The advantage of porous bearings is
- that the pores can be filled with lubricating oil.
- so that the bearing requires no further lubrication during the whole life of the machine in which it is used.
This feature accounts for the use of the term 'self-lubricating bearings ', and typically these are made of bronze:
- Starting materials may be mixed elemental powders of copper and tin , fully pre-alloyed bronze powder, or mixtures of the three.
- If pre-alloyed bronze is used shrinkage on sintering normally results, while with elemental powder a significant growth can be obtained.
- Clearly it is possible by using the appropriate proportion of pre-alloyed bronze to achieve a situation in which the dimensions of the finished part are approximately the same as those of the die.
- In this way the compacting tools may be used for the sizing operation which is essential with bearings in order to 'true-up' the part.
- In addition to the normal lubricant graphite is often added to the mixes used for porous bronze bearings.
- This is thought to assist in pore formation and, in addition, it acts as a lubricant in its own right.
It is essential that the pores form an interconnected system of controlled size and volume, so that oil is supplied to the entire bearing surface.
- The rate of oil supply automatically increases with temperature and, therefore, with increasing speeds of rotation, to achieve optimum working conditions.
- For most applications, the porosity should lie between 20% and 35% by volume, the upper limit being imposed by strength considerations which varies inversely with its porosity.
- Self-lubricating bronze bearings were among the first PM products, and are still essential in many applications.
- Additionally porous bearings made of iron, iron copper , or iron mixed with bronze may be used in non-critical applications, but 90/10 bronze is the most widely used, having a low coefficient of friction, high wear resistance, and also resistance to seizure with the steel shaft.
The processing steps are compacting at between 150 and 230MPa, sintering at approximately 800°C for between 5 and 10 minutes, followed by sizing and oil-impregnation.
- Very high production rates are possible because of the short sintering times required.
- Iron-based bearings have the advantage of greater strength , and in addition their coefficient of thermal expansion is of the same order as that of the steel shaft.
- Compacting is carried out between 230 and 380MPa, followed by sintering in combusted gas atmospheres or cracked ammonia at 1100°C to 1150°C for approximately 30 to 45 minutes, sizing and impregnation.
Non Porous Bearings
Bearings of copper/lead or copper/tin/lead (leaded bronze) made from powder.
For heavy duty applications backed bearings of copper/lead or copper/tin/lead (leaded bronze) made from powder are widely used.
- Large-scale production is carried out under continuous processing conditions, whereby the powder is loose-sintered onto a steel backing-strip, rolled to a density approaching theoretical and to the required thickness, and finally annealed.
In addition to the economic advantage of this production method in comparison with cast alloys, a superior microstructure and better properties are obtained.
- Lead, although soluble in molten copper and bronze, is practically insoluble in the solid metal.
Thus when such alloys are made as castings the lead is rejected and appears as a coarse, non-uniformly distributed second phase.
- In the production of powders, which is done by atomization, the droplets are cooled very rapidly so that a uniform distribution of very fine lead inclusions results.
For this application spheroidal powders are used because they are more-free flowing and pack to a higher density than irregular particles.
The demand for materials that can operate under conditions where the presence of oil or grease is unacceptable, has led to the development of the dry bearing.
The PTFE bearing is an outstanding example in this class.
Polytetrafluorethylene (Fluon or Teflon) is a polymer of carbon and fluorine, which has an extremely low coefficient of friction and is stable up to 300°C under almost all corrosive and oxidising conditions.
- It is possible to overcome the inherent disadvantages of massive PTFE namely low strength, poor heat conductivity, and high thermal expansion, by incorporating a metal powder in the plastic.
Bronze is normally used in this application.
- Alternatively a porous metal bearing, generally bronze, may be impregnated with PTFE. This applies especially to steel-backed bearings.