Q.1: Describe a lubricant problem in an industrial gear drive and how you solved it.

TLT Sounding Board January 2019


© Can Stock Photo / WSkG

Gear Lubrication

Coking, water ingress, foaming, high-temp varnish issues and short oil life were among the most common problems cited by readers responding to this month’s survey. Improper viscosity was the No. 1 cause of tribology-related causes, followed by improper lubricant, contamination, poor maintenance and a plant classic—no oil in the unit. But readers also say that only 30% of gear problems they encounter are lubricant related. “The worst failures I see tend to be related to assembly or parts errors and not lubricant,” said one reader. “I’ve seen gears unbolt from the carrier and cause lots of binding damage to mating gears and housings.” No matter the cause, gear issues can be among the costliest of mechanical problems. 62% of readers said they had seen gear sets ruined beyond repair. 

The multigrade lubricant we were using would shear substantially in service, resulting in bearing spalling and gear frosting. We switched to a higher-viscosity lube with different viscosity improvers.

Oil transferring from another compartment was causing the gear oil to drop viscosity and go out of grade. Identified the dilution via oil sampling and had maintenance techs identify the source. Changed the offending seal via dealer warranty support and changed the oil.

Customer was using EP gear oil in worm gears. The EP additive attacked the softer metals in the gear. We switched them to the proper lubricant.

Oil coking in a worm gear speed reducer. Removed mineral oil, replaced with ISO 460 PAG.

Water intrusion. Small side stream filtration.

Large Falk, single-reduction, helical box running hot with ISO 220. Switched to ISO 150, lowered temp due to less heat generated from large gear churning oil.

Short oil life. Solved by changing from a mineral-based gear oil to a synthetic and reducing ambient temperature by improving ventilation in the area.

Had an excess foaming issue in a used gearbox that was put back into service. Found out it had contaminants in it from before. Just performed flush and refill, and that seemed to solve the problem. Should have done this to begin with.

Gearbox filled with apparent hydraulic oil of too low viscosity. Replaced with proper viscosity for load and speed, but damage was done.

External contamination. Replaced bent pipe serving as vent pipe with a desiccant breather.

Adjusted preventive maintenance charts to reflect actual wear and timing for lubrication.

Water/debris intrusion into gearbox in beef-processing plant. Replaced gear oil with food grade grease to keep water out.

Clutch-based marine winch requiring an operating temperature range outside manufacturer’s specifications for the military. Emailed the manufacturer to get approval for a synthetic alternative.

Increasing wear debris levels led us to look at the oil used and we found it was the wrong viscosity. (That has happened more than once.)

We had a helicopter gearbox with a turbine oil that was causing premature wear of free wheel duplex bearings. We solved by using a gear oil.

High-temp varnish issues solved with a PAG gear fluid.

Customer had excessive water contamination in a wet manufacturing environment causing corrosion in gearbox. Replaced competitor’s gear oil with a faster demulsifying lubricant and periodically removed water layer. Corrosion stopped being an issue.

Adhesive wear. Solved via fluid/material/failure analysis.

A 1,000-hp-rated reducer with a service factor of 1.2, powered by a 1,200-hp motor turning a 144-ton DF9 ball mill. There were several modes of failure in the beginning. The reducers had a range of lubricant from ISO VG 320 - ISO VG 680. The reducers were running over 200 F most of the time causing shear down of the lubricant and, thus, gear pitting and adhesion welding. If the higher viscosity was used, since these were splash feed reducers, the bearings would be damaged from lack of lubrication until the oil warmed up to a flowable condition. Circulation coolers were installed with suction and discharge filtration included. This dropped the operating temperature to an acceptable range of 120-150 F. We still were getting micropitting on the reducer teeth from excessive loading during starts. The lube was switched from a conventional EP to a Borate additive EP oil. This solved many of the problems considering the reducer was still overloaded. Unfortunately the borate additive has been discontinued/banned. Compatibility testing has been completed between the new oil and current oil, and all is good. Time will tell if it is as good as the Borate lube was.

I’ve personally encountered an open gear drive in a SAG mill that has been having scuffing and adhesive wear in a mine site. As these defects are mostly lubricant related, we checked all possible factors such as spray pattern, quantity and so forth. Everything checked out, and as we continued to monitor it we found that water from inside the SAG was leaking and contaminating the lube on the gear flanks. It was then that we recommended to have the mill seals checked and corrected.

I couldn’t get a tractor cart wheel to turn for a major OEM’s machine, so I sprayed the #$@%*! out of it with WD-40.

Oil sprayed on gears under too much pressure creating foam. Pressure reduced.

Rapid degradation of a mineral-based gear oil used in the paper machine gear drives caused the oil to have to be replaced every three months due to the high operating temperatures and high ambient temperature. Changed the customer to a high quality SHC synthetic gear oil that enabled us to extend the service life first to a year and then ultimately to two years. Oil cost was five times the cost/gal, but total operating cost including labor was reduced to less than 50% of the previous cost. In addition, we were able to document reduced wear on the gears from the oil sampling program.

Cross-contamination of mineral-based oil with PAG. It was necessary to flush the gear oil and filtration system. It also was better to replace the cooler modules proactively.

No oil in unit. Trained staff to check and refill oil as needed.

Problem: Accelerated wear on bronze gear teeth. Solution: Changed to a non-EP gear oil as it was likely the sulfur or chlorine additives that were reducing gear life.

Very cold-weather application. Used low base viscosity lubricant.

