Material of Shaft – Types, Properties, and Practical Applications
The material of shaft plays a critical role in mechanical engineering, as shafts are responsible for transmitting torque and power in machines such as pumps, compressors, turbines, automobiles, and industrial equipment. Choosing the right shaft material ensures strength, durability, and cost-effectiveness.
Cast Iron Shafts – Rarely Used Today
A century ago, cast iron was commonly used for shafting. However, it is rarely used today due to its brittleness. Brittle materials fail suddenly without warning, which can cause serious secondary damage in machinery.
- Advantage (historical): Cast iron shafts were close to final form when cast, requiring less machining.
- Disadvantage: High brittleness and poor shock resistance make them unsuitable for modern applications.
Practical Example: In early steam engines, cast iron shafts were used, but frequent breakages led to costly downtime. Modern industries now avoid cast iron for critical rotating components.
Wrought Materials – Preferred Choice
With the availability of wrought materials, shafts can be easily machined with minimal material loss. This cancels out the machining advantage of cast iron.
- Benefit: Wrought steel shafts are stronger, more ductile, and reliable.
- Application: Used in pumps, compressors, and automotive drive shafts.
Forged Shafts – Strong and Economical
For large shafts where machining removes significant material, forgings are economical and very strong. Forged shafts have improved grain structure and mechanical properties.
- Benefit: High strength and toughness.
- Application: Turbine shafts, marine propeller shafts, and heavy-duty industrial equipment.
Malleable Iron (SG Iron / Spheroidal Graphitic Iron)
A modern development of cast iron is malleable iron or SG iron. It has better ductility and toughness compared to traditional cast iron.
- Limitation: Despite its versatility, SG iron is not widely used for shafts because steel and forged materials offer superior performance.
Commonly Used Shaft Materials – EN Series Steels
The most widely used shafting materials today are EN series steels in rolled condition:
- EN-8 Steel – Medium strength, suitable for general engineering applications.
Example: Shafts in small pumps and gearboxes.
- EN-9 Steel – Higher strength than EN-8, used in automotive and mechanical components.
Example: Crankshafts and machine tool spindles.
- EN-24 Steel – High strength alloy steel with nickel, chromium, and molybdenum.
Example: Heavy-duty shafts in turbines, marine engines, and aerospace applications.
For higher strength requirements, forged EN steels are preferred.
Heat Treatment – Normalizing Process
During rolling or forging, residual stresses are introduced in the shaft material. These stresses can cause distortion or failure during operation.
- Solution: Normalizing heat treatment is applied to relieve stresses and refine grain structure.
- Result: Improved mechanical properties and longer service life of shafts.
Practical Example – Shaft Material in a Pump System
Consider a centrifugal pump used in an industrial plant:
- The motor shaft is made of EN-8 steel for general strength.
- The pump shaft is made of EN-24 forged steel to handle heavy loads and vibrations.
- Heat treatment ensures durability and prevents premature failure.
This combination balances cost, strength, and reliability.
Conclusion
The choice of shaft material has evolved from brittle cast iron to strong and reliable EN series steels. Forged shafts and heat-treated steels dominate modern applications due to their strength, toughness, and durability.
