In mechanical engineering, on a general level, a dog can be any piece of material that prevents movement or imparts movement, in either case by offering physical obstruction or engagement of some kind. It may hold another object in place by blocking it, clamping it, or otherwise obstructing its movement. Or it may couple various parts together so that they move in unison - the primary example of this being a flexible drive to mate two shafts in order to transmit torque. This word usage is a metaphor derived from the idea of a dog (animal) biting and holding on.
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Subtypes and examples of applications
Shutter dogs
Anyone who has seen functional exterior window shutters (which can be swung shut whenever storms approach in order to protect the window glass from impact by wind-blown debris) may have noticed that the shutters are held open during pleasant weather by wrought-iron or cast-iron dogs, which are called shutter dogs.
Ladder dogs
Ladder dogs are the parts of a ladder that hold the ladder at a certain height, and articulate against pawls to allow adjustment.
Hatch dogs
The doors that allow passage through bulkheads between compartments inside a ship can be closed during emergencies to seal off one compartment from another, thus sequestering water leaks, fires, or waves of air pressure and preventing them from compromising the rest of the ship's interior. The objects that are wedged against the door to hold it closed against the water or air pressure are an example of dogs. To dog the hatches means to close the hatches and dog them down (fasten them closed).
Clutch dogs
The clutch that mates the engine to the transmission in a modern manual-shift automobile is a friction clutch whose disc and pressure plate are smooth; they lock up simply through friction. However, some kinds of clutches (including the ones inside an automobile transmission) may lock up via the engagement of dogs, rather than only through friction. These clutches are called dog clutches and the dogs used within them are called clutch dogs.
Lathe dogs
The lathe dog (or lathe carrier) is essentially analogous to a clutch dog. It is used to provide positive drive to a workpiece turning on centers in a lathe. Without the dog, the cutting tool would tend to stop the workpiece from turning while the headstock center continued to rotate.
