AIEL MARKET
Purchase Guide

Caster Guide

	In the touring business, crews move a great deal of equipment on a regular basis, and much of it is on wheelss. Choosing the correct caster for the purpose is the subject of this article.     Among the topics discussed are types of casters, their mounts, bearings, and their wheels and tire/tread materials. To close, will be additional criteria and recommended uses.
Not all casters will be discussed because the focus here will be the typical ones used by the majority in the entertainment industry. Due to this focus, some advantages and disadvantages of certain caster types and their tire materials, or combinations of materials, will not be discussed because they are not relevant to this article.

THE FOLLOWING MAY NOT BE REPRODUCED
WITHOUT PERMISSION FROM THE AUTHOR ©

Topics

  BASIC CONSIDERATIONS

    Choice of a caster is based upon the weight to
be carried, ease of rolling and turning, the location
and terrain where it is to be used, and suitability for
the object on which the casters are to be installed.

    Load: In general, the heavier the load, the greater the diameter of the caster required. Larger wheels reduce the effort of the person pushing the attached object because they ride more easily over surfaces with cracks, hollows, or bumps. They can also facilitate turning when the load weight is high. Another way to increase load handling without increasing wheel diameter is to have more wheels per caster.

    Stress: Another consideration is the drop stress (sometimes called "shock force") that occurs when a caster hits a hard surface under load. This is worst when a road case is dropped and the first to touch down is a single caster at an angle. The case weight plus the energy built up during the fall will all come upon that one caster first.

 TYPES of CASTERS

    Caster Categories: Casters are divided into two basic groups: Rigid and Swivel. As the names suggest, the former is locked to the two directions it rolls, while the latter can rotate up to 360 degrees to allow movement in any direction of the object to which it is attached.

    Subdivision Groups: A division of the swivel type is the canted caster, often referred to by the brand name of "Shepherd". These casters have a wheel that is basically in the shape of a sphere which is offset from its stud mount. It has an off-centre tire that leans away from being perpendicular to the surface on which it rests. This lean, called "negative camber" (because the wheel bottom tilts outward during rolling), gives more stability. The main idea of this design though, is to improve turning ability. When an object on these casters is pushed, all the wheels will align to the direction of travel because each caster will quickly turn to trail its stud mount.

    Further Subdivision: The above categories can be divided into Locking and Non-Locking. The latter are always free to rotate and are most useful on objects that must be precisely manoeuvred into position from any angle to any other. They also allow easy, 360-degree rotation.

The Locking types have a lever that when engaged prevents the wheel from rotating. For many models, this means a treadle across the wheel hub that levers in two directions -- one way locks when pushed downward. Foot pressure determins the degree of locking; the other side will release the lock when stepped on. Another type works by stepping on to a flat lever to operate a bar that directly presses against the tire. Releasing this type is done by lifting up on the lever with the top of the toes.

    Some types can also prevent the action of the swivel. This can be done via a spring pin, while others use a push/pull knob, or foot-operated flat bar. These are sometimes found on a cart where one wants two wheels free to swivel, while two others can only roll back and forth without turning.

Then there are those casters where both actions can be arrested. These dual locks are specified when casters must not allow the object to which they are attached to shimmy during use. Think of racks containing equipment that is handled during a show. Another situation which calls for both wheel and swivel locks is for the small platforms on which musicians or other persons perform. These cannot shimmy when in use -- both for safety reasons and for esthetics in front of an audience. A shop application regards mobile work benches where pounding, pulling, or vibration-causing work must not cause movement of the work surface.

  TIRE/TREAD TYPES

    The typical tire is shaped as a circle, but there are also ball tires, as well as roller tires that are on cylindrical wheels. The latter is used when a heavy weight is to be carried, and closeness to the rolling surface is desired. Rarely used in our industry, a common purpose is for beds. Heavy loads can also be borne by doubling up on the wheels and tires, as is often seen in the light-duty plastic casters used on cheaper office furniture, but also available in the more robust, double-wheeled caster seen to your right.

    Tire shapes vary. Some contact the floor or ground evenly across the width of the tire, while others curve up away from that point. The "Shepherd" style treads are slanted so that the canted tire lies flat on the rolling surface.

    Materials for tires vary widely. Those typically
used for casters in the entertainment industry are:

• Neoprene
• Nylon
• Polyurethane
• Rubber
• Thermoplastic

    Neoprene: A substance that is a rubber substitute, it is quiet and absorbs shock well, and it wears better than rubber. It won't damage floors, and is less likely to leave skid marks than rubber.

