Caster
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A caster 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 as swivel casters, such that the wheel will automatically swivel, aligning itself to the direction in which it is moving. Swiveling casters are sometimes 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.
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[edit] Designs and applications
Casters provide furniture mobility and shock absorption. Furniture casters, colloquially known as furniture wheels, are used on beds, chairs and tables in homes, offices, schools, hospitals, banquet halls, restaurants, retail stores, truck garages and industrial manufacturing plants and warehouses. Casters are advantageous for moving vehicles or platforms such as shopping carts along straight or curved paths. Furniture glides are attached to the bottoms of furniture legs to provide friction reduction, like casters; however, unlike casters, glides have wheel-less, smooth nylon or metal surfaces which allow furniture to sit or be moved across the floor with minimal friction and scratching. Casters are typically used on carts and furniture and are mounted underneath these platforms.[1]
Swivel casters allow caster wheels to pivot, giving furniture on swivel casters greater ease of movement than furniture attached to rigid casters. Being able to roll tables, chairs and equipment platforms around smoothly is important in manufacturing warehouses, where equipment, workstations and chairs are frequently moved around the facility. It is also very important in hospitals, where computer stations, beds, IV racks and other equipment are constantly in transit.
A caster is defined as a wheel mounted to a fork; swivel casters have an additional offset steering joint. The steering joint allows swivel caster wheels 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.[2]
[edit] Swivel Casters
Swivel casters allow caster wheels smooth motion multiple directions, giving furniture on swivel casters greater ease of movement than furniture attached to rigid casters. During straightforward motion, the swivel caster will tend to rotate parallel to the direction of travel, causing the vehicle to naturally maintain travel in a straight line. It is important to consider aspects such as load carrying capacity, swivel caster and wheel dimensions, flooring materials and environmental conditions before selecting a swivel caster for any application. Being able to roll tables, chairs and equipment platforms around smoothly is important in the manufacturing industry, in workshops where equipment, workstations and chairs are frequently moved around the facility. It is also very important in the medical industry, such as hospitals, where computer stations, beds, IV racks and other equipment are constantly in transit.[3]
[edit] Materials and Construction
Caster wheels, axles, wheel centers and housings are fabricated from different types of polymers, stamped metals and rubber materials. Wheels are most often made from a grey, non-marking thermoplastic rubber, although rubber, polyamide and tread polyurethane are used for heavy-duty casters. Rubber-wheeled casters in both medical and industrial applications provide quiet, shock-absorbing movement when moving heavy tables, workstations or beds. Caster brackets are the hardware which attach to the furniture, while caster housings are the hardware which connect the caster wheel axle to the bracket. Brackets and housings for industrial casters are made from stamped steel, while lighter-duty office and household casters are usually thermoformed polypropylene. Hospital bed casters, equipment table casters and similar equipment use locking casters to safely secure furniture in place; locking casters have an additional piece of locking hardware attached to the wheel housing.
Industrial caster applications require heavy duty casters which can withstand heavy loads, uneven floor surfaces and being jarred against ledges. For this reason, industrial furniture casters are fabricated from stamped steel housings and brackets, aluminum or cast iron wheel centers and axle nuts and thermoplastic rubber or tread polyurethane caster wheels. Some caster assemblies are two-wheeled, providing a broader, more stable support for heavy equipment. Medical and hospital furniture casters are medium-duty with soft rubber of polyamide wheels and polypro injection molded housings for smooth, quiet operation. Office and household chairs are made from lower-duty thermoplastic materials, although bed casters typically have strong metal housings.
A swivel caster consists of a caster frame and a wheel, constructed so that the caster frame houses the wheel. When the wheel is installed into the caster frame, two legs are attached to either side of the wheel and the wheel is held in place between these two legs by an axle or bolt. Positioned above the legs is a swivel bearing, a component that provides the swivel caster with a full 360º range of movement. The swivel caster is commonly attached to equipment through a mounting plate, or top plate, that is connected to both the swivel bearing and to the two legs. Mounting plates generally contain four holes used to bolt the swivel caster to the equipment. Typical materials used for swivel casters include: metal and plastic coated metal, which has the greatest load capacity; rubber, which allows for silent movement; hybrid rubber, which provides both silent movement and a high load capacity; and thermoformed plastic, which offers a high load capacity.
[edit] Caster Wheels
Caster wheels are positioned between the legs of the caster frame, which can be mounted to objects by an axle or bolt. Since the function of the caster wheel is to roll on different flooring surfaces, it is situated as the lowest part of the caster assembly. Caster wheels can be made specifically for different industries or applications. Some of the more common wheel types include: pneumatic, or "air tire", caster wheels which are designed for fragile, heavy cargo or rough outdoor surfaces; dual wheel, or twin wheel casters, which are durable in high load applications; and durable service and furniture wheel casters, which are specifically designed to add mobility to furniture. Caster wheels range from standard to heavy duty and can have hard or soft tires; they vary in size and are able to accommodate different load capacities on various floor types such as carpet, plastic or hardwood.
Caster wheels can be solid or spoked. The most common caster wheel materials include: thermoplastic rubber, which is ideal for clean environments; rubber, which absorbs shock well and is noiseless; and molded urethane. Molded urethane is one of the most common caster wheel materials, as urethane may be molded to varying degrees of hardness to increase or decrease shock absorption. Urethane wheels perform well in high-moisture applications and are more shock-absorbent and wear-resistant than rubber. HDPE and other polyethylene resins are cost-effective materials that are widely used in industries such as industrial warehouses and high temperature food processing plants. Wheels may also be nylon, which provides excellent resistance to chemicals and are ideal under wet conditions.[4]
[edit] 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.[5] 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.
[edit] 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 with side a retraining fixture.
[edit] Kingpin Casters vs. Kingpinless Casters
In the past, all swivel casters used kingpins both to attach the caster housing to the caster bracket and to provide pivot motion. Kingpins absorb a significant amount of stress, particularly in high-impact industrial applications, and they are consequently the number one cause of swivel caster failure. "Kingpinless" casters have become quite common among medical casters, office casters and even industrial casters. In kingpinless casters, a raceway of ball bearings connect the upper bracket to the caster housing in place of a kingpin. These are not only far more wear-resistant, but provide smoother swivel motion as well.
[edit] See also
[edit] References
- ^ Industrial Casters Info Page IQS Directory
- ^ Siegwart, R. and Nournakhsh, I. "Introduction to Autonomous Mobile Robots", MIT Press, Cambridge, MA , 2004.
- ^ Swivel Casters Info Page IQS Directory
- ^ Caster Wheels Info Page IQS Directory
- ^ 12 Making the Front Wheels, Center for International Rehabilitation Chapter
