Balancing Shafts, Drive Shafts and Couplings
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!
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.
Shaft balancing
Instruments and services for balancing shafts with critical speeds in mind
Balanset-1A instrument
An instrument for balancing shafts, propeller shafts and couplings
Buy the instrument