Flash Point Takes a Dive

On February 12, 2003 we asked ...

"This winter you sent a sample of a 220 cSt EP gear oil to your lab for routine analysis. The oil was used in an outdoor gearbox for about two years in Canada.

Most of the analysis results appear normal - viscosity is 217 cSt @ 40 deg C, only 100 ppm water, and acid number is 0.65. However, the flash point is 76 deg C which is well below the new oil level of 190 deg C.

A second sample sent to the lab confirms the results of the first test. What could have caused the flash point to drop so dramatically, while the viscosity and acid number are at near normal levels? What inspections of the gearbox could have confirmed your suspicion? What risks could this condition present? How could the root cause be corrected?"

Winning answer by Tang Ai Jun, Technical Service Engineer, Shell (China) Limited:

Cause: overheating of oil will lead to cracking of gear oil.

Inspection: you will find soot on the surface of the gearbox.

Risks: 1. HSE problem, may lead to fire if oil met air.
2. Could cause damage to the gearbox if problem is not fixed.

Correction: 1. Reduce the capacity of the heater to a lower temperature that the gear oil can endure. 2. Correct any operation conditions that will cause hot spots of the gear and the overheating of the oil.

Editor's Note: We received a LOT of responses this week. Below are just a few that we received:

Even a small amount of cleaning solvent will cause the flash point to go down substantially. It will have little impact on viscosity.

The fact that the oil has been in use 2 years with some water present might raise the viscosity enough to partially compensate for the effect of the solvent on the oil's viscosity. A quick sniff would reveal if the hypothesis is correct.

There is risk of explosion, especially in presence of fire (eg: someone smoking). A good breather will help block entry of extraneous fluids of all types. Tom Muckian, Technical Service Manager, Whitmore Mfg. Co.

The oil may have been cracked due to point or spot loading. Carbon fines will be produced as the oil "burns" in the loaded zones and the temp at this point would be about 550 degrees Celsius.

The carbon will keep the VI index up and compensate the shearing of the oil chains ( which would normally reduce VI index ). You would be looking for bearings out of adjustment, gear teath point loading, discolouration of the metal surfaces blue/ straw.

One more may be if no setting errors are found, misapplication of the transmission ( hp and or speed rating, incl low speed overloading) as the area of contact on the meshing gears could be to small and crushing the oil film.

For worm drives, vast ammounts of metal usually bronze/cast iron would be found in the oil sample and easily sighted, if the oil lost the lubrication properties in service. So an assumption is made that it is a Spur/ helical geartrain. Alastair Geddes, Technician, Energy Power Systems CAT

Assuming that the flash point tester is not faulty, I would suspect that this gearbox is in an ennvironment where either gases or moisture could get drawn into the gearbox. One needs only small quantities of these volatile substances to drop the flash point. Suggest heating the sample to 100 Degress Celsius for few minutes and then repeat the test. Tony Casas, Business Support Manager, Talcor Pty Ltd.