Industrial Casters
Casters are the wheels of industry, although technically, the wheel is only part of the caster assembly. The caster assembly is a wheeled device within a housing or framework used to provide mobility to an otherwise stationary object.
Casters come in many sizes, to accommodate any load capacity. They may be manufactured from a wide variety of materials, may swivel, lock, run on tracks, or work in tandem with other casters. They may be configured as traditional wheels or as bearings that facilitate smooth and/ or multidirectional movement. Some casters are designed to absorb shock.
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Casting Through History
The history of the caster is basically the history of the wheel. Some evidence has been produced showing the use of a wheeled device around 5,500 B.C. The configuration of the crude wheelbarrow meets the criteria for a caster and wheel, thereby pre-dating the cart wheel by about 1,000 years, although historical purists do not necessarily agree.
Revisionist theories suggest that casters of some sort were use in the building of the pyramids (ca. 2,600 B.C.) and the erection of Stonehenge (ca. 2,000 B.C.).
Leathern bowls, or glides were used under the legs of furniture in the 1700s. These devices allowed easy movement of desks and tables to take advantage of the light from windows.
Most modern historians credit David A. Fisher with the invention of the caster as we know it today. Fisher was granted a patent in 1876 for his heavy duty industrial caster which consisted of a simple metal wheel, mounted on an axle, attached to a frame that could be mounted to the bottom of furniture or equipment to provide mobility.
By 1900, casters with glass wheels wrapped in leather were widely used to move furniture without scuffing floors. Wheeled chairs, bicycles, tricycles, and carts were being made with rubber wheels, which provided traction and a bit of shock absorption.
John Weigel developed and sold small shoe rack casters from his garage in Hamilton Ohio, in 1907.
1925 saw the implementation of the kingpinless caster.
Designing Casters
A good caster design incorporates all aspects of the end use of the product to create a custom caster that suits each individual need. Many workplace injuries stem from improper lifting, carrying, pushing or pulling. Providing the right casters for each application will make the ergonomics of moving more conducive to employee well being, reducing accidents and injuries.
Heavier loads will require larger diameter wheels. Loads over 250 pounds should be fitted with bearing casters. Shock and impact loading should be considered when calculating overall load ratings. Using the largest diameter wheel that is feasible for the job increases the rollability, or ease with which the caster can be moved. Additionally, the larger the swivel radius, the distance from the center of the kingpin to the outside edge of the tread, the easier the turning capability of the caster.
Medical and food service uses will require that safe standards are met with regard to product materials and function. ANSI and OSHA have both set standards for reliability and cleanliness of casters used in these settings. A reputable caster manufacturer will provide products that meet and exceed these standards.
Parts of a Caster
A basic caster is made up of several parts. The frame, also known as the yoke, rig, fork, or housing, provides the main support for the caster, often a "U" shaped metal bracket. The legs of the bracket are fitted over a wheel, through which the axle is secured, to hold the wheel in place. The frame also provides a mounting surface for the caster plate or swivel head.
The axle may be a tubular rivet or a hardened, zinc plated, or stainless steel bolt with a nut. It may be forged or construction molded for additional strength.
Caster wheels may be of any diameter or width. They may be made of urethanes, polymers, plastics, rubber, composites, glass, wood, metal, or leather. For many heavy duty purposes, pneumatic tires are necessary. The wheels are purposely designed to withstand the weight of the equipment they support, the distance they must move, the surface on which they do move, and the frequency with which they are utilized.
Bearings are another rolling mechanism for casters. Bearings, packed in a round housing called a race, can be used vertically, to provide smooth rolling of the wheels, horizontally, as a "skate", or placed into the swivel head, to facilitate horizontal turning motion, or swiveling of the caster.
Rotation of the caster head is a secondary action of the swivel caster. The bearing race attached to the mounting plate or stem allows a piece of furniture or machinery to be moved easily in any direction. A swivel lock may be incorporated to stop the turning motion, but still allow the caster to roll.
Bearing casters are used to support extremely heavy loads or to move equipment omni-directionally. These bearings may be ball or roller types and must typically be lubricated at intervals to maintain operation. Many of these bearing assemblies are fitted with a receptacle called a zerk fitting, or simply zerk, into which grease is pumped periodically.
The caster may be screwed, bolted, welded, or pinned to the foot of an object. A mounting plate is made of heavy duty material, usually metal, that may be pre-drilled to receive a fastener, or fitted with a hollow kingpin, a sleeve that receives a locking pin attached to the object. It may have a caster stem that fits into a hole on the bottom of the object. These stems may be square. They may be tapered for use in wooden furniture, have a locking grip ring, an expanding adapter, or may be threaded.
