bearings are manufactured by sintering, e.g. powder is pressed to
parts at high temperature and high pressure.
Despite of the powder is melted together capillary channels between the
resulting in a porous material. By impregnation of these channels with
liquid or solid lubricants, the bearing material may contain enough
lubricant for life. As
a fact of the capillary channels porous materials are brittle and sensitive
Examples of sintered porous bearing materials are
carbon brushes of electric drives,
porous bronze bearing bushes,
engineering ceramics, products made by rapid prototyping and sintered polymers.
Porous bronze bearings are frequently applied in consumer products.
Those bearing materials consist of 90%
bronze, 10% tin and often some addition of graphite and lead to improve
dry running properties. Porous
iron bearings can take up higher bearing loads but have a lower
permissible sliding speed.
About 10 to 35% of the porous material consists of lubricant impregnated
channels. The porosity avoids full separation of the shaft and the
hydrodynamic pressure buildup but effectively creates lubricant
circulation. The pressure
build up forces the oil into to pores of the loaded part of the bearing.
Then the oil flow to the unloaded section of the porous bearing and
replenish the gap [Morgan, 1957]. The lubrication regime can be
established by determining the so-called Stribeck curve,
which gives the relation between the friction and the shaft speed.
Depending on rotational speed and load porous bearings almost invariably
operate in the region of mixed lubrication.
circulation of oil in the bearing>>>
In the exceptional case that the pores of the bearing are filled with
oil for 100%, it is when the bearing operates in an oil bath, full
hydrodynamic lubrication can be maintained. In all other circumstances
escape of oil result in some air in the pores causing mixed lubrication.
During stand still the bearing gap can be filled by capillary action.
When the shaft starts to rotate a pressure is build up in the oil film and
full-film lubrication can be maintained for some
►Running in: With stationary load a perfect
smooth slide surface can be formed as a result of the favorable
lubrication conditions in the mixed lubrication regime. At high
stationary loading the pores in
the slide surface can disappear locally by smearing. Both effects
hydrodynamic lubrication during running in. Start-stop cycles
during running in can accelerate the running in
period. Without stationary load the running in effects are negligible
and the initial friction and wear persists.
►Friction and wear.
In the table below some porous sinter materials are compared to PA6.6.
The results are obtained using pin on disc
tests, i.e. spherical sintered pin
Rsphere=4 mm, F=10 N and a rotating disc of tool steel v=0.03 m/s,
Ra<0.1, H=60Rc; test duration t=2 hours.
porous iron bronze