Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
OIL
Although several grease products have been successful at dN values
as high as one million, oils are generally suggested for bearings
operating at high speeds or to provide more cooling and dissipation
of heat than is possible with grease. High-grade spindle oil having
a viscosity of 100 seconds Saybolt at 37° C (100° F) is suggested
for use in drip-feed oilers, oil bath lubrication arrangements and oil
mist systems. In heavily loaded applications, oil in relatively large
quantities must be supplied, and where temperatures run higher
than normal, oil coolers will be suggested. Churning of a large pool
of oil is to be avoided if speed is significant.
Oil Bath
The conventional oil-bath system for lubricating the bearings is
satisfactory for low and moderate speeds. The static oil level must
never be higher than the center of the lowermost ball. When the
shaft is rotating, the running level may drop considerably below the
standstill level, depending on the speed of the revolving parts. A
sight gauge or other suitable method should be provided to permit
an easy check.
Drip-Feed Oil
Where the speeds are considered high for oil bath and the bearings
are moderately loaded, oil introduced through a filter-type, sightfeed
oiler is suggested. This assures a constant supply of lubricant.
The feed in drops-per-minute is determined by closely observing
the operating temperatures.
Oil Jet
In applications where the ball bearing is heavily loaded and
operating at high speed and high temperatures, or where the
operating conditions are severe with high ambient temperatures,
oil jet lubrication may be required. In such cases, it is necessary
to lubricate each bearing location individually and to provide
adequately large drain openings to prevent excessive accumulation
of oil after it has passed through the bearings. (1)
Oil Mist
Oil mist lubrication is often used for spindles running continuously
at high speeds. With this method of lubrication, oil of the proper
viscosity is atomized into finely divided particles, mixed with clean,
filtered, dry compressed air and directed to pass through the
bearings in a constant stream. This oil is metered into the air under
pressure. This system not only lubricates the bearings, it affords
some cooling due to the air flow. This continuous passage of air
and oil through the bearings and the labyrinth seals also serves to
prevent the entrance of contaminants into the bearings.
To ensure the “wetting” of the bearings and to prevent possible
damage to the balls and raceways, it is imperative that the oil mist
system be turned on for several minutes before the spindle is started.
The importance of wetting the bearings before starting cannot be
overstated and has particular significance for spindles that have
been idle for extended periods of time. To avoid such effects, most
oil mist systems have interlocks that make it impossible to start
the spindle until the lubricating system is working properly and the
bearings are thoroughly wetted. (1)
Metered Air/Oil
This method is similar to the oil mist; however, the oil is fed by
periodic pulses to the lubrication line providing a higher air-tooil
ratio. Therefore, this method lowers the operating bearing
temperature and lubricant shear effects, enabling higher operating
speeds. (1) System cost Typical speed (dN) (2)
Grease Low 500000
High-speed grease Low 750000
Oil bath Low 400000
Oil drip Low 600000
Oil mist Medium 1000000
Metered air/oil High >1000000
Oil jet High >1000000
(2) Speed value is an approximation and assumes proper mounting and preload
techniques along with average loading conditions. For more specific guidance
contact your Timken representative.
The speed, “dN,” value is obtained by multiplying the bearing bore size in
millimeters by the shaft RPM.
Table 14. Lubrication system comparison.
(1) For further information, refer to the lubrication specification tables found on pages 243-244.
60 TIMKEN MACHINE TOOL CATALOG