Castor & Wheel Guide

a) Swivel Castors - an assembly in which a housing containing a wheel is free to swivel without restriction about the vertical axis of the swivel bearing with the castor wheel axle offset (see Fig. 1)
b) Fixed Castors - an assembly housing a wheel which cannot swivel about its vertical axis.
c) Wheels -a revolving centre rotating freely on an axle of which the external part (in contact with the ground) can be constituted by the material of the wheel itself or by various other materials.
d) Offset - the horizontal distance between the centre of the wheel axle and the vertical axis of the swivel bearing: this may sometimes be known as trail (see Fig. 1).
e) Tractive Resistance - the effort required to move a piece of equipment fitted with castors: this is usually expressed as a percentage of the total load carried.
a) 2 Swivel Castors and 2 Fixed Castors
The most practical configuration for industrial use. This solution provides good load capacity with good manoeuvrability and ensures accurate steering, even on long straight runs. The trolley should normally be pushed with the fixed castors leading.
b) 4 Swivel Castors
This arrangement provides good load capacity with excellent maneuverability suitable for winding runs, and where side motion is frequently required. However, trolleys with this configuration may be difficult to guide in straight runs, particularly with heavy loads and uneven ground. Not recommended for ramps. If two castors are equipped with directional locks, this arrangement is then suitable for long straight runs, a very versatile arrangement.
c) 1 swivel Castor and 2 Fixed Castors
An economical solution for lightly loaded trolleys requiring good manoeuvrability. The trolley most be fairly small in size and it is essential that the load is evenly distributed to ensure stability on the unit.
d) 3 swivel Castors
The arrangement provides good load capacity with excellent manoeuvrability. However, equipment with this arrangement will be difficult to guide on straight runs particularly over uneven ground. This configuration is ideal for barrel dollies and small portable machines.
e) 4 Swivel Castors and 2 Fixed Castors centrally pivoting
Besides providing a very high load capacity, this arrangement also assures greater manoeuvrability and stability when compared to type f). This configuration is best used with very long trolleys designed to carry heavy loads. The fixed castors can be replaced by wheels mounted onto a central axle. The base of the unit must of a robust construction. The swivel castors are mounted in such a way as to pivot the trolley on the central wheels, it is usual to put approximately 25 mm of packing above the two fixed castors (wheels) and thus provide alternating load support, depending on which pair of wheels are in contact with the floor. However, the swivel castors are subjected to shock loads if the trolley is tipped or the load is not evenly distributed. The entire load thus rests on the 2 central, fixed castors / wheels.
f) 2 Swivel Castors and 2 Fixed Castors Centrally Pivoting
This solution provides good load capacity with excellent maneuverability, in confined areas. The fixed castors can be replaced by wheels mounted in such a way as to pivot the trolley centrally, it is usual to put approximately 25 mm of the packing above the two fixed castors (wheels) and thus provide the alternating load support. However, the fixed castors are subjected to shock loads if the trolley is tipped or the load is not evenly distributed. The entire load rests on the 2 central, fixed castors / wheels.
g) 4 Fixed Castors Centrally Pivoting
An economical solution for moderate loads suitable for long, straight runs with occasional changes in direction. The fixed castors can be replaced by wheels mounted onto a central axle. The two end castors are mounted in such a way as to pivot the trolley on the central wheels, it is usual to put approximately 25 mm of packing above the two fixed castor, (wheels) and thus provide alternating support. However, the end castors are subjected to shock loads if the trolley is tipped or the load is not evenly distributed. The entire load rests on the 2 central, fixed castors / wheels.
In general castors can be subdivided into two main types:
a) Pressed Steel - typical pressed steel type castors incorporate single and double ball race constructions. The balls run in tracks formed in the fork and top plate pressings which are secured together by a heavily riveted steel king pin.
b) 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.
Castor applications can be divided into two main categories:
a) Castors 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) Castors 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 obviously essential that castors 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 100 mm diameter and preferably 150 mm or more, regardless of the rated load capacity. For category (b) smaller castors may be acceptable and are often used at their full rated capacity.
Great care has been taken in choosing ratings which will ensure satisfactory life, performance and ability to withstand reasonable abuse under normal conditions.
There are, however, many occasions when it is necessary to under rate castors to ensure the correct degree of mobility for the application.
When castors are fitted to a trolley which is used regularly for transportation of goods or products, it is necessary to specify a minimum of 150 mm wheel diameter and choose swivel heads for this type of application.
