Key (engineering)
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mechanical engineering Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, and ...
, a key is a
machine element Machine element or hardware refers to an elementary component of a machine. These elements consist of three basic types: # '' structural components'' such as frame members, bearings, axles, splines, fasteners, seals, and lubricants, # '' mech ...
used to connect a rotating machine element to a shaft. The key prevents relative rotation between the two parts and may enable
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 ...
transmission. For a key to function, the shaft and rotating machine element must have a keyway and a keyseat, which is a slot and pocket in which the key fits. The whole system is called a keyed joint. A keyed joint may allow relative axial movement between the parts. Commonly keyed components include
gear A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic pr ...
s,
pulley A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that ...
s,
coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
s, and
washer Washer most commonly refers to: *Washer (hardware), a thin usually disc-shaped plate with a hole in the middle typically used with a bolt or nut *Washing machine, for cleaning clothes Washer may also refer to: *Dishwasher, a machine for cleani ...
s.


Types

There are five main types of keys: ''sunk'', ''saddle'', ''tangent'', ''round'', and ''spline''.


Sunk key

Types of sunk keys: ''rectangular'', ''square'', ''parallel sunk'', ''gib-head'', ''feather'', and ''Woodruff''.


Parallel keys

''Parallel keys'' are the most widely used. They have a square or rectangular cross-section. Square keys are used for smaller shafts and rectangular faced keys are used for shaft diameters over or when the wall thickness of the mating hub is an issue. Set screws often accompany parallel keys to lock the mating parts into place. The keyway is a longitudinal slot in both the shaft and mating part. File:Passfedernut.jpg, The keyseat in a shaft for a parallel key File:Sprocket parallel keyway.png, A sprocket with an internal parallel keyway File:Parallel keys.png, Cross-section of a parallel keyed joint : ''W'' = : ''H'' = where * ''W'' is the key width * ''H'' is the key height * ''d'' is the shaft diameter


Woodruff keys

Woodruff keys are
semicircular In mathematics (and more specifically geometry), a semicircle is a one-dimensional locus of points that forms half of a circle. The full arc of a semicircle always measures 180° (equivalently, radians, or a half-turn). It has only one line of ...
, fitting partly into a
circular segment In geometry, a circular segment (symbol: ), also known as a disk segment, is a region of a disk which is "cut off" from the rest of the disk by a secant or a chord. More formally, a circular segment is a region of two-dimensional space that is ...
keyway with the remainder fitting into a longitudinal slot keyway in the mating part. The circular segment can be cut directly by plunge cutting with a circular Woodruff cutter without any reliefs. The main advantage of the Woodruff key is the elimination of milling near shaft shoulders, where
stress concentration In solid mechanics, a stress concentration (also called a stress raiser or a stress riser) is a location in an object where the stress is significantly greater than the surrounding region. Stress concentrations occur when there are irregularitie ...
s, and
concentricity In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...
would be affected. The latter is particularly important for high speed operation. The more exact fit of the key and keyway also reduces play, and stress concentrations in, and improves the reliability of the key. An additional advantage is a stuck key can be removed from a shaft with a hammer blow, the circular profile will push the key out of the slot, as opposed to a standard key which will need to be pushed axially, or pulled out of its slot. Common applications include
machine tool A machine tool is a machine for handling or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformations. Machine tools employ some sort of tool that does the cutting or shaping. All m ...
s, automotive applications,
snowblower A snow blower or snow thrower is a machine for removing snow from an area where it is problematic, such as a driveway, sidewalk, roadway, railroad track, ice rink, or runway. The commonly used term "snow blower" is a misnomer, as the snow is ...
s and marine
propeller A propeller (colloquially often called a screw if on a ship or an airscrew if on an aircraft) is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon ...
s. This type of key was developed by William N. Woodruff of Hartford
Connecticut Connecticut () is the southernmost state in the New England region of the Northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, New York to the west, and Long Island Sound to the south. Its cap ...
. In 1888, he was awarded the John Scott Medal by the
Franklin Institute The Franklin Institute is a science museum and the center of science education and research in Philadelphia, Pennsylvania. It is named after the American scientist and statesman Benjamin Franklin. It houses the Benjamin Franklin National Memori ...
for his invention. Image:Spieverbinding.jpg, A Woodruff key installed Image:Spieverbinding (2).jpg, A Woodruff key and keyway Image:Woodruff2.png, Gear G is positively located on shaft S by Woodruff key N


Tapered keys

The tapered key is tapered only on the side that engages the hub. The keyway in the hub has a taper that matches that of the tapered key. Some taper keys have a ''gib'', or tab, for easy removal during disassembly. The purpose of the taper is to secure the key itself, as well as, to firmly engage the shaft to the hub without the need for a set screw. The problem with taper keys is that they can cause the center of the shaft rotation to be slightly off of the mating part. It is different from a tapered shaft lock in that tapered keys have a matching taper on the keyway, while tapered shaft locks do not.


