Finding the right caster

Casters 101: Key Measurements & Terminology

 At B & C Industrial Products, we know that choosing the right caster for your application is important and understanding Caster 101 will make this task easier.

  • Wheel Diameter: Measurement of the wheel measured vertically from one side to the other. The wheel diameter is often used synonymously with the size of a caster. 
  • Tread Width: The measurement of the face or tread of the wheel, horizontally. 
  • Overall Height: The Total distance the caster raises the equipment from the floor, measured from the top of the fastener to the ground. 
  • Swivel Radius: Horizontal measurement from the tread of the trailing side of the wheel to the centerline of the kingpin.
  • Load Capacity: The amount of weight a individual caster or wheel can safely operate under without failure. There is always at least a 25% safety factory built into all printed weight capacities. 
  • Hub Length: Measurement Through the bore of the wheel from side to side. This measurement is critical to know, when replacing a wheel. 
  • Offset: Horizontal distance between the centerline of the kingpin and the centerline of the axle. As a General rule, the longer the offset the easier a caster should swivel, But if it is too long the caster will wobble and the capacity is reduced. A shorter offset does not swivel as well and the equipment it is attached to may become difficult to steer, however the shorter the offset is a stronger fork design. 
  • Outer Diameter O.D.: The measurement of the outside diameter of any given item, usually refers to the measurement of bearings to see it will fit into a wheel bore.  
  • Inner Diameter I.D.: The measurement of the inside diameter of any given item, usually refers to the measurement of wheel or bearing bores. 
  • Overall Top plate size: Complete measurement of the top plate both directions horizontally and vertically when looking down on it. 
  • Bolt Hole Pattern: Measurement from the middle of the attaching bolt hole to the opposite hole, both directions; horizontal and vertical. Many bolt holes are slotted and fit a range of hole patterns. 
  • Attaching Bolt: The recommended and maximum size of the attaching bolt that will fit into the bolt holes.
  • Bore Diameter: The I.D. (Inner diameter) or measurement of the bore of a wheel.
  • Capacity: The Maximum load a wheel or caster can safely sustain while in use. Capacity of a caster is always based on the part with the lowest capacity. If a wheel is rated at 700 lbs and the caster yoke it is set in, is rated at 1200 lbs, the capacity of the complete caster is 700 lbs. Also to determine the capacity needed for a application, divide the expected maximum load by the number of wheels or casters used. When a customer asks for a capacity, always clarify that there desired capacity is per caster or the overall capacity of the application.
  • Rollability: Force required to begin or maintain motion. Rollability is affected by the wheel size, the type of wheel (particularly the tread material and design) and the bearings. As a general rule, the larger the wheel the easier it will roll. A harder durometer wheel rolls easier than a soft wheel on a hard surface but may be hard on the floor surface, could be noisy and may stop or be inhibited on obstructions. Soft treads protect floors, are quite and roll over obstructions more easily. A crowned tread reduces friction and rolls and turns easier than a flat tread.
  • Shock or impact load: A momentary load imposed on a caster or wheel when a load is dropped on the equipment or it strikes an obstruction or experiences vibration from a cart being pushed, pulled or mechanically driven into or over thresholds, expansion joints, curbs, cracks, potholes, tracks, and debris. The affects can overload or cause excessive strain on the kingpin of a caster and may result in the premature failure of the kingpin. Certain wheels may also not be able to handle impacts and may crack, deform or break apart. Bearings may also be affected. Stem Casters are particularly vulnerable to failure in shock loading applications.
  • Side Thrust: Strain imposed on a caster when the equipment it is mounted to, is hit in a sideways motion and the casters cannot turn fast enough to react to the movement. The affects can cause caster forks to bend (rigids are especially vulnerable), Wheel treads to wear irregularly or de-bond, wheel cores to crack at the hub, and bearings to bend, deform and malfunction. In applications were Side thrust is a issue, it is recommended to use heavier casters than a normal application may require.
  • Chemical Bond: Refers to the type of bonding of a wheel thread being glued or chemically fused to the core of the wheel. Some Chemical bonds may be broken when exposed to certain chemicals, oil, grease and other liquids, causing the tread to de-bond.
  • Mechanical Bond: Refers to the design of the tread material of a wheel being interlocked to the material of the core without the need of chemicals. Mechanical bonds are not affected by the same problems as a chemical bond.
  • Static Load: The load imposed on a caster or wheel while it is stationary.
  • Dynamic Load: The load imposed on a caster or wheel while rolling. Most capacities of casters and wheels listed in catalogs are this type.
  • Centered Hub: Refers to the configuration of a wheel hub, in this case the hubs are of equal length on each side. Wheels used in caster yokes require a centered hub.
  • Offset Hub: Refers to the configuration of a wheel hub, in this case, one hub is longer and protrudes past the side and tread of the wheel, the other side is short and is normally flush with the rim. This configuration is used on hand trucks and equipment were the wheel rides on a axle bar and the long hub causes the wheel to protrude from side of the equipment. Wheels with an offset hub are not used in casters.
  • Keyway bore: A square or rectangular channel cut through the length of the wheel bore that would match with a similarly shaped channel on a drive bar (steel bar or rod connected to a motor that turns it). A "key" (a square or rectangular piece of steel) just the right size to fit tightly, is placed in the channel between the wheel and bar, to secure the wheel to the drive bar. When the bar is mechanically turned, the wheel turns with it. The wheel would be called a drive wheel.

CONTACT B & C Industrial Products, Inc. for all your caster needs!