Some casters are fitted with shock absorbing coiled steel, elastomeric, or hydraulic spring mechanisms. They reduce the vibration and noise during rolling, and lessen the impact of rolling across uneven or rough surfaces.
Caster Types
- Ball Bearing Casters
- Tools that, when affixed to an object, improve the mobility of that object. In contrast to wheeled casters, ball bearing casters feature a rolling, spherical bearing enclosed in a housing.
- Caster Wheels
- Positioned between the legs of the caster to enable mobility.
- Chair Casters
- Wheel assemblies that are attached to the bottoms of chair legs to improve mobility and shock absorption.
- Dual Wheel Casters
- Have resilient tread wheels that help to minimize overall height and maximize load capacity. The differential action of dual wheel casters reduces the “scrubbing” effect of soft treads against the floor, thus improving stability and swivel action.
- Furniture Casters
- Attached to a variety of furniture for industrial, business and consumer use. Furniture caster applications include beds, T.V. stands, chairs and portable tables.
- Furniture Glides
- Protect floors from damage caused by movement of furniture.
- Furniture Wheels
- Added to the bottom of various pieces of furniture to enable mobility.
- Heavy-Duty Casters
- Used in heavy weight applications involving large or heavy loads and significant shock. Heavy-duty casters can withstand loads of more than 300 pounds.
- Hollow Kingpin Casters
- Held together by a tubular rivet that can accommodate a stem or bolt to attach the housing or yoke to furniture or equipment.
- Institutional Casters
- Used for the transportation of furniture and equipment in the pharmaceutical, scientific and medical industries.
- Kingpinless Casters
- Constructed as a one piece unit without a separate axle. They are built to withstand extremely heavy loads and abusive situations.
- Leveling Casters
- Designed with a height adjustment feature. Often a simple screw mechanism, the height of each individual caster can be adjusted to accommodate differences in floor levels. The mechanism may be operated by use of a ratchet, thumbscrew, or turned by a wrench.
- Light-Duty Casters
- Designed to handle light loads and low levels of shock and are used for small racks and stands. Load capacities for light duty casters usually range from approximately 75 to 190 pounds per caster.
- Locking Casters
- Designed with some sort of braking device to stop rotation of the wheel or turning of the swivel. The locking device may be a simple friction sleeve or a positive locking pin.
- Medium-Duty Casters
- Designed to handle moderate loads and shock. Used for storage racks, trash cans and office furniture, medium duty casters have load capacities that usually range from approximately 200 to 290 pounds per caster.
- Pneumatic Casters
- Contain air that aids in the absorption of vibration and shock and provides the caster with a smooth roll on bumpy and uneven surfaces. Pneumatic casters are useful in the transportation of sensitive items like medical, electronic and computer equipment.
- Rigid Casters
- Do not swivel but provide only forward and backward motion. Rigid casters, often used in conjunction with swivel caster pairs, are useful in applications involving the transportation of items through an aisle or in a straight path, as they facilitate vertical (up/down) movement. Rigid casters are made with wheels that travel forward and back, but do not turn from side to side.
- Shock Absorbing Casters
- Aid in protecting delicate cargo from shock and reduce noise levels and wear on floors.
- Specialty Casters
- Custom-made casters that fulfill a particular need, such as shock absorption and inversion.
- Stainless Casters
- Made of stainless steel. Stainless casters have great corrosion, rust and chemical resistance and are easy to clean.
- Steel Casters
- Mobility enhancing tools that are constructed with steel components.
- Stem Casters
- Made with a pin or "stem" attached to the top of the yoke. This stem is fitted into a hole on the bottom of the article to be moved. Plate casters operate on a similar principle, but with a flat plate that may be welded or with pre-drilled holes for mounting with screws or bolts, instead of the pin.
- Swivel Casters
- Contain an extra bearing that allows the caster to rotate 360º. Swivel casters, commonly found on office chairs, provide convenient vertical, horizontal and diagonal movement.
Uses of Casters
Rolling chairs and grocery carts are perhaps the most familiar use of casters. They are found on heavier appliances, such as refrigerators and freezers, and on lighter furniture, such as tea carts or portable filing cabinets. Many pieces of luggage are fitted with casters, as are boxes and shipping crates used for transporting delicate items, such as musical instruments or electronic equipment. Smaller aircraft can be towed by caster dollies, allowing a single person to move an entire plane. Hospital gurneys and medical equipment are placed on casters that are antimicrobial, quiet, and capable of total or directional locking.
In industrial settings, nearly every piece of machinery or equipment that isn't welded down is set on casters for ease of movement and maintenance, as are the maintenance carts brought round to service them.