When castors are to be fitted to a piece of equipment for occasional movement, it is often possible to use quite small castors provided they have sufficient load capacity.
The dimensions and load capacities detailed in this catalogue are in metric i.e. mm and kg.
Working conditions for castors may vary enormously both with regard to the type of floor and also the severity of the actual application. In recognition of the variation some castors and wheels are given separate load capacity ratings for Ideal and Average working conditions. Ideal Working Conditions indicate that overloading and shock loading are impossible, and that the floor is reasonably level and free from cracks, guIlies, door guide rails, etc., and that the floor surface is not of an abrasive nature. Average Working Conditions are typified by many factors where one or more of the above hazards may be present to a limited degree. Although there are many variables in the selection of the correct castor for the particular application, the total load to be carried on the castors is generally known,
It must be appreciated, however, that load capacity is not the only factor to be considered in choosing castors for a specific job. It may often be necessary to choose a castor having given a load capacity several times greater than the conditions appear to warrant to ensure that the castors are capable of giving the desired performance.
In accordance with the relevant ISO standard, top plate fixing holes have a working clearance of I mm for all bolts up to 12 mm, and 2 mm for larger sizes. This accommodates minor positional errors in the manufacture of the trolley but unfortunately also allows the castors to be misaligned to the point where drag and tyre wear could be excessive. It is therefore necessary to align the castors correctly before the bolts are finally tightened.
For manual applications the choice of the correct wheel is very closely related to the effort which a man can exert. 18 kgf is the generally accepted figure for moving from a standing start, but this must be reduced to 12 kgf once the truck is on the move. A man is able to maintain this effort for reasonably short distances, but for longer distances of travel he cannot be expected to maintain a force of more than 6 kgf. In choosing the type of wheel to specify, those figures relating to tractive resistance must be borne in mind. The tractive resistance, which is usually expressed as a percentage of the total load carried, is dependent on a number of factors and will vary for each application. It is impossible to publish tractive resistance figures for particular applications, but typical tractive resistance figures are given below.
Resilient or Soft Tread Wheels, such as rubber or polyurethane, give the ultimate in quietness and floor protection but introduce a penalty from the point of view of ease of movement, as the tractive resistance or effort required to move them is generally up to three times greater than that of the equivalent size hard tread wheels. Polyurethane, however, has the remarkable property of being able to carry loads approaching that of cast iron and has a great resistance to tearing and to abrasive wear (many times the life of rubber.) It also has a lower tractive resistance than rubber for the same load and is impervious to mineral oils and greases. For many high load manual applications soft tread wheels must be ruled out as, although they may be capable of carrying a very high load, the force required to move the equipment would require the efforts of several men. However, the quiet running and floor protecting properties of the resilient tread wheels make them particularly suitable for power towing applications. For applications where the weight of the finished trolley is important micro-cellular foam tyres manufactured from EVA are an excellent option providing a cushioned ride while being durable and puncture proof and are particularly well suited for sensitive applications where floor marking may be a concern.
In the range of Hard Tread Wheels it is usual to consider castors with cast iron wheels as a basic standard, as these are generally the strongest and have the longest life. They are also certainly the easiest to push and in many cases will therefore, be the first choice for maximum mobility. They have the disadvantage, however, of being rather noisy and can sometimes cause excessive floor wear. As an alternative to cast iron, Injection Molded Solid Nylon wheels have been developed. Nylon is unaffected by water and its use in the wet further enhances its self lubricating properties. The load capacity of nylon approaches that of cast iron and these wheels also have the additional advantages of being quiet running, light in weight and almost incapable of damaging floors. They can be used in the temperature range -40ºC to 80ºC, although it may be necessary to derate the load capacity of the wheels by up to 25% for very arduous conditions. For higher temperature applications reinforced Phenolic wheels are also available.
The load ratings detailed on this site for wheels fitted in castors and for independent use on customer's own mountings are based on intermittent use on normal truck, trailer and similar mechanical handling applications. Specialised application such as turntables, drive rollers for drum rotation, drive wheel, and similar unusual types of use can introduce stresses on the wheels far in excess of those normally encountered and therefore the load ratings specified on this site will no longer apply.
We offer 4 main types of wheel bearing. Plain bearings are adequate for many 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 Journals absorb axial loads and are essential for power towing applications.
