chapter 3 - keys and pins

7/28/2019 CHAPTER 3 - Keys and Pins

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CHAPTER 3:

DESIGN OF MECANICAL JOINT


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DefinitionAMechanical joint is a part of a machine which is

used to connect another mechanical part ormechanism

2 types of mechanical joint:-

Temporary

Permanent


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Topic to be covered Key and pin joint

bolt and nut joint

Welding joint


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Keys and Pins Keys and pins are used on shafts to secure rotating

elements, such as gears, pulleys or other wheels

Keys are used to enable the transmission of torquefrom the shaft to the shaft-supported element

Pins are used for axial positioning and for the transferof torque or thrust or both


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a. Square keyb. Round key

c. Round pind. Round pin

e. Taper pinf. Split tabular spring pin

Keys and Pins


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Split tabular spring pin


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keys Stock key material is typically made from low carbon

cold-rolled steel, and is manufactured such that itsdimensions never exceed the nominal dimension.

This allows standard cutter sizes to be used for thekeyseats.


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Keys Square and rectangular keys are commonly used for shaft size up to

6.5inch

Advantages of square and rectangular key

1. Cost effective

2. Easy to replace damaged component

3. Easy to installation


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Function

The shaft and rotating machine element must have a keyway, also known asa keyseat, which is a slot or pocket for the key to fit in. The whole system iscalled a keyed joint


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Keyways Key ways can be milled horizontallyorvertically


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Classification of keys Saddle keys

Hollow saddle key

Flat saddle key

Sunk keys Taper sunk keys

Parallel sunk keys

Feather keys

Woodruff key (adjustable keys)

Round keys Parallel pin

Taper pin


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Sunk keys Sunk keys are sunk in the shaft and the hub. These keys are

suitable for heavy duty since they rely on positive drive


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Tapered sunk key


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Tapered sunk key


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Tapered sunk key The gib-head keyis tapered so that, when firmly

driven, it acts to prevent relative axial motion

This also gives the advantage that the hub positioncan be adjusted for the best axial location.

The head makes removal possible without access tothe other end


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Taper sunk keys The key is sunk in the shaft to a depth of half its

nominal thickness when measured at the side. Rectangular cross-section

let D = diameter of the shaft

width of the key W = D/4

nominal thickness T = (2/3)W = (1/6)D

Square cross-section:T = W


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Taper sunk keys


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Parallel sunk key It is uniform in width and thickness throughout. It is thus taper less and is used where the pulley or other mating

piece is required to slide along the shaft.

It may be rectangular or square cross-section and their endsmay be squared or rounded


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Parallel sunk keyThe keyway in a shaft for a parallel key

A sprocket with an internal parallel keyway

Cross-section of a parallel keyed joint


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Feather keys

It is a key attached to one member of a pair It is a particular kind of parallel key that permits axial

moment

A feather key is secured either to the shaft or to the hub,the key being a sliding fit in the keyway of the machineelement on which it moves.

Double

Headed Key

Peg Key


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Woodruff keys


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Woodruff keys It is an adjustable sunk key in the form of a semi-circular disc of uniform thickness.

The key fits into a semi-circular keyway in the shaftand the top of the key fits into a plain rectangular keyway in the hub of the wheel.

Since the key and the key seat bear the same radius , ithas the advantage of adjusting itself to any taper of theslot of the hub or boss of wheel

Used in feed gear box of lathe, other machine toolsand in automobiles


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Woodruff keys


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Hollow saddle keyThis key has curved underside so that it can be placed on the curvedsurface of the shaft. The keyway is cut in the mating piece (hub) only


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Hollow saddle keySaddle key are taper keys and are sunk into the hubonly

Taper prevents axial movement along the shaft

Saddle key is suitable for lightduty, since they rely on a frictiondrive alone and are liable to slip on

the shaft under load


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Flat saddle key

holding force is comparative large

than the hollow saddle key

Flat surface on the

shaft

Let D = diameter of the shaftWidth of the key, W = D/4Nominal thickness at largeend , T = W/3 = D/12Length of the key , L = D to1.5DTaper on the top surface =

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Sunk taper key


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Round key or Pin Key:

l h f d h b


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Spline shaft and hub

A spline shaft is used when the hub is

required to slide along the shaft.These shafts are used mostly forsliding gear application as inautomotive gear box and propellershaft of aircraft.

A spline shaft in which are cutequiangular longitudinal groove, themetal between these groove formingsplines or feathers of uniform depth.

By this means the power transmittedis equally divided amongst thenumber of keys giving great strengthand security against total failure thanby using a single key.


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Serrated shafts A spine shaft - disadvantage of

reduced strength A serrated shaft gives

maximum strength for a givenweight of material.

number of correspondinggrooves are cut in the shaft andmating piece, the two beingtightly pressed, one over theother

the bottom of the serrationsand the crests of the teeth areflat.

these are used in aircraftassemblies

chapter 3 - keys and pins

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