A layshaft is the middle shaft within a gearbox (transmission). When the clutch is engaged, the layshaft turns constantly. All gears on the layshaft turn at the same rate and are permanently attached. It carries gears but does not transfer the primary drive of the gearbox in of the gearbox or out of the gearbox. Layshafts are mostly known through their use in car gearboxes. The oil level in a gearbox is set to layshaft height. That way the spinning gears distribute oil to all the gears.