Summary

This is a well-known rule of thumb passed down by experienced machinists and mechanical engineers. The meaning is quite straightforward:

• Using 40Cr for small shafts often results in excessive performance and unnecessary cost.

• Using 45 steel for large shafts may lead to insufficient hardenability and inadequate strength.

As a result, a common material selection guideline has gradually formed in mechanical design:

• Small-diameter shafts → 45 Steel

• Medium-diameter shafts → 40Cr

• Heavy-duty or large-diameter shafts → 42CrMo

Since most shaft components in machinery fall within the medium size and load range, an industry-wide phenomenon has emerged:

Approximately 80% of shaft components are made from 40Cr steel.

When selecting a shaft material, engineers typically need to consider only three factors:

1. Shaft size

2. Applied load

3. Consequences of failure

1. Start with Size: The Larger the Shaft, the Higher the Material Requirements

Whether a shaft can truly achieve higher strength after heat treatment largely depends on its hardenability.

Simply put, hardenability refers to whether the material can harden effectively from the surface all the way to the core.

Different materials have different effective hardening capacities:

Shaft Diameter Selection Guide

Why?

• 45 Steel has relatively low hardenability. Once the shaft diameter becomes larger, the core cannot be sufficiently strengthened.

• 40Cr offers moderate hardenability and can achieve through-hardening in most shaft applications.

• 42CrMo has excellent hardenability and is well suited for large-diameter shafts.

2. Consider the Load: How Heavy Is the Job?

Shafts are primarily subjected to:

• Torsional loads

• Bending loads

• Fatigue loads

Based on loading conditions, shaft applications can generally be divided into three categories:

Light-Duty Shafts

Examples:

• Small electric motor shafts

• Small pump shafts

Recommended material: 45 Steel

Advantages:

• Lower cost

• Easy machining

• Sufficient strength for light-duty applications

Medium-Duty Shafts

Examples:

• Transmission shafts

• Gear shafts

• Gearbox shafts

Recommended material: 40Cr

Advantages:

• Good balance of strength, toughness, and fatigue resistance

• Widely used in general mechanical equipment

Heavy-Duty or Impact-Loaded Shafts

Examples:

• Construction machinery shafts

• Large crankshafts

• Critical power transmission shafts

Recommended material: 42CrMo

Advantages:

• Higher strength

• Better toughness

• Superior resistance to shock and fatigue loading

3. Finally, Evaluate the Consequences of Failure

Another practical principle in material selection is:

The more critical the component, the more conservative the material choice should be.

If shaft failure only results in routine maintenance and downtime, standard materials are usually sufficient.

However, if failure could cause:

• Severe equipment damage

• Safety incidents

• Significant production losses

Then engineers often upgrade the material by one level:

• 45 Steel → 40Cr

• 40Cr → 42CrMo

The reason is simple:

A slightly more expensive material costs far less than a major accident.

Quick Comparison of the Three Materials

Conclusion

The saying "Small shafts shouldn't use 40Cr, and large shafts shouldn't use 45 steel" reflects decades of practical engineering experience.

A simple decision rule is:

• Choose 45 Steel when the shaft is small and lightly loaded.

• Choose 40Cr for most standard shaft applications.

• Choose 42CrMo when the shaft is large, heavily loaded, or safety-critical.

This is why 40Cr has become the most widely used shaft material in the machinery industry, accounting for roughly 80% of shaft applications.