    Nylon: This synthetic material wears very well and is quiet, but can take a `set' when left for periods under load. That is, it develops flat spots that then cause a rough ride and increased rolling resistance until the tire returns to round with use. This can be overcome by forming a hard nylon tire which makes for more rolling noise, but is extremely robust. This version can handle heavy loads and abuse. Nylon can withstand very wide temperature extremes from well below freezing to the boiling point of water.

    Polyurethane: A preferred material because it wears better than rubber but is still a good choice for cushioning and shock absorption. Because this substance has some flexibility, it is kinder to floors. As with Nylon, it can be made harder for more durability, but with a trade-off of greater rolling noise.

    Rubber: This material is liked because it is quiet and absorbs shock well. It won't damage floors, but can leave skid marks. Rubber does not wear as well as the other substances discussed here.

    Thermoplastic: A cheaper choice for tires, it wears well, but is noisy when it rolls, and it is more susceptible to drop stress damage. This is not a good choice for carpet usage because thermoplastic is hard enough that carpet tends to pile up in front of the tire, thus increasing rolling resistance.

 BEARING TYPES

    All caster wheels must revolve around an axle. How
the load is borne on that axle makes a difference.

    Sleeve Bearing: Basic casters use a Sleeve Bearing that allows the axle to turn within. Friction is great with this type of wheel bearing, so one must be sure to lubricate it on a regular basis using grease or a heavy oil.

    Ball Bearing: A better method is to surround the axle with Ball Bearings. These roll across one another and the axel so that rolling friction is greatly reduced. Still, one must employ an oil heavy enough to the bearings so as to create a film that lubricates and separates each from the ones beside it, and also from the contacting surfaces of the axel. Better casters house each ball bearing in individual holders to keep them apart.

    Ball Bearings are also used for the swivel action of a caster. They typically sit in a circular groove between the top of the wheel mount and the underside of the caster's mounting plate or stud base. As just mentioned, good quality casters have separate wells for each bearing so as to prevent interactive friction where bearings would otherwise touch.

    Roller Bearing: This style employs pins (rollers) within individual housings which hold each in place, but allow freedom of rotation. A series of these surround the wheel hub where the full width of each roller touches the axle. Such a setup means less friction because the rollers only touch the axel and never one another. It also allows for a greater load to be placed on the caster for a given size because a roller has a larger surface area than a ball bearing; thus there is less stress on each one.

  MOUNTING STYLES

• Rectangular Plate
• Stud - Socket
• Stud - Threaded
• Detachable

    Plate: For most purposes in our industry, the rectangular plate is the typical caster mount. These give the most strength and cause the least stress to the points where the casters attach to the object; the size of the plates and their multiple mounting points spread out the force.

    Choose a plate that is thick enough to not deform easily, is nickel-coated for corrosion resistance, and when possible, fits the object to which it will be attached without allowing the wheels to extend beyond the case at any time or position.

    Studs: Stud mounts are meant for attaching to items that don't have much bottom surface area that touches the floor. This would occur with objects that have legs or large feet as opposed to one that rests entirely on its bottom. A work bench would be an example. With some stud mount typess, each slips into a socket that has been inserted into a cavity that has been hollowed into each leg of the object. When fully inserted, they lock into place, often via one or more spring-loaded, captive balls or half-sleeves that allow the stud to pass, then drop into a groove in the stud.

    Others have a threaded stud that screws into a threaded female socket which has been installed into the cavity. The threaded mounts are best for correcting when an object sits in an unlevel position on a level surface. These may be adjusted to achieve a level stance of the object when castered.

    Locking nuts can be added to prevent "thread creep" over time during usage, thus maintaining the level position. Be sure to have as much thread as possible into the socket, and thus into the object, so as to minimise side stress at the mounting point.

    Detachable: For those that need easily removed casters, a slide mount fits the requirement. Typically, a rectangular plate is attached to the object. It has two parallel rails, one along each long side that act as guides. A caster with a smaller plate is slid inside the guides of the slide-way, and upon full insertion, the caster plate locks in place. Manipulating a latch or pin allows for later removal.

    Detachable casters are typically used on objects that must be tipped off their wheels when in use. Large speaker cabinets are one example. They can be rolled on & off a vehicle in a horizontal orientation, but when then raised vertically for use, the casters can be removed to eliminate sympathetic frequency (resonant) rattles.

  OTHER CRITERIA

• Thread Guards
• Tread Pliability
• Tire Thickness
• Tire Width
• Swivel Clearance

    Thread Guards: When casters are to be used around carpet or in dirty conditions, guards are a must. These are slightly flattened cups that cover each side of the axel to prevent infiltration of unwanted dirt and fibres. Each cup's edges often sit in a groove on the side of the wheel for a better seal. In addition are thread guards for the swivel portion. A ring surrounds the dish where the swivel bearings reside so as to keep out unwanted debris.

