Review: IPMD 14th Edition 2010-2011, 5 pages, 2462 words

Author:  Dr Enrico Mosca, Consultant, Italy

The advent of electric motors in the early years of the 20th century led to the development of oil-impregnated, self lubricating PM bronze bearings which would last the life of the motor without any maintenance. The following review outlines the functions of PM self-lubricating bearings, the wider range of materials now being used, and their properties. Case studies give some examples of applications.

Introduction

Self-lubricating bearings were the first PM components to be mass-produced. Since their introduction, in the beginning of the 1920’s, they represent by far the highest volume part output of our PM industry: every year billions of oil-impregnated PM bearings find their applications in mechanical and electromechanical devices, where sliding processes, especially rotary motions, are occurring.

When two solids slide one against the other phenomena like friction and wear must be controlled, otherwise it becomes impossible to predict the life of the coupling. The solution is the introduction of a gaseous, liquid or solid lubricant between the surfaces: friction and wear are reduced, and liquids can carry away both frictional heat and wear debris generated during the sliding process.

Liquid and solid lubrication represent the most common way to avoid direct contact between mating surfaces. From the functional point of view porous PM bearings represent an exceptionally appealing solution: pores are filled with suitable lubricants and act as oil reservoirs in service, so that the supply of lubricant in the clearance between sliding surfaces is guaranteed for the entire life: the device becomes maintenance-free.
This approach is different from that of solid bearings, where lubricant has to be supplied in some way from outside, i.e. by dripping, use of pressurised circuits, etc.

Composite PM bearings represent another interesting solution: in this case solid lubricants are present in the structure, so that during sliding they form a film which minimises solid-solid friction and controls all tribological functions.

The mode of functioning of PM porous bearings may be summarised as follows: when the shaft is at rest all the oil is retained in the pores and the load is sustained by direct metallic contact between the two surfaces. At the beginning of movement the direct metallic rubbing between bearing and journal with its high friction coefficient raises the temperature rapidly.

As the expansion coefficient of the oil is higher than that of the alloys used for the bearings by an order of magnitude, the oil is pushed out of the pores and, as rotation continues and load and speed are kept within certain limits, it almost instantaneously forms a thin film, which prevents further metallic contact................

Further sections of this article include:

• Bearing  compositions
• Production sequence
• Properties and quality testing
• Choice of material, porosity and lubricant
• Design and tolerance specifications
• Application examples
• CASE STUDY 1 Fuel pump bearings with improved corrosion resistance to impure fuels
• CASE STUDY 2 Sintered aluminium alloy sliding bearings
• CASE STUDY 3 PM Bearing Impregnated with Magnetic Fluid

Figures and Tables:

Fig. 1  Operating principle of PM porous bearings.

Fig. 2  Stribeck curve showing relation of friction coefficient to shaft speed.

Fig. 3  Pressure distribution of lubricant in PM bearings

Fig. 4  Mechanical parameters in designing PM bearings

Fig. 5  A sintered bearing produced by Porite Corp, Japan, has replaced  an hydrostatic air bearing in a high speed polygon scanner motor operating at over 40,000 rpm.   (JPMA Award Winner 2006)

Fig. 6  Hitachi Powdered Metals Ltd received a JPMA Award in 2006 for this PM thrust bearing used in a copier machine where the side porosity of the bearing has been closed by ultrasonic welding..

Fig. 7  Thick-wall self-lubricating bearings are fully compacted PM bearings containing an evenly distributed solid lubricant (graphite, MoS2, WS2, etc) in the matrix. These bearings are capable of dry running at low sliding speeds and high loads in applications such as foundries and steelworks, furnaces, waste water cleaning plans, water, steam and gas turbines, pump and compressors, machinery, etc.  Bronze and lead bronze are used for applications up to 350OC and Fe- and Ni-base alloys above this temperature. (Photo courtesy Federal-Mogul Deva GmbH, Germany)

Table 1 Properties of PM porous bearing materials