Purchasing gear lubricants: be careful when playing the numbers game
Written by John Sander
4. Determine Type of Load
Figure 1 illustrates how gear oils are characterized according to their formulation and/or usage application. It especially shows how industrial gear oils are characterized according to their additive type, specifically the wear-related chemistry used. For example, a gear lubricant may be described as R & O, EP or AW. Certain enclosed gear applications have little to no load applied. The appropriate oil in this case might be R & O oil only. As the amount of loading increases, so will the amount of metal-to-metal contact between the gears. For gears operating in the mixed film wear regime, AW oil may be needed, while for those operating in the boundary regime, EP oil may be needed.
Sometimes the OEM specifically describes the type needed either in the equipment user's manual or on a plate attached directly to the gearbox itself. If not, then it is up to the user to determine whether the application subjects the gearbox to either heavy loads or shock loads during operation. In that case, it will require EP oil that contains active sulfur and phosphorous compounds that form a protective chemical layer on the gear surfaces when the fluid is compressed out of the meshed gears. In extreme loading cases, it might be helpful to use gear lubricant that contains solid EP additives. However, it is important to note whether fine filtration is in use, because it can remove solid additives and make this specific product ineffective. Some OEMs specifically recommend against the use of solid additives, so it is important to know this.
