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Shaft keys
➢ are small machine member
inserted between a shaft
and a hub to prevent
angular rotation. Keys
require a key seat both in
the shaft and the hub. Keys
also acts like a fuse, in
which it is designed to fail
first before the shaft is
damaged completely.

A key is usually made from steel and is

inserted or mounted between the shaft and
the hub of the component in an axial
direction to prevent relative movement.
Keyseat is a recess in the shaft, and
the Keyway is the recess in the hub to
receive the key and thus securely lock the
component.
Generally, the term keyseat is rarely used as
keyway is referred to both recesses in the
industry.

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Key types
Types of Keys
Shaft keys come in a wide variety of types and shapes and can
be divided into four categories and subcategories.

1.Sunk Keys
1. Rectangular & square keys
2. Parallel keys
3. Gib head keys
4. Feather key (sliding clearance with keys)
5. Woodruff key
2.Saddle keys
1. Flat & Hollow saddle keys
3.Tangent keys
4.Round/Circular keys

Shift variations
Shaft and hub keyways are often cut on key seating machines but can also be
made using broaching, milling, shaping, and slotting Electrical Discharge
Machining (EDM).

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There are various advantages, and disadvantages of using shaft keys hence proper consideration
must be given to the finer details of the overall design to evaluate the suitability of the keyed
joint.

Advantages of shaft keys & keyed Disadvantages of shaft keys & keyed joint
joint • Not suitable for alternating directional loads and shocks
• Possible axial displacement of hub unless locked by an extra
• Cheap manufacturing cost component such as a set screw or retainer rings
• Well-standardised ( ISO, BS, DIN and ANSI) • Over time keyed joints might become very difficult to
• Medium to High torque transmission dismantle
• Easy mount and dismount, hence easily • Keyways introduce stress points due to the notch effect and
reusable reduce shaft strength
• Introduces shaft imbalance
• Difficult to calculate and combine the load-carrying and the
tolerance stack analysis hence keyed joints are over-
dimensioned
• To transmit axial force, it needs a stop lock

Double key
◦ Because of manufacturing tolerances and to avoid double fits, only one parallel key is used, but Double
keys are occasionally used for very high infrequent loads. This should only be considered if the material
is ductile. The calculations should be based on one and a half-parallel key.

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Rectangular/square keys
Sunk keys are sunk into the shaft for half its thickness, where the
measurement is taken at the side of the key. Not along the centre
line through the shaft.

Rectangular/square keys
Rectangular keys, as shown, are wider than their height and are sometimes called flat
keys. These are used on shafts up to about 500 mm or 20″ in diameter. The extra key
width allows it to transmit greater torque without increasing the depth. An increase in
depth means a weaker shaft due to a reduction in effective shaft cross-sectional area.

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As their names suggest, square, cross-sectional keys are

generally specified for shafts up to about 25mm or 1″.
They can be used for larger shafts when deeper key depth
is desirable than rectangular keys. An increase in depth
means a weaker shaft due to reduced effective shaft
cross-sectional area.

Square and rectangular keys may have a taper of 1 in 100

along the length of the key, as shown above.

Parallel sunk keys

Parallel sunk keys can be either rectangular or square sections but without the taper. These keys
are inexpensive and readily available. It is one of the easiest to install. But the keys must ideally
be held by a set screw through the hub. Because vibration or rotational direction reversal often
pushes the key out.

These keys are generally fitted tightly to the bottom of the

shaft keyway and the sides of the keyed joint, leaving a
clearance at the top of the hub keyway.

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Gib head sunk keys

A Gib head-on sunk key is added to make it easier to remove. As shown in figure 8, Gib head
sunk keys are generally rectangular or square keys with a taper on the top surface to ensure a
tight fit.

Feather keys
Feather keys are attached to the
shaft or the hub to permit relative
axial movement. As shown in the
picture, there are three main feather
keys. Double-headed, Peg feather
and Feather key. This enables power
transmission between the shaft and
hub with their parallel opposite faces
while simultaneously allowing it to
slide.

