European Powder Metallurgy Association

* 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.