Shaft Design | Material , Types , How to Design Shaft

Introduction to the Shaft:-

• A shaft is a rotating member, usually of circular cross section, used to transmit power or motion.
• It provides the axis of rotation, or oscillation, of elements such as gears, pulleys, flywheels, cranks, sprockets, and the like and controls the geometry of their motion.
• Carbon steels of grade 40C8, 45C8, 50C4, 50C12 are normally used as shaft materials.

Shaft Sizing

• Stress Analysis
– In design it is usually possible to locate the critical areas, size these to meet the strength requirements, and then size the rest of the shaft to meet the requirements of the shaft-supported elements.
• Deflection and Slope
– They are a function of inertia. Inertia is a function of Geometry. For this reason, shaft design allows a consideration of stress first. Then, after tentative values for the shaft dimensions have been established, the determination of the deflections and slopes can be made.

Shaft Materials:-

• A good practice for material selection:
– Start with an inexpensive, low or medium carbon steel for the first time through the design calculations. If strength considerations turn out to dominate over deflection, then a higher strength material should be tried, allowing the shaft sizes to be reduced until excess deflection becomes an issue.

The cost of the material and its processing must be weighed against the need for smaller shaft diameters.

Material properties

• It should have high strength
• It should have good machinability.
• It should have low notch sensitivity factor.
• It should have good heat treatment properties.
• It should have high wear resistance.

Manufacturing of Shafts

• For low production, turning is the usual primary shaping process. An economic viewpoint may require removing the least material.
• High production may permit a volume conservative shaping method (hot or cold forming, casting), and minimum material in the shaft can become a design goal.

Shaft Layout

• In most cases, Only two bearings should be used in most cases.
• Load bearing components should be placed next to the bearings to minimize the bending due to large forces.
• Shafts should be kept short to minimize bending and deflection.

Shoulder
• It allows precise positioning
• Support to minimize deflection.
• In cases where the loads are small, positioning is not very important, shoulders can be eliminated.

TYPES OF SHAFT:-

Transmission shaft:

These shafts transmit power between the source and machines absorbing power. The counter shafts, line shafts, overhead shafts all shafts are transmission shafts.

Machine shafts:

These shafts from an integral part of the machine itself.

DESIGN OF SHAFTS

The shaft may be designed on the basis of
1. Strength
2. Rigidity and stiffness

In designing shaft on the basis of strength the following cases may be consider
1. Shafts subjected to twisting moment only.
2. Shaft subjected to bending moment only.
3. Shaft subjected to combined twisting moment and bending moment.
4. Shaft subjected to fluctuating loads.

Shaft Design for Stress
• It is not necessary to evaluate the stresses in a shaft at every point; a few potentially critical locations will suffice. Critical locations will usually be on the outer surface.
• Possible Critical Locations, axial locations where:
1- The bending moment is large and/or
2- The torque is present, and/or
3- Stress concentrations exist.

DESIGN OF HOLLOW SHAFTS

Explanation

The shaft may be designed on the basis of
1. Strength
2. Rigidity and stiffness

In designing shaft on the basis of strength the following cases may be consider
1. Shafts subjected to twisting moment only
2. Shaft subjected to bending moment only
3. Shaft subjected to combined twisting moment and bending moment
4. Shaft subjected to fluctuating loads

Solid and Hollow shaft

When the shaft is subjected to combined twisting moment ad bending moment then the shaft must be designed on the basic of two moments simultaneously.

DESIGN OF SHAFT FOR RIGIDITY:

In many cases, the shaft is to be designed from a rigidity point of view.

For a shaft subjected twisting moment, the angle of twist is given by,

the angle of twist = TL / GJ 

Where T  = Torque applied

L    = Length of the shaft

J     = Polar moment of inertia of the shaft about the axis of rotation = πDˆ4 / 32

G    = Modulus of rigidity of the shaft material. The calculations need the modulus of rigidity of the material that makes up the shaft. This varies depending on the material, and the values of G for many kinds of materials can again be found in charts in design handbooks and from manufacturers.

Therefore for the known values of T, L, and G and allowable value of angle of twist, the diameter of the shaft can be calculated.

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