In a sliding-mesh gearbox, individual gears are mounted so they always engage the shaft, but gears on shaft can be moved axially. To engage a specific pair of gears, gear is slid axially until it fully engages a gear on the other shaft. If the gear shafts are spinning so the gears have the same surface velocity, the gears are comparatively simple to engage. However, if velocities are mismatched, the gears tend to "bounce" off each other at first contact & resist engagement. Thus, gear engagement depends on the driver carefully matching speeds, usually through practice & intuition.
crash gearbox, often known as a crash box, is a transmission type used in elderly cars, trucks, & other automotive vehicles. It is more properly called a "sliding mesh" gearbox & has the nickname "crash" because it is difficult to alter gears, so gear changes are often accompanied by loud noises. The etymology of "crash" is probably "clash".
In contrast, newer "constant mesh" transmissions use gears that are held axially, but gears on shaft spin freely on the shaft. Gear pairs in the transmission are always in mesh, though at most 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. Plenty of constant mesh transmissions use a sliding-mesh gear for reverse, but since reverse is only engaged from near a cease, it is still simple to engage.
A constant-mesh transmission offers several advantages over a sliding-mesh design. First, the dog clutch is designed for the task, than asking the gear to do dual duty of power transmission & sliding engagement. Second, the dog clutch is usually 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 depends on the operator to match speeds, gears are simpler to engage. Third, a constant-mesh transmission can basically use spiral gears which are smoother, quieter, & can carryover more torque for a given size of gear. Fourth, a constant-mesh transmission can use synchromesh for simpler shifting; while plenty of heavy vehicle transmissions do not use it, most medium- & light-duty automotive transmissions do.
crash gearbox, often known as a crash box, is a transmission type used in elderly cars, trucks, & other automotive vehicles. It is more properly called a "sliding mesh" gearbox & has the nickname "crash" because it is difficult to alter gears, so gear changes are often accompanied by loud noises. The etymology of "crash" is probably "clash".
In contrast, newer "constant mesh" transmissions use gears that are held axially, but gears on shaft spin freely on the shaft. Gear pairs in the transmission are always in mesh, though at most 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. Plenty of constant mesh transmissions use a sliding-mesh gear for reverse, but since reverse is only engaged from near a cease, it is still simple to engage.
A constant-mesh transmission offers several advantages over a sliding-mesh design. First, the dog clutch is designed for the task, than asking the gear to do dual duty of power transmission & sliding engagement. Second, the dog clutch is usually 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 depends on the operator to match speeds, gears are simpler to engage. Third, a constant-mesh transmission can basically use spiral gears which are smoother, quieter, & can carryover more torque for a given size of gear. Fourth, a constant-mesh transmission can use synchromesh for simpler shifting; while plenty of heavy vehicle transmissions do not use it, most medium- & light-duty automotive transmissions do.
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