Oil lubrication methods

Oil bath
The simplest method of oil lubrication is the oil bath. The oil, which is picked up by the rotating components of the bearing, is distributed within the bearing and then flows back to a sump in the housing. Typically, the oil level should almost reach the centre of the lowest rolling element when the bearing is stationary. Oil bath lubrication is particularly suitable for low speeds. At high speeds, however, too much oil is supplied to the bearings, increasing friction and causing the operating temperature to rise.
Circulating oil

In general, high-speed operation increases frictional heat, elevates operating temperatures and accelerates ageing of the oil. To reduce operating temperatures and avoid frequent oil changes, the circulating oil lubrication method is generally preferred (fig 1). Circulation is usually controlled by a pump. After the oil has passed through the bearing, it generally settles in a tank where it is filtered and cooled before being returned to the bearing. Proper filtering decreases the contamination level and extends bearing service life. In bigger systems with several different bearing sizes, the main volume flow from the pump can be split into several smaller flows. The flow rate in each sub-circuit in the system can be checked by SKF flow monitoring devices.

Guideline values for oil flow rates are listed in table 1. For a more accurate analysis, contact the SKF application engineering service.

For information about the SKF CircOil system and SKF flow monitoring devices, refer to Lubrication solutions.

Oil jet
The oil jet lubrication method (fig. 2) is an extension of circulating oil systems. A jet of oil under high pressure is directed at the side of the bearing. The velocity of the oil jet should be sufficiently high (≥ 15 m/s) to penetrate the turbulence surrounding the rotating bearing. Oil jet lubrication is used for very high speed operation, where a sufficient, but not excessive, amount of oil should be supplied to the bearing without increasing the operating temperature unnecessarily.
Oil drop
With the oil drop method, an accurately metered quantity of oil is supplied to the bearing at given intervals. The delivered quantity may be relatively small, keeping frictional losses at high speeds to a minimum. However, it is difficult to ascertain whether the oil is able to penetrate the bearing at high speeds and, therefore, individual testing is always recommended. Whenever possible, the oil-air method should be preferred over the oil drop method.
Oil mist
Modern application specific oil mist systems, such as those offered by SKF, matched with a suitable non-toxic and non-carcinogenic oil formulated for minimum stray mist emissions and suitable sealing arrangements, address environmental and health concerns. These systems, when well maintained, provide a cost-effective, environmentally clean way to continuously and effectively atomise oil and deliver metered minimum required quantities to the bearings. Modern oil mist systems suspend oil droplets 1 to 5 μm in size in dry instrument air. The oil to air ratio, which is typically 1:200 000, creates a very lean but effective mixture that is delivered under 0,005 MPa pressure.

Oil-air lubrication systems are appropriate for high-precision applications with very high operating speeds and requisite low operating temperatures. For information about the SKF Oil+Air lubrication systems, refer to Lubrication solutions.

The oil-air method (fig. 3) also called the oil-spot method, uses compressed air to transport small, accurately metered quantities of oil as small droplets along the inside of feed lines to an injector nozzle, where it is delivered to the bearing (fig. 4). This minimum quantity lubrication method enables bearings to operate at very high speeds with relatively low operating temperature. The compressed air serves to cool the bearing and also produces an excess pressure in the bearing housing to prevent contaminants from entering. Because the air is only used to transport the oil and is not mixed with it, the oil is retained within the housing. Oil-air systems are considered to be environmentally safe, provided that any residual used oil is disposed of correctly.

For bearings used in sets, each bearing should be supplied by a separate injector. Most designs include special spacers that incorporate the oil nozzles.

Guideline values for the oil quantity to be supplied to an angular contact ball bearing for high-speed operation can be obtained from

Q = 1,3 dm

Guideline values for the oil quantity to be supplied to a cylindrical roller bearing or double direction angular contact thrust ball bearing can be obtained from

Q=oil flow rate [mm3/h]
B=bearing width [mm]
d=bearing bore diameter [mm]
dm=bearing mean diameter [mm]
= 0,5 (d + D)
= 1 to 2 for cylindrical roller bearings
= 2 to 5 for double direction angular contact thrust ball bearings

Individual testing is, however, always recommended in order to optimize the conditions.

Different bearing designs show varying sensitivity to oil quantity changes. For example, roller bearings are very sensitive, whereas for ball bearings, the quantity can be changed substantially without any major rise in bearing temperature.

A factor influencing temperature rise and reliability of oil-air lubrication is the lubrication interval, i.e. the time in between two measures from the oil-air lubricator. Generally, the lubrication interval is determined by the oil flow rate generated by each injector and the oil quantity supplied per hour. The interval can vary from one minute to one hour, with the most common interval being 15 to 20 minutes.

Feed lines from the lubricator are 1 to 5 m in length, depending on the lubrication interval. A filter that prevents particles > 5 μm from reaching the bearings should be incorporated. The air pressure should be 0,2 to 0,3 MPa, but should be increased for longer runs to compensate for the pressure drop along the pipe’s length.

To maintain the lowest possible operating temperature, ducts must be able to drain any excess oil away from the bearing. With horizontal shafts it is relatively easy to arrange drainage ducts on each side of the bearings. For vertical shafts the oil passing the upper bearing(s) should be prevented from reaching the lower bearings, which would otherwise receive too much lubricant. Drainage, together with a sealing device, should be incorporated beneath each bearing. An effective seal should also be located at the spindle nose to prevent lubricant from reaching the work piece.

The oil nozzles should be positioned so that oil can be introduced into the contact area between the rolling elements and raceways without interference by the cage. For the diameter (measured on the bearing) where oil injection should take place, refer to the product tables. For bearings equipped with alternative cages that are not listed, contact the SKF application engineering service.

The attainable speeds listed in the product tables for oil lubrication refer specifically to oil-air lubrication.

Direct oil-air lubrication
For super-precision angular contact ball bearings operating at very high speeds, the injection of small amounts of oil-air directly through the outer ring is beneficial. With this method, lubricant dispersion is prevented, as the lubricant is supplied directly and safely to the ball/raceway contact area. As a result, lubricant consumption is minimized and bearing performance is improved. The different variants (fig. 5) for direct oil-air lubrication provide different benefits:
  • Bearings with an annular groove and O-rings in the outer ring (designation suffix L or L1) prevent lubricant leakage between the bearing and its seat in the housing. For bearings without these features (designation suffix H or H1), SKF recommends machining the housing bore and incorporating O-rings into the bearing arrangement design.
  • Bearings with lubrication holes on the thick side of the bearing shoulder (designation suffix H1 or L1) enable the lubricant to be supplied very close to the ball/raceway contact area. The locations of these lubrication holes enable the bearings to achieve maximum speeds.
Direct minimum quantity lubrication with minimal air consumption

The use of a continuous air flow in an oil-air lubrication system includes some system related disadvantages like the high cost of compressed air, high noise levels and a complex dosing and control process. The SKF Microdosage system (fig. 6) virtually eliminates these disadvantages and offers better control and a lower cost of ownership.

Designed for ultra high speed spindles where the speed factor A ≥ 2 000 000 mm/min, this system delivers precisely metered amounts of oil to each bearing based on the machine tool’s CAM program. The SKF Microdosage system also automatically re-calibrates when conditions like temperature or oil viscosity change. With this technology, oil consumption can typically be reduced to 0,5 to 5 mm3/min with a minimal amount of compressed air.

For information about the SKF Microdosage system, refer to Lubrication solutions.

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