Reliable Plant July August 2008

APPLIED
RELIABILITY
A COMPARISON, EXAMINATION
OF SELF-LUBRICATING BEARINGS
These days, maintenance departments do not need to
spend time and money lubricating bearings on the plant
floor. Bearing lubrication technology has overtaken traditionally
accepted procedures still practiced in many
industrial facilities. Self-lubricating plain bearings offer plant-floor
decision-makers maintenance-free options to maximize productivity
and minimize costs.
There are three types of self-lubricating plain bearings used today.
Oil-filled sintered bronze bearings were first conceived in the 1930s,
with self-lubricating metal-polymer bearings developed around 1950
and lubrication-free solid polymer bearings appearing in the 1960s.
Wear rate and operation under specific loads, temperatures and
speeds are all important considerations when selecting the best
bearing for your application. The effects of dust, dirt or chemicals on
the bearing are also often decisive factors, as well as noise levels and
performance alongside different shaft materials.
Oil-impregnated, sintered bronze bearings rely on a capillary action
to create a lubricating oil film. Benefits include a low coefficient of
friction, high precision and rotational speeds up to 1,200 surface
feet per minute (sfm). Critically, however, high speed and
rotational motion are both required to draw the oil out and
maintain a full film of lubricant. Shaft oscillation, slow speed and
Figure 1. The benefits of solid polymer bearings include resistance
to dirt and impact.
intermittent use can all inhibit this process. If movement stops, the oil
on the surface of the bearing dries up; this can lead to squeaking and
an increase in the coefficient of friction. In addition, high temperatures
can break down the oil. Further disadvantages with this type of
bearing include low chemical resistance and sensitivity to dirt, edge
pressure and impacts.
Metal-polymer solutions consist of a metal backing and a thin
polymer (such as PTFE) liner. This plastic liner contains a solid lubricant
which is displaced from the bearing onto the shaft during
movement. The advantages conferred by this self-lubricating layer
include a low coefficient of friction and high load capacity — up to
36,260 pounds per square inch (psi) compared to the 1,450 psi
achievable with sintered bronze bearings. The bearings are also suitable
for high temperatures. However, they are very sensitive to dirt,
susceptible to corrosion and do not readily absorb vibrations. The
polymer liner can’t withstand high edge loads or oscillating movements
and is gradually stripped off.
The third, lubrication-free option is to use injection-molded solid
polymer bearings. These bearings are made with a mixture of
thermoplastic materials, embedded reinforcing fibers and solid lubricant.
The fiber-reinforced materials maintain the bearing’s strength
and resistance to high forces and edge loads.
Self-lubricating plastic plain bearings offer technical advantages in
comparison to metal or metal-backed bearings. A low wear rate, superior
chemical resistance and low noise are a few examples. Solid
polymer bearings are also impervious to corrosion and have a high
load capacity. They are usually underestimated at high temperatures
because they are made from plastic, yet short-term temperatures
greater than 572 degrees Fahrenheit and long-term temperatures of
482 F are possible.
The dry-running nature of these bearings means dirt particles do
not stick to the surface, but instead deflect off it. For this reason,
optimal performance can be maintained even in extremely dirty
environments.
After the plain bearing itself, the shaft is the most important
parameter for a bearing system. This is because it is in direct contact
with the bearing and is affected by relative motion. The shaft surface
is critical due to the fact a shaft that is too rough acts like a file and,
during movement, separates small particles from the bearing surface.
On the other hand, shafts that are too smooth can lead to higher wear
because not enough lubricant can be transferred onto the shaft to
36 July - August 2008 www.reliableplant.com