Focus on Casters & Wheels: Practical tips for selecting the right casters
By Ken Otmanowski & Al Rounds
Selecting the right caster for an application is quite similar to buying the right car. The choices seem endless: make, model, colour, style, price range, reliability, who has the best service, who ha...
Selecting the right caster for an application is quite similar to buying the right car. The choices seem endless: make, model, colour, style, price range, reliability, who has the best service, who has the nicest salespeople, and so on. There’s too many choices for the typical person to make. Selecting a caster can seem like that too.
Except with casters, the choices sound more like: What is the load capacity, what are the floor and environmental conditions, what type of wheel should be used, what speed will they be used at, and so on. It can seem very confusing.
However, the task of caster selection can be made straightforward if the correct questions are asked. Let’s step through some of the questions and hope to learn something in the process.
First, is this a replacement or new application? If it is for replacement, ask “Why did the old casters fail?” They may have just worn out, or they may have broken because they weren’t properly selected initially. To be completely sure the replacements fit, the mounting bolt-hole pattern and overall height are two critical items.
Often the person doing the specifying is less than enthusiastic about gathering complete application details because they think “It’s only a caster, and they’re all the same.” Making this type of assumption is a serious mistake. Although casters can by no stretch of the imagination be considered high technology, their design has progressed in the past several decades. Literally thousands of designs are now available for most applications.
To select the best caster for any application, complete information about the job requirement is necessary. There are marketing/sales and engineering/manufacturing type factors to be considered. If you put on your marketing hat, you should ask questions like: what caster is currently being used, what is the target price for the caster, what is the required delivery, how many casters are required, and has this been quoted in the past? Engineering considerations are: What is the load to be carried, what are the floor conditions, speed of travel, operating environment, and load handling? Options should also be considered, like: will a brake be necessary, will a swivel lock be used, and should the casters be spring loaded? All these factors plus others must be considered. Let’s concentrate on the engineering aspects.
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 DC10.
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 tend to be the most damaging to the floors. On the other hand, 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, although they are 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: Most casters are rated at manual speeds, which means no faster than walking speed. Yet, in some cases where low-speed operation had been specified, the caster could actually travel at 10 miles per hour. For a caster, 10 mph is extremely fast.
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 1-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 environment: The caster supplier must be informed of unusual environmental conditions also. For example, the casters may have to travel into an oven. At about 500F, 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 lock up.
Temperature also affects the type of wheel material used. Polyurethane wheels begin to melt at temperatures above 200F. 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 will 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 handling: Special 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 will design a special one for your needs exactly.
Ken Otmanowski is Engineering Mana
ger and Al Rounds is Sales Engineering Manager of Albion Industries. This article was prepared with contributions from Colson Casters Limited.
Editor’s note: Addtional information on casters, including a detailed chart provided by Colson Casters on the Relative Ability of Various Wheel Types to Solve Caster Problems, can be found on our web site at www.mro-esource.com.
Caster plating offers many advantages
Plating of casters serves two main functions–it prevents corrosion and enhances the appearance.
Corrosion resistance is most commonly measured by the Salt Spray Test, an accelerated corrosion test used to compare the relative protection of different metal finishes. It uses a five per cent salt solution. This is sprayed over the parts to be tested in an environmentally controlled, closed cabinet.
Different plating processes produce different results. Electro Zinc plating (Clear Zinc), which is commonly used, provides a minimum of 30 hours of salt spay protection. Yellow Dichromate, used on some heavy duty lift truck casters, provides a minimum of 60 hours salt spray.
For tougher applications, a minimum of 96 hours of salt spray is available with a nickel chrome plating. Black Zinc, such as used on some medical caster components, has very good corrosion resistance at over 100 hours of salt spray, but scratches easily. Other black coatings, such as Black Oxide or Black Matte, provide a black look but their corrosion resistance is minimal.
For the toughest wet conditions, a powder coating produces over 300 hours of salt spay protection, with excellent scratch resistance. However, this is an expensive process. Most customers require powder coating for appearance reasons. Bearing seals can raise the bar even further, providing over 500 hours of salt spray protection.
Regarding appearance, chrome plating and powder coating are the most common options. There are even choices for decorative applications, such as piano casters. These include Brushed Brass, Antique Bronze and Bright Brass. And then there are painted casters.
Finally, there also are a few more plating processes available to handle very specific applications. A caster manufacturer can provide details about these.
Rob E. Hilborn of Darcor Limited provided material for this discussion of caster plating. It covers plating processes used by the company.