    Tread Pliability: Softer treads roll more easily over surfaces such as carpet or those with small bumps or debris, while a hard material rolls best on smooth, hard surfaces. Even then, some prefer softer treads for their cushioning ability of the load. Plus, softer means less rolling noise.

    Tire Thickness: Thicker tires can handle more weight per wheel size; however, they perform less well under very heavy loads because some materials deform more easily when thickened, thus increasing friction.

    Tire Width: The wider the tire, the less stress per tread area. This makes for easier rolling and lower wear on the tire because there is more tread contacting the rolling surface, spreading out the force. However, wider treads make for a wheel that is harder to turn because the extra surface area means more friction.

    Swivel Clearance: This was touched upon earlier. It means mounting a caster so that in no orientation will the tire ever jut out beyond the bottom of the castered object. This is to prevent a trip hazard and to facilitate very tight case packing in a truck.

    An exception is for locking casters. The locking treadle must protrude enough to allow one's foot to lock and unlock the caster as necessary. For large casters that can allow industrial footwear (such as steel-toed boots) to fit under the object and contact the treadle, the casters may be able to be mounted farther in under the object. Test to see if this might be accomplished. Casters that don't protrude are usually the better mounting method whenever that can be achieved.

  SUGGESTED CASTER USES

    Any object that needs to be
moved is a candidate for casters.

• Cases
• Benches
• Rolling Shelves
• Scenery Trucks
• Shop Workstations
• Stage Management Desks

    Cases: This is the obvious one because most entertainment industry casters are used for this putpose. Select ones that will suit the load, but also the environment where they will most often be used. An audio-visual company that works trade centres and hotels would choose casters that are quiet, roll easily on carpet, and do not marr floors. These criteria need not be considered by companies that work arenas with concrete floors.

    Benches: In the shop, some work benches might benefit from casters if they need to be moved out of the way from time to time, or brought to another bench as a side work surface. Select large-wheel, locking casters so as to stablise the bench during work, and ones that can handle pounding or vibration produced by workers. Have on-bench power outlets or strips wired with flexible cable and plugs so that they can be disconnected before movement.

    Rolling Shelves: With castered shelving units, they can be placed in front of other shelves for double-space storage. Be sure that shelves are stable enough to be castered without tipping. Locking casters are a plus here to prevent movement during item placement or removal.

    Scenery Trucks: Another obvious usage. On stage, scenery that needs to be quickly moved or rotated into view would be placed on platforms, called "trucks" or "wagons". These are often mounted on locking swivel casters. Use ones that are quiet and roll smoothly with minimal tire flexing. Mount them far enough under the object so that tires do not swivel beyond the edges of the trucks. For very large platforms, more than four casters might be specified.

    If actors, musicians, or stage crew are to be on these during a performance, employ casters with quality locks/brakes that will not allow platform shimmy -- especially important when drum kits are used. This is where a swivel-lock feature might be employed as well as wheel locks.

    Some set or platform designers employ `drop feet' that can be released after placement so as to drop or be levered down to the stage from the platform in order to stablise it even more. If these are the type that raise the platform off its wheels, then non-locking casters can be employed. If casters must not rattle when in this position, less-critical locks/brakes may be specified since they are not needed to lock under load.

    Shop Workstations: In a large workshop it is often handy to bring the work surface, tools and accessories to the work. This unit might only need space for one seated person, so the rest of the under-counter space could have drawers or shelves. If no one is to be seated, all this space could be completely filled with storage facilities.

    This station must be very stable and not shimmy when in use. Select quality, locking casters for all four. Should soft tires be needed, do not have them be too pliable if they are to support heavy weight. This is to prevent too much deformation. Thread guards are recommended if shop conditions are dirty.

    Stage Management Rehearsal Desk: For theatres with open floor space where a stage management team and/or director might sit during rehearsals, a dedicated station is a must. It would consist of a platform serving as a desk where two or three people can sit. On top at the back would be a bridge enclosed at the front, and often the sides. It is under lit by dimmable LED or fluorescent strips. There would be a valance to keep the light out of the faces of the seated persons so that light falls only on to the surface where scripts and notes would rest. Below the light bar would be a power strip of outlets for the lights, laptops, mobile device chargers, and so on. A cable reel or brackets at one end and underneath would store the extension cord for the station's power.

    Select casters that are stable when locked. Thread guards and soft tires are a must for this station if the rehearsal space or theatre is carpeted. Guards are not critical if this station is only ever to be used off- or back- stage where there is a smooth floor.

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