Plain Bearing- In the case of cast iron wheels and cast iron centred wheels, frequent lubrication is essential and except in the 75 mm size, a grease nipple is provided. In the case of nylon centred wheels and those with nylon bushes, 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 Bearing- For light duty applications, roller bearings are greased on assembly and should under normal conditions not require replenishment. Castors designed for heavy duty applications are aIways provided with a means of replenishing the lubricant in the roller bearing. On cast iron wheel, 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 therefore omitted.
Ball Journal Bearing- 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, etc.. 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 Bearing- 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 customers' axles - our castors are already adjusted on assembly. Seals are fitted to retain grease and exclude grit, dirt, etc., and grease nipples are provided for replenishment under arduous working conditions.
It is vitally important to be able to predict the amount of effort required to either push a truck along in a straight line or to manoeuvre it in confined places. When this information is known it should be possible to predict how many men are required to move and control a truck in all normal workshop conditions. As an aide memoire, the following table is sufficient:-
Wheel Type Tractive Resistance
Nylon, Phenolic, Cast Iron Less than 1%
Polyurethane Approx. 2%
Rubber Approx. 3.5%
All the above figures are based on roller bearing wheels of 200 mm diameter. For minor diameter changes, the tractive resistance should vary approximately in inverse proportion to the diameter, down to a minimum of 125 mm. To put the above figures into perspective, it should be remembered that a typical human being is capable of maintaining a horizontal force of 12 kg increasing to about 18 kg from a standing start. Polyurethane wheels, if left standing under full load for a substantial time, could require at least twice the above force to initiate movement, its tractive resistance will increase momentarily up to about seven times the figures given above.
All moving parts of our castors are liberally treated with a high performance grease containing specially prepared additives. This forms a protective layer which minimises wear by metal to metal contact. The Majority of our heavy duty castors incorporate head seals which eliminate the need for subsequent lubrication except under extreme working conditions. Where lubrication is necessary, this can be done using a pressure oil gun, ensuring that the oil reaches the king pin head on single ball race castors.
Most roller bearing wheels and heavy duty cast iron centred wheels of 100 mm diameter and above can be lubricated using either the dimpled and drilled axle bolt or the grease nipple fitted in the boss of the cast iron centre.
Nylon wheel bushes are virtually self lubricating but will benefit from occasional lubrication, particularly in very dusty conditions.
Grease nipples are fitted in the swivel heads of many of the heavy duty fabricated castors.
A general purpose lithium based grease is used in most of our castors and this has a normal temperature range from 130ºC down to -30ºC. At the low temperature end the consistency of the grease increases up to a point where the swivel and rolling performance is reduced.
For low temperature applications, it is recommended that double ball race castors are used in conjunction with low temperature grease. This combination is available in the 22 Series castors with thermoplastic based wheels of 100 and 125 mm diameter. The grease used has an extended temperature range from 130ºC down to -50ºC and will work satisfactorily at the lowest known temperature for a cold store of -40ºC.
At the top end of the temperature range the grease will operate satisfactorily but will need to be replenished frequently. This particularly applies when used in forced draught ovens which causes the oil component of the grease to be volatilised which can lead to the formation of "coke".
GT Series castors are supplied with an extended temperature range grease up to 200'C and can be used to this temperature in conjunction with the equivalent high temperature version of the Cast Iron roller bearing wheel.
This grease is compatible with normal lithium based greases. It is not good practice to try to convert standard castors and wheels for high temperature applications simply by pumping in the high temperature grease unless all of the original grease has been purged. At the higher temperatures, the original grease is liable to carbonise, resulting in damage to the bearings. Special purpose grease is also available to special order for applications in excess of 200'C on certain ranges of castors.
All our pressed steel castors are designed specifically for manual propulsion. Due to the greatly increased stresses exerted on various components on castors in power towing, our pressed steel castors are not recommended in power towing applications. In the case of fabricated castors, the load capacities are much higher and it is obvious that power towing must be the norm. The castors are therefore designed on the assumption that power towing will take place but at a maximum of 7 kph and under good working conditions. Obstructions such as curbs and gullies and even relatively small steps, can exert enormous impact loads which can destroy a castor. Steps such as lift sills, drain covers and joints in concrete slabs, present a particular problem if they are not approached squarely. An oblique approach will almost certainly result in the castor turning at right angles to the obstruction instead of turning in such a way that it can climb over it. In these circumstances the destruction of the castor is inevitable.
