Higher Technological Institute HTI
Key Design
Dr. Ahmed Nabhan M. Elkholy
Associate Professor
Faculty of Engineering - Minia University
Lecture slides to accompany
A Textbook of
Machine Design
by
R. S. Khurmi
J. K. Gupta
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Quiz
The simply supported beam ABCD is loaded by a weight W = 27 kN
through the arrangement shown in the figure. The cable passes over a
small frictionless pulley at B and is attached at E to the end of the vertical
arm. Calculate the axial force N, shear force V, and bending moment M at
section C, which is just to the left of the vertical arm. And determine the
cross-section area of the beam.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Keys
A key is a machinery component placed at the interface between a shaft
and the hub of a power-transmitting element for the purpose of
transmitting torque [Fig. 3-l]. The key is demountable to facilitate
assembly and disassembly of the shaft system. It is stalled in an axial
groove machined into the shaft, called a key seat. A similar groove in the
hub of the power-transmitting element is usually called a keyway, but it
is more properly also a key seat. The key is typically installed into the
shaft key seat first; then the hub key seat is aligned with the key, and the
hub is slid into position.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Types of keys
The following types of keys are important from the subject point of view:
1. Sunk keys, 2. Saddle keys, 3. Tangent keys, and 4. Round keys.
Sunk Keys
Square and Rectangular Parallel Keys
The most common type of key for shafts up to 65 inches in diameter is the
square key.
The rectangular key is recommended for larger shafts and is used for
smaller shafts where the shorter height can be tolerated.
Both the square and the rectangular keys are referred to as parallel keys
because the top and bottom and the sides of the key are parallel.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
The usual proportions of this key are:
Width of key, b = d / 4;
And thickness of key, t = 2 / 3 (b) = d / 6
Where d = Diameter of the shaft or diameter of the hole in the hub. The key
has taper 1 in 100 on the top side only.
The only difference between a rectangular sunk key and a square sunk key
is that its width and thickness are equal, i.e. b = t = d / 4
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Gib Head Keys
It is a rectangular sunk key with a head at one end known as gib head. It is
usually provided to facilitate the removal of key
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Feather key
A key attached to one member of a pair, and which permits relative axial
movement is known as feather key. It is a special type of parallel key which
transmits a turning moment and permits axial movement. It is fastened
either to the shaft or hub, the key being a sliding fit in the keyway of the
moving piece.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Woodruff key
The woodruff key is an easily adjustable key. It is a piece from a cylindrical
disc having segmental cross-section in front view as shown in Fig. A
woodruff key is capable of tilting in a recess milled out in the shaft by a
cutter having the same curvature as the disc from which the key is made.
This key is largely used in machine tool and automobile construction.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Saddle keys
The saddle keys are of the following two types
A flat saddle key is a taper key which fits in a keyway in the hub and is
flat on the shaft as shown in Fig.. It is likely to slip round the shaft
under load. Therefore, it is used for comparatively light loads.
A hollow saddle key is a taper key which fits in a keyway in the hub and
the bottom of the key is shaped to fit the curved surface of the shaft.
Since hollow saddle keys hold on by friction, therefore these are
suitable for light loads. It is usually used as a temporary fastening in
fixing and setting eccentrics, cams etc.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Round keys
The round keys, as shown in Fig., are circular in section and fit into holes
drilled partly in the shaft and partly in the hub. They have the advantage
that their keyways may be drilled and reamed after the mating parts have
been assembled. Round keys are usually considered to be most appropriate
for low power drives.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Strength of a Sunk Key
There are two basic modes of potential failure for keys transmitting power:
(1)Shear across the shaft/huh interface
(2)Crushing failure due to the bearing action between the sides of the
key and the shaft or hub material.
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
The magnitude of the shearing force can be found from
2 𝑀𝑡
𝐹=
𝑑
Considering shearing of the key, the tangential shearing force acting at the
circumference of the shaft,
F = Area resisting shearing × Shear stress = b × L × τ
2 𝑀𝑡
𝐹= =𝑏 ×𝐿× 𝜏
𝑑
Then the required length of the key is
𝟐 𝐌𝐭
𝐋=
𝐛 ×𝐝× 𝛕
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
The failure in bearing is related to the compressive stress on the side of the
key, the side of the shaft key seat, or the side of the hub key seat. The area in
compression is the same for either of these zones, L × (t/2).
Considering crushing of the key, the tangential crushing force acting at the
circumference of the shaft,
F = Area resisting crushing × Crushing stress = t/2 × L × σ
2 𝑀𝑡 𝑡
𝐹= = ×𝐿× 𝜎
𝑑 2
Then the required length of the key is
𝟒 𝐌𝐭
𝐋=
𝐭 ×𝐝× 𝛔
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Splines
A spline can be described as a series of axial keys machined into a shaft,
with corresponding grooves machined into the bore of the mating part
(gear, sheave, sprocket, and so on: see Figure). The splines perform the
same function as a key in transmitting torque from the shaft to the mating
element. The advantages of splines over keys are many. Because usually
four or more splines are used, as compared with one or two keys, a more
uniform transfer of the torque and a lower loading on a given part of the
shaft/hub interface result.
D = 1.25 d and b = 0.25 D
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
Production Engineering and Mechanical Design Department
Faculty of Engineering - Minia University Dr. Ahmed Nabhan M. Elkholy
