Caster

Typical swivel caster

A caster (also castor according to some dictionaries^[1][2]) is a wheeled device typically mounted to a larger object that enables relatively easy rolling movement of the object. Casters are essentially special housings that includes a wheel, facilitating the installation of wheels on objects. Casters are found virtually everywhere, from office desk chairs to shipyards, from hospital beds to automotive factories. They range in size from the very small furniture casters to massive industrial casters, and individual load capacities span 100 pounds (45 kg) or less to 100,000 pounds (45 t). Wheel materials include cast iron, plastic, rubber, polyurethane, forged steel, stainless steel, aluminum, and more.

Types of casters

Casters comprise two major categories: a swivel caster pivots around a kingpin, so it rotates as well as rolls, while a rigid caster has its wheel mounted in a fixed frame, or rig, and only rolls.

Swivel caster

This type of caster allows for movement in multiple directions. They can have one or two sets of raceways that allow the caster to swivel 360 degrees under a load. The different types of swivel casters include the following:

• Locking casters: There are several devices that can be added to casters to prevent the wheel from rotating or the swivel assembly from turning.
• Kingpin-less casters: This caster does not have a bolt and nut kingpin. The raceways are a one piece construction forged together. This design is extremely durable and can be used in abusive applications and shock load applications where kingpin type casters may fail.
• Hollow Kingpin casters: This type of caster has a tubular rivet that holds the caster together. The hole in the rivet can accept a bolt or a customized stem for any type of mounting requirement.
• Plate casters: This is the most common type of means to mount a caster to a unit and is sometimes called the top plate. Most mounting plates contain four holes used to bolt the caster to the unit. Top plates are offered with various hole patterns to match numerous types of mounting requirements.
• Stem casters: This type caster can have various stem styles to be used to mount the caster to a unit. Some common types of stems are threaded, round or square with mounting holes, friction ring & expandable stems.

Rigid caster

This style of caster only allows forward and backward movement. Rigid casters tend to be stronger than swivel casters. They are rated at the same capacity as the swivel casters for safety reasons. They can be a one piece construction or a two piece construction that is riveted or welded.

Caster design

The key dimensions to consider when determining the proper type of caster and caster size for a particular type of equipment and application are its overall height, swivel radius, and swivel offset (see Caster Dimensions diagram). The key elements of a caster include the following:

• Mount: Casters mount to equipment or carts in three common manners. A caster top plate allows the caster to be bolted to the underside of the equipment. A caster stem may screw into place or ‘snap’ into place. Casters may also have a bracket allowing the caster to be mounted to vertical panels.
• Swivel head: A rigid caster allows single back and forth direction. A swivel caster allows for 360 degree directional movement. The swivel action of a caster depends on bearings and lubrication. Bearing designs include ball bearings and raceway bearings. Grease fittings serve to inject grease into the axles, caster raceways and wheels. Grease fittings may also be known as "zerks."^[3]
• Yoke: The part of a swivel or rigid caster and can be considered a frame. The caster yoke serves to hold the wheel in place. The yoke, working with a swivel head allows the caster wheel to operate in a 360 degree manner. The yoke is also known as the fork, rig or housing.
• Spring mechanism: Certain casters serve a shock absorbing or vibration dampening function so there needs to be a spring mechanism in the caster design. The typical spring mechanism is a coiled steel spring. There are also hydraulic and elastomeric springs.
• Wheel: Caster wheel materials include elastomers (rubber and polyurethane), phenolic, nylons and even steel. There are numerous grades of all of these materials. The proper wheel selection is dependent on application factors such as floor conditions, load, rollability, speed and climate.
• Wheel bearings: Most caster applications require the wheel to function with a bearing. There are numerous bearing options depending on the wheel design and the application being considered. Common bearings used for caster wheels include: roller bearings, tapered roller bearings, ball bearings, precision ball bearings, Delrin^[4] and self-lubricating sleeves.
• Axle bolt and nut: The axle bolt locates the wheel into the caster yoke. The nut secures the axle to the yoke. In some applications the axle bolt and nut may be hardened or zinc plated for corrosion resistance. Stainless steel may occasionally be used.

Applications

Casters are available in a large selection of various rigs and yokes, wheel materials, swivel offsets, and wheel configurations. In many cases, it can become extremely difficult to choose the right caster for the application. In order to help the user to determine the right caster to use, it's important to take a couple of factors into consideration, which include:

1. Load capacity (the total load applied on the casters)
2. The number of casters to be used on the equipment (usually four or six casters)
3. Floor type (concrete, steel, linoleum, carpet, etc.)
4. Floor condition (are there cracks, bumps, unlevel floors?)
5. Environment (is the equipment operating in high temperatures, wet or humid conditions, etc.)
6. Floor cleanliness (are the floors clean or contain debris such as metal chips, grease, gravel, etc.)

Many casters are specifically designed for each of the following applications.

Typical caster found on an office desk chair.

Industrial and automotive

Casters can be designed to meet the ergonomic needs of an industrial or automotive plant setting, which typically means floor conditions can range from being relatively clean, to having some debris. For these applications, casters can be designed using a variety of wheel materials, including thermoplastic elastomers, polyurethane, and soft rubber wheels. Harder wheels, such as the elastomer and polyurethane can be used on smoother concrete plant floors to give easy rolling for plant equipment, and the softer wheels such as those made with rubber, can be used on various floor surfaces with debris. The increased swivel offset on many rigs can also be designed into the caster to reduce the swiveling force of casters (the force required to turn a caster in the direction of travel).^[5] Many industrial casters are also designed with multiple wheels to allow for an increased load capacity to be handled by the equipment.

