Rolling resistance determines the effort necessary to move the equipment from rest and to keep it rolling. If a wheel, so hard that it will not flatten under load, rolls on an absolutely flat hard surface, perfect rolling is possible.
The nearer you get to this ideal, the lower the rolling resistance becomes. In view of this, the first choice should be hard tread wheels, but the average floor is far from smooth and clean and is often littered with small particles of some description. Once the wheels are rolling, they will pass over these small items with no problem, but if the equipment starts from rest with minor obstructions, such as swarf, in front of the wheels, then the starting effort includes that which is necessary to lift the equipment to the height of the obstructions.
In cases where hard wheels are used on soft floors such as carpeting, or even asphalt, the surface deformation causes the wheels to sit in small ruts, and extra effort is needed to overcome such resistance. Resilient wheels move more easily over obstructions as they deform and roll over them instead of climbing. On the other hand, as a tire flattens under load, an area instead of a line is always in contact with the floor, and rolling resistance to rolling is greater.
Individual characteristics of our wheels are in the “Caster Wheels” section of this website; however, because of variations in floor conditions, hardness, wheel diameter, tread profile, etc., it is not practical to give rolling resistance figures in the tables. We will always be pleased to advise when given the necessary information, but the only accurate assessment is one based on the user making his or her own tests on site.
We do not give load capacities for “ideal working conditions” as these rarely exist. Capacities given in this catalog are based on normal working conditions on reasonably level floors free from grooves, breaks, door sills, lift entrances, and other hazards. They are also based on a speed of 3.5mph unless otherwise stated.
Although Revvo Casters and wheels have a high factor of safety-to-capacity ratio, we recommend the use of the following formula to determine the load capacity for uneven floors:
Weight of Transport Equipment + Load
Uneven floors only have three wheels in contact with the floor on a 4-wheel trolley.
Caster and Wheel Lubrication
We recommend regular greasing of all grease nipples with a lithium-based grease of the type used on car axles. Where grease nipples are not provided, a few drops of light machine oil should be used.
Revvo’s standard finish for pressed steel casters is bright zinc electro-plating to BS1706. Wheels and fabricated casters are paint sprayed to a high finish. Other finishes can be supplied to special order, subject to economical batches.
Caster and Wheel Selection
The following factors all have a bearing on which wheel or caster is suitable for a particular application, including the performance to be expected.
Apart from the gross weight of the load and equipment, an allowance should be made for uneven distribution of weight as this often results in one wheel carrying more than the rest. Mechanical towing can also impose severe shock loading.
Rough, uneven floors often go unnoticed, but they cause severe overloading in some cases simply because one wheel leaves the ground, and the load is thrust upon the remainder. Impact loads due to obstructions and holes in the floor also cause problems.
If a truck moved by hand meets an obstruction, it becomes harder to push, and the truck slows down, takes an easier route, or the load is reduced. All these reduce the strain on the casters as well as the person pushing. This does not happen with mechanical towing as there is usually plenty of power in hand; this, combined with higher speed, results in considerably more caster stress.
All Revvo pressed steel casters are designed specifically for manual propulsion. As power towing of casters places increased stresses on various components, these casters are not recommended for this application. Fabricated casters have much higher load capacities, making them suitable for power towing. H Series casters are specifically designed for power towing but must be fitted to wheels with Precision Ball (BJ) or Taper Roller (TP) bearings only. Obstructions can exert enormous impact, which can damage a caster. Steps such as lift sills, drain covers, and joints in concrete slabs, present a particular problem if they are not approached squarely. Approaching such obstacles obliquely makes the caster turn at right angles to the obstruction instead of turning in such a way that it can climb over it, this damages the caster.
Towing trailers in train increases the problem as only one caster may have to withstand the force generated by the mass of the whole train, including the tractor.
As power towing becomes more common, it is often desirable to tow several trucks or trailers one behind another. Traditional turntable axles are commonly used, but are expensive and have two disadvantages:
- If the turntable is at right angle to the axis of the trucks, then the stability is dramatically reduced;
- There is a risk of jack-knifing if the trailers stop on a sharp corner.
Because of this, casters are becoming increasingly popular for this application. They work satisfactorily in these conditions provided suitable precautions are taken. “Cutting-in” must be avoided, as this prevents the use of truck trains in narrow gangways. If trucks are rigidly coupled together with pin couplings at each end, they will follow the track most accurately if the distance from the fixed caster centerline 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 casters forward. However, this reduces the truck’s wheelbase and causes the total weight to be shared unevenly between the swivel and rear fixed casters. Therefore, it is advisable to extend the drawbar at the rear of the truck.
All metal wheels will damage floors if the load is heavy. Resilient wheels are not so easy to move, but they are certainly kinder to floors. It should be remembered that the floor will cost more to repair than the price of the wheel running on it.
Revvo supplies two-way or four-way locks across the range of heavy-duty casters. More positions are available on request.
Designed to steady the equipment to which they are fitted and stop horizontal movement, floor locks are never intended to act as a jack. Any attempt to vary the fitting height from the catalog specification will result in failure and/or damage. There is some built-in resilience in the footpad, which will allow some variations in floor level. Nonetheless it is essential that the floor lock is fitted as closely as possible to one of the swivel casters
Please note that the floor lock can be severely damaged if the linkage is operated while the floor lock is moving.
Casters and wheels are usually fitted to equipment that is used intermittently and is often stationary for much of the time. Revvo’s products are specifically designed to operate well under such conditions.
For casters and wheels on applications such as conveyors that run continuously, the normal load capacities do not apply as a life factor has to be introduced. Each case should be considered individually, and we will be pleased to advise you on receipt of the relevant information.