SELECTING THE CORRECT GEAR OIL*
There are two primary considerations for selecting the correct gear oil for an application:
The now superseded Industrial Gear Lubrication Standards, AGMA 250.04, used center distance as the primary criterion for gear lubricant selection. The new version of this standard, designated AGMA 9005-D94 Industrial Gear Lubrication, has adopted pitchline velocity as the primary selection criterion. The pitchline velocity determines the contact time between gear teeth. High velocities are generally associated with light loads and very short contact times. For these applications, low-viscosity oils are usually adequate. In contrast, low speeds are associated with high loads and long contact times. These conditions require higher viscosity oils. EP additives may be required if the loads are very high.
Ambient and operating temperatures also determine the selection of gear lubricants. Normal gear oil operating temperature ranges from 90 F to 100 F (50 C to 55 C) above ambient. Oils operating at high temperatures require good viscosity and high resistance to oxidation and foaming. Caution should be exercised with abnormally high temperatures. High operating temperatures indicate oils that are too viscous for the application, excess oil in the housing or an overloaded condition. Each of these conditions should be investigated to determine the cause and correct the problem. Oil for gears operating at low ambient temperatures must be able to flow easily and provide adequate viscosity. Therefore, these gear oils must possess high viscosity indices and low pour points.