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Woodruff keys
The Woodruff key is a semi-circular disc and
fits into a circular recess in the shaft machined
by a woodruff keyway cutter. These woodruff
keys are mostly used in machine tools and
automobile shafts from ¼” to 2½” (6 mm to 60
mm) in diameter. Woodruff keys cannot carry
the same load as long parallel keys.
The advantage of the Woodruff key is that it
can accommodate any taper in the hub
keyway, and its captive and depth prevent the
key from turning over.
The disadvantages or drawbacks of woodruff
keys are that the depth of the keyway
weakens the shaft, these cannot be used as a
feather key, are difficult to install, are short,
and can’t carry too much load.

Saddle keys
Compared to sunk keys, saddle keys are
not sunk into the shaft and hub instead,
they are only sunk into the hub. They
either sit on a flat or the circumference
of the shaft. Power transmission is
achieved through friction between the
shaft and the key.
As shown in the below figure, Saddle
keys can be subdivided into Flat saddle,
and Hollow saddle keys and are only
suitable for light loads to avoid slipping
along the shaft.

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Flat saddle key

A flat saddle key is tapered at the top and flat at the bottom, as shown in the figure
below. The key fits into a tapered hub keyway pushing down on the flat face of the shaft

Hollow saddle key

A hollow saddle key is tapered at the top and curved at the bottom, as shown in
the figure below. The key fits into a tapered hub keyway and is pushed down on
the curved circumferential surface of the shaft.

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Tangent keys
The tangent keys, sometimes
called Tangential keys, are fitted
as a pair at right angles, as shown
in the figure below, where each
key only withstands torsion in
one direction. These are used in
large heavy-duty shafts.

Round / Circular Keys

The round keys are circular in section and fit into holes
drilled partly into the shaft and the hub. They have the
advantage of easy manufacturing as 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.

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Power transmitted by the key

2𝜋𝑇𝑁 𝑤ℎ𝑒𝑟𝑒: 𝑃 = power transmitted, kW
𝑃= 60 𝑇 = torque or torsional moment
𝑁 = angular speed, rpm

Torque transmitted by the key

𝑇=𝐹 𝑟 𝑁𝑜𝑡𝑒: 𝑇𝑘 = 𝑇
𝐷
𝑇=𝐹 2
◦ Torque capacity of one key = total torque
transmitted

Design of keys:
1. Based on torsional stress in the shaft

𝟏𝟔𝑻
◦𝑺 = 𝝅𝑫³
(𝒑𝒖𝒓𝒆 𝒕𝒐𝒓𝒔𝒊𝒐𝒏 𝒐𝒇 𝒂 𝒔𝒐𝒍𝒊𝒅 𝒔𝒉𝒂𝒇𝒕)

𝟎. 𝟔 𝑺𝒚 𝟏𝟔𝑻 𝑺𝒚 = yield stress based on shaft material

=
𝑵 𝝅𝑫³

The value 0.6 is estimated due to the reduction of strength in the keyway because of the torsional
effect in the shaft.

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Design of keys:
2. Based on the compressive stress between the key/hub or key/shaft.

𝑭
𝑺𝒄 =
𝑨

𝐷 𝑡
𝑤ℎ𝑒𝑟𝑒: 𝑇=𝐹 2
and 𝐴 = L
2

𝟒𝑻
𝑺𝒄 =
𝑫𝒕𝑳

𝑺𝒚
𝑺𝒄 = 𝑤ℎ𝑒𝑟𝑒: 𝑺𝒚 = yield stress based on the weakest material, either from the key, hub, or shaft.
𝑵

Design of keys:
3. Based on shearing of the key

𝑭
𝑺𝒔 =
𝑨

𝐷
𝑤ℎ𝑒𝑟𝑒: 𝑇=𝐹 2
and 𝐴 =bL 𝐹𝑎𝑐𝑡𝑜𝑟 𝑜𝑓 𝑠𝑎𝑓𝑒𝑡𝑦:
N = 1.5 for smooth loading
𝟐𝑻 N = 2.0 – 2.5 for minor shock loading
𝑺𝒔 =
𝑫𝒃𝑳
N = 4.5 for severe shock loading

𝟎. 𝟔𝑺𝒚
𝑺𝒔 = 𝑤ℎ𝑒𝑟𝑒: 𝑺𝒚 = yield stress based on the material of te key
𝑵

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Number of keys used:

For heavy loads, two or more keys can be employed.

