A. Grease Lubrication

Grease lubrication is used in 85% if all roller bearing element applications. The main advantages of grease lubrication are simple design, grease acting as a sealing medium, long service life with maintenance free lubrication for five years or more, negligible lubrication equipment and lower operating temperatures yielded in caparison to oil under similar working conditions.

The long lubrication life is generally found in horizontal spindles equipped with cylindrical roller bearings, or angular contact ball bearings. Taper roller bearings are generally more sensitive in operation and will require a life reduction factor or more frequent re-lubrication than ball bearings.

Vertically mounted spindles should have grease which has the highest mechanical stability because the grease will have a tendency to flow away from the bearings. Proper selection and design are major factors when life is required.

Grease re-lubrication is required when the service life is shorter than the bearing anticipated life. General grease failure is due to high temperature contamination, or stressing of grease film. R.A.S. uses a modified technique outlined by Kluber to estimate the life intervals.

     B. Oil Lubrication

Oil lubrication can be utilized over the entire speed ranges of machine tool spindles and while grease is inherently simpler than oil lubrication, oil lubrication is the only medium that can remove heat from the bearing.

Oil viscosity must be properly selected to suit the working conditions. To reduce frictional resistance light oils are used for bearings operating at high speeds. For heavy loads and slow speeds highly viscous oils are utilized. The regimes in which oil is utilized; copious (oil bath, circulating and oil jet) and minimal lubrication (oil mist and oil air) has great influence on the operating temperature of the bearing.

1. Oil Bath
   1. Oil bath lubrication is a simple method of lubricating bearings at slow speeds. General application is less than 1% of spindles. The oil reservoir should be of fairly large capacity to ensure minimal volumetric fluctuation of the oil. The level of oil must not be above center of the lowest rolling element during static ocnditions
   2. A slinger ring can be incorporated into the system to splash lubricant from the reservoir to the bearings and generally high speeds are achievable, however during slow speeds a slinger ring is less effective and should be used cautiously.
   3. During the initial run-in period, the oil should be changed after 1/2 hour and then again after the run-in is complete. Purging of the oil will ensure removal of any abrasive particles released during rotation of bearings. Normally the oil changes will then only be required once a year.
2. Forced Circulation Lubrication
   1. Forced circulation of oil is generally pumped to a higher level and the oil passes through the bearing on the return to the sump. The circulation system consists of a reservoir, pump, filters, valves, and flow restrictions and in some cases when excessive heat is generated, oil chillers are required.
   2. When large quantities of oil are utilized, provisions must be made to drain off excess oil to minimize power consumption and heat gain. The pumping direction of the bearings whether angular contact or taper roller should also be considered to ensure the correct direction of flow.
3. Oil Jet Lubrication
   1. Oil jet lubrication injects large volumes of oil into the gap between the cage and bearing race by means of properly sized nozzles and discharge ducts positioned carefully to provide sufficient drainage. Oil jet lubrication may be necessary at extremely high speeds to remove frictional heat and maintain low bearing temperatures. Large amounts of lubrication create oil churning and substantial amounts of power may be required to overcome the resistance to rotation therefore cooling of the lubricant medium is necessary
4. Oil Mist
   1. Oil mist conveys atomized oil suspended in air, to the lubricating point where it is partially precipitated. Care must be exhibited during design to ensure that the oil mist can flow through the bearings because air currents created by high speed rotation may prevent the oil from passing through the bearings. The spindle seals must allow for the passage of air within the spindle which may restrict lubrication flow. R.A.S. has used the oil mist system in approximately 10% of it's applications however, the environmental concerns limits it's use because of the exhaust of more precipitated lube escapes into the atmosphere. The most common systems utilize are the Trabon and Alemite consisting of a central piping and sometimes a heater to maintain the operational viscosiity of the oil.
5. Oil-Air Lubrication
   1. Oil-air lubrication is a minimum quantity lubrication. The air flow is passed over small amounts of oil in a tube which breaks up the oil from the tube walls and transports it to the bearing surfaces. Careful regard to porting and excess oil removal is required to ensure the systemic lubrication.
   2. Oil-air systems can be suited to high speed bearing applications up to 1.5 x 10 to exponent 6 mm/min. Where a small finely dispersed oil flow must be continuously fed to the bearing. The lubrication requirements can be exactly metered and adapted to the construction of the bearings. Typically these systems represent about 3% of the spindle lube requirement.
   3. The oil-air system component requirement is an oil-air metering unit, mixing tees, pressure gauges, switches, pumps sand check valves. Willy Vogel, Orsco and Spindle Guard are the products of choice for oil-air systems.