Multi-wheel swivel caster used in heavy industrial applications

Heavy industrial

A broad range of industrial applications require the movement of large, heavy objects. Aerospace is one such category, in which large airframes or engine parts are moved throughout manufacturing plants. Casters in these plants are high capacity, with load limits of several tons each or more. The caster wheels must be robust, and are normally forged steel, nylon, or metal with a polyurethane tread. In some cases they may be pneumatic or solid rubber, and the casters may be multi-wheel (i.e., having two or more wheels per caster).

High temperature

Casters used in and around furnaces and ovens must withstand extreme heat. General purpose casters cannot survive excessive heat because tread materials, lubricants and other critical components can fail. For example, at 200 degrees F, common resilient tread materials are at or above their functional temperature limits. High temperature casters employ special lubricants and materials that can withstand higher heat. Additionally, for very high temperatures, special sleeve bearings may be used that have no lubricant whatsoever.

Medical

Central locking caster

Casters are widely used in the medical industry for applications such as hospital beds, equipment carts, surgical tables, and IV poles. Medical casters must tolerate exposure to the various cleaning and disinfectant fluids used in hospitals. Hospital beds and stretchers are typically equipped with a central caster locking system that allow all casters to be simultaneously locked by one person. Medical equipment casters must often have high load capacities, with low profiles and swivel resistance, to accommodate high patient or equipment weight while providing ease of operation over different floor types and through tight spaces.

Safety and reliability are paramount for casters that support (e.g., hospital beds) patients. Consequently, medical casters are subject to industry requirements such as ANSI, BS EN12531,^[6] and IEC 60601.

Health and safety regulations

The primary example of the benefits of using casters is to reduce the risk of workplace injuries for its users, particularly overexertion. Overexertion^[7] occurs when the caster being used is not suited for the application, mainly due to the wrong wheel material or rig, causing injury to the user.

There are many health and safety organizations that enforce and regulate the allowable forces and noise levels that casters can make on a plant floor, and include the Canadian Centre for Occupational Health and Safety (CCOHS), Occupational Safety and Health Administration (OSHA). For example, CCOHS recommends that the maximum horizontal force someone should exert is 50 lbf.^[8] Liberty Mutual has also produced a Snook Table that provides the percent population of male and female able to push at a given horizontal force.^[9] This force can be adjusted to a safe level by providing the right caster wheel material and rig.

To ensure the design integrity of casters, the Institute of Caster and Wheel Manufacturers^[10] (ICWM) working in collaboration with the American National Standards Institute (ANSI) has developed the ANSI ICWM: 2012 The ICWM Performance Standard for Casters and Wheels^[11] which is intended to provide manufacturers, specifiers and users with a common basis for evaluating the safety, durability, structural adequacy and technical requirements for group specific casters and wheels.

Caster flutter

One major disadvantage of casters is flutter. A common example of caster flutter is on a supermarket shopping cart, when one caster rapidly swings side-to-side. This oscillation, which is also known as shimmy, occurs naturally at certain speeds, and is similar to speed wobble that occurs in other wheeled vehicles. The speed at which caster flutter occurs is based on the weight borne by the caster and the distance between the wheel axle and steering axis. This distance is known as trailing distance, and increasing this distance can eliminate flutter at moderate speeds. Generally, flutter occurs at high speeds.

What makes flutter dangerous is that it can cause a vehicle to suddenly move in an unwanted direction. Flutter occurs when the caster is not in full contact with the ground and therefore its orientation is uncontrollable. As the caster regains full contact with the ground, it can be in any orientation. This can cause the vehicle to suddenly move in the direction that the caster is pointed. At slower speeds, the caster’s ability to swivel can correct the direction and can continue travel in the desired direction. But at high speeds this can be dangerous as the wheel may not be able to swivel quickly enough and the vehicle may lurch in any direction.

Electric and racing wheelchair designers are very concerned with flutter because the chair must be safe for riders. Increasing trailing distance can increase stability at higher speeds for wheelchair racing, but may create flutter at lower speeds for everyday use. Unfortunately, the more trail the caster has, the more space the caster requires to swivel. Therefore, in order to accommodate this extra swivel space, lengthening of frame or extending the footrests may be required. This tends to make the chair more cumbersome.

Caster flutter can be controlled by adding dampers or increasing the friction of the swivel joints.^[12] This can be accomplished by adding washers to the swivel joint. The friction increases as the weight on the front of the chair increases. Anytime the caster begins to flutter, it slows the chair and shifts weight to the front wheels. There are several online anti-flutter kits for retrofitting wheelchair casters in this manner. Other methods of reducing caster flutter include increasing swivel lead, using heavier grease, reducing the mass of the wheel, or increasing friction with the ground by changing materials.

Casters are also stopped completely using caster cups.

See also

• Wheel
• Material handling
• Material handling equipment
• College-Industry Council on Material Handling Education
• Automated storage and retrieval system

Notes and references

Further reading

• Kulwiec, R.A., Ed., 1985, Materials Handling Handbook, 2nd Ed., New York: Wiley.
• Mulcahy, D.E., 1999, Materials Handling Handbook, New York: McGraw-Hill.

External links

Look up caster in Wiktionary, the free dictionary.

• Institute of Caster and Wheel Manufacturers (ICWM)
• Casters and Wheels
• Material Handling Industry

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