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 materials | Lubrication regimes | Running in | friction and wear |

►Porous bearings are manufactured by sintering, e.g. powder of the material components is pressed together at high temperature and high pressure. Despite of the powder is melted together capillary channels remain resulting in a porous material. By impregnation of the channels with liquid or solid lubricants, bearing materials acquires that are lubricated for life, they have not to be lubricated during lifetime. As a fact of the capillary channels porous materials are bros and sensitive for cracks.

Examples of sintered porous bearing materials are the ceramic materials, carbon brushes of electric drives, porous bronze bearing bushes, aluminum matrix composites consisting of a mixture of aluminum powder and ceramic granules and sintered polymers.

The most widely used type of porous bearing in consumer apparatus is a porous bronze consisting of 90% bronze, 10% tin and often some addition of graphite and lead to improve dry running properties. Furthermore a frequent use is made of porous iron bearings, which can take up a higher bearing load but have a lower permissible speed of the shaft in the bearing.

►Lubrication regime: About 10 to 35% of the porous material consists of lubricant impregnated channels. As a consequence of the porous bearing surface a sufficient hydrodynamic pressure distribution to avoid mechanical contact between shaft and bush as in solid bearings cannot be created. 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 gab [Morgan, 1957]. The type of lubrication  that occurs can be assessed 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.

 
Only if the pores of the bearing are 100% filled with oil, it is when the bearing operates in an oil bath, hydrodynamic lubrication can be maintained. In all other circumstances escape of oil result in some air in the pores causing mixed lubrication. During a stand still the bearing gab can be filled by capillary action. When the shaft starts to rotate a pressure is build up in the oil film, as in the case of the solid journal bearing and hydrodynamic pressure can be maintained for some minutes.

►Running in: With stationary load a perfect smooth slide plane can be formed as a result of the favorable lubrication in the mixed lubrication regime. Besides that, the pores in the slide plane can disappear by smearing. Both effects stimulate the hydrodynamic lubrication component during running in with an increasing friction as a result. A start-stop cycles 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. A number of porous sinter materials are mutually compared by pin on disc tests with spherical sintered pin and a rotating disc of tool steel. (pin: F=10 N, Rspherical=4 mm; disc: v=0.03 m/s, Ra<0.1, H=60Rc; test duration t=2 hours).

material	µ	k·10-15 m2/N
porous bronze porous iron bronze porous iron	0.06...0.1 0.13 0.1	0.3...0.9 0.17 0.08

From sinter bronze a range is presented from a set of four different bronze materials tested. The porous iron appear to be much more hard wearing than porous bronze.
  

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