MACHINE ELEMENTS IN
MECHANICAL DESIGN
Chapter 11:
Keys, Coupling and Seals
https://www.youtube.com/watch?v=F3c6G
PAFZMI
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Common Types of Shaft Keys.
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Taper keys/ Gib head keys
Designed to be inserted from the end of the shaft after the hub
is in position.
Gib head keys have an extended head for easy extraction.
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Common Types of Shaft Keys.
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Pin keys
Cylindrical pin place in a cylindrical groove in the shaft and
hub, shown in the diagram below.
Pin keys
For lower stress concentrations
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Common Types of Shaft Pins.
Woodruff keys
Shaft’s circular groove holds the key in position while the
mating part is slid over the key.
For light loading and easy assembly/ disassembly.
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Design procedures for Parallel Keys
(Summarized)
1. Complete the design of the shaft into which the key
will be installed and specify the actual diameter of
the location of the keyseat.
2. Select the appropriate key size.
3. Specify the material of the key.
4. Determine the yield strength of the materials for the
key, the shaft and the hub.
5. Specify the actual length of the key to be equal to or
longer than the computed minimum length.
6. Complete the design of the key seat in the shaft and
the keyway in the hub.
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Splines
Transmit torque from shaft to
another machine element
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Spline Types
Straight
SAE
4, 6, 10 or 16 splines
Involute
Pressure angles of 30, 37.5, or 45 deg.
Common Types of Splines.
Two types of splines:
Straight Sided Involute:
SAE Spline Sizes
A: Permanent Fit
B: Slide without Load
C: Slide under Load
Pg 504
Advantages
Uniform transfer of torque
Lower loading on elements
No relative motion between “key” and shaft
Axial motion can be accommodated (can cause
fretting and corrosion)
Generally hardened to resist wear
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Torque Capacity
Torque capacity is based on 1000 psi bearing
stress on the sides of the splines
T = 1000*N*R*h
N number of splines
R mean radius of the splines
h depth of the splines
Torque Capacity Cont’d
1D d D d
R
2 2 4
1
h (D d)
2
Substituting R and h into torque
equation: D d D d D 2 d2
T 1000N 1000N
4 2 8
Torque Capacity Cont’d
Further refinement can be done by substituting
appropriate values for N and d.
For 16 spline version, with C fit,
N = 16 and d = .810D
D 2 (.810D)2
T 1000(16)
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T 688D 2 Torque in IN-LBS/INCH of spline
D T / 688 Required D for given Torque
Use this for spline
design – SAE
formulas based on
1,000 psi bearing
stress allowable!!
Use this to get
diameter. Then
table 11.4 to get W,
h, d
Torque Capacity for Straight Splines
Torque Capacity for Straight
Splines
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Torque Capacity for Straight
Splines
Torque Capacity for Straight
Splines
2.5
4076/3.25
https://www.youtube.com/watch?v=SPnTA3H2G7g
https://www.youtube.com/watch?v=SPnTA3H2G7g&t=382s
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Rigid Couplings
NO relative motion between the shafts.
Precise alignment of the shafts
Bolts in carry torque in shear. N = # of bolts.
8T
d
DbcN( d )
Flexible Couplings
Transmit torque smoothly
Permit some axial, radial and angular
misalignment
Flexible Couplings
Flexible Couplings
https://www.youtube.com/watch?v=lqVkaJwJ7z4
https://www.youtube.com/watch?v=F7-
RXzB1Qb8
https://www.youtube.com/watch?v=NI4D4X
BXvIw
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Flexible Coupling
Lord Corp. Products
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Universal Joints
Large shaft misalignments permissible
Key factors in selection are Torque, Angular Speed
and the Operating Angle
Output not uniform wrt input Output IS uniform wrt input
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Retaining Rings
Common Types of Retaining or Snap Rings.
Axial Constraint Methods
Spacers
Retaining ring
Retaining ring Shoulders
Locknuts
Universal Joints
Used to accommodate misalignment between mating
shafts that is greater than three degrees typically
provided by flexible couples.
Can be single or double.
Diagrams below show universal joint component and
a double universal joint:
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3. Seals
Function:
To protect critical elements of a machine from
contamination due to dust, dirt, water or other fluids while
allowing rotating translating machine elements to
accomplish their desirable functions.
https://www.youtube.com/watch?v=dO7AJiUZTzU
Types:
O-rings and T-rings for static conditions
Diaphragms, bellows and boots for sealing a container
while allowing relative movement of some part.
Lip seal, U-cup seal, V packing and split ring seals for
sealing around a continuously reciprocating rod or piston.
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Lip seal, wipers and scrapers and face seals for sealing around a
rotating shaft.
Circumferential seals: labyrinth seals, adradable seals and
hydrostatic seals.
An example: Lip seals, U-cup seals, wiper and O-rings applied
to a hydraulic actuator shown below:
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Seal materials
Properties to consider, e.g.: weather resistance,
petroleum fluid resistance, acid resistance, high/low
thermal property, tensile strength, abrasion resistance,
impermeability, etc.
Examples of Elastomers
Neoprene, butyl, fluorocarbon, silicone, etc
Examples of rigid materials
Metals, plastics, carbon/ceramics, etc
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