The self lubricating bearing is a typical application of metal powder parts or powder metallurgy process. The self lubricating bearings made from brass, iron, aluminium bronze etc, which are later impregnated with lubricants. High porosity is required in porous oil impregnated bearings which are self-lubricating.
The basic operations involved in manufacture of self-lubricating bearings using powder metallurgy are :
(i) Preparation of powder
For making powder from a metal, it is extracted from its ore or converted into an oxide. There are many methods for manufacturing powders. Some of the common processes are
- Mechanical Pulverisation
- Electrolytic process
- Chemical Reduction
- Atomization
Atomization is mainly used for self-lubricating bearings. In this method the molten metal is forced through a small orifice and broken up by a stream of compressed air. This process is used for metals like tin, zinc, lead, aluminium, cadmium etc.
(ii) After powder is made, it is washed before grinding and then grinding is done.
(iii) Blending of Powder or Mixing : If only one metal powder of correct
particle size distribution is being used, no blending is required. But blending becomes essential when different metal powders are used or when non-metallic particles are added to impart some specific properties.
(iv) Pressing or Compacting or Briquetting of Powder : It is the process of converting loose powder into a green compact of accurate shape and size. It is done in steel dies and punches.
(v) Presintering : Presintering means heating the green compact to a temperature below the sintering temperature. It is done to increase strength of green compact and remove the lubricants and binders added during blending.
(vi) Sintering : Sintering of briquetted parts is done in large continuous furnaces having controlled atmosphere for protection against oxidation and other chemical reactions. The main objective of Sintering are
- Achieving high strength
- Achieving good bonding of powder particles
- Producing a dense and compact structure
- Producing parts free of oxides etc.
and some secondary processes are
(i) Sizing (ii) Coining (iii) Machining
(iv) Infiltration (v) Impregnation (vi) Plating
(vii) Heat treatment