1.1 Rubber wheels
A rubber wheel covering consists of an elastomer made from natural and/or synthesised rubber. The rubber used to build industrial wheels can be vulcanised or injection moulded.
- Vulcanised rubber: special mineral loads and vulcanising agents are added to the rubber that undergoes a process called “vulcanising”. During this process, the rubber’s molecular structure changes significantly: the “pasty” material at the beginning of the process becomes a non-fusible product that acquires and, over time, maintains the form of the mould in which the reaction occurs.
The ring obtained is mechanically assembled to the wheel centre body. Vulcanised rubber has enhanced elastic deformability properties within relatively broad ranges of applied traction and compression loads.
The physical-mechanical characteristics of vulcanised rubber vary according to the quality of the natural and/or synthesised rubber used, the type and quantity of mineral loads added and the conditions under which the vulcanisation process takes place.
- Injected rubber: the rubber goes through a chemical synthesis process. The material obtained is injected into a mould in which the wheel centre body has already been inserted. The injected rubber maintains its fusibility even after moulding.
Normally, the elastic properties of injected rubber are worse than those of the best quality vulcanised rubber, even though they are comparable to those of medium and low-quality vulcanised rubber. The following are some of the main physical-mechanical parameters relative to the quality of rubber (for the definition of each parameter see the standards indicated next to that parameter):
- hardness UNI EN ISO 868:1999; ASTM D 2240-2004
- specific density UNI 7092:1972; ISO 2781:1988
- impact strength UNI 7716:2000; ISO 4662:1986
- abrasion loss UNI 9185:1988; DIN 53516:1987
- ultimate tensile strength UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998
- ultimate elongation UNI 6065:2001; ISO 37:1994; ASTM D 412c-1998
- tearing resistance UNI 4914:1987; ASTM D 624b-2000
- compression set UNI ISO 815:2001
These parameters are not independent; in other words, changing one of them usually leads to a change in other parameters (to varying degrees). Hardness is the easiest parameter to determine: in general, increased hardness reduces the elastic properties (impact strength, ultimate elongation, compression set) and lowers overall wheel performances. Instead, parameters such as tearing resistance and abrasion loss depend mainly on the composition of the vulcanised rubber and, to a lesser extent, on hardness.