Assembly: Mill having a power input of more than 1000 KW
Sub-assembly: Gear box
Location: Around the area of the torsion shaft on the output side
Failure Mode: Casing crack in semi-elliptical pattern (a probabilistic failure mode)
Description: semi-eliptical crack — the pink area on the photo
Phenomenon: Thermal Fatigue (such semi-elliptical cracks are distinguishing patterns for thermal fatigue; but not necessarily the only pattern of failure due to thermal fatigue).
Such a crack is a combination of many interactions taking place simultaneously.
However, the initiator of such a problem with casing is the likelihood of blowholes or casting defects in the casing. On examination this was confirmed.
1. Thermal fatigue then accelerates the development of such a crack and especially so, if the casing thickness is not much.
2. Induced by rotating bending of the torsion shaft. Hence the crack appeared around the middle of the casing around the torsion shaft area.
3. Temperature of the gearbox was higher on the output side. Around 70 degrees C.
Nature of Crack growth and development:
When the depth of the crack is around 0.025 to 0.25 mm the shape of the crack becomes semi elliptical (check depth of crack). The threshold limit is around 0.16 mm, when initial crack starts to propagate.
When such a crack starts to develop and propagate a cyclic noise is audible.
Other areas where Thermal Fatigue can cause failures:
1. Rolls of rolling mills — can be experienced anytime during operation
2. Gear Boxes — usually experienced in winters or relatively cold ambient temperature
3. Motor Windings — usually experienced in topical summers
4. Anti-friction bearings — can be experienced anytime during operation
Method of Montoring: (Varied): But some of the methods might be as follows:
1. Infra-red thermal imaging — especially for motors
2. Visual — powerful technique — both sight and sound
3. Vibration analysis — especially for rolls and anti-friction bearings
4. Temperature monitoring (differential or temperature distribution) — for gearbox casings and motor casings.
Precaution during purchase: (Design Review)
1. Casing thickness
2. Allowable temperature rise
4. Cooling system
One thought on “Strange case of Semi-elliptical cracks”
So Dibyendu, could such failures be anticipated – and prevented – by monitoring visual and sound and processing through and AI-aided detector? Would that reduce down time and maintenance costs?
Have a wonderful new year’s eve, and all best for 2020.