caster

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Casters on a desk chair

A caster (or castor) is an undriven, single, double, or compound wheel mounted on an object to make movement easier. Found on shopping carts, office chairs and material handling equipment, casters may be fixed to roll in one direction, or mounted on a pivot, such that the wheel will automatically swivel, aligning itself to the direction in which it is moving. Swiveling casters are sometimes themselves attached to handles, so users can turn the caster into the desired direction.

Casters are used in many industrial applications. Heavy duty and high capacity casters are used on platform trucks, carts, assemblies, and tow lines in plants.

Designs and applications

A caster is defined as a wheel mounted to a fork, but has an additional offset steering joint. The steering joint allows the wheel to rotate freely in 360°. This allows for easy turning of objects without changing the direction of the chassis that the casters are mounted to. The angle and distance of the wheel axles and steering joint can be adjusted for different types of caster performance.^[1]

Casters are typically used on carts and furniture and are mounted underneath these platforms. Casters are advantageous for moving vehicles or platforms in both straight and turning motions. Casters are commonly used on supermarket shopping carts.

During straightforward motion, the swivel caster will tend to rotate parallel to the direction of travel. This can be seen on a shopping cart when the caster rotates backwards during forward motion down an aisle. A benefit of this caster rotation is the vehicle naturally tends to travel in a straight direction. Precise steering is not required because the casters tend to maintain straight motion.

This is also true during turning. The caster rotates again parallel to the turning radius and provides a smooth turn. The can be seen on a shopping cart as the wheels rotate differently depending on how tight a turn is made. Because of these two qualities, casters are often used on dollies, office chairs, and wheelchairs.

Casters can be designed in many different sizes and materials depending on application. Generally, they are made from rubber, plastic, nylon, aluminum, or stainless steel. Generally, casters operate well on smooth and flat surfaces.

Caster flutter

One major disadvantage of the casters is flutter. A common example is on a supermarket shopping cart, when one caster rapidly swings side-to-side. This oscillation is known as caster flutter and occurs naturally at certain speeds. The speed at which a caster flutters is based on the weight on the caster and the distance between the wheel axle and steering joint. This distance is known as trailing distance. 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 it 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 maybe required. This tends to make the chair more cumbersome.

One method for removing caster flutter is by increasing the friction of the swivel joints.^[2] A simple method for completing is 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 wheel chair casters in this manner.

Other related wheels

There are four main classifications of wheels: Standard Wheels, Casters, Omni-directional Wheels, and Spherical Wheels. A standard wheel has a center rotating hub (or bearing) and a compliant material of the outer side. A caster is a wheel mounted to a fork and has an additional offset steering joint.

The omni-directional wheel (Mecanum wheel, or Swedish wheel) is made of a large central hub with many additional smaller wheels mounted along the perimeter such that their axes are perpendicular to the central wheel. The central wheel can rotate around its axis like traditional wheels, but the smaller wheels can also enable movement perpendicular to the central axis.

A spherical wheel is a truly omni-directional wheel and is generally a spherical ball mounted inside a retraining fixture.

See also

• Caster angle
• Caster board

References

External links

• Caster patents