ROOT CAUSE FAILURE ANALYSIS OF HIGH SPEED GEAR

ROOT CAUSE FAILURE ANALYSIS OF HIGH SPEED GEAR (A case study)

Sequence of the paper • • • Incident Physical analysis Maintenance status Operational status Failure possibilities Helical gear and its advantages Oil circuit and oil function in high speed gears Factuality of gear failure Learning curve

Incident • • High speed reduction gear of Mill # 3 turbine 400 k. w power made by KKK Germany damaged at the end of the crushing season 2017 -18. Physical analysis show the metal to metal contact in absence of the oil film. Sunk key of the mechanical oil pump found broken but the mechanical pump was working due to the lock nut grip on the driving wheel. How this incident occured in the presence of low oil pressure safeties, this is investigated in the following proceeding.

Physical analysis of the metal Deformed metal

Physical analysis of the Broken pinion metal Damaged pinion teeth

Mechanical pump driving wheel

Maintenance status of the gear: • Back lash of the gear 0. 2 mm • End play of the gear 0. 15 mm • Bearing clearance 0. 15 mm

Backlash “The maximum distance or angle through which any part of a mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence",

End play is the total distance the shaft can move between the two thrust bearings. The end play allows room for the formation of an oil film, misalignment, and thermal expansion of the bearing components.

Bearing clearance Bearing • clearance essential is form tofilm oilbetween the surfaces, where the shaft sink in 50 percent of the bearing diameteral clearance. • diameter. • 1 mil = 0. 001 mm

Operational status • Mill operating @ of 3200 TCD (Capacity 4000 TCD) • Due to low turbine speed turbo pump was operated to compensate the low oil pressure • The bearing lubrication pressure was maintained at 1. 5 bar and the throttle valve control pressure was 10 bar. • Temperature and other operational parameters were maintained

High speed gears failure • Increase of noise or vibration • Excessive increase in temperature • Increase of backlash • Low viscosity lubricant • The load is over large • Excessive end play and bearing clearance

Introduction - high speed gears • • Gearing can change the speed, torque and direction of motion from a drive source. Typically, the torque increases in direct proportion to the reduction of rotations per unit of time The term “speed reducer” is applicable to the units operating at pinion speeds below 3, 600 rpm or pitch-line velocities below 1500 M/m Reducers operating at speeds higher than these are called high-speed units. “American gear manufacturing association”

Advantages of helical gears Gear teeth contact is gradual, beginning at one end of the tooth, and staying in contact with the gear, which rotates until full contact is achieved. The typical helix angle ranges from 15 to 30 degrees. On the other hand, the thrust load varies in direct proportion to the magnitude of the tangent of the helix angle.

Advantages of helical gears Thrust Helix angle 15 Rotation

Advantages of helical gears • The angled teeth work more gradually, allowing for smoother and more silent gear operation when compared to spur gears. • Helical gears last longer and are ideal for high-load applications, since they have a higher amount of teeth in contact. • Load is at all times distributed among several axes, which produces less wear.

Advantages of helical gears Helix angle 15

Oil circuit Oil Filter Oil cooler Distributor Emergency trip Auxiliary oil pump Turbo oil pump Pressure spray for gears Bearings lubrication Mechanical oil pump ON OFF Governor Throttle valve Sump tank Low pressure trip

Factuality of failure 4000 TCD capacity plant was being operated on 3200 TCD, The turbine speed was 930 rpm, slightly below the lowest permissible speed i-e 950 rpm, below this limit, the low pressure safety shut-off the turbine. To avoid low lubrication pressure tripping, Turbo auxiliary pump was operated, designed for emergency shut off when both mechanical and auxiliary pumps are not possible to be operated.

Factuality of failure Turbo lubrication pump supplies oil to the main oil distributor, from distributor one channel supply oil to the dial gauge and another one to throttle valve to control the turbine speed, both the 10 mm diameter channels receiving enough quantity of oil and are closed loops. The high speed reduction gear oil supply channel is of 6 mm diameter open end channel, receiving oil less than reduction gear requirement.

Factuality of failure High speed reduction gear lubrication requirement is 392 ml/s, mechanical pump provides oil to the reduction gear @ 562 ml/s through 12 mm diameter channel. While the turbo auxiliary pump supply 250 ml/s oil to the reduction gear which is too less to energize both the oil loops of high speed reduction gear.

Oil channels in distributor Mechanical oil pump ᶲ 12 mm to gear Flow rate 562 ml/s ᶲ 10 mm to Throttle valve Pressure 10 bar ᶲ 10 mm to Pressure gauge Pressure 2 bar ᶲ 5 mm to gear Flow rate 250 ml/s Turbo auxiliary oil pump

Oil channels in distributor Throttle valve oil channel Reduction gear oil channel

Reduction gear’s oil circuit Main functions of the lubricant in a reduction gear is: 1. Pressure lubrication of the gears through 1 mm diameter 3 x nozzles. 2. Bearings lubrication through 5 mm diameter 4 x port. In case of insufficient lubrication the lubricant rushes towards the low pressure area i-e bearing lubrication.

Reduction gear’s oil circuit Gear pressure lubrication nozzles Bearing lubrication circuit

Learning curve • Turbo pump should be operated when the machine is shut off, as it mainly lubricates the gear bearings, the oil quantity is insufficient for the pressure lubrication of the gear teeth • Turbine and its accessories should be operated / kept functional as per designed parameters. • Machines operation as per operating manual is safe and result oriented

Questions?