Gear drive operating at sub-Hertz speeds by design. Catastrophic failure of reducer. Replaced reducer, calculated pitch live velocities of gears and increased gear lubricant viscosity from IOS 220 to ISO 460. Unit operating at sub-Hertz speed left the gear sets in contact long enough to rupture and squeeze out the lubricant film.

Shorter than normal life on a set of coal pulverizer gearboxes due to use of an active sulfur (rather than inactive sulfur or alternate additive technology) EP oil causing extensive pitting corrosion. Solved problem by changing to a different oil.

Heating problems. Correct lubrication recommendation.

Foaming. Solved by adding an antifoam.

Are most gear failures you encounter lubricant related?
Yes 30%
No 70%
Based on responses sent to 15,000 TLT readers.

© Can Stock Photo / speyeder

Q.2: Describe the worst gear failure you encountered or heard about.
An overspeed of an ultra-class haul truck powertrain when software caused a shift from 7th to 1st while traveling at 50 kph.

The worst failures I see tend to be related to assembly or parts errors and not lubricant. I’ve seen gears unbolt from the carrier and cause lots of binding damage to mating gears and housings. Root cause involved threaded holes not being formed properly during manufacture allowing fasteners to loosen.

Brass gear teeth on a large worm gear completely sheared/broken on a coal mill in a power plant.

Gear teeth braking on large mixer gear reducer.

Total gear failure. Bull gear lost all drive teeth due to bearing failures.

New inline helical at a paper mill. Installed and ran without oil. Complete failure throughout.

Agitator gearbox catastrophically failed. All of the teeth on the intermediate pinion shaft were completely stripped off.

Worst one I saw was on the final drive of a Class 8 truck. Customer used non-EP gear oil, and gears were destroyed shortly after the incorrect product was used.

Hot metal crane with badly worn seals had lube replaced with open gear lube to stop leaks.

Gearbox in a meat plant where the seal failed and the gearbox filled with meat product. The bacteria had a party in this moist and warm environment as they could only get to open it every six months or so. They had the last mechanic hired open it as part of the initiation. Between the smell and the sight, most got violently sick upon opening it.

We had nylon gears on a stacked pulp dryer. Some metal nuts came lose at the top of the stack of gears and worked their way to the bottom, imprinting themselves in every nylon gear on the way down.

Gear lubricant had become contaminated with sea water. The marine gear drive was completely chewed up inside and was a total loss.

Many years ago, the problem was with a 1:1 ratio drive for a twin-screw extruder. The gears were of great length due to extremely high forces (torque) and axially supported by a pair of double self-aligning spherical sliding bearings (two series in parallel). The malfunctioning alignment was the cause of extreme wear. Solution: hydrostatic bearings.

They installed a badly aligned pinion on a 300-hp industrial conveyor drive. They knew it was misaligned, but “had to get back in production.” Twenty-four hours later the pinion broke a tooth and the plant was down for almost 24 hours.

Broken teeth on a helicopter main gearbox input bevel gear.

An EP gear oil recommended by a gear manufacturer for use with a sacrificial brass gear. Catastrophic failure.

High-speed small gear in an assembly tool, grease lubricated. Poor bearing choice and excessive loads on gear teeth caused almost immediate galling failure of gears on first duty cycle.

Plastic deformation in wind turbine gearbox. Heard about it at a wind turbine symposium following STLE’s Tribology Frontiers Conference a couple of years ago.

We have a repeat offender with two failures in four years. It is usually caught first with oil analysis, then vibration. After the last failure it was decided to make it to a shut down. At this point it was given to operations to monitor and listen as failure was imminent. A few days later the reducer was very noisy then stopped turning the output. Upon inspection there were no teeth left on the input pinion gear. None. Since the base was never corrected after installation, we contributed the failure to angular soft foot and reducer distortion.

We had a bull gear tooth break off, which caused the machine to crack the case and drop parts and fluids to the ground—about 80 feet.

Worst I’ve heard about is a gear pinion cracking way before its expected life.

The engine in our 240 froze up, and none of the gears in the transmission would work.

Small gearboxes filled with PAG-based grease left in long-term storage lost too much oil due to bleeding, the oil leaked out, it was not noticed. Gearboxes seized when put to work.

Drive fear on a large cement kiln that was failing every two years at a very large cost.

Bearing damage at planetary carrier of gearbox, which destroyed the housing. The gear oil spilled out of gearbox and contaminated the environment.

Helicopter main drive transmission.

Plastic flow due to overheating, resulting in severe damage to gear teeth.

Teeth completely stripped due to overload failure.

Bearing cage corrosion to the point where the rolling elements fell out. To say the shafts went out of alignment is an understatement, but the gears were torn apart as the failure progressed.

An automotive pinion gear that failed due to inadequate EP protection in the lubricant.

Large pinion gear on ball mill had major pitting/cracks and plastic deformation. Wrong application.

Which statement best describes the worst gear failure you have encountered?
All gears showed severe pitting. 11%
Entire gear set ruined beyond repair. 62%
Contamination and improper maintenance led to failure. 27%
Based on responses sent to 15,000 TLT readers.
Editor’s Note: Sounding Board is based on an informal poll of 15,000 TLT readers. Views expressed are those of the respondents and do not reflect the opinions of the Society of Tribologists and Lubrication Engineers. STLE does not vouch for the technical accuracy of opinions expressed in Sounding Board, nor does inclusion of a comment represent an endorsement of the technology by STLE.