Timken Super Precision Bearings for Machine Tool Applications

ENGINEERING
A
LUBRICATION
TAPERED ROLLER BEARINGS
The selection of the lubricant and lubricant delivery method is directly
linked with the speedability of a bearing. It is strongly suggested
that the section on permissible operating speed be reviewed by the
customer in addition to this section on lubrication.
Fig. 57. Filling
a bearing with
synthetic grease.
Fig. 58. Simple
radial oil inlet
hole with oil
collector.
Fig. 59. Axial oil
jet to direct oil
at small end of
the rollers.
GREASE
Grease lubrication speed limits are lower than limits for oil
lubrication because all the heat must be carried away by conduction
through the shaft and housing.
Mineral Grease
When conventional (mineral) greases are used, the rib speed should
be limited to 5 m/s (985 fpm). This limit can be increased under pure
radial loads up to 13 m/s (2560 fpm) provided that the bearings
remain in end play under all operating conditions. Generally, No.
2 consistency greases are used with medium- to low-viscosity
base oils.
Lubricant quantity (V mg) may be approximated by using the following
equation:
V mg = f mg x V = f mg x [
x T x (D 2 - d 2 ) x 10 -3 M
- (cm3)
7.8 x 10 -3] 4
Where:
f mg = factor depending on speed: 0.3 < f mg < 0.5
V = free volume of the bearing (cm 3 )
T = overall bearing width (mm)
D = cup outer diameter (mm)
d = cone bore (mm)
M = bearing weight (kg)
= 3.1416
Synthetic Grease
The use of “low-torque” greases (or synthetic greases) can be
considered for rib speeds over 13 m/s (2560 fpm), up to maximum
of 25 m/s (4920 fpm). Experience has shown that stabilized
temperatures, around 15° C to 20° C (27˚ F to 36˚ F) above ambient,
can be obtained at the maximum permissible speed.
It is important to follow these procedures to help achieve the
above performance:
• All corrosion protection is removed from the bearing surfaces
by using an organic solvent.
• Very small initial quantity of grease is applied to prevent
excessive churning.
• Initial run-in period to evacuate unnecessary grease from
the bearing.
• Good spindle design to retain grease around the bearings.
• Efficient sealing to protect against external contamination.
Lubricant quantity (V sg) may be approximated by using the following
equation:
V sg = f sg x V = f sg x [
x T x (D 2 - d 2 ) x 10 -3 M
- (cm3)
4
7.8 x 10 -3]
Where:
f sg = factor depending on speed: 0.15 < fsg < 0.3
= 3.1416
When using synthetic greases, the limiting factor is the "lubrication
for life” concept (without re-greasing). Depending on load and
speed conditions, the grease life will typically be limited to 5000
to 8000 hours.
The Timken Company's suggestions for the use and run-in of
synthetic greases are illustrated later in this section.
A normal way to fill the bearing with grease is to do it by hand before
heating and fitting the components. For the cone, the free volume
corresponding to the first third of the rollers, starting from their large
end, is filled with grease; an additional quantity is provided below
the cage. For the cup, a thin film of grease is spread all around the
race as shown in Figure 57.
OIL
Grease lubrication of spindle bearings is generally preferred by
machine tool builders over oil circulation lubrication due to its
simplicity and low heat generation. For high loads or high speeds,
however, circulating oil is probably the most widely used method
because of its capability to remove heat from the spindle.
56 TIMKEN MACHINE TOOL CATALOG