Crash gearbox

A crash gearbox, also known as a crash box, is a transmission type used in old cars, trucks, and other automotive vehicles. It is more properly called a "sliding mesh" gearbox and has the nickname "crash" because it is difficult to change gears, so gear changes are often accompanied by loud noises. The etymology of "crash" is probably "clash".

In a sliding-mesh gearbox, individual gears are mounted so they always engage the shaft, but gears on one shaft can be moved axially. To engage a particular pair of gears, one gear is slid axially until it fully engages a gear on the other shaft. If the gear shafts are spinning so the two gears have the same surface velocity, the gears are relatively easy to engage. However, if velocities are mismatched, the gears tend to "bounce" off each other at first contact and resist engagement. Thus, gear engagement relies on the driver carefully matching speeds, typically through practice and intuition.

In contrast, newer "constant mesh" transmissions use gears that are held axially, but gears on one shaft spin freely on the shaft. Gear pairs in the transmission are always in mesh, though at most one is engaged at any time. Each free-spinning gear has a dog clutch which is engaged by an axial sliding collar that transfers power to the shaft. The dog clutch may be plain, also called "non-synchromesh", or may use an additional synchromesh mechanism that helps get the parts moving at the same speed to assist engagement. Many constant mesh transmissions use a sliding-mesh gear for reverse, but since reverse is only engaged from near a stop, it is still easy to engage.

A constant-mesh transmission offers several advantages over a sliding-mesh design. First, the dog clutch is designed for the task, rather than asking the gear to do dual duty of power transmission and sliding engagement. Second, the dog clutch is typically smaller in diameter than the gear it controls, so absolute speeds of the engaging parts are lower, aiding engagement. Thus, while a non-synchromesh transmission still relies on the operator to match speeds, gears are easier to engage. Third, a constant-mesh transmission can easily use helical gears which are smoother, quieter, and can carry more torque for a given size of gear. Fourth, a constant-mesh transmission can use synchromesh for easier shifting; while many heavy vehicle transmissions do not use it, most medium- and light-duty automotive transmissions do.