Casters are pretty simple at first glance–a wheel mounted in a frame, basically. But closer inspection reveals many individual components that must work together for safe, effective operation of the caster under load. That’s why the selection process isn’t as simple as it first may appear.
This guide to the technical aspects of caster and wheel selection, compiled by Kee Industrial Products (and supplanted with other resources from companies such as Darcor, Colson, Algood and Bassick), discusses the most important engineering considerations you need to be concerned about.
In general, casters can be divided into two main types: pressed steel and fabricated steel.
Pressed steel: Typical pressed steel type casters incorporate single- and double-ball race constructions. The balls run in tracks formed in the fork and top plate pressings, which are secured by a heavily riveted king pin.
Fabricated steel: These usually consist of heavy steel forgings which are precision machined and house combinations of tapered roller bearings or ball races. The fork legs are securely welded to the body forging, giving an extremely strong construction suitable for extra-heavy loads.
Caster applications can be divided into two main categories:
a.) Casters for use on trucks, trolleys, trailers, etc., which are used as a means of transportation (i.e. they are loaded with goods of some description and then regularly moved from one place to another)
b.) Casters fitted to a machine or other device to enable it to be delivered to the place where it is used or to enable it to be moved occasionally for very short distances.
In the case of (a) it is essential that the casters should have the lowest tractive resistance and should also be able to swivel freely so that the trolley can be manoeuvred without too much effort. To achieve this, wheels should be at least 4-in. in diameter and preferably 6-in. or more, regardless of rated load capacity. For category (b), smaller casters may well be acceptable and are often used a their full rated capacity.
Swivel caster: An assembly in which a housing contains a wheel that is free to swivel without restriction about the vertical axis of the swivel bearing, with the caster axle being offset.
Fixed caster: An assembly housing a wheel which cannot swivel about its vertical axis.
Wheel: A revolving centre, rotating freely on an axle of which the external part (the tire) may be composed of the same material as the centre or of various other materials (i.e. rubber, polyurethane, etc.).
Offset: The horizontal distance between the centre of the wheel axle and the vertical axis of the swivel bearing. This is sometimes known as trail.
Tractive resistance: The effort required to move a piece of equipment fitted with casters. This is usually expressed as a percentage of the total load carried.
Code numbers of casters
Caster code numbers may look confusing but they are derived in a straightforward manner. The caster and wheel codes are separated by hyphens, e.g. 4SL-CIRB (SL Series), 3XL-PSW (XL Series), 591-RCRB (91 Series).
All pertinent caster information lies before they hyphen. Subsequently, all pertinent wheel information lies after the hyphen. The first one or two digits represent the caster size in inches. The following characters represent the series. Any available caster options will follow the series.
The first two or three letters following the hyphen will be the wheel code. Next comes the bearing code, which may be followed by the tread width. Only casters having identical wheel and bearing components but different tread widths will have a tread width designator. For example: 5XLB-PSWRB is a 5-in. XL series caster with a bolt hole fitting and a PSW wheel with a roller bearing.
Working conditions for casters may vary enormously, both with regard to the type of floor and also the severity of the actual application. Although there are many variables in the selection of the correct caster for each particular application, the total load to be carried on the casters is generally known.
It must be appreciated that load capacity is not the only factor to be considered in choosing casters for a specific job. It may often be necessary to choose a caster by specifying a load capacity several times greater than the conditions appear to warrant, to ensure that the caster is capable of giving the desired performance.
Correct alignment of fixed and directional lock casters
Some casters are designed with top plate fixing holes that have a working clearance (for example, 1 mm for all bolts up to 12 mm and 2 mm clearance for all larger bolt sizes). This accommodates minor positional errors in caster mounting; however proper care should be taken to avoid such errors, as improper caster alignment may lead to premature tire wear.
Wheels can be divided into two main types: resilient- tread wheels and hard-tread wheels.
Resilient tread wheels: Resilient- or soft-tread wheels, such as rubber or polyurethane, give the ultimate in quietness and floor protection, but introduce greater tractive resistance than hard-tread wheels. Polyurethane, however can carry loads approaching those of cast iron.
For many high-load manual applications, soft-tread wheels must be ruled out, as the force required to move the equipment would require the power of several people. However, the quiet-running and floor-protecting properties of resilient-tread wheels make them an ideal choice for power towing applications.
Hard-tread wheels: Cast iron is the standard hard- tread wheel as it is generally the strongest, has the longest life, and is the easiest to push. It also is the loudest and potentially the most damaging to floor surfaces.
As an alternative to cast iron, injection molded nylon wheels are available. Nylon wheels are near the strength of cast iron and are not as loud or damaging to floor surfaces as cast iron. Nylon wheels also have the advantage of being light in weight and unaffected by water.
For high-temperature, high-load applications, reinforced phenolic wheels are also available.
Code numbers of wheels
Wheel coding follows a similar pattern to caster coding. The series identification always consists of the letter “W” for wheel, followed by up to three letters to identify the type of wheel, e.g. WSU, WPSW, WNY, etc.
