Shaft design

What are mechanical shafts?

A mechanical shaft is a mechanical power transmission element, usually circular in cross-section, either solid or hollow, which transmits torque and rotational motion from one device to another.

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Machine elements such as gears, pulleys, flywheels, clutches, and sprockets are mounted on various shaft types and are used to transmit power from the driving device such as motor or engine. Vehicle crankshaft is a prime example of a mechanical shaft as shown in the above figure.





Types of shafts?

Mechanical shafts are broadly categorized into the following four types.

Transmission shaft – The transmission shaft is one of the essential machine components that provides the axis of rotation, oscillation, and regulates the motion geometry. ex

Axle shaft – An axle is a non-rotating version of a shaft that supports elements such as rotating pulleys and wheels but carries no torque. An axle is a static beam and can be analyzed as one simply supported beam.

Spindle shaft – A spindle is a revolving shaft with a fixture for retaining a tool (or workpiece in the case of milling, grinding, or drilling spindle) (in the case of a turning spindle). The spindle shaft acts as a tool or workpiece support, positioner, and rotational drive.

Machine shaft – These shafts are an inherent element of the machine and are located inside the assembly. A crankshaft of an automobile engine is an example of a machine shaft.

Failure modes for shaft design

  • Fatigue failure
  • Force-induced elastic deformation failure
  • Wear failure

Key principles of shaft design

During the design stage of the shaft, the following key principle should be taken into consideration by the product designer.

  • Keep the shaft as short as possible and the bearing supports as close to the load vectors as possible. This will keep the shaft deflection and bending moments and also increase resonance and critical speed
  • Place shaft stress concentration points away from stressed regions of the shaft. Add fillet radii, smooth surface finish
  • Only use hollow shaft if weight is critical





Shaft design consideration

  • Form, fit, and function including tolerancing – Within the embodiment of the design during the
  • Shaft material and treatment
  • Shaft deflection and Rigidity – Deflection based calculation
  • Shaft strength and stress – Strength bases calculation
  • Frequency response and critical speed

Shaft design process

  • Material selection
  • Geometric layout design
  • Stress and strength
    • Static strength
    • Fatigue strength
  • Deflection and rigidity
    • Bending deflection
    • Torsional deflection
    • The slope at bearing and shaft supported elements.
    • Shear deflection due to transverse loading of short shafts
  • Vibrational due to natural frequency
  • Manufacturing method
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