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:
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.
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.
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
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.
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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