Bearing Oil Mist
Oil mist lubrication is widely used, especially in high speed
applications, as it offers the following advantages:
- satisfactory efficiency even with complex bearing arrangements
- low temperature, reduced power absorption
- low cost assembly
- simple construction (channels, spacers, etc.)
- good protection against outside contamination (pressurized
environment)
Oil mist lubrication systems must be designed to precise
standards taking account of the features and speed of the
bearings to be lubricated. (Oil mist equipment manufacturers
are generally in a position to supply the required specific
data).
Oil recommended for use in oil mist lubrication systems is
ISO VG32.
Air – Oil
An important peculiarity of this system is the use of a high
viscosity synthetic oil (generally ISO VG68) which, even when
applied in small amounts, ensures the presence of a resistant
film between rolling parts and bearing races.
This means simultaneous reduced ball rolling resistance and
excellent bearing behavior even under high stress. The system
is moderately polluting for the following reasons:
- low oil consumption
- reduced misting effect
The conveyor element air and the oil reach the bearing via
side nozzles (Figure 5) or through holes in the bearing outer
ring without being mixes during their passage.
Air – oil lubrication has led to outstanding results
in high frequency, high power electrospindles.
Series VEB and VEX high speed bearings in the air-oil lubricated
version, with ceramic material balls and outer ring with radial
lubrication holes (H1/NS execution) can reach high speeds
of over 2,500,000 ndm.
Oil quantity (Q) required can be calculated, roughly, with
the following formula:
Q = 1.2 x dm mm3/h (for each bearing)
*where dm is the bearing pitch diameter in millimeters
Air-oil flow to the bearings must be homogeneous and without
losses along its path. Individual air-oil supply to each bearing
is therefore strongly recommended, even if a more elaborate
supply system may be required in certain instances.
Minimum Oil
Any stems capable of supplying a bearing with the amount
of oil required for proper operation can be considered as
minimum.
This type of lubrication can be used for high speed bearings
too, by direct injection of small amounts of oil directly
into the bearings, through the outside ring (H1 version bearings).
Oil type and dosage are monitored by a central control and
a circuit ensuring continuous pressure and flow.
Nozzle Position
Maximum efficiency is achieved for all oil lubrication systems
when the lubrication flow reaches the bearing contact areas
with minimum turbulence.
Nozzle positioning, as indicated in the table below, is therefore
strongly recommended.
“P” and “S” values of nozzle position
(INSERT CHART)
Lubricant Contamination Effect on Bearing Life and Behavior
Cleanliness affects bearing life and efficiency. It is necessary
to achieve an application where bearings operated in free
of external contamination.
It is important, therefore, to keep bearings clean during
greasing and to provide good sealing during operation.
Addition precautions must be taken for oil lubricated bearing
to monitor lubrication contamination levels. In the case of
oil re-circulation systems in particular, this depends on
the rate of contamination, the filtration standard and the
size of the filter.
Contamination by the ingress of coolants, cutting oils, etc.,
alter lubricant properties thereby effecting bearing life.
This should be eliminated at the design stage by providing
adequate sealing.
Contaminating particle classifications are available, which
specify limit sizes and amounts per unit volume of 100 cm3.
With reference to ISO standard 4406 (Figure 6) for high precision
applications as, for instance, high performance electrospindles,
contamination should be restricted to level 10/7, for new
spindles and to level 13/10, after long use. In any case,
the maximum particle size should not exceed 5 micrometers.
Figure 6 shows the area with acceptable pollution levels.
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GRW
Product Desc.