Outfitting manual material handling carts with the wheel or caster that best matches a given application is an ideal way to achieve both higher efficiency and improved ergonomics for workers. Because the wheel’s construction and materials are optimized for the load and facility, the risk of musculoskeletal injuries is greatly reduced while productivity rises. But how do you figure out which wheel or caster — among the thousands of possible configurations available — is the most ergonomic choice for your operation? Consider the following seven areas:
1. Wheel or caster type and mounting method. Wheels are simply that — a wheel. Casters come in swivel styles that rotate a full 360-degrees so the wheel can roll in any direction, or fixed (also called rigid or stationary) styles that only roll in a straight-line path. Depending on the desired motion, application and style of cart to be used, casters can be mounted in one of five different ways: top plate; bolt hole; stem; expanding stem; or a plug-in pin.
2. Load capacity. The maximum amount of weight each wheel or caster can bear affects how easily the cart will roll, as well as how much force the operator will need to apply to push, pull or stop it. Calculating the total load capacity of the wheels or casters includes factors such as the total weight of the transport cart itself, the maximum additional load and the total number of casters or wheels to be used in the design.The load capacity calculation also takes into account the safety factor, or deviation from standard application conditions (such as obstacles like uneven surfaces that must be crossed). Indoor manual transport has the least amount of deviation, followed increasingly by outdoor manual transport use, indoor powered transport, and outdoor powered transport. Additionally, load capacity is impacted by the number of wheels that maintain contact with the floor surface. When a loaded cart travels over an uneven surface, there is a high chance that not all the wheels will maintain contact with the floor. Therefore, the selected wheels or caters will need to be strong enough that only two or three of them can safely support the full weight of the load.
3. Tread material. There are a broad range of wheel and caster tread materials available with attributes ranging from softness to hardness and operating noises from silent to noisy. Types include rubbers (pneumatic, elastic and solid composites), polyurethane, nylons, polypropylene, phenolic resin, steel and cast iron. Generally speaking, harder treads (nylon, polypropylene, steel, cast iron) have low starting resistance and roll more easily making them ideal for the heaviest loads — but also making them more difficult to control and stop. They are also prone to damaging floors. They also resist chemicals. Softer materials (solid rubber and polyurethanes) are comfortable to push and preserve flooring but have less capacity to carry heavy loads.
4. Starting, rolling and swiveling resistance. A variety of factors influence the degree of resistance a wheel or caster must overcome when moving a load — and therefore the amount of effort an operator must expend to start and control the cart’s movement. Factors include the surface of the floor, tread material the wheel is made of, and the size and weight of the load itself. Depending on the hardness, geometry and elasticity of the tread, it will be more or less likely to be compressed by the load. Softer treads experience a higher degree of compression, which equates to higher starting and rolling resistance. Conversely, harder treads experience considerably less compression and therefore have less starting and rolling resistance on smooth floors; however, harder materials cannot easily traverse over obstacles. Factors that impact a caster’s ability to swivel include the geometry and hardness of the tread material, as well as the hardness and evenness of the floor surface.
5. Bearing style. There are three primary types of bearings used in casters: plain bore (which tends to be the least expensive but have the highest degree of rolling resistance); roller bearing (which offer less rolling resistance but are more prone to damage by uneven floors); and ball bearings (the most expensive but with the least amount of rolling resistance — and therefore the most ergonomic solution).
6. Environmental factors. The surrounding environment in which the cart is to operate can have a considerable impact on wheel and caster function. Considerations include the smoothness or unevenness of the floor; indoor or outdoor use; application temperature (ambient, extreme cold in a freezer, or extreme heat in an oven); dust or dirt; caustic or washdown applications; and more.
7. Available options. Depending on the intended use of the cart, a variety of options may be specified to customize the wheels and casters for specific needs. These include a variety of locking systems to prevent both rolling and swivel motions; construction materials for bearings and fittings; anti-static and spark-proof materials; and more.
Ultimately, when determining which wheels and casters will create the most ergonomic, cost-effective, and efficient cart system for your operation, work with your equipment supplier to review these seven considerations. For assistance, the members of the Ergonomic Assist Systems & Equipment (EASE) Industry Group of MHI are available to answer questions about ways to best optimize your operations’ ergonomics.