Many people first learn about casters when they see a piece of equipment mounted on four wheels. It rolls so easily in any direction! Often, if they have their own equipment, which could benefit from being mobile, they decide to do the same.
“I’d like to buy a set of wheels!”
At Douglas Equipment, this a request we get very often. It’s at that point, we know we’ve got some explaining to do, because there’s more to casters than just wheels!
Casters may seem simple – and sometimes they are, but most casters are composed of many different parts all working together to produce smooth, fluid motion. When we talk to our customers, we find that if they have some understanding about the general anatomy of all casters, they can make wise decisions about the best ones to purchase for their application.
Below, we’ll discuss the basic components that go together to make up a caster. We hope that this information, will aid you in choosing between the numerous options available to you when buying casters.
Basic Caster Anatomy 101
Casters are composed of three parts: a top, a bottom, and the middle that connects the top and bottom.
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Top Plate
– The top of the caster is called the top plate. It’s usually rectangular, with four holes, one in each corner. The top plate mounts underneath equipment that will be placed on casters.
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Wheel
– The bottom of the caster is the wheel. Its job is to roll and enable the equipment to move.
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Fork
– The middle part of the caster is the fork, an inverted U-shaped bracket. Its top connects to the top plate. Both sides of the fork point downward, each has a hole drilled in its lower end. The wheel’s axle is inserted into these two holes, connecting the wheel to the bottom end of the fork. (You’ll hear other names for the fork like yoke, horn, legs, and rig are a few examples.)
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Top plate, fork and wheel:
These are the basic components. Every caster has them. If it’s so simple, why are there tens of thousands of caster models each different from the other? The great variety is a result of modifying these three components, adding other subcomponents, or putting everything together in different ways.
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Rigid Caster
– If the wheel is mounted to the fork, and the fork is attached directly to the top plate, the assembly forms a rigid caster. It is capable of rolling forward and back in a straight line. Most mobile equipment will have two rigid casters mounted on one end, and two swivel casters attached to the other end.
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Swivel Caster
– When components such as kingpins, ball bearings and their raceways are added between the top plate and the yoke, the assembly is called a swivel caster. The yoke and wheel can rotate 360°, swiveling in any direction. This allows equipment to be easily maneuvered in tight spaces.
Swivel casters are usually more complex than rigid casters. We’ll talk about them next.
Swivel Caster Anatomy
Swivel casters work their magic – rolling easily in any direction – thanks to one or more sets of ball bearings.
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Ball Bearings
– Are mounted in a ring called a raceway that is placed between the yoke and the top plate. In a caster with a single raceway, the ball bearings carry the weight of the load, and absorb any side thrusts. In casters with double raceways – two sets of ball bearings, one above the other – the top bearings carry the load, and the lower bearings absorb side thrusts. Double raceway casters are usually more expensive than the more economical single raceway models.
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Threaded Kingpin
– To connect the top plate, ball bearings and yoke into an assembly, a threaded bolt called a kingpin is inserted downward through the top plate, the center of the ball bearing raceways, and the top of the fork. A nut threads onto the bottom end of the kingpin bolt. When tightened, the nut and kingpin hold everything together. As the components wear, the nut can be tightened to maintain appropriate tolerances.
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Riveted Kingpin
– In some casters, the kingpin is a solid rivet. These casters are more economical, but can’t be tightened if the components wear.
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Hollow Kingpin and Stem Casters
– Some casters make use of a hollow kingpin. In particular, stem casters don’t have a top plate. Instead, a “stem” is inserted upwards through the hollow kingpin and forms a post that can be mounted to equipment where space for a top plate is not available. A good example is a chair: stem casters mount in a variety of ways to the bottom of the chair legs.
- The items discussed above comprise the basic components of a caster – except for the wheels.
Caster Wheel Anatomy
Caster wheels have three important parts – the tread, the hub and an axle.
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Tread
– The wheel’s tread is the part that contacts the floor or ground. The entire wheel may be composed of the tread material; or the tread may be molded onto a hub. For example, solid polyurethane wheels are available. Moldon polyurethane wheels are also available, in which a layer of polyurethane tread is molded to a hub made of steel, cast iron, nylon or other material.
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Hub
– The center section of the wheel, to which the tread is mounted.
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Axle
– The axle goes through the center of the hub. Its ends are inserted into the two holes in the bottom end of the fork legs, to attach the wheel to the fork, and complete the caster assembly.
For All Your Caster Anatomy Questions – Contact Douglas Equipment!
Now that you know the names of the essential caster parts, it’s time to discuss the details! For information about axle bearings, accessories like brakes, swivel locks, thread guards and more, the best way to continue is to contact Douglas Equipment’s team of caster experts at 800-451-0030, or online through our contact form.
