How to Choose the Right Caster
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 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
deteriming 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 few 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 the unusual circumstance.
When selecting casters, load capacity is always measured reciprocally against floor
protection when selecting casters. 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.
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 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 rollability; 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 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 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.
Three measurements are important with plate casters: (1) the bolt hole pattern, the
distance between the centerlines of the bolt holes; (2) the outside perimeter dimensions
of the plate ; and (3) the size of the bolt holes themselves. The most important
measurement in most cases is the bolt hole pattern. The are eight different types of
caster stems: wood stems, straight stems, grip ring stems, threaded stems, hollow
kingpin (or stemless) stems, octagonal stems, pipe stems, and rubber applicator stems.
It is important to know not only what type of stem is required but what exactly the
dimensions, the diameter and length, are of the stem.
Please check out our caster identification guide to assist with some of these questions.
Fill out the questionaire, and then email it to us.