Towing trailers in train exacerbates the problem as only one castor may have to withstand the force generated by the mass of the whole train including the tractor.
As power forcing becomes more common it is often desirable to tow a number of trucks or trailers one behind the other. Traditional turntable axles have often been used for this purpose but in addition to being costly they have two disadvantages:
1) If the turntable is at right angles to the axis of the trucks the stability is dramatically reduced.
2) There is a risk of jack-knifing if the trailers stop on a sharp corner.
Castors are, therefore, becoming very popular for this application and work satisfactorily provided suitable precautions are taken. It is essential to obviate 'cutting-in' as this would prevent the use of truck trains in narrow gangways. Experience has shown that if trucks are rigidly coupled together by means of pin couplings at each end they will follow the track most accurately if the distance from the fixed castor centre line to the rear coupling pin is 25% - 33% of the total length of the truck from front to rear coupling pins.
This can be achieved by moving the fixed castors forward as shown in arrangement (ii) but this incurs a penalty in that it reduces the wheel base of the trucks and also causes the total weight to be shared disproportionately between the swivel castors and the fixed castors. In order to distribute the weight of the truck and its contents evenly between the front swivel and rear fixed castors it is advisable to extend the draw bar at the rear of the truck as shown in illustration (iii).
ISO 2184 sets out a range of standardised fixed hole centres as follows:-
Class 2 80mm x 60mm
Class 3 105mm x 80mm
Class 4 140mm x 105mm
Wherever possible our castors conform to the above standard. In some cases the plates are already slotted to accommodate alternative hole centres such as those to the DIN standard and in other cases to the popular standard used throughout North America. Whenever possible customers are advised to conform to the ISO standard and always to avoid placing bolts anywhere except at the ends of the slots.
Swivel head seals are fitted as standard on some of our range of castors. The seals are produced in polypropylene, nylon and glass filled nylon and are effective over the temperature range from -40ºC to 100ºC. Head seals effectively reduce maintenance to a minimum but some castors are fitted with grease nipples for replenishment purposes under arduous working conditions.
Generally we offer 2-way directional locks on certain castors. Other known competitors offer 4-way locks and it is necessary for customers to clearly understand the reasons for this decision.
If 4-way locks are fitted and a trailer is moved with the automatic locks in 'on' position but not engaged and the castors are facing the wrong way, they will almost inevitably lock automatically as the castor swivels through the 90º position. This will in effect brake the trailer and could cause a sudden swerve injuring anyone in the way.
If 2-way locked castors are fitted, they will function exactly as before but without risk of locking in the 90° position. We will still supply 4-way locks for special applications when we are satisfied that the customer understands the risks involved and knows the limitations in the use of these castors.
These are specifically designed to steady the equipment to which they are fitted and prevent horizontal movement. They are never intended to act as a jack and any attempt to vary the fitting height from their original specification may result in failure and/or damage. In each case there is a degree of built-in resilience in the foot pad. This will, to a certain degree, accommodate variations in floor level, but it is still essential to fit the truck lock as close as possible to one of the swivel castors. The truck lock can be severely damaged if the linkage is operated whilst the truck is moving.
The table below (still in preparation) gives a general 'rule of thumb' guide to the resistance of the wheels to certain chemicals and oils. The resistance is graded on a scale of I to 6 with 1 being high resistance and 6 being low resistance. The resistance to corrosion indicated in the table refers to a wheel being immersed in the chemical for a prolonged period.
In the case of splashes or very short periods of exposure the expected life of a wheel may be extended.
The bearings are correctly adjusted when the castor is built and should never require attention other than possibly greasing if the conditions are particularly arduous. Tapered Roller Bearings in swivel heads are deliberately pre-loaded as this eliminates any risk of damage even in conditions of severe abuse. No attempt should be made to strip down or readjust these bearings.
Tapered Roller Bearings in wheels are set up with a predetermined amount of axial clearance in exactly the same way as with road vehicle front hubs, and should never require readjustment. If, however, a wheel has to be replaced due to abuse or accidental damage, it is essential that the wheel is correctly set up with absolutely no risk of any pre-load of the bearings. In the absence of a dial indicator gauge (the correct measuring instrument) it is difficult to judge the degree of axial clearance, which should not exceed 0.1 mm. As with a bicycle front hub, it is possible to judge this reasonably accurately by feeling the amount of "rim-rock" at the head of the wheel.