Believe it or not, lubricant performance at high temperatures is not all that matters in gearbox oils. To prove this, we put lubricants to the test in critical, low-temperature 40°C conditions when the gearbox is not fully loaded.
Why is oil performance at low temperatures important? After a prolonged standstill, it takes time for the oil to heat up from ambient temperature to service temperature. Similarly, when a gearbox is operating at reduced speed below its full load capacity, or with frequent stop-and-go, the operating temperature of the oil is also lower than it would be under full load. Reduced oil temperature causes a substantial increase of oil viscosity during operation.
Standard lubricant film thickness calculations for typical gearbox applications still prove that even at temperatures as low as 40°C, the gearbox will operate under mixed or boundary layer lubrication. This means that the formation of a gear oil reaction layer by means of proper additive chemistry is vital to protect the friction bodies against damage.
Today’s high-performance lubricants are tested to determine their low-temperature performance against three typical gear failures:
- Micropitting failure of gears: Micropitting is fatigue failure in hardened tooth flanks of highly loaded gears. This failure consists of very small cracks and pores on the surface of tooth flanks. Micropitting causes material loss and a change in the profile form of the tooth flanks, which can lead to gear breakdown.
- Wear failure of gears: Wear is the result of abrasive material removal on the tooth flanks of gears. This condition is continuous, also causing material loss and a change in the profile form of the tooth flanks, eventually leading to gear breakdown.
- Failures of rolling bearings: High rolling bearing wear or premature fatigue of rolling bearings results in gear damage. Impending roller bearing failure can be detected by fatigue cracks and wear.
Test results show that failure conditions can be mitigated by high-performance lubricants that use advanced additive technologies. When advanced additives are used in mineral oil, polyalphaolefin, or polyglycol base oils, then micropitting formation and abrasive wear were prevented even at low oil temperatures. Test results also show that the advanced additive technologies in the high-performance gear oil reacted at the surface of the tooth flanks to build up a reaction layer that prevented wear failure.
How is your oil performing? Let us know in the comments below.