Casters
When selecting and ordering casters for a particular application, the following information must be considered and specified.
Load Capacity
How much weight will each caster bear in the worst case (for example, the shock load when unloaded from a truck onto the ground can easily exceed four times the weight of the cargo).
Wheel Diameter
The size of the wheels determines how easily the rack will roll, not only over cracks and ridges but also on flat surfaces. Large wheels of 3 inches (76 mm) or greater are preferred for regularly moved loads.
Wheel Material
The wheel material is one of the most important aspects of any caster.
Hard materials (greater than 85 Durometer Shore A scale), such as polyolefin, hard rubber, and phenolic, roll more easily on carpet, but can mar (physically damage) hardwood and linoleum floors.
Medium materials (70-80 Durometer Shore A scale), such as urethane and polyurethane, are long lasting, environmentally resistant, and usually nonmarking. They are recommended for portable or touring sound systems. Medium rubbers are also available but often leave marks.
Soft materials (less than 65 Durometer Shore A scale), such as neoprene and soft rubber, absorb some shock and vibration. They also provide maximum floor protection, although rubber can leave marks. In control rooms, wheels made of soft materials stay put, helping to eliminate the annoying problem of dollies and effects racks rolling away inadvertently.
From a cost standpoint these materials compare as follows:
Most expensive: Urethane, Neoprene, rubbers, phenolic, and polyurethane
Least expensive: Polyolefin
Bearing Type
Casters that are used daily in rough conditions should have heavy-duty ball or roller bearings to ensure long life and ease of rolling. For casters that are less often used, a simple sleeve type bearing will suffice.
Swivel or Rigid
Rigid casters allow movement only in the direction of wheel roll, which tends to make maneuvering difficult in a room. They are less expensive than bearing types but not recommended for most audio systems.
Locking
Any piece of equipment that could be placed on a stage or platform should be equipped with at least two braking casters.
Mounting
Casters are available in many different mounting schemes. The top plate and the threaded stud are the two most appropriate types for custom applications.
Top Plate
The top plate can be bolted directly to the underside of the rack or, as is popular in touring applications where casters are often replaced, to a piece of 0.75-inch (19-mm) plywood, which is then secured to the underside of the rack or dolly.
The advantages of the latter scheme are twofold: (a) the weight is distributed by the plywood and (b), if carriage bolts are used, with the nut at the caster, only a crescent wrench is needed to make a replacement. In cases where the nut is on the caster side, the bolt must be short enough not to interfere with the swivel action of the caster, even after it has become slightly bent!
Threaded Stud
The threaded stud is appropriate for light applications, such as tape machine dollies and signal processing racks. If the dolly is metal, a bolt is welded over a hole in the underside and the caster simply threaded in. If it is wood, a hole is put in the underside and a nut and washers are used to hold the caster in place.
Applications
In control rooms and other light-duty areas, neoprene casters are appropriate for hard and carpeted floors. Hard rubber may also be used on carpet but may mark hard floor surfaces.
For portable equipment, such as amplifier racks and console cases, an appropriate type of caster is of neoprene, 3 to 3.5 inches (76 to 89 mm) diameter with ball bearings. For equipment that is seldom moved but must be on casters, polyolefin 3-inch (76-mm) sleeve bearing casters are suitable and inexpensive.
Excerpted from “Audio Systems Designs and Installation” by Philip Giddings, P. Eng. Trade paperback (xxvi + 574 pp, index, ISBN 978-0-9920244-0-6), available exclusively online from the publisher, Post Toronto Books, at http://posttoronto.com/.