Top plates with four bolt holes spread the load over the swivel head and are the commonest fixing. Single bolt hole with a fastener through the centre of the swivel head are limited to loads up to 320 kilogrammes. Bolt hole castors can be fitted with threaded stems and round or square expanding sockets to fit into tubes.
When assessing the load rating of a castor remember that a smooth floor – free from cracks, uneven joints, gullies etc – will be needed to overcome the shock loads created by the defect. The configuration of the castors on the trolley also makes a difference to load calculations. Because floors are not perfectly flat, as a trolley moves, one castor may float and not be supporting any load. An allowance must be made for this and below are the castor load calculations for each configuration.
- 2 Swivel and 2 Fixed. Pushed with the fixed castors leading this offers the best control of the load with good steering on both straight runs and cambers. Suitable for most loads. Castor capacity required is at least 33% of the total gross load.
- 4 Swivel. Offers maximum manoeuvrability but is difficult to control on straight runs, cambers and uneven surfaces, particularly with heavy loads. Fit directional locks to the front castors to overcome these difficulties. Castor capacity required is at least 33% of the total gross load.
- 1 Swivel and 2 Fixed. Easy to manoeuvre but only suitable for small trolleys with light loads. Stability can be a problem and loads must be distributed evenly. Castor capacity required is at least 40% of the total gross load.
- 3 Swivel. Maximum manoeuvrability but is difficult to control on straight runs, cambers and uneven surfaces. Only suitable for small trolleys with light loads. Castor capacity required is at least 40% of the total gross load.
- 4 Swivel and 2 Fixed Centrally Pivoting. Used on long trolleys to optimise control and manoeuvrability. The two central fixed castors are usually one size larger or the same as the swivel but fitted with a 25mm packing under the top plate. Castor capacity for the fixed castors is at least 50% of the total gross load. Swivel castors load will vary.
- 2 Swivel and 2 Fixed Centrally Pivoting Similar to above but less stable if the load is not evenly distributed. The two central castors are usually one size larger or the same as the swivel but fitted with a 25mm packing under the top plate. Castor capacity for the fixed castors is at least 50% of the total gross load. Swivel castors loads will vary.
Castor wheels are designed to be supported at both ends. They are intended for use in units with manual propulsion with speeds up to four miles per hour. Do not use on power towing without specialist advice as this application introduces many extra stresses and strains. Wheels can be categorised into two groups, hard treads and soft treads.
Hard Tread Wheels
Are the easiest to push because they have the least tractive resistance. The disadvantage of them is that they are noisy and are liable to cause excessive floor wear. The main types of hard tread wheels are as follows:
- Cast Iron and Steel combine strength and shock resistance with long life and economy. Temperature range – 40°c to +250°c
- Nylon has a high load capacity, is light and clean and causes little floor damage. Temperature range – 40°c to + 130°c
- Polyproylene has a good load capacity but does not have the abrasion of fracture resistance of nylon. Temp – 20°c to + 80°c
- Phenolic is very hard, abrasion resistant, fracture resistant but liable to wear and chip. High temperatures up to + 350°c
Soft Tread Wheels
Are resilient, making the quieter with generally less marking and floor wear. Tractive resistance however is much higher. Recent developments in new grades of polyurethane and rubber have now increased load capacities and reduced tractive resistance. There are many types of soft tread wheels, which is increasing all the time the advance in rubber and elastomer technologies.
- Solid Rubber is the basic range although new thermoplastic grades are hard wearing with low coast. Temp – 20°c to + 60°c
- Elastic Rubber has a high load capacity, high wear resistance and gives a smooth, soft, cushioned ride. Temp – 20°c to + 60°c
- Polyurethanes have very high load capacity, abrasion and tear resistance, soft ride and chemical resistance. Temp – 30°c to + 90°c
- Pneumatic are excellent for shock absorbing and work well on difficult surfaces like grass, gravel and rough uneven surfaces.
- High temperature wheels are now also available with high temperature rubber types suitable for temperatures up to 250°c
The choice of bearings is determined by the application, working environment and load.
- Plain bearings are mainly for intermittent use and low loads, Risks include axle wear and squeaking in dusty and wet conditions.
- Roller bearings are simple in construction, robust and excellent for regular use on low/medium loads working at low speeds.
- Ball Journal precision bearings are for higher loads with high radial and moderate forces. Low speed towing and manual.
- Taper Roller bearings are in larger diameter wheels with the highest loads and most arduous applications including towing.
- Sintered Bronze or other metals are a plain bearing used on high temperature application to reduce friction and bearing wear.
- Teflon Sleeve bushes are another plain bearing to reduce wear and friction, but are used on lighter loads than sintered bronze.