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No one knows who invented the wheel. The oldest wheel in existence was discovered in Mesopotamia and dates back to 3,500 B.C. For thousands of years, man has known that wheels can make a job a whole lot easier.

In modern lab applications, equipment and heavy loads are moved on casters. A caster is an assembly consisting of a housing (called fork or rig) and one or more wheels. The wheel is affixed to the housing with an axle. This allows it to rotate freely, either directly or with the use of bearing(s). Casters are affixed underneath carts, material handling trucks, or animal cages, to help facilitate their movement within a lab or throughout a facility. Swivel casters have the ability to rotate 360 degrees, whereas rigid casters permit straight-line travel only.

The Right Caster for the Job
When transporting a load on casters, it is important to match the caster to the job. Choosing the right caster requires an assessment of the load to be moved, the floor surface, and environmental conditions. For lab animal facilities, ergonomics, maintenance, and cleaning also play an important role in caster selection and operation. Identifying the applications and conditions where casters will be used can increase efficiency and safety.

Determining Caster Load
There are several important questions that are starting points in caster selection. The first of these is the load or capacity that the casters will have to bear. It is recommended to first determine the maximum weight to be moved inclusive of the load and weight of the cart, equipment, or object. Even if a maximum load were only going to occur occasionally, averaging the weight or underestimating the load would only result in choosing casters that won’t roll easily. Incorrectly chosen casters may collapse, bend, or break causing serious problems.

The rated load of a caster is the maximum weight that may be imposed on it for manual operation. The load capacity of a caster is calculated by adding the weight of the object to be moved, plus the apparatus that has to carry that object, divided by the number of affixed casters. In lab animal facilities, the carrying load on casters would consist of the cage system when fully loaded with animals, feed, and related weights. The caster load would be the full load of the cage system divided by the number of casters affixed to the cage unit.

Wheel Selection
When determining wheel size, typically, the largest wheel possible is the best choice. Larger wheels move more easily, carry more load, and can handle uneven surfaces better. Of course, it is important to make sure that the wheel size does not affect the overall height. After the casters are added, the wheeled item still needs to fit through doorways or into smaller openings, depending on the equipment.

Wheel hardness must also be considered. In laboratories, equipment is generally manually pushed or pulled. Thus, both caster load and wheel diameter are important considerations regarding the object to be moved. For a given load, the force required to push or pull decreases with an increase in wheel diameter, and the hardness of the wheel. Durometer is the measure of the hardness of the wheel tread material. The higher the durometer, the harder the wheel. Hard tread wheels provide greater rollability than soft tread wheels.

Hard tread wheels are the common choice for heavier loads. Hard tread wheels roll easily on smooth surfaces and require less starting force - the amount of energy needed to get a stationary object into motion. Soft tread wheels move more easily over bumps or rough surfaces. These are also generally quieter and provide better floor protection.

Wheel Material and Floor Surface
Wheel material comes in a wide variety, such as polyurethane, rubber, nylon, and metal. Choosing the appropriate wheel material depends on the floor surface and the environment. When caster wheels and floors are a poor match, they can do damage to each other. Rough floors can damage some wheel materials and some wheel materials can leave marks and abrasions on floors. For most lab animal facilities, smooth flooring is standard. The wheel should be able to bear a certain load while not marring the floor.

Rubber wheels provide a resilient tread on wheels and may be of natural rubber, synthetic rubber, or a combination of both. Rubber provides a softer durometer rating and offers maximum floor protection. Rubber wheels transmit less vibration, and this may be very important in laboratory environments. Nylon wheels are much harder in durometer, but offer higher load capacity and roll easier. Because they are harder, floor protection is diminished over time with these types of wheels. Similarly, steel wheels cause floor damage in lab environments, but do carry very heavy loads. Polyurethane wheels provide maximum rollability, long life, and wear well in abrasive conditions while offering properties of both hard and soft tread materials, and the advantages associated with them. There are many new synthetic materials emerging in the caster industry that now offer quick, quiet, and clean wheels for laboratory applications.

Caster Accessories and Add-Ons
Casters can be equipped with wheel brakes, total lock on both wheel and swivel, directional lock, and other features. These options depend on how the wheeled equipment is to be operated or stored. For example, a directional lock caster will act like a swivel rotating 360 degrees, but once the directional lock is engaged, the caster will perform like a rigid and travel in a straight line only. For lab animal equipment, total lock and directional lock casters are a very good combination.

