821047 to Off-Highway Equipment COPVRIGHT. SOCIETY OF AUTOMOTIVE Richard C. Perlberg and James H. Cooper, Jr. ENGINEERS, INC. Garlock Bearings, Inc. ABSTRACT The usage of self-lubricated and pre- lubricated bearings is growing in many- industries. The necessary parameters for specific application of these bearing types will be examined and how each effects the bearing operating life. A method will be presented that will enable the design engineer to choose a bearing material and subsequently prescribe lubricating intervals and wear life. A review of maintenance procedures will also be outlined in comparison to conventional type bearings with primary concentration on pinned joints and other highly loaded joints. MOST APPLICATIONS FOR off-highway, earth moving equipment, agricultrural and mining equipment external to the power train are pivot points for hydraulic cylinders and control linkages. These move at a very low speed, change direction on the return cycle and have short arc movements. Bearing heat generated is considered inconsequential, although, at times, the pinned joint is located close to a prime mover and input heat may be experienced. These applications may be handled as special cases. There are four basic questions that plague equipment designers today concerning self- lubricated bearings and they are why? when? where? how? While the answers to these questions might overlap in certain areas, it is intended here to answer these questions and give direction to the designer on usage of self-lubricated bearings. The following concepts will be confined to journal type plain bearings. Of the many types of bearings that can be considered for pinned joints, two specific types become extremely useful. First the self-lubri­ cated bearing and secondly the pre-lubricated bearing. Being self-lubricated means the bearing provides its own lubricate from within the composite and does not require external lubrication. The pre-lubricated journal bearing is one that is pre~wiped with a grease upon initial installation. It is a form of self- lubricating bearing since it has its own lubricant in the liner, but may require relubri- cation at some extended interval. THE CONCEPT Quite often, applied loads, running speeds, temperatures and desired unit life are known, however, the designer must be aware of the concept of the self-lubricated bearing and why it should be considered for the application: when it should be considered, where it can be applied, and finally how to use a self-lubricated bearing. The first concept to be aware of is that the bearing must wear slightly itself in order to lubricate itself. The method that a self- lubricated bearing uses to lubricate itself is essentially the same for most products, however, the capability of the different materials widely varies. Sliding bearings can be classified according to their method of lubrication: 1. hydrostatic 2. hydrodynamic 3. boundary lubricated (oil/grease) 4. dry film lubricated 5. self-lubricated Lubrication is the prime influence on the load carrying capacity, and also the bearing life, of a conventional metal sleeve bearing. If an oil film of sufficient thickness can be maintained between the bearing's surface and a loaded rotating shaft no damage will occur at either interface. This is hydrodynamic lubrication where the load capacity of the bearing is related to and governed by the fluid viscosity and speed of the shaft to generate an oil film. If external pressure is added to the bearing, the oil film 23 Downloaded from SAE International by North Carolina State Univ, Monday, September 17, 2018becomes pressurized and will lift the shaft away from the bearing surface and the shaft will rotate on a pressurized oil film. This is hydrostatic lubrication. The above two classes do not lend themselves to pinned joints. Under inadequate lubricating conditions, the oil or grease film becom