• Two keys are usually separated by 90 degrees and each key can
are sized to carry 20 percent more than one-half of the total
torque.
• Three keys are usually separated by 120 degrees and each key
can carry 15 percent of the total torque.

Design of 2-keys separated by 90°

Torque transmitted per key:

𝑇𝑜𝑡𝑎𝑙 𝑡𝑜𝑟𝑞𝑢𝑒 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑

𝑇𝑘 = ∗ 1.2
𝑛𝑜.𝑜𝑓 𝑘𝑒𝑦𝑠

𝑇
𝑇𝑘 = ∗ 1.2
𝑛𝑘

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1. Based on the compressive 2. Based on the shearing

stress between the key/hub or stress of the key
key/shaft
𝟐𝑻 𝟏. 𝟐
𝟒𝑻 𝟏. 𝟐 𝑺𝒔 =
𝑫𝒃𝑳𝒏𝒌
𝑺𝒄 =
𝑫𝒕𝑳𝒏𝒌
𝒘𝒉𝒆𝒓𝒆: 𝑫 = shaft diameter
𝑻 = torque transmitted
𝒕 = thickness of the key
𝑳 = length of the key
𝒃 = width of the key
𝒏𝒌 = no. of the keys

Design of 3-keys separated by 120°

Torque transmitted per key:

𝑇𝑜𝑡𝑎𝑙 𝑡𝑜𝑟𝑞𝑢𝑒 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑

𝑇𝑘 = ∗ 1.15
𝑛𝑜.𝑜𝑓 𝑘𝑒𝑦𝑠

𝑇
𝑇𝑘 = ∗ 1.15
𝑛𝑘

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1. Based on the compressive 2. Based on the shearing

stress between the key/hub or stress of the key
key/shaft
𝟐𝑻 𝟏. 𝟏𝟓
𝟒𝑻 𝟏. 𝟏𝟓 𝑺𝒔 =
𝑫𝒃𝑳𝒏𝒌
𝑺𝒄 =
𝑫𝒕𝑳𝒏𝒌
𝒘𝒉𝒆𝒓𝒆: 𝑫 = shaft diameter
𝑻 = torque transmitted
𝒕 = thickness of the key
𝑳 = length of the key
𝒃 = width of the key
𝒏𝒌 = no. of the keys

Ex. 1
A rectangular key was used in a pulley connected to a
lineshaft to transmit a power of 130 kW at 1000 rpm. The
shearing stress of the shaft and the key is 45 Mpa and 25
Mpa, respectively. Calculate the length of the key if the
width is one-fourth of the shaft diameter.

Answer: L = 147 mm

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Example 2.
A key 2.5 cm width and 5 cm length is fastened thru a
3-in diameter shaft. The yield stress of the key is 70
Mpa. If the factor of safety is 4, find the power
delivered by the shaft at a speed of 380 rpm.

Ans: 20 kW

Example 3.
A keyed gear deliver a torque of 915 N-m thru its
shaft of 64 mm outside diameter. If the key has
thickness of 16mm and width of 12 mm, find the
length of the key. Assume the permissible stress
value of 62 Mpa for shear and 100 Mpa for tension.

Ans: Lk = 38.4 mm

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Ex. 4
A key with a size of ¾ x ¾ x 6 inches is used in a 4-in
diameter shafting of SAE 1040 grade, cold rolled, having a
yield point of 50 kSi. Calculate the minimum yield point in
the key to transmit power of the shaft. Use a factor of safety
of 3, Sys = 0.5 Sy.

Answer: Sykey = 69.81 kSi

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