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Caster Terminology


Capacity – The maximum load per caster or wheel recommended for intermittent operation over smooth floors at speeds of 3 m.p.h. or less. (Gross weight should be divided by the number of casters or wheels on which it is distributed.)

Caster Size – A loose term for classifying casters, based on the nominal diameter of the wheel. For example: a “6 inch caster“ is any caster having a 6 inch diameter wheel.

Component Thrust – Designates side forces exerted on a swivel caster and properly counteracted by the secondary load bearing.

Direct Thrust – Designates those downward forces exerted on a caster by the load, and counteracted by the main load bearing.

Durometer – A measure of the hardness of resilient tread wheels. Face – The width of the wheel tread cross-section, measured at the base of the tread rather than at the point of floor contact.

Horn – The caster part which comprises legs, plus a base (horn base of swivel caster or mounting plate of rigid).

Horn Base – The flat part at the top of the horn from which the legs extend. Serves as lower raceway for main load bearing, and may provide a seat or upper raceway for secondary load bearing.

King Pin – A rivet, or threaded stud or bolt with nut, that holds the mounting plate and horn assembly of a swivel caster together. Forged as an integral part of mounting plate on higher quality casters for maximum rigidity and strength.

King Pin Nut – The nut on the bottom of a threaded king pin or bolt that permits the disassembly or adjustment of swiveling components; a maintenance feature of better casters not found with rivet type king pins.


Legs – The axle support brackets extending down from the horn base of a swivel caster, or from the mounting plate of a rigid caster.

Main Load Bearing – The row of steel balls or rollers that swivels the horn base in relation to the mounting plate of a swivel caster, and counteracts direct thrust.

Mounting Plate – The flat base, usually with four bolt holes, that forms the top of a caster and permits attachment by bolting or welding to a flat surface.

Offset – (swivel lead) The perpendicular distance between the vertical centerlines of the king pin and the axle of a swivel caster. Larger offsets afford easier swiveling, shorter offsets greater strength.

Overall Height – The vertical distances from the top of the caster mounting plate to the bottom of the wheel.

Raceway – A surface in which bearing balls or rollers rotate.

Retaining Washer – A steel washer pressed into the hub ends of some roller bearing wheels to hold the bearing in place.

Rig – A swivel or rigid caster assembly less wheel, that may or may not include axle, nut or spanner bushing.

Rockwell C – System for measuring hardness of metals.

Rollability – Ease of starting and rolling, measured by drawbar pull. This is determined by load, floor conditions, and type and size of wheels and bearings.

Secondary Load Bearing (Commercial) – The row of steel balls that rides in the raceway between the horn base and secondary bearing retainer of a swivel caster to counteract component thrust. It is this bearing that characterizes the popular “double ball race” caster.

Secondary Load Bearing (Precision) – A precision self contained bearing, usually a tapered roller bearing, operating between the horn base and the king pin of a swivel caster. Constitutes the most efficient means of assisting swivel action by counteracting component thrust.

Spanner Bushing – A non-rotating sleeve of seamless steel tubing that fits over the axle in many casters. Serves the dual purpose of providing a smooth inner raceway for the wheel bearings, and strengthening the caster by permitting the legs to be tightened against it.

Swivel Radius – The horizontal distance from the centerline of the caster king pin to the outer edge of the wheel (a measure of clearance required to swivel a caster 360°).

Thrust Washer – A steel washer between the hub ends of the wheel and the inside of the caster legs.

Yoke – Alternate term for horn.

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