Sintered bearings
 

The shaft of a jigsaw is guided in porous bearings. A turning mechanism is obtained by an at a gear eccentric fixed ball bearing that moves horizontally in a sleeve.

Porous 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 particles remain 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 to cracks.

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.

Lubrication regime: About 10 to 35% of the porous material consists of lubricant impregnated channels. The porosity avoids full separation of the shaft and the bearing by 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 minutes.

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 improve the 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.
 
material k10-15 [m2/N] PV [MPam/s]
porous bronze
porous iron bronze
porous iron
PA6.6
0.06...0.1
0.13
0.1
0.35
0.3...0.9
0.17
0.08
5.7
1.75
1.25
1.05
0.1


Morgan, V.T. and Cameron, A, 1957, "Mechanics of lubrication in porous metal bearings". Proceedings of the conference on lubrication and wear. London, p.151-157.

Braun, A.L.,  "Investigations on porous bearings", Philips Nat. Lab. Technical note no. 155/77

www.tribology-abc.com