Design of Shafts
MaE 4043 – Design of Machine Elements
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Outline
• Introduction
• Types of shafts
• Standard sizes of shafts
• Design of Shafts
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Design considerations
Stresses in shafts
Design of shafts on the basis of strength
Design of shafts on the basis of rigidity
Design of shafts for fatigue
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Introduction
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Shaft is a machine element used for power transmission
Transmit power by twisting
Other machine elements such as Pulleys and gears are mounted on shafts
Shafts usually have circular cross-section
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Transmission Shafts
• Transmit power between the source and the machine
e.g. Line shaft, propeller shaft
Line Shaft
Propeller Shaft
Machine Shafts
• Shafts which are integral part of a machine
e.g. Crank shaft
Mounting of Shafts …
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Standard Sizes …
The following are the common sizes (in mm) of the
transmission shafts:
25, 30, 35, 40, 45, 55, 60, 70, 80, 90, 100, 110, 125, 140, 160,
180, 200, 220, 240, 260, 300, 320, 340, 360, 380, 400, 420,
440, 460, 480, 500.
Machine shaft sizes vary from:
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10 to 75mm by increment of 5mm
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40 to 100mm by increment of 5mm
The maximum length of shaft usually does not
exceed 7m
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Design of Shafts
Shafts must be designed so that:
• The deflections are within acceptable levels
• High deflections cause noise and vibration while in operation
• Critical speed
• Start vibrating violently
Design Considerations:
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Size and spacing of components, tolerances
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Type of material to be used
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Deflection and rigidity
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Stress and strength (static strength, fatigue, reliability)
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Type of loading (static, shock or cyclic)
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Manufacturing constraints
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Typical Shaft Loading and Deflection
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Design of Shafts Based on Strength: Stresses in Shafts
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Shear stresses due to torsional load
Bending stresses due to the forces coming from gears, pulleys, etc.
Stresses due to combined torsional and bending loads
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Shafts Subjected to Twisting Moment Only
A line shaft rotating at 200 r.p.m. is to transmit 20 kW. The shaft
may be assumed to be made of mild steel with an allowable shear
stress of 42 MPa. Determine the diameter of the shaft, neglecting
the bending moment on the shaft.
If a hollow shaft is to be used in place of the solid shaft, find the
inside and outside diameter when the ratio of inside to outside
diameters is 0.5.
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Shafts Subjected to Bending Moment Only
A pair of wheels of a railway wagon carries a load of 50 kN on each axle box,
acting at a distance of 100 mm outside the wheel base. The gauge of the
rails is 1.4 m. Find the diameter of the axle between the wheels, if the stress
is not to exceed 100 Mpa.
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Combined Twisting Moment and Bending Moment
• Shaft should be designed by taking two moments simultaneously
• Maximum shear stress theory – ductile materials
• Maximum normal stress theory – brittle materials
Example
A solid circular shaft is subjected to a bending moment of 3000 N-m and a torque
of 10 000 N-m. The shaft is made of 45 C 8 steel having ultimate tensile stress of
700 MPa and an ultimate shear stress of 500 MPa. Assuming a factor of safety
as 6, determine the diameter of the shaft
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