The remainder of the wheel code always contains a figure denoting the wheel diameter, a figure denoting the bearing type, a figure demoting the bore size, and possibly a figure denoting the tread width. Only wheels having identical composition, bearing type, and bore size, but with different tread widths, will have a figure denoting tread width.
The wheel with the wider of the two tread widths will have an “L” as the last letter in the code. For example, WSU-8YM127 is an 8-in. Flexello Superthane on a cast-iron centre wheel with a tapered roller bearing and 12.7 mm bore size and standard tread width of 2 in. WSU-8TM127L is an 8-in Flexello Superthane wheel with a tapered roller bearing, a bore size of 12.7 mm and a wider tread width of 3 in.
Wheel bearing selection
Plain bearings are adequate for applications where maximum mobility is not essential. Roller bearings do not increase load capacity but keep rolling resistance to a minimum and prolong working life. Tapered roller bearings and ball journal bearings absorb axial loads and are essential for power towing applications.
Plain bearings: In the case of cast iron wheels and cast iron centred wheels, frequent lubrication is necessary and except in the 3-in. size, a grease nipple is provided. In the case of nylon centred models and those with nylon bushings, the axle tube is greased on assembly and in good conditions will run for a very long time without additional lubrication. There is, however, a risk of squeaking and excessive axle tube wear in dusty and gritty conditions.
Roller bearings: For light-duty applications, roller bearings are greased on
assembly and should, under normal working conditions, not require regreasing. Casters designed for heavy duty applications are also provided with a means of re-greasing the bearing. On cast iron wheels and cast iron centred wheels, the roller bearing runs on a hardened steel split sleeve, which gives extended life and reduced rolling resistance. When wheels have plastic centres, the split sleeve is not necessary and it is therefore omitted.
Ball journal bearings: Precision ball journals are ideal for applications where high radial and moderate axial loads are present, and give very low rolling friction. The bearings are shielded to retain the grease and to exclude grit and dust. A spacer inner tube is used to support the inner races, which must be clamped endwise on assembly. No adjustment is necessary. Ball journals are suitable for both manual and low-speed power towing applications.
Tapered roller bearings: Precision tapered roller bearings are suitable for the heaviest applications, where high radial and axial loads are present. The bearing cups and cores are hardened for extended life and the bearing must be adjusted on installation to give optimum service life. This adjustment is only necessary on wheels fitted to retain the customer’s axles. Seals are fitted to retain grease and exclude grit and dirt, and grease nipples are provided for re-greasing under severe working conditions.
Caster mounting formations
Various combinations of swivel casters and fixed brackets are in general use. The type of truck used, operating space and floor conditions determine the choice of formation. The most common mounting configurations are:
a) Four swivel casters
b) Two swivels and two fixed
c) Two swivels and two fixed in a diamond formation
d) Four swivels at corners and two central fixed brackets.
Four swivel casters: Essential where maximum manoeuvrability is required in restricted spaces, this mounting configuration can be easily converted to two swivel and two fixed by implementing casters with swivel locks.
Two swivels and two fixed: Allows accurate steering, negotiates ramps safely and is ideal for use on long, straight runs. The truck should be pushed with the fixed casters leading.
Two swivels and two fixed in a diamond formation: Either rocking or centrally fixed brackets or with casters equalling the height of the fixed brackets, this configuration will turn in its own length and steer accurately. This can be a dangerous arrangement as the truck may tip when negotiating a ramp or is unevenly loaded, or if the load has a high centre of gravity.
Four swivels at the corners and two central fixed brackets: Either equal height or rocking on fixed brackets, this is the best arrangement for a long wheelbase, is easily steered in a straight line and can turn in its own length. This configuration is best suited for very long trucks and trolleys.
Power towing applications
For power towing of truck or trailer trains, turntable steering is often used. But in addition to being costly it has the disadvantage of serious loss of stability when the axle is at right angles to the truck axis. Also, there is also the possibility of jackknifing on sharp turns.
The use of casters for this application has proved very satisfactory. Providing suitable precautions are taken, excellent stability, weight distribution, and tracking can be obtained.
To allow truck trains to operate in narrow gangways, good tracking is essential and experience has shown that if trucks are rigidly coupled together by means of pin couplings at each end, they will follow the tractor most accurately if the distance from the fixed caster centre line to the rear coupling pin is 25-33% of the total length of the truck from the front to rear coupling pins.
This may be achieved by moving the fixed casters to reduce the wheelbase of the truck, although it may not be possible, depending on material conveyed, to obtain a reasonable load distribution over the swivel and fixed casters. In order to be able to distribute the weight of the truck and its contents evenly between the casters, it is advisable to extend the drawbar at the rear of the truck.
Pressed steel casters are designed specifically for manual propulsion. Due to the greatly increased stresses exerted on various components on casters in power towing, only fabricated casters should be used.
Truck locks are specifically designed to steady the equipment to which they are fitted and prevent horizontal movement. Trucks locks should never be used as a jack and any attempt to vary the fittings height from the catalogue specifications may result in failure or damage. It is essential that truck locks be fitted as close as possible to one of the swivel casters.
This article was provided by Kee Industrial Products, Concord, Ont., and is adapted from the Technical Information section of Flexello’s Caster and Wheel Catalogue, for which Kee is a master distributor.