Worm gearing has a high slide-to-roll ratio when compared to other types of gearing. Because of a high sliding component, it relies heavily on the generation of an oil film between the worm and gear. The oil film produces an effect similar to what happens when a speeding car hits a rain puddle. The car tire has a tendency to float on a wedge of water. In a car, this is called hydroplaning: in gear, it is called elasto-hydrodynamic lubrication (EHL). This is a simplistic description with other modes of lubrication coming into play, depending on conditions, but it gives a general idea.
For EHL to be the only lubrication mode, it must generate a film thickness greater than the surface roughness of the contacting parts ..Film thickness is proportional to the sliding velocity and lubricant viscosity and inversely proportional to the unit load. High unit loads possible at the relatively low speed of worm gearing require a very high viscosity lubricant. Viscositie of over 400 cSt at 40°C are normally used to prevent premature wear and high contact temperatures. Under high loads the film can collapse, causing the surfaces to contact, This is called “boundary lubrication .” In this lubrication mode, other properties (i.e., lubricity or slipperiness) of the lubricant become more important than the viscosity. In a worm gear set, a mixture of EHL and boundary lubrication are at work.
A satisfactory lubricant for most gear motors applications is an AGMA 7 compounded oil, Low speeds require the higher viscosity of AGMA 8 compounded oil. Both are petroleum-based mineral oils compounded with 3% to 10% fatty oils. These lubricants are sometimes referred to as steam cylinder oils. The compounded oil provides lower friction and better wear characteristics than straight mineral oil. At the high pressures and temperature in the contact area, a chemical reaction occurs on the tooth surface, forming protective skin.
Extreme pressure oils (EP oils) are another type of lubricant that uses surface-acting chemistry. Most EP oils use sulfur, phosphorus, and/or chlorine additives, and are designed to work in steel-an-steel applications. When these oils are used with bronze under high temperature and pressure, conditions common ill theme contact, the chemical reaction can go awry. The surface of the bronze can begin to flake off, causing massive wear, and intergranular stress corrosion call. cause the teeth to break. There are EP oils designed for use with bronze that use a different additive package. and in certain applications a standard EP oi] may work very wen. When selecting an EP oil for bronze gearing make sure it was carefully reviewed.
Synthetic lubricants are also very common. They are more viscosity-temperature stable than mineral oils. This allows one lubricant to provide adequate service over a broader temperature range, They have a longer service life. reducing the number of oil changes required. They reduce wear and friction, increasing gearbox life ..Efficiency increase of 20% of the lost power are possible. Under severe conditions properly selected synthetic oils are outstanding. Many companies have found cost advantages in using the more expensive synthetic oil for normal applications.