In a manual transmission, the gears and shafts are supported on bearings or bearing surfaces. This allows for rotation and also maintains the alignment of the components.The type of bearing used at each locating point depends on the load which must be sustained.
Radial loads try to force the gears and shafts apart. The bearing therefore carries the load along its radius.
Thrust loads are applied along the length of the components, so the bearing must cater for side thrusts.
In many cases these loads are combined and some bearings designed for radial loads will also accept light thrust loads. This applies to most ball bearings.
Single-row, deep-groove ball bearings normally support the input shaft and mainshaft in conventional rear-wheel drives. They can also be used at each end of the countershaft.
The helical gearing creates radial and thrust loads as the gears try to force each other apart and move end wise.
A circlip, in a groove on the outside of the bearing, can locate each shaft in the casing and maintain the alignment of the gears and engagement hubs.
Roller bearings are common where loads are purely radial. Needle roller bearings can be caged, or as a loose number assembled to operate directly on a hardened gear or shaft.
Typical applications are the spigot on the front of the mainshaft, supported in the rear of the input shaft, and, in some cases, on the countershaft, and the freely rotating gears on the mainshaft.
Separate thrust washers can retain them and cater for thrust loads. They are often a selective thickness to allow for end-float adjustment.
Thrust washers are plain bearings They are usually bronze, but can be steel, with a bronze facing on one side.
They can be dimpled or grooved to allow for lubrication.
A plain bearing can also be used in the extension housing, to support the front of the propeller shaft. Machined and hardened surfaces on the mainshaft can provide bearings for the freely rotating gears.
Flanges and snap-rings on the mainshaft locate the gears and cater for end-thrust.
A plain bearing is also used for the reverse idler.
Taper roller bearings are normally used in pairs and can sustain heavy radial and thrust loads in either direction.
They are commonly used in trans-axles, where they support the front and rear of the primary and secondary shafts, and the final drive assembly.
Shims or selective washers can be used to provide for adjustment of pre-load.