Environment
Chemicals, dust, temperature, and moisture or water are environmental factors that will affect wheel selection, operation, and maintenance. Chemicals, mostly used for cleaning, can affect the wheel material. Dust and other small particulates can accumulate in the moving parts of the caster. The temperatures used for cleaning, as in rack washers, need to be factored in to prevent damage from high heat exposure. Rust and corrosion can occur from exposure to high levels of moisture on a continual basis. These items are not difficult to determine. Taking the time to make the assessment will ensure a smooth ride and a longer life for the casters. If a piece of equipment will be used or moved to multiple areas, the environmental conditions of each area should be factored into the equation.

For lab animal facilities, the most suitable casters are made of stainless steel or high strength composite nylon material. These properties, over time, ensure long life and functionality of casters in laboratory facilities. Unlike zinc-plated metal, rust is also minimized with stainless steel or nylon casters.

Safety and Ergonomics
Unless there is some form of motorized operation, moving equipment on casters requires people. The purpose of using casters is to make heavy loads easy to transport. Avoiding strain is one of the most important ergonomic considerations. To keep workers safe from excess strain in pushing or pulling equipment, the loads must be kept appropriate. Exceeding the capacity of the casters means that the equipment is harder to move.

Sprains and strains are the most common type of injury that result from poorly specified casters on laboratory equipment. Starting effort is the point of greatest exertion. Taking an item from a stationary position into motion can put the most stress on workers. It is important that the casters roll smoothly and that the operators are trained to not overload the capacity. Making one trip with a load in excess of capacity can be more harmful than multiple trips with appropriate or lighter loads. As noted earlier, the largest wheel for the application is the easiest to push/pull. Where equipment is to be pushed or pulled, it is most desirable to specify wheels with roller or ball bearings. Plain bearings, or busing inserted in the wheel, are suitable for infrequent movement of objects.

Maintenance
In order to keep casters rolling smoothly and maneuvering well, the caster should be inspected for frame tightness, lubrication, tread wear, free movement of the swivel assembly, and for corrosion or dirt. It is not difficult to make a quick assessment of any maintenance issues. Repairing and replacing casters as needed is an important safety item in a facility. Preventing operational problems doesn’t require much time or effort and can reduce potential injury to personnel. Remember, it is not just the squeaky wheel that gets the grease.

Caster Configurations on Laboratory Animal Equipment
To facilitate maximum maneuverability when pushing or pulling laboratory equipment, various caster-mounting configurations may be used. Several types of mountings are illustrated in Figures 1, 2, and 3.

The most common combination of casters on any moving object is two swivel and two rigid casters. This combination provides the most practical and inexpensive arrangement for straight and long distances. Good load capacity is achieved. The maximum load of the moving object is divided by four to calculate the load capacity of each caster.

Where side motion is frequently needed, the four-swivel arrangement is excellent. Exceptional maneuverability is facilitated by this combination, while maximizing load capacity per caster. Again, the full load is divided by four to ascertain the maximum capacity of each caster. In lab animal facilities, this combination facilitates ease of movement in tight corridors or where sideways action is required. However, having four swivels on lab equipment is not desirable for straight line pushing as it becomes difficult to guide. But, a set of directional lock casters can solve the sideways running problem without compromising the 360-degree maneuverability.

The combination of four swivel casters and two rigid casters, with equal load height, provides very high load capacity, with good maneuverability and stability. The two rigid casters help to distribute and reduce the load on the swivel units while the four swivel casters provide easy steering. With six casters having level contact with the floor, the maximum load capacity is equally distributed on all six units. An alternative combination is achieved by having the rigid casters mounted in the middle to allow the load-carrying object to pivot on the central wheels. In such a case, the full load is distributed on only four casters at a given time: the two rigid casters in the middle, plus only one pair of swivel casters that are in contact with the floor. The load height of the rigid set of casters in the middle must be marginally higher to allow pivoting and added maneuverability.

Where the Rubber Hits the Road

Anyone who has pushed a grocery cart with a broken or wobbly wheel has gotten first hand experience in what a poorly operating caster can mean. The noise and difficulties in maneuvering the aisles with a damaged wheel can make carrying the groceries seem like an easier option. Matching the caster to the application in a facility can improve transport operations, extend the life of the caster, extend the life of flooring, and, most importantly, reduce the risk of injury to personnel. When casters work, casters make work easier.

Mike Titizian is General Manager at MedCaster, 1600 Bishop Street, Cambridge, ON N1R 7N6; 800-330-8411 (within US); 800-643-5515 (within Canada); titizian@medcaster.com; www.MedCaster.com.

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