In engineering, a dog is a

tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates ba ...

or part of a tool, such as a pawl, that prevents or imparts movement through physical engagement. 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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

. Some devices use

dog clutch A dog clutch (also known as a positive clutch or dog gears) is a type of clutch that couples two rotating shafts or other rotating components by engagement of interlocking teeth or dogs rather than by friction. The two parts of the clutch are de ...

es to lock together two spinning components. In a

manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canada, the United Kingdom, and the United States), or stick shift (in the United States), is a multi-speed motor vehicle transmission system, where gear change ...

, the dog clutches, or "dogs" lock the selected gear to the shaft it rotates on. Unless the dog is engaged, the gear will simply freewheel on the shaft. This word usage is a

metaphor A metaphor is a figure of speech that, for rhetorical effect, directly refers to one thing by mentioning another. It may provide (or obscure) clarity or identify hidden similarities between two different ideas. Metaphors are often compared wi ...

derived from the idea of a

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

(animal) biting and holding on, the "dog" name derived from the basic idea of how a dog jaw locks on, by the movement of the jaw, or by the presence of many teeth. In engineering the "dog" device has some special engineering work when making it – it is not a simple part to make as it is not a simple bar or pipe, and the metal used in its construction is likely to be special rather than regular steel. There is potential for confusion as "dog tensioners" are levers that are named due to the shape of the lever appearing as a dog leg, as the lever is in a

pantograph A pantograph (, from their original use for copying writing) is a mechanical linkage connected in a manner based on parallelograms so that the movement of one pen, in tracing an image, produces identical movements in a second pen. If a line dr ...

arrangement, or "dog trailers", which are named due to the use of multiple trailers for transporting animal cages.

Subtypes and examples of applications

Chainsaw dog

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.

Shutter dog

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''.

Bench dog

A bench dog is an accessory used on a woodworking workbench to allow clamping of wooden items whilst being worked.

Ladder dog

Ladder dogs secure the sections of an extension ladder at a given height by engaging on its rungs, and are secured in place by spring-loaded pawls.

Door dog {{anchor, Hatch dog

The watertight doors that allow passage through bulkheads between compartments inside a ship can be closed during emergencies to seal off one compartment from another, thus isolating flooding, smoke and heat from adjoining compartments. The objects that are wedged against the door to hold it closed are ''dogs''. To ''dog the doors'' means to close the doors and secure them in the closed positions. Similarly, hatches are the horizontal openings between decks, and are secured with ''hatch dogs''.

Firedog

A firedog, fire dog, or dog iron is a form of andiron used in a pair to secure wood for a fire or skewers for cooking over one.

Clutch dog

A clutch dog 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 those inside an

automatic transmission An automatic transmission (sometimes abbreviated to auto or AT) is a multi-speed transmission used in internal combustion engine-based motor vehicles that does not require any input from the driver to change forward gears under normal driving ...

) may lock up via the engagement of dogs, rather than only through friction. These clutches are called ''

es'' and the dogs used within them are called ''clutch dogs''.

Lathe dog

lathe dog A lathe dog is a mechanical device typically made of cast iron, steel or aluminum that transmits rotary motion from a faceplate to a workpiece mounted between centers in a lathe. The tail of the dog is rotated by a slot in a driving faceplate, ...

(or "lathe carrier") is a form of clutch dog used to secure a workpiece to a metal lathe.

Feed dog

The

feed dogs A feed dog is a movable plate which pulls fabric through a sewing machine in discrete steps between stitches. Action A set of feed dogs typically resembles two or three short, thin metal bars, crosscut with diagonal teeth, which move both fro ...

of a sewing machine feed the fabric in a linear stepping motion past the needle.

Log dog or timber dog

In carpentry log dogs were used to repair

timber frame Timber framing (german: Holzfachwerk) and "post-and-beam" construction are traditional methods of building with heavy timbers, creating structures using squared-off and carefully fitted and joined timbers with joints secured by large wooden ...

joints. In

hewing In woodworking, hewing is the process of converting a log from its rounded natural form into lumber (timber) with more or less flat surfaces using primarily an axe. It is an ancient method, and before the advent of the industrial-era type o ...

(shaping with an axe) timbers or sawing with some types of water powered sawmills log dogs are used to hold the timber in place.

Drive dog or reel dog

In film projection, reel arms and film rewinds employ drive pins adjacent to the spindle shaft, which are used to drive the reel as the spindle rotates. These are referred to as drive dogs or reel dogs.

Gallery

Image:twobenchdogs.jpg, A bench vise with its dog extended, holding (without using the dog) a wooden bench dog being worked. Image:dog_and_pawl.jpg, A ladder dog Image:LatheDrivePlate-carriers.jpg, Straight-leg lathe dogs (carriers) File:Timmerhakar.jpg, Log dogs are used to hold timbers in place while

or as a repair to

joints. Tools