Keys and Coupling

Keys

It is defined as, "a machine element used to transmit motion or torque to other components".

• It is usually made up of mild steel, carbon steel.
• Mount on shaft and placed parallel to shaft axis.
• Act as temporary fastener.
• Subjected to crushing (compression) and shearing stresses.
• Hub (Coupling of shaft)  ↠  Internal diameter of pulley that connects with shaft.
• Keyway  ↠  Space in which key is inserted.
• Broaching  ↠  Removal of keyway from hub.
• Milling  ↠  Addition of key with shaft.

Types of Keys

Key selection depends on torque magnitude transmitted, type of loading, cost. Types of keys are discussed ahead:

1. Sunk Keys

• Most commonly used key.
• Power is transmitted due to shear resistance of key.
• For heavy duty applications (positive drive due to no slip).

Types of Sunk Keys

a. Rectangular Sunk Key

• Flat key, more stable than square key and have taper.

b. Square Sunk Key

• Key is in square shape and have taper.
• Use for mounting pulleys, gears on shafts.
• If diameter of shaft > 17 mm  ↠  use square sunk key.

c. Parallel Sunk Key

• It can be rectangular or square but having no taper (taperless key).
• Used for mating piece (pulley, gear, etc).
  

d. Gib-head Taper Sunk Key

• Rectangular key with a head at one end.
• Head is provided to facilitate removal of key.
• Taper surface increases frictional force and tighten joint.

e. Feather Sunk Key

• Parallel key (fixed on shaft with screw or hub) which permits rotation as well as relative axial movement between them as well.
• Used in clutches, gear shifting devices.

f. Woodruff Sunk Key

• Semi circular key.
• Extra depth in shaft and hub  ↠  increase stress concentration which reduces strength.

2. Tangent or Kennedy Keys

• Rectangular or parallel keys in pairs placed at angles and diagonally.
• Each key withstand torsion in one direction.
• Used in large, heavy duty shafts.
• Due to diagonally added keys  ↠  area decreases shear forces increases.

3. Round Keys

• Circular section key which fit into holes drilled partly in shaft and partly in hub.
• For low power drives.

4. Splines

• Keys are made integral with shaft and keyway with hub so called Splined Shafts.
• Used when there is relative axial motion between shaft and hub like gear shifting mechanisms.
• For heavy duty shaft (no slip), 100% power transfer.

Force Acting on a Sunk Key

Shaft Coupling or Coupling

It is defined as, "an element used to connect two shafts so that power from driving shaft is transmitted to driven shaft without any change of speed".

• Used in connecting motor and generator.
• Hold shaft in perfect alignment.
• Reduce the transmission of shock loads.

There are misalignment's between driving and driven shafts which are discussed ahead

1. Offset  ↠  used when shafts axes are parallel.
2. Angular   ↠  used when shafts axes are intersecting at their center.
3. Offset and Angular  ↠  used when shafts are intersecting other than their center.

Types of Couplings

There are two types of coupling which are described below:

1. Rigid Coupling

• It is used to connect perfectly aligned shafts.
• No lubrication required so no movable parts.
• Can absorb shocks and vibrations.

Types of Rigid Couplings

Following are the types of rigid couplings.

a. Sleeve or Muff Coupling

• Consist of hollow cylinder (made of cast iron) which is fitted over the ends of two shafts by means of gib-head key.
• Used up to 70 mm diameter shafts.
• If using 2 keys (general case)  ↠  Length of Hub/Sleeve = 0.5 x Length of Key.
• If using 1 key  ↠  Length of Hub/Sleeve = Length of Key.

b. Clamp or Split Muff or Compression Coupling

• A type of muff and rigid coupling in which muff is divided into 2 halves and are bolted together.
• Stud Bolts  ↠  placed in radial direction for the alignment of upper and lower surface.
• Single key is fitted in keyways of both shafts.
• Used for low and moderate speeds and not for shock loads.

c. Flange Coupling

• It have two separate flanges of cast iron mounted on shaft and keyed to it.
• One flange  ↠  projected portion (spigot) maintain alignment with another flange called Socket.
• Bolts and nuts are used to join flanges in axial direction.
• Most commonly used coupling, for high speed and large diameter shafts.

Types of Flange Coupling 

1. Unprotected Type Flange Coupling  ↠  Nuts and bolts are placed on the outside of coupling.
2. Protected Type Flange Coupling  ↠  Increase the thickness of coupling provided nuts and bolts within the coupling.
3. Marine Type Flange Coupling  ↠  Shaft and coupling are integral part and pinned to assemble.

Design of Flange Coupling

2. Flexible Coupling

This coupling is used to join ends of shafts having lateral and angular misalignment.

• Chances of power loss due to misalignment of shafts.
• It has flexible elements which can absorbs shocks and vibrations.
• Misalignment causes temperature change, bearing wear, vibrations.

Types of Flexible Coupling

 a. Bushed-Pin Flexible Coupling

Modification of flange coupling in which we use bush and pin to join flanges rather than nuts and bolts.

• Pin is fixed on output flange bu nuts.
• Rubber bush is provided to input flange to absorb shocks and vibrations. To avoid wear, it is Brass lined at the inner.
• Clearance should be placed between two halves (5 mm) to accommodate misalignment.
• Require more axial space because Outer diameter of flange = 4 x Diameter of Shaft.

b. Oldham Coupling

It is used to join 2 shafts having lateral misalignment or connect two parallel shafts which are at a small distance apart.

• It has 2 flanges: Hub and Central floating part (mostly black) with 2 keyway.
• Central floating part is held together by pin and passed through flanges and floating part.
• It can allow maximum lateral misalignment of 5% of the diameter of shaft and allow 1-degree of angular misalignment.
• Shaft diameter should be slightly greater whole number.

c. Universal or Hooke's Coupling

It is used to connect two shafts whose axes intersects at small angles.

• It can allow maximum misalignment of 30-degrees (safe ↠ 5-15 degrees).
  

References:

• Material from Class Lectures + Book named Mechanical Engineering Design by Shigley (8th Edition) + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.