Caster Selection: Not as Simple as You Think

By: Dave Lippert, Contributor

A spring loaded shock absorbing caster. Image
courtesy of Hamilton Caster.

Casters seem pretty simple. If the total load is known, simply divide by four (or the number of casters to be used) and select one with sufficient rated capacity, right? Well, maybe not.

Let’s start with capacity. Frequently, one may see a loaded cart with one wheel not touching the floor. This can be the result of uneven floors or a cart that is out of alignment. Also, particularly common outdoors is uneven pavement, including concrete or asphalt. As loaded carts roll across these pavements, the load on each caster can shift considerably. This happens even though it may not be visible. That load can greatly exceed the rated capacity.

Then there is shock loading. This happens when a load is dropped onto a cart but is more likely to occur when a rolling cart encounters an object on the floor (such as stray hardware) and the wheel tries to go up and over the obstacle. This encounter can easily create a load up to three times the static load on the caster, which means it can skyrocket past the rated load capacity.

Incidentally, when a wheel encounters an obstacle on the floor, it can roll up and over, push the obstacle to the side, or “catch” on the object. In this last situation, the wheel suddenly stops turning and begins skidding or dragging. For treaded wheels, such as polyurethane, this can quickly lead to a flat spot. This can become the cause of that very annoying “thump, thump, thump” we hear (and feel) with every wheel rotation.

Six Takeaways to Help You Select the Right Caster

The lesson from all of this is that selecting the proper caster is not a simple process. Getting casters that will serve well and for a long time requires some planning. Lesson number one is to overstate the capacity. In the industry, we suggest assuming three casters will carry the load of four.

When carts will roll across uneven surfaces and the load capacity is critical, the use of spring-loaded casters can help to spread the load evenly. Think of an automobile on an uneven surface, and how the four wheels stay in contact with the pavement even as the surface changes. The suspension serves to keep all the wheels on the pavement, and to spread the load across all the wheels. Spring-loaded casters help in a similar way.

Lesson number two is planning the casters’ overall height. In some cases, this is not important. But in many cases, it is crucial. For example, are there low doorways that must be cleared? Determine the tallest item(s) to be carried and add the height of the cart or dolly. If the required capacity results in casters that will make the load too high, consider multi-wheeled casters. Typically, these have two wheels (most standard casters have one wheel), and the load capacity can exceed that of taller single-wheeled casters.

Green Machine – built by Hamilton Caster – is an 8’ wide, 20’ long trailer that can support 30 tons of industrial recycling machinery. Image courtesy of Hamilton Caster.

Another reason to opt for multi-wheeled casters is to keep the center of gravity of a load lower – and safer. Taller loads can be unsteady, posing a safety hazard. (Imagine what happens when a tall load on wheels encounters debris or hardware on the floor.) One way to lower the center of gravity is to keep the dolly or cart height to a minimum.

Lesson number three is protecting the factory floor. Steel and metal wheels carry heavy industrial loads, and also roll relatively easily. Yet they can do damage to concrete floors, resulting in expensive repairs or premature replacement. Wheels with polyurethane treads can have high load ratings and also protect the floor. Have a maximum load pressure limit on your floor? Caster manufacturers can determine which treaded wheels can spread the load sufficiently to protect the floor. “Contact pressure” curves can be determined in a lab, resulting in data to help select appropriate floor-preserving wheels.

Lesson number four is floor surfaces. Many industrial environments have multiple floor types encountered during load transport, including smooth, level indoor concrete floors, outdoor pavement of either concrete or asphalt, and even gravel. Selecting the optimum wheel type is key to success. Besides load capacity, know that softer tread wheels such as pneumatics or solid rubber can traverse even gravel, while the more traditional industrial wheels are normally too narrow and too small for that. Consult the manufacturer for good advice.

Lesson number five: environment. Is there an oven (excessive heat) in play? Outdoor applications include sunlight (can accelerate the demise of some polyurethanes), rain, sand, and dirt. Chemicals in plants, even cleaning solutions, must be considered when selecting casters and wheels. As before, consult the manufacturer to assist.

Lesson number six is power requirement. Even when heavy loads will be moved by a tugger or other powered device, wheel type plays a significant role in the power required to begin and sustain movement. How much horsepower and torque should the tow vehicle have? Optimum wheel selection can make the difference between success and failure (for the tow vehicle) or enable the purchase of a less expensive tugger.

Still think caster selection is simple? There are two ways to assure success. One is to become a caster expert. The other is to consult with a capable industrial distributor or caster manufacturer. We think the second choice makes the most sense! WMHSDave Lippert has worked in the industry since 1982 and has served as President of Hamilton Caster since 1995. Hamilton Caster is a family-owned and managed manufacturing company located in Hamilton, OH. In business since 1907, the company designs and makes industrial wheels and casters for industry, and produces industrial carts and trailers (www.hamiltoncaster.com).