5.4 SPEED AND MEANS OF TRACTION
Equipment speed is an important factor when choosing a wheel. In fact, if the speed is 0, and thus the use is mainly static, it is enough to compare the load capacity for each wheel with the static load indicated in the manufacturers’ catalogues. If the speed is other than 0, then the means of traction must be taken into consideration. The means of traction is the tool used to exert the force that moves the body. In industry, traction devices can be manual or mechanical. Manual moving refers to the situation in which the force is exerted by one or more persons, while mechanical refers to the situation in which such force is exerted by a mechanical device (on-board drives or by using towing devices).
For manual moving, the speed is generally less than or equal to 4 km/h. Choosing a wheel that allows only one operator to move a load should be based on a wheel rolling resistance value determined by the following formula:
where: S = rolling resistance Pu = weight to transport Pc = equipment tare (equipment weight) n = number of equipment wheels (maximum 4). The value obtained should be compared to the wheel rolling resistance value indicated in the manufacturer’s catalogue.
Mechanical moving with towing devices
In case of towed mechanical moving, the wheel should be chosen based on the equipment’s operating speed. The wheel rated dynamic load capacity normally refers to a speed of no more than 4 km/h (1.1 m/s).
If the speed is higher than 4 km/h, a correction factor must be applied to the load capacity value since the materials forming the wheel undergo chemical-physical changes during which their performances decrease with an increase in operating speed. The indicative percentages of load capacity variation with an increase in speed for different types of wheels are shown in the following table.
On-board mechanical movement
For equipments with an on-board drive (equipments with drive wheels – self-propelled equipments), the wheels are subjected to particular stress and strain.
In fact, the drive wheels not only support the load, but also must transmit the tangent stress that allows the wheel and therefore the equipment to advance. In addition, the drive wheel covering is subjected to even greater stress. In particular, when choosing wheels and castors for self-propelled equipments, the following factors must also be taken into consideration:
- type of plain or ball bearing applied in the bore;
- shaft/bore coupling tolerances;
- bore material in relation to shaft material;
- start and stopping frequency of the motion transmission part;
- direction reversals;
- presence of even temporary overloads.
Since many factors have to be evaluated, it is recommended to contact ELESA S.p.A. to choose the wheels and castors to apply to self-propelled equipments.