A key and keyway unbalance: when chamfer size changed the vibration
A high-speed coupling hub test pointed at unbalance. The real source was a few grams of missing material in the keyway.
The machine
A steam turbine drove a compressor through a gearbox, around 3300 hp over 6,370 to 7,495 rpm, with hydraulically fitted hubs on the compressor side and single-key fits elsewhere. The gearbox maker found high vibration on a solo run, added field balance weights to finish the test, and shipped it on. Later in the string test the turbine showed 18 micrometre and the gear 29 micrometre vibration.
The wrong suspect
Everyone assumed the coupling was unbalanced. Hubs were checked on mandrels with dummy keys. The first results looked high: 408 g-mm at one hub and 479 g-mm at the other, against limits of 9.8 and 13.25 g-mm per plane in the standard. The dummy keys, however, did not represent the mass of the missing material. Recalculated with a 3D model, correct dummy keys brought both hubs within the limits.
The real source
The installed key had been turned down to the shaft diameter but its chamfer was almost four times too big. A chamfer of 3 mm instead of the 0.6 to 0.8 mm recommended removes about 7 g from the key. At the keyway radius of about 56 mm that is roughly 392 g-mm of unbalance, more than the whole allowable balance of the hub.
| Item | Value |
|---|---|
| Unbalance from key chamfer difference | 392 g-mm |
| Mass of the difference | 7 g |
| Chamfer used vs recommended | about 3 mm vs 0.6 to 0.8 mm |
| Vibration after correct key | about 13 micrometre, same as solo run of the turbine |
Why it matters
Hubs are balanced before the keyway is cut, so the key has to replace the removed material accurately. The heavy turbine rotor hid the same error; the lighter gear pinion was sensitive. A rotor analysis only predicts the response if the amount and location of unbalance fed into it are right.
- Cut keyways after balancing and fit the key to the recommended chamfer and tolerance.
- Fit keys to the standards for key and keyway fits on light, high-speed equipment.
- Better still on high-speed trains: use a hydraulic fit keyless connection. No key fit has zero clearance, and the clearance itself is an unbalance.
- Record key weight and chamfer on the balance sheet.