Balancing Shafts, Drive Shafts and Couplings

In shortA shaft with L/D under 3 behaves as a rigid body and takes standard two-plane balancing; above L/D 10 it bends as it turns and may run above its first critical speed, where standard balancing fails. Keep the working speed at least 20% away from the critical speed, found by a coast-down test.

When this applies

Short rigid shafts (L/D < 3) needing two-plane balancing
Long shafts (L/D > 10) that bend when rotating
Jointed propeller shafts with multiple sections
Couplings, pulleys or sprockets mounted on a shaft

When it doesn’t

Standard balancing fails above the first critical speed
Flexible shafts need bending-mode-aware methods
Operating speed too near critical speed is unsuitable

Types of shaft

1. Short rigid shafts

L/D < 3: These behave as rigid bodies. Standard two-plane balancing applies.

2. Long flexible shafts

L/D > 10: These bend as they rotate. Watch out for critical speeds!

Warning: Long shafts can run ABOVE their first critical speed. Standard balancing will not work in that case!

3. Propeller (jointed) shafts

The specifics: These consist of several sections joined by universal joints. Balance each section separately.

4. Shafts with mounted components

Couplings, pulleys and sprockets — balance these as an assembly together with the shaft.

Critical speeds of shafts

Definition: The speed at which a shaft enters resonance and begins to bend severely.

The danger: As the shaft passes through a critical speed, vibration rises sharply.

How to identify it:

Coast-down test (measuring during run-down)
The graph will show a sharp vibration peak
The phase shifts by 180° as the shaft passes through resonance

What to do:

The operating speed must be ABOVE or BELOW the critical speed by at least ±20%
If operating speed = critical speed → change the support stiffness or the shaft mass
For flexible shafts: use specialised balancing that accounts for the bending mode shape

Balancing couplings

Types of coupling:

Rigid: Balance separately, then install
Flexible: May contain rubber elements — balance as an assembly
Gear: Balance each half separately

Rule of thumb: Balance the two coupling halves diametrically opposite to one another so that they cancel out.

Conclusion

Shafts and propeller shafts demand particular attention to critical speeds. Long shafts (L/D > 10) may be flexible, in which case standard balancing is unsuitable. A coast-down test is mandatory.