When the cart is in motion, and a turn is initiated over some arcing distance, the inertial forces are restricted to how much acceleration the operator applies in the new direction. When performing fine positioning, which is often a series of stops and star ts, the inertial forces may have a greater effect due to the accelerations and decelerations inherent in these motions. However, friction at the floor (or in the swivel housing for inferior or poorly maintained casters), while the wheel surface pivots on the floor, can add considerable force to a turning or positioning task.

Consider the contact area between the wheel material and the floor. A smaller diameter wheel, or a compliant wheel that “flattens” somewhat under the weight of the load, will have a larger contact area than a large diameter, or hard wheel material. The smaller the contact area, the lower the resistance as the wheel pivots in place. A compliant wheel that has a large contact area under loaded conditions is sometimes said to “stick” or “grip” the floor if it is pivoted in place.

Manufacturers design casters with an offset to reduce the force required to turn and swivel (see Figure 6a). The offset design, meaning the wheel is laterally offset from the point where the caster housing connects to the equipment, provides a horizontal lever arm between the equipment and the point where the wheel contacts the ground. Without this offset, a swivel caster would not swivel unless the equipment was moved in an arc. With the offset lever arm, a horizontal force applied to the equipment acts through the lever arm to pivot the wheel with much greater ease and with a much smaller arc of travel. When fine positioning a piece of equipment, the small travel arcs are very desirable.

Figure 6b shows an innovative caster design that eliminates the gripping effect all together. When the double wheel design pivots, the wheels roll in opposite directions and no gripping or pivoting occurs directly on the wheel surface. The twin wheel design reduces turning forces, thereby protecting the life of the caster. Also, some caster companies offer extended offset swivel designs that make positioning easier.

Carts and equipment with four casters are often designed with two swivel casters and two rigid casters. In such cases, the handhold should be on the side with the swivel casters, which reduces the twisting forces and motions necessary to maneuver the cart.

Physical Interference

Physical interference (i.e., a physical barrier or interference to rolling) occurs when wheel or floor materials deform over time or when a wheel must roll over debris or an uneven surface.

"Flat Spots" and Wheel Damage

One form of mechanical interference is due to “flat spots” or other wheel irregularities. For example, if a loaded cart is left stationary for some time, the wheel or floor material may slowly deform, creating a small flat spot at the wheel-floor interface. On a smaller scale, this can also occur in the bearings. Thus, when a person begins to push the cart, she must overcome the “flat spots,” as well as the initial forces due to static friction and acceleration.

Permanent flat spots and other wheel material damage can occur with “wear and tear.” In particular, flat spots may develop when a non-swivel wheel is slid across a surface perpendicular to the rolling direction or when a swivel wheel is pivoted in place. Caster designers use the offset caster to reduce this effect, but when a wheel pivots in place, there will still be some gripping between the wheel and the ground, and friction can wear the material to create flat spots and reduce wheel life. Inferior wheel material or a mismatch between wheel material and expected operating conditions can result in accelerated deterioration and resultant increases in rolling resistance.

A wheel with permanent flat spots or physical damage not only has a greater resistance to rolling, but also can be very noisy and create vibration, which may damage equipment, and in severe cases, may contribute to human vibration related injury.

Uneven Surfaces, Debris, and Embedding

Rough or uneven surfaces, debris, and other contaminants can create physical barriers to rolling. When a wheel encounters such physical barriers it must roll up and over that barrier. The forces required to do this depend upon the size of the barrier relative to the diameter of the wheel. For example, a small diameter wheel encountering a small stone will experience great resistance. As the diameter of the wheel increases, the resistance will become lower and lower, until the relative difference in size is so great that the small stone is more like a grain of sand in relation to the wheel. Wheel diameter is one of the most important factors, yet it is often overlooked.

The resilience of the wheel material, or how compliant it is, is another important factor when a wheel rolls over physical barriers. If the wheel is “soft,” it will deform and absorb the barrier to some extent. In this case, the wheel does not have to rise up and over the barrier, and the resistance is therefore lower. Resilient wheels also absorb shock, resulting in less vibration and quieter operation.

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