Others

A ''Scotch key'' or ''Dutch key'' features a circular keyway hole (instead of rectangular), produced by drilling axially into the assembled hub and shaft, with a metal dowel pin serving as the key. If the hole and key are tapered, the key is referred to as a ''Dutch pin'', which is driven in and optionally finished by cutting or grinding flush with the end of the shaft. If a straight Dutch keyway hole is optionally tapped with a thread, then an ordinary screw serves as the threaded Dutch key.
Spring pin A spring pin (also called tension pin or roll pin) is a mechanical fastener that secures the position of two or more parts of a machine relative to each other. Spring pins have a body diameter which is larger than the hole diameter, and a chamf ...
s are an alternative Dutch key component, instead of solid dowel pins. A spring pin is self-fastening and does not work loose under vibration. Hollow spring pins provide a weaker shear strength than a solid dowel pin, and the strength may be varied by varying the wall thickness. This limited shear strength specification is designed to sustain normal operation, but then give way in the event of excessive shaft torque, thus protecting the rest of the machine from damage. Introducing an additional bushing component between hub and shaft improves the performance and convenience of keyed joints. ''Taper-Lock'' bushings are keyed hub fittings which provide three threaded Dutch keyways and two setscrews as Dutch keys, in addition to the rectangular keyway. The Dutch keyways are threaded only on the alternate hub side or shaft side, with a thread clearance hole form on the opposite side. By simply driving setscrews into selected holes, the hub mechanism conveniently operates to rigidly lock or definitely release from the shaft, without hammering or hub-pulling. Quick-disconnect (''QD'') bushings work similarly, but place a circular pattern of three unthreaded and three fully threaded holes further out from the shaft axis on a bushing flange, instead of across the bushing-to-hub interface. A
Hirth joint A Hirth joint or Hirth coupling is a type of mechanical connection named after its developeAlbert Hirth It is used to connect two pieces of a shaft together and is characterized by tapered teeth that mesh together on the end faces of each half sha ...
is similar to a spline joint but with the teeth on the end of the shaft instead of on the surface.


Saddle keys

These types of keys are generally attached to the driving member (e.g. shafts). These types of keys have less strength as compared with the sunk keys. These are rarely used keys, to transmit lower power to the driven members (e.g. couplings)


Tangent keys

Tangent keys are used in high-torque heavy-duty applications. What would have been the side of each keyway forms heels against which the key sits, and transfers force compressively. This latter point means that for reversible motion of the shaft, another key along a tangent outwards in the opposing direction is needed. Typically this will be offset by 90° or 180° on the shaft. The key may be wedge, rectangular, or square shaped, but particularly rectangular double-taper keys are used.


Spline key

This type of key uses multiple keyways in the hub to transmit high power.


Keyseating

''Keyseating'' is the creation of the slots in the mating items. Keyseating can be done on a variety of different machines including a broach, a keyseater, wire-cut EDM, a shaper or vertical slotting machine, either a vertical or horizontal mill, or with a chisel and file. File:Werkzeuge001.jpg, Keyway cutters File:Werkzeuge006.jpg, Special cutters File:Stosswerkzeug002.jpg, Slotting tools File:Stosswerkzeug001.jpg, Different slotting tools


Broaching

Broaching The BROACH warhead is a Tandem-charge, multi-stage warhead developed by Team BROACH; BAE Systems Global Combat Systems Munitions, Thales Missile Electronics and QinetiQ. BROACH stands for ''Bomb Royal Ordnance Augmented CHarge''. Development of ...
is primarily used to cut square cornered internal keyways. The specific broach, bushing and guide are used for each given keyway cross-section, which makes this process more expensive than most of the alternatives. However, it can produce the most accurate keyway out of all the processes. There are three main steps in broaching a keyway: First, the workpiece is set on the
arbor press An arbor press is a small hand-operated press. It is typically used to perform smaller jobs, such as staking, riveting, installing, configuring and removing bearings and other press fit work. Punches, inserters, or other tools/dies may be add ...
and the bushing is placed in the opening of the workpiece. Next, the broach is inserted and pushed through, cutting the keyway. Finally,
shim Shim may refer to: * Shim (spacer), a thin and often tapered or wedged piece of material ** CPU shim, a spacer for a computer heat sink ** Shim (fencing), a device used in the sport fencing ** Shim (lock pick), a tool used to bypass padlocks * Sh ...
s are placed between the bushing and the broach to achieve the correct depth necessary for the key.