Casters that can support several tons each are used in the aerospace industry to move parts and equipment. Industrial manufacturing plants rely on casters to move a wide variety of items of various weights across varied surfaces under a variety of conditions, including high heat, or humidity.
Choosing a Caster
Casters are chosen on the basis of their application. There are several factors to consider when shopping for casters or having custom casters designed for a project. The material used in the manufacture of the caster wheel and body will depend greatly on the load capacity of the caster, which is the total amount of weight that will be placed on the caster.
Light duty caster wheels, often purchased as replacement casters for furniture, are typically made from plastics. Historical reproduction casters may offer glass or wooden wheels.
Heavy duty casters are typically made from hardened metals such as stainless- or zinc plated- steel, although many of the new composite materials are finding their way into this sector. Heavy duty caster wheels may also have a rubber or polymer coating, be made from solid rubber, polymers, or urethane, or may be solid or pneumatic tires mounted on metal or composite rims. The type, composition, and density of the tires will affect the weight capacity of the caster.
The type of wheel should be matched to the surface of the floor on which it will be operating. Consideration should be given to whether the floor is carpeted, steel, concrete, or wood. Additional concerns include whether it is bumpy, smooth, cracked, unlevel, or potentially strewn with debris. The environment in which the equipment is housed, such as humidity levels, extreme temperatures, and toxic or caustic products will also be a deciding factor when choosing the composition of the wheel.
Preventing Problems in Casters
Caster flutter, or shimmy is a disadvantage that takes place when the caster wheel oscillates as the speed increases. Sometimes called "speed wobble", the phenomenon occurs commonly on shopping carts and wheelchairs, and can be reduced by increasing friction on the swivel joint. In other applications, caster flutter can be controlled by increasing the distance between the front and rear wheels, known as the trailing distance, or placing more weight on the front end.
Bad housekeeping habits or exposure to weather can cause caster wheels to malfunction. Failure to grease the bearings or allowing debris to build up around the wheels or swivels will slow or even halt progress. Regular cleaning and greasing will keep the wheels moving. If the caster is beyond repair, standard replacement casters are available for many applications.
Caster Terms
- Axle
- Caster mechanism that, in conjunction with the nut or bolt, connects the caster wheel to the leg.
- Bore Size
- The amount of space required for caster wheel insertion.
- Caster Wheel
- The rolling caster part that provides movement to the equipment to which the caster is attached. Caster wheels are positioned between the legs of the caster and connected by the axle.
- Dust Cap
- The enclosure in which the hard cap sits. The dust cap protects the raceway from foreign materials.
- Frame
- Caster part, also referred to as a “fork,” “yoke” or “rig,” inside of which the caster wheel rests. The frame consists of two legs to which the caster wheel is attached by means of an axle, nut or bolt and a top plate; swivel casters also contain a swivel bearing between the top plate and the caster legs.
- Hardcap
- The upper raceway that bears the thrust.
- Hub
- The caster wheel core.
- Leg
- Caster part that is connected to the top plate. Legs are also attached to the caster wheel by the axle.
- Kingpin
- In a swivel caster, the rivet is inserted into a hole in the center of the top plate to connect the top plate to the rest of the swivel assembly.
- Load Capacity
- The recommended load that an individual caster can accept during standard operation conditions.
- Mounting Height
- Distance measured from the bottom of the unit to the rolling surface.
- Offset
- Distance measured from the center of the axle to the center of the attachment method.
- Shock Load
- The largest load a caster can handle under conditions of shock, such as bumps, uneven surfaces and the dropping of items onto the caster.
- Spanner Bushing
- The round, cylindrical stationary sleeve between the bracket legs, within the bearing and over the axle, through which the axle runs.
- Static Load
- The largest load that a caster can accept while stationary and under no exposure to shock.
- Swivel Lock
- Mechanism on a swivel caster that prevents the swivel from rotating but not from moving forward and backward like a rigid caster.
- Swivel Raceway
- Also known as a “swivel bearing,” it is the plate of a swivel caster located between the top plate and the legs that facilitates the swivel movement.
- Thread Guard
- Caster accessory that encases the wheel bearing and shields the caster wheel hub and frame from substances and materials, like threads, that might otherwise gather between the two caster mechanisms.
- Top Plate
- Also referred to as the “base plate” or “mounting plate,” it is a part of the caster that connects the caster to the equipment and is located on top of the legs or, in the case of swivel casters, the swivel bearing. A rigid top plate contains four holes, one in each corner of the plate, while a swivel top plate contains an additional central hole through which the kingpin is inserted.
- Yoke
- A component that supports the upper and lower raceways.