In engineering, a dog is a

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

or part of a tool, such as a

pawl A pawl is a movable lever that engages a fixed component to either prevent movement in one direction or restrain it altogether. As such, it is a type of latch and can also be considered a type of dog. It typically consists of a spring-loaded ...

, 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 analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

. Some devices use

dog clutch A dog clutch (also known as a dog box, dog gears, dog ring, clutch dog, or positive clutch) 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. T ...

es to lock together two spinning components. In a

manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canadian English, Canada, British English, the United Kingdom and American English, the United States), or stick shift (in the United States), is a multi-speed ...

, 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 usually meant to cr ...

derived from the idea of a

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the gray wolf. Also called the domestic dog, it was selectively bred from a population of wolves during the Late Pleistocene by hunter-gatherers. ...

(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 Linkage (mechanical), 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 se ...

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

chainsaw A chainsaw (or chain saw) is a portable handheld power saw, power saw that cuts with a set of teeth attached to a rotating chain driven along a guide bar. Modern chainsaws are typically gasoline or electric and are used in activities such as t ...

s, 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 A ladder is a vertical or inclined set of rungs or steps commonly used for climbing or descending. There are two types: rigid ladders that are self-supporting or that may be leaned against a vertical surface such as a wall, and rollable ladd ...

at a given height by engaging on its rungs, and are secured in place by spring-loaded

Door 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 An andiron, firedog, fire-dog, fire dog or iron-dog is a bracket support, normally one of a pair, on which logs are laid for burning in an open fireplace, so that air may circulate under the firewood, allowing better burning and less smoke. T ...

used in a pair to secure wood for a fire or skewers for cooking over one.

Clutch dog

Clutch dogs are the individual lugs within a mechanism called the ''

'', which comprises a pair of unconnected discs aligned axially, each with a radial arrangement of lugs which oppose one another and which interlock when the plates are in contact. Examples of dog clutches can be found in many vintage automobiles and are implemented in some modern automatic transmissions. The dog clutch contrasts with the '' friction clutch'', the most basic form of which comprises a smooth disc and pressure plate: when pressed together they lock up simply through friction, which permits some degree of slip as the mating pressure is modulated and thus permits some difference in the rotational speeds of the discs. A dog clutch, however, is a binary system, thus requiring that the rotating speeds of the two discs be matched prior to mating to avoid damage by jerk within the system - this can be achieved automatically, such as by implementing a synchromesh mechanism, or manually, as in a crash gearbox.

Lathe dog

lathe dog A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, Deformation (engineering), deformation, facing (machining), facing, Threading (manufac ...

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

metal lathe In machining, a metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, an ...

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 () and "post-and-beam" construction are traditional methods of building with heavy Beam (structure), timbers, creating structures using squared-off and carefully fitted and Woodworking joints, joined timbers with joints secure ...

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 the sawmills, it was a standa ...

(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.

References

{{Reflist Tools