Five Important Issues When Ordering New or Replacement Casters:
1. What is the load capacity?
2. What wheel type is needed for floor protection?
3. What wheel diameter will give the necessary load height and mobility?
4. In what type of environment will the casters or wheels operate?
5. How are the casters to be mounted?
The first and most important consideration in selecting a caster is load capacity. The load capacity of a caster is the maximum weight per caster that can be supported for intermittent operations over smooth floors at walking speeds. To determine the load capacity for each caster, divide the combined weight of equipment and maximum load by the number of casters to be used. This is the weight that each caster and wheel must support. There are a couple of important considerations in determining load capacity. Manufacturers distinguish between static and dynamic loads. Static load is the weight on a caster at rest; dynamic load is the weight on a rolling caster. With few exceptions casters are rated by dynamic load. Business machine casters, for instance, are one of the casters rated by static load, since they are not intended to be moved except for maintenance. The static load capacity of a caster is often much higher than the dynamic load. The reason for this is that dynamic loads are subject to more stress and possible abuse than static loads. The weakest link in a caster is usually not the wheel or the fork, but the king pin, the pin or bolt that holds the bearings in the swivel together. Wheels and forks as a rule will take a lot more abuse than this pin. Most caster failures occur when a near capacity load bangs into some obstruction on the floor, causing the king pin to bend; when this happens the bearings fall out, and the swivel caster ceases to swivel. Just to be on the safe side, we always recommend casters with extra load capacity, to allow for unusual circumstances.
When selecting casters, load capacity is always measured reciprocally with regard to floor protection. Generally speaking, the heavier the load capacity, the harder the wheel; the harder the wheel, the more possible abuse to your floors is a concern. Wheels with softer treads will carry lighter loads, but offer the most protection to your floors, because they are more resilient; whereas wheels with harder treads will carry heavier loads, but offer more abuse to your floors, because they are less resilient. We urge you to protect your floors. It is always less expensive and much less time consuming to replace worn out wheels than to replace your floors. To insure that this is the case, the material that the wheel is made of should not be harder than that of the floor. On wood and other relatively soft floors, where scratches and the like are a concern, always use some kind of rubber. Rubber is the best for floor protection and noise reduction, but the worst for load capacity. The new thermoplastic rubber (TPR) wheels go some way to overcome this limitation; they will carry more weight than other rubber compounds with not much of a reduction in resiliency. Polyurethane provides an effective compromise between rubber and the hard tread materials such as steel, phenolic and polyolefin, since it offers some floor protection with relatively large load capacity. Depending on the application, polyurethane may scratch soft floors. It will support a lot more weight than most rubbers, but is not nearly as resilient. Indeed, is not much more resilient than hard wheels. The big difference is that polyurethane is much quieter than hard wheels on hard surfaces. For concrete, especially smooth concrete, with heavy loads, use polyurethane, phenolic, or polyolefin. We do not recommend steel wheels on concrete. If pushed or pulled abruptly to one side or the other, before the swivels have a chance to turn in the appropriate direction, or if moved laterally with a forklift, steel wheels may gouge the floors. Steel wheels, forged or ductile steel in particular, will carry extremely heavy loads in very abusive conditions. Because of this we do not recommend their use on any floor material other than flat or diamond steel plate. Wheels used outside on natural surfaces have wide and resilient treads to minimize the possibility of their digging in. We recommend either semi or full pneumatic or the new cushion rubber wheels.
There are several factors determining roll-ability; that is, how easy or hard it is to move a load. First, and by far the most important factor, is the diameter of the wheel. In short, the larger the wheel the easier the load will move. Always pick the largest diameter wheel possible. This is particularly important if there are obstructions that the wheels need to roll over, such as door thresholds, uneven or pitted floors, rugs, or the debris that may collect on the floors in some industrial environments. Second, though not nearly as important as wheel diameter, is the hardness of the wheel material: the harder the wheel the easier it will be to move. Wheels with hard treads perform better on smooth surfaces, whereas soft tread wheels perform better on uneven or pitted floors and debris. Third, it is always easier to roll wheels with bearings. Other conditions being equal, we recommend ball or roller bearings for all applications with loads over 500 lbs. Fourth, the shape of the wheel makes some difference: rounded treads roll easier than flat treads, especially on carpeting, because they have less Contact with the floor. But, by the same token, round treads usually have less load capacity than flat treads.
Environmental issues (other than the ones already mentioned) are concerns when purchasing the proper caster. First, the presence of water, steam, oils or grease, animal fats, solvents, and other corrosive chemicals must be considered when selecting casters and wheels. Natural rubber is not recommended when corrosive materials are found in the environment. Stainless steel is the material of choice for caster forks when hazards of this sort are present, and plastic wheels such as polyolefin, polyurethane and thermoplastic rubber (TPR) are virtually impervious to most solvents and chemicals. Second, exposure to extreme heat or cold is another important issue. Some wheels subjected to high heat will degrade rapidly, thereby shortening their usefulness. High-temp phenolic or high-temp polyolefin and steel wheels are designed for hot environments such as autoclaves and bakers ovens. Steel wheels will take a lot of heat, but care must be taken with floor materials, because steel wheels also hold a lot of heat. We have seen floors in bakery ovens melt or burn because someone choose steel wheels for their durability over the high temp plastics which, though much less durable, do not hold heat. Third, floors with debris such as metal shavings present special problems for caster selection. We recommend polyurethane for applications such as this. Wheels with soft treads such as rubber or polyurethane tend to roll over small obstructions, such as metal shavings. The softer and more resilient the material, the easier it is for it to roll over debris. This prevents abuse to the floors, but tends to chew up the wheel to some degree depending on the material. Hard wheels, on the other hand, tend to push debris along in front of them, grinding, scratching or gouging the floor along the way. Polyurethane again provides a compromise material; it is resilient enough to roll over most debris, and tough enough to withstand much of the abuse.
Casters are mounted to a piece of equipment usually in one of two general ways: by a plate or by a stem of some sort. There are many different plate sizes for casters. There are eight different types of caster stems: wood stems, straight stems, grip ring stems, threaded stems, hollow kingpin (or stem less) stems, octagonal stems, pipe stems, and expandable rubber applicator stems. Again, we stock most common plate and stem sizes. If you do not see what you need either in the catalogue or on the web site, please call or email; we may have it in stock, and if we do not, we can certainly get it in a timely manner and at a reasonable price.