Transmissions and gearbox systems are used in cars, trucks, and busses to transmit the mechanical power produced by the engine to the wheels. Similar transmission systems are used in propeller aircraft to transmit power to the propeller(s) from the engine(s) or turbine(s). Transmission gearboxes are also used in some railroad systems and ships. Some modern high‐speed rail vehicles, however, use motive power systems (electric motors) mounted directly onto the axles of each rail vehicle, resulting in quieter operation and reduced noise problems.
The gearbox can be the source of vibration and radiated noise and should be suitably soft mounted to the vehicle structure, wherever possible. Shaft misalignment problems must be avoided, however, with the mounting system chosen. The principal components of a gearbox are comprised of gear trains, bearings, and transmission shafts.
Unless substantial bearing wear and/or damage have occurred, gear meshing noise and vibration are normally the predominant sources in a gearbox. The vibration and noise produced depend upon gear contact ratios, gear profiles, manufacturing tolerances, load and speed, and gear meshing frequencies. Different gear surface profiles and gear types produce different levels of noise and vibration. In general, smaller gear tolerances result in smoother gear operation but require increased manufacturing costs.
Gearboxes are often fitted with enclosures to reduce noise radiation, since the use of a low‐cost gearbox combined with an enclosure may be less expensive than the use of a high‐performance gear system and gearbox without an enclosure. Gearbox enclosures, however, can result in reduced accessibility and additional maintenance difficulties. A better approach, where possible, is to try to utilize a lower noise and vibration gear system so that a gearbox enclosure is unnecessary. Reference [29] deals with gearbox noise and vibration and Ref. [30] specifically addresses gear noise and vibration prediction and control methods.
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