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In engineering a dog is a tool that prevents movement or imparts movement 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.
Although not seen on all chainsaws, when present chainsaw dogs are mounted where the bar meets the power head. Chainsaw dogs provide stability and serve as a sort of fulcrum for swinging the bar through the item being cut.
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) are held open during pleasant weather by wrought-iron or cast-iron dogs, which are called shutter dogs.
A bench dog is an accessory used on a woodworking workbench to allow clamping of wooden items whilst being worked.
Ladder dogs are the parts of a ladder that hold the ladder at a certain height, and articulate against pawls to allow adjustment.
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).
Andirons, which hold up the firewood in a fireplace, are sometimes called dogs, firedogs, or dog irons.
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.
The lathe dog (or lathe carrier) is essentially analogous to a clutch dog. It is used to provide positive drive to a workpiece turning between centers on a lathe. Without the dog, the cutting tool would tend to "catch", e.g. stop the workpiece from turning while the headstock center continued to rotate, possibly causing damage to the workpiece, or the lathe.
The feed dogs of a sewing machine feed the fabric in a linear stepping motion past the needle.