Layshaft

A layshaft is an intermediate shaft within a gearbox that carries gears, but does not transfer the primary drive of the gearbox either in or out of the gearbox. Layshafts are best known through their use in car gearboxes, where they were a ubiquitous part of the rear-wheel drive layout. With the shift to front-wheel drive, the use of layshafts is now rarer.

The driving shaft carries the input power into the gearbox. The driven shaft is the output shaft from the gearbox. In car gearboxes with layshafts, these two shafts emerge from opposite ends of the gearbox, which is convenient for RWD cars but may be a disadvantage for other layouts.

For gearboxes in general, gear clusters mounted on a layshaft may either turn freely on a fixed shaft, or may be part of a shaft that then rotates in bearings. There may be multiple separate clusters on a shared shaft and these are allowed to turn freely relative to each other.

The term layshaft originates with watermill machinery. The layshaft is the gear-carrying shaft that links the wallower (the small spur gear driven at increased speed by the waterwheel) to any upright shafts that carry the millstones. The term, layshaft, was also used by millwrights, in both wind- and watermills, to refer to a shaft that drove secondary machinery such as sack hoists, rather than the main milling machinery.

The term layshaft was also applied to back-geared lathes. These were lathes with a slow-speed mechanism in addition to their usual belt drive. This used two gears on a layshaft behind the , giving a double reduction gear.

In the typical manual gearbox for a RWD car, the driving shaft (input) is in-line with the driven shaft (output), but not permanently connected to it. A reduction gear on the driving shaft drives the layshaft. In car transmissions, the term countershaft is also used. A number of gears on the layshaft may then be connected, one at a time, to the driven shaft. Selecting each of these gears in turn gives the various ratios of the gearbox. All of these gear ratios are reduction gears, the engine speed being higher than the input speed to the final drive of the rear axle.

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Wikipedia