The Science of Motion – Designing Caster Wheels to Function Efficiently
Caster wheels (they may also be correctly spelled as “castor”, both words are interchangeable) are non-powered wheels fitted to the feet or undercarriages of large objects. Their purpose is to allow those objects to be moved safely and with control.
Casters are seen in many varieties in everyday life. A supermarket trolley features a caster wheel, for example; as does a pushchair. The materials used to make a caster are dictated by the weight and form of the object moved; and of course by the requirements of the human user.
In all cases, there is a health and safety element inherent in the design considerations. The caster wheel must be able to come into contact with the human foot (in appropriate clothing for the circumstances) without causing injury. Where heavy loads are concerned, it is obviously impossible to design any type of wheel that will prevent injury – but one that can minimise damage, for example a rubber coated wheel, should be preferred.
There are three basic types of caster wheel – the rigid caster; the swivel caster; and a brake or lock caster. The third type may also be defined as a subset of the first two – i.e. you can have a swivel caster with a brake or lock; and you can also have a rigid caster with braking functions.
Rigid casters have no directionality other than forwards or backwards to their permanent alignment. They are typically found on two wheeled trolleys )such as a sack trolley), where steering is done by human agency rather than mechanical assistance.
Swivel casters include a swivel above the fork setting for the wheel. The swivel effectively allows the wheel to be pointed in any direction. Where more than two of these wheels are situated, the operator may then orient the object’s motion along any plane necessary: and may execute quick turns in a relatively short space.
A brake or locking caster (which may, as noted, refer to either swivel or rigid caster wheels) has a braking mechanism applied. The most common form of braking mechanism for a brake caster is a simple foot switch. The switch is depressed using the toes, which locks the wheel and prevents the wheeled object from moving any further. This safety function is seen on casters ranging from supermarket trolleys to large stock movement cages.
More complex or heavier duty brakes may be operated using handheld levers, which apply greater braking force to a larger wheel.
All swivel casters are prone to “fluttering”. This is the motion where the wheel appears to stutter as the wheeled object is moved along. Fluttering is a motion naturally expressed by caster wheels travelling at specific speeds, and may be exacerbated by any unevenness in the ground over which the wheeled object is travelling. If a wheel moves in mid-air, for instance, and hits the ground sideways, then the object’[s motion may jerk suddenly in an unexpected direction.
As casters are used on hospital equipment (such as beds and operating tables) the control of flutter is one of the primary concerns of the designer or engineer. The benefits of having 360 degree directionality are huge – particularly when manoeuvring a bed-ridden patient in an emergency, to an operating theater – and therefore are not willingly discarded in favour of rigid casters. The problem of flutter is usually controlled using dampers.