INTRODUCTION: Metallic (rolling element and sliding element) thrust bearings are commonly used in automotive and industrial applications to support dynamic loads and relative rotation between components. Depending on the specifics of the application (load, speed, temperature, lubrication, etc.), engineered polymeric bearings can be a feasible alternative to metallic bearings and thrust washers. In addition to providing design flexibility and being axially compact (1.5 mm to 2 mm thick), polymeric bearings are less sensitive to surface finish, surface hardness and angular misalignments, and also provide weight/cost savings and damping characteristics. The choice of base polymer and filler system drives the physical, mechanical and tribological properties of the polymer composite. Although considerable work has been done in understanding tribological properties of polymeric materials 1-4, their use in bearing type applications has been limited. PTFE (Polytetrafluoroethylene) is commonly used as a coating on metallic thrust washers to lower friction and wear 5,6, while PEEK (Polyether ether ketone) and PI (polyimide) polymers are finding applications in tilting pad bearings to replace traditionally used Babbitt type alloys 7-9.
In the current work, two fixed profile polymeric bearing designs developed specifically for automotive transmission applications are evaluated for frictional performance using a modified test method that maintains ‘close to isothermal’ conditions at the interface. The Stribeck curves for these tests indicate that these designs operate as fluid film bearings and exhibit friction coefficients in the same order of magnitude as rolling element bearings. Results also indicate that that isothermal Stribeck curves generated at multiple load / speed combinations tend to collapse to one curve which is indicative of the performance of the specific design.
METHODS: Thrust bearings in two patent pending designs (LevitorqTM D4 and LevitorqTM D7) were manufactured using Torlon 4275 polyamide-imide engineering polymer. Torlon 4275 is an injection moldable polymer with desirable mechanical and tribological properties and exhibits good chemical compatibility and a glass transition (Tg) temperature in excess of 280°C. The thrust bearings have an inner diameter of 56.25 mm, an outer diameter of 77.95 mm, a thickness of 1.75 mm and have twelve load supporting pads with 0.3 mm deep grooves in between the pads to allow for flow of lubricant (Dex VI transmission fluid). The difference between the two designs is in the cross-section of the grooves. While the groove profile is constant for the LevitorqTM D4 design, the LevitorqTM D7 has a tapering groove with diminishing cross-sectional area along the radial direction.
The performance of these designs was evaluated using a specialized thrust bearing test stand that was designed, developed and built in-house at Freudenberg-NOK. The test stand is capable of applying an axial load of up to ~22kN (5,000 lbs.) and a has maximum rotational speed of 10,000 rpm. Lubricant can be supplied at a maximum flow rate of ~11 liters/minute (3 gallons per minute) / pressure of 2 MPa (300 psi) and up to a maximum temperature of 150°C (~300°F). The data acquisition system records temperatures, fluid pressure, fluid flow rate, axial thrust force, rotational speed, wear to the bearing and frictional torque. Fluid flow can be arranged either from the ID to the OD of the thrust bearing or vice versa. Tests can be set-up such that a constant fluid temperature is maintained either at lubricant inlet or at the interface (measured 1 mm below the running surface at four locations 90 degrees apart) by varying fluid inlet temperature based on the friction condition at the contact.