Keyseater

''Keyseaters'', also known as ''keyseating machines'' and ''keyway cutters'', are specialized machines designed to cut keyways. They are very similar to vertical shapers; the difference is that the
cutting tool In the context of machining, a cutting tool or cutter is typically a hardened metal tool that is used to cut, shape, and remove material from a workpiece by means of machining tools as well as abrasive tools by way of shear deformation. The major ...
on a keyseater enters the workpiece from the bottom and cuts on the down-stroke, while the tool on a shaper enters the workpiece from the top and cuts downward. Another difference is a keyseater has a guiding system above the workpiece to minimize deflection, which results in a closer tolerance cut. The process starts by clamping the workpiece to the table with a fixture or
vise A vise or vice (British English) is a mechanical apparatus used to secure an object to allow work to be performed on it. Vises have two parallel jaws, one fixed and the other movable, threaded in and out by a screw and lever. A vise grip is no ...
. The workpiece is properly located and then the reciprocating arm is started. Some models have a stationary table so the cutter is fed horizontally into the workpiece, while others have a movable table that feeds the workpiece into a fixed cutter. These machines can cut other straight sided features other than keyways (see the picture). They can also produce blind slots, which are slots that do not extend through the whole workpiece.


Wire-cut electrical-discharge machining (EDM)

Wire-cut electrical-discharge machining (EDM) is primarily used for small production lot sizes where either extreme precision is required or other cutting technologies are not readily available. Wire-cut EDM cuts keyways by eroding material away from the workpiece through a series of rapid electric current discharges between a spooling wire and the workpiece through a dielectric liquid.
Computer numerical control Numerical control (also computer numerical control, and commonly called CNC) is the automated control of machining tools (such as drills, lathes, mills, grinders, routers and 3D printers) by means of a computer. A CNC machine processes a pie ...
(CNC) wire-cut EDM machines allow for a wide variety selection of keyways to be cut, inclusive of multiple keyways on the same hub. The main limitations of CNC wire-cut EDM is the time it takes to cut a keyway as well as the size of parts a given wire-cut EDM machine can accommodate.


Shaping or slotting

Shaping or slotting is largely used for cutting keyways that do not extend through the full length of the part. Like keyseating, shaping uses a single-point cutting tool for cutting, however, shapers are not guided through the cut on a fixed post. As such, shaper cuts are generally more susceptible to
deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...
than keyseater cuts.


Milling

Parallel, tapered, and Woodruff keyways can be produced on a
milling machine Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done by varying direction on one or several axes, cutter head speed, and pressure. Milling covers a wide variety of d ...
.
End mill An end mill is a type of milling cutter, a cutting tool used in industrial milling applications. It is distinguished from the drill bit in its application, geometry, and manufacture. While a drill bit can only cut in the axial direction, most m ...
s or slotting cutters are used for parallel and tapered keyways, while a
Woodruff cutter Milling cutters are cutting tools typically used in milling machines or machining centres to perform milling operations (and occasionally in other machine tools). They remove material by their movement within the machine (e.g., a ball nose mill) ...
is used for Woodruff keyways. For internal keyways that are not too long, the keyways can be milled if a radius is acceptable.


Chiseling

One of the earliest forms of keyseating was done by
chisel A chisel is a tool with a characteristically shaped cutting edge (such that wood chisels have lent part of their name to a particular grind) of blade on its end, for carving or cutting a hard material such as wood, stone, or metal by hand, stru ...
ing. The keyway is roughed out using a chisel and then filed to size; the key is tried frequently to avoid over filing. This technique is long, tedious, and rarely used anymore.


Keyed joints

A ''shear key'' is a feature intended to fail and avoid further damage should the machinery be accidentally operated in excess of its design limits. Shear keys may be any of the designs described above, but are made from a weaker material than the shaft. The shear key is easily and inexpensively replaced, and avoids more serious damage to the mechanism that would be costly or difficult to repair. For example, a steel shaft and pulley may employ a brass key. When excessive torque is applied to the joint, the steel edges shear the brass key into two pieces, leaving the pulley spinning loosely on the shaft and relieving the rest of the machine from possible damage. Two parallel keys can be used if the shaft connection requires a higher torque rating. Improperly machined keyways that had cutter deflection or drifting occur, may not be strong enough for the required application.Romig, J. V. (1926). The Popular Science Monthly. ''The Popular Science Monthly''., 110(5), 72, 124.


See also

*
Coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...


References


Bibliography

*


External links

{{commons cat, Keys and keyways
Standard Metric Keys & Keyways

ASME recommended key sizes for inch based units



Key joint article
from the 1979 ''
Great Soviet Encyclopedia The ''Great Soviet Encyclopedia'' (GSE; ) is one of the largest Russian-language encyclopedias, published in the Soviet Union from 1926 to 1990. After 2002, the encyclopedia's data was partially included into the later ''Bolshaya rossiyskaya e ...
''
Drawing and standard dimensions of keys Gardette
Hardware (mechanical) Mechanical power transmission