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Selecting The Right CasterSelecting the right caster for an application is quite similar to buying the right car.By Ken Otmanowski and Al RoundsThe selections seem endless: make, model, color, style, price range, reliability, who has the best
service, who has the nicest salespeople... to many choices for the typical person to make. Selecting a
caster can seem like that.
Load Determining the operating load is not as simple as it may seem because of the necessity of determining load distribution. In most cases, no matter how many casters are involved, there is no guarantee that the load will be distributed over more than three casters. Thus, each caster should be able to handle at least one-third of the load. Of course, the load can be distributed evenly among more than three casters. The most common method is to use an articulating axle or spring-loaded casters that maintain contact with the ground in spite of uneven floors. Either of these features increases the cost of the caster. If the load is too heavy to be handled by standard casters, custom casters can be designed. For example, special casters have been designed that support a 140,000 lb. steel aircraft maintenance hangar that is wheeled into position over the tails of large aircraft such as the Boeing 747 and McDonnell Douglas DC-10. Surface Conditions The rougher the floor, the more likely it is that the load will be supported on less than the full number of casters. Protecting the floor from damage may also be an important consideration. In most cases, it is less expensive to replace a set of casters than to refinish a floor. Tradeoffs are often required. Steel, which provides the best mix of high capacity and low cost, also tends to be the most damaging to floors. On the other hand, the lower capacity polymer materials provide the best load distribution properties and are the least likely to damage a floor. Also, larger diameter wheels work better on rough floors, albeit more expensive. Floor marking can also be a problem, especially when conventional rubber wheels are used. For example, a pharmaceutical plant with white vinyl floors would allow no marks on the floor. In this instance, a harder polyurethane wheel would provide sufficient floor protection. But for applications where the harder polyurethane wheel does not work, a non-marking polymer compound nearly as soft as rubber could be used. In the most demanding surface conditions, unimproved outdoor terrain for example, pneumatic wheels may be necessary.
Travel Speed
Nearly all casters can travel at power-driven speeds, but their load-carrying capacities are significantly reduced. In fact, the capacity of a caster is somewhat inversely proportional to the speed at which it is used. Higher speed reduces caster capacity because it reduces the capacity of the wheel bearing. Furthermore, higher speed greatly increases the forces experienced in traversing an uneven surface, or hitting an obstacle. For example, a steel wheel dropped one inch can sustain a momentary load of 10 times the actual load the wheel is carrying. The caster supplier must be informed when operating speed is to exceed a few miles per hour. Operating EnvironmentThe caster supplier must also be informed of unusual environmental conditions. For example, the casters may have to travel into an oven. At about 500°F, the temperature begins to reduce the load-carrying capacity of metallic caster components, forcing the use of a heavier model. Other temperature-related effects that must be considered are the tendency of lubricants to run off or burn up at higher temperatures, and thermal expansion, which can cause bearings to freeze. Temperature also affects the type of wheel material used. Polyurethane wheels begin to melt at temperatures above 200°F. Low temperatures also present problems that are addressed by special lubricants and wheel materials. Acids may erode many metals, and common polymer materials can be attacked by a variety of chemicals such as aircraft hydraulic fluids. In the most corrosive environments, stainless steel is a possible alternative, although it costs much more than conventional materials. The presence of dust or other abrasive materials must be considered. Double-sealed bearings exclude particles from casters used in applications such as sand-blast units. Several design alternatives address casters that will be submerged in water. One option is to use stainless steel and leave the bearings open so that water serves as the lubricant. Another option is packing the bearing with a lubricant that will not wash out. Load HandlingSpecial cargo handling requirements sometimes come into play. If steel shavings are being moved, delicate handling is not a major concern. However, if the cargo is a sensitive instrument, a soft wheel material such as natural rubber, or even a spring-loaded caster may be required. Spring-loaded casters provide the ultimate in shock absorption when combined with pneumatic tires. In the most extreme cases, additional provisions may be necessary. For example, explosives are frequently hauled using a special container that suspends them in liquid. If a caster with a shock absorber is ruled out for cost reasons, the specifier should look for a wheel with some shock absorbing properties. Examples include natural rubber wheels, semi-rubber, semi-pneumatic (zero pressure wheels) or mold-on rubber wheels. The best overall rule for the person selecting casters is to be sure to inform the supplier of every aspect of the application. Always try to consider both sides of the application issue: the Sales/Marketing side and the Engineering/Manufacturing side. In most cases, the supplier will help you select the best standard product or design a special one exactly for your needs. |
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