Layshaft

[1][2] Layshafts are best known through their use in car gearboxes, where they were a ubiquitous part of the rear-wheel drive layout.

There may be multiple separate clusters on a shared shaft and these are allowed to turn freely relative to each other.

This gives a 'direct drive' top gear, which has advantages for both efficiency and quietness at cruising speed.

The gear lever enters through the top of the casing and so it is more convenient for the sliding components of the dog clutches to be mounted on the driven shaft, rather than the layshaft.

The bearings may be plain phosphor bronze bushes, or for high-load applications needle rollers.

[8] Where a power take-off is required, usually for industrial vehicles to drive winches, hydraulic pumps etc., this is often driven from one end of the layshaft, as this is more accessible shaft than the main shafts, already in use by the drivetrain.

[3] All-indirect were used for some very early cars, before the advantages of the direct-drive top gear were recognised.

Once the dog clutch came into use, the further advantage of a direct-drive top gear was immediately recognised.

These have been used for both transverse engine layouts, where the offset of two shafts is more convenient than the in-line arrangement of a layshaft gearbox,[9] and for longitudinal transaxle designs, such as the VW Beetle or many Renaults,[iv] where the requirement was to have both driven and driving shafts at the same end of the gearbox.

One of the first mass-market cars to offer such a feature was the VW Polo and the Formel E, with a deliberately-labelled '-E' for 'economy' overdrive ratio.