Load Capacity of Casters and Wheels
The ease with which carts, laboratory equipment, mobile tables, or animal cages are maneuvered in any application are dependent upon the casters affixed them. But are the casters suitable for the load carrying capacity of the equipment?
More often than not, the selection of casters and wheels becomes an afterthought once the application of the laboratory equipment is designed. Sometimes, the overall height of the equipment is an issue and, unfortunately, the fix is usually addressed by simply having smaller casters installed on the equipment. While this allows the equipment to pass through lower threshold door openings, or enter a desterilizing chamber, the capacity of the casters become a critical functional and safety issue.
Knowing the appropriate load carrying capacity of casters and wheels in any laboratory setting is important for the continued safe operation of carts and equipment in the work place. There are several factors that influence the overall weight that the casters should carry. The determination of the right caster always requires an assessment of:
- The load to be moved
- The condition of the floor surface
- The number of affixed casters
- The size of wheels installed
Assessing the gross load to be moved is a first step in calculating the load rating of casters. The load to be transported includes the weight of the equipment or cart, plus the load being carried on it (i.e. total gross load). The load being transported and the weight of the equipment can be easily determined by weighing the specific load, or checking the equipment manufacturer’s specifications. The load per caster is then calculated as the total gross load divided by the number of casters affixed to the cart or equipment.
Sharing the Load
With relatively smooth floors, the presumption is that all casters share an equally distributed load factor. In research laboratory environments, smooth floor conditions tend to be the standard in the industry. Generally there are either four or six casters mounted to most primate housing systems or transport equipment. Some mobile diagnostics stands have up to five casters, while other maintenance specific apparatus may have only three. In any case, dividing the total gross load by the number of mounted casters equals the imposed load on each caster. The manufacturer’s stipulated load capacity for each caster must be, at a minimum, equal to or, preferably, higher than this calculated imposed load per caster. Consult the caster manufacturer and ask for their test criteria, methodology, and specific caster test results prior to use or specifications of caster application. The safe application of casters is paramount when it comes to caster load capacity of manually propelled carts or equipment in any work place environment. (For more information about caster tests and standards, consult industry sites such as www.icwm.org or www.mhia.org.)
Workplace situations with uneven floors, or carts transported outdoors over uneven ground, require different calculations in determining imposed load on casters. The determined load of casters traveling over uneven floors is calculated by dividing the total gross load by the number of affixed casters, less the number of casters that may not be in contact with the ground while the equipment is in motion. Generally, with four casters affixed to the cart or equipment, the total load is then divided over three casters (the presumption being that at least three casters are in contact with the floor at a given time in a four caster set configuration).
To optimize load height of animal laboratory equipment, the smaller the wheel size, the lower the gravity threshold of equipment. However, smaller wheels tend to carry less weight, and have difficulty rolling over floor obstacles and rolling onto sterilizer ramps. The creative resources of the caster manufacturer are the key to solving such weight mobility optimization challenges. The product designer or engineering team of the caster manufacturer can usually solve low threshold issues by matching an oversized wheel with an undersized caster housing to minimize overall caster load height, while concurrently retaining both caster load capacity and optimizing equipment height. Such hybrid casters are designed, engineered, and pre-tested to optimize the dynamic load capacity of casters to meet the needs of the animal laboratory industry. The load determining component is usually the wheel, since caster housings are made of steel or stainless steel material that have the capacity for higher carry loads than wheels made of various plastic, urethane, or related nylon composite materials. Again, rely on the caster manufacturer’s design and engineering team to create smaller diameter wheels, with quicker push pull features that make the work day pushing and pulling carts and equipment a much lighter load.