Design of Shafts | Design of Shaft Based on Strength and Rigidity

By Mohit Uniyal|Updated : June 1st, 2022

Design of Shafts: Before we go into the details of the design of shafts, let us first define what a shaft is. A shaft is a spinning machine element with a circular cross-section that is used to convey power from one part to another, or from a power-producing machine to a power-absorbing machine. Shafts are an essential component of machinery. They support rotating components such as gears and pulleys and are supported by bearings in the stiff machine housing. 

The shafts' job is to transfer power from one rotating element to another that is supported or connected to it. As a result, they are subjected to torque from power transfer as well as bending moments from the reactions of the members they support. Axles, which likewise support rotating components but do not transmit power, are not to be confused with shafts. The design of shafts is a common task in machines. Although common, the amount of attention that goes into the design of shafts can be surprising. Let's look at the design of shaft in further detail.

Table of Content

Design of Shafts 

Shafts are always circular in cross-section, and they can be solid or hollow. Straight, cranked, flexible, or articulated shafts are the different types of shafts. Straight shafts are the most prevalent type of power transmission shaft. Although constant diameter shafts would be simple to construct, such shafts are often built as stepped cylindrical bars, that is, they have varied diameters along their length. The stepped shafts represent the varying degree of stress along the length of the shaft. 

Shafts are built for strength, stiffness, or a combination of the two. In some circumstances, rigidity is also significant. For example, if the shaft is deflected, the position of a gear mounted on the shaft may change, and if this value exceeds some permissible limit, it may cause excessive dynamic loads and noise in the gears. The shafts can be designed based on two criteria: 

  • Strength 
  • Rigidity

Design of Shafts on the Basis of Strength

The goal of a strength-based design shaft is to ensure that stress at any point on the shaft does not exceed the yield stress of the material. When designing shafts based on strength, take into account the following scenarios:

(a) Shafts that are only subjected to a twisting moment or torque.

(b) Shafts that are solely susceptible to bending moments. 

(c) Shafts that have been exposed to a combination of twisting and bending moments. 

(d) Shafts that are subjected to axial loads as well as torsional and bending loads. 

Design of Shafts on the Basis of Rigidity 

The goal of the rigidity-based design of the shaft is to keep the shaft's maximum deflection (due to bending) and maximum twist (due to torsion) within acceptable bounds. Rigidity is sometimes a factor to consider when designing shafts. The two sorts of rigidity that we'll look at are as follows.

Torsional Rigidity 

The torsional rigidity of a camshaft in an internal combustion engine is essential because it affects the valve timing. The maximum amount of twists allowed per meter length of such shafts is 0.25°. Deflections of 2.5 to 3 degrees per meter length can be utilized as a limitation value for line or transmission shafts. The most commonly utilized shaft deflection is 1 degree over a length equal to twenty times the diameter of the shaft.

Lateral Rigidity

It's crucial for transmission shafts and shafts moving at high speeds because even a slight lateral deflection can result in massive out-of-balance forces. Maintaining optimum bearing clearances and gear tooth alignment is also dependent on lateral rigidity. The lateral deflection of a shaft can be calculated using the deflection formula found in Strength of Materials if the shaft has a uniform cross-section. However, if the shaft has a variable cross-section, the lateral deflection can be calculated using the fundamental equation for a beam's elastic curve.

Design of Shaft for Flywheel

This is about the design of shaft for a flywheel. The flywheel is utilized to keep the engine's speed fluctuation to a minimum possible value. The maximum torque transferred is used to determine the diameter of the shaft for the flywheel. We know the maximum torque that can be transmitted. 

Tmax = (π/16) * τ(d1)3

where 

d1 = Diameter of the shaft

  = Allowable shear stress for the shaft's material

For maximum torque transmission, the hub is designed as a hollow shaft. We know the maximum torque that can be transmitted.

where 

Tmax = (π/16) *  τ[(d4 - d14)/d]3

d = Outer diameter of the hub 

d1 = The diameter of the shaft or the inner diameter of the hub.

 

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FAQs on Design of Shafts 

  • The goal of rigidity-based design is to keep the shaft's maximum deflection (due to bending) and maximum twist (due to torsion) within acceptable limits. The following are the two types of rigidity that are considered.

    • Torsional rigidity
    • Lateral rigidity
  • CNC turning is the most prevalent method for manufacturing shafts. Workers or automated processes attach or clamp material bars to chucks and rotate them using this approach. Cutting and other subtractive operations are used by specialized instruments to manufacture and shape the ultimate result while spinning.

  • The shaft is a rotating element with a circular cross-section that supports transmission devices such as gears and pulleys and transmits power, whereas the axel is a supporting rotating element such as wheels that is attached to the housing via bearings.

  • Materials with the following desirable qualities are used to construct a shaft:

    • High static strength.
    • High fatigue strength.
    • High resilience.
    • Good machinability.
    • Ductility.
  • Mild steel is the material of choice for typical shafts. Alloy steel, such as nickel, nickel-chromium, or chromium-vanadium steel, is utilized when great strength is required. Shafts are usually made by hot rolling and then cold drawn or turned and ground to size.

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