Title of Invention

"POLYMER BASED PADS FOR LARGE THRUST BEARING OF HYDROGENERATORS IN HYDROELECTRIC POWER PLANT"

Abstract

This invention relates to a polymer based pads for large thrust bearing of hydrogenerators in hydroelectric power plant, having improved friction and wear resistance comprising a steel backing (1) consisting of plurality of thrust pads (7) being adhesively bonded (6) to said steel backing, the said pads having an inside diameter of 160 mm and outside diameter of 320 mm and is consisted of PTFE (polytetra fluoroethylene) composite having 40% PTFE, 55% Bronze and 5% graphite, the said pads resulting lower friction than the existing white metal pads (5) during start and stop operation of the generator and eliminating need of any supporting lubrication system. Fig. 4

Full Text

The invention relates to development of a polymer based pads for large thrust bearings of hydrogenerators in hydroelectric power plant. Thrust bearings are the most essential component of large vertical rotating machinery such as hydrogenerators, vertical motors and pumps. In hydrogenerators, the bearings are used for supporting large axial loads due to weight of the rotating parts and the hydraulic load on turbine runner. The bearings are specially designed for specific application, depending on the operating requirement of the machine. These bearings are custom-made for individual and are designed for trouble free operation without wear or loss of efficiency. Presently the material used for thrust bearings is tin based white metal. The tin based alloy is universally used for the bearing lining because of its excellent anti-seizure property and good thermal conductivity. Also the process of manufacturing and repair are very simple and well established. There are disadvantages associated with the present system of using tin based white metal alloy as bearing lining in hydrogenerator. The main disadvantage with the present system of using white metal is that co-efficient of friction of white metal in boundary lubrication regime is 0.30 which leads to generation of more heat during start and stop. Another disadvantage associated with the present system of using white metal is that the loading pressure is usually restricted to 3.5 MPa, Still another disadvantage with the present system of using white metal is that the softening temperature is low, therefore the operating temperature is limited to 90°C, Yet another disadvantage associated with the present system of using white metal is that the hydrogenerators, where these thrust bearings are used are subject to frequent 'start' as and "stop' operations,as these bearings operate in boundary and mixed lubrcation regime. Due to high friction of traditionally used bearing with white metal lining, during such lubrication regimes, a significant amount of heat is generated which maydamage the bearing. To overcome this problem, the bearing is hydrostatically lubricated during these transient operations. Further disadvantage associated with the present system of using white metal is that when the bearing operates under boundary lubrication regime during starting the machine, when rotor is not jacked up before starting the asperities contact weld together and break away due to relative motion. As the coefficient of friction is high and consequent generation of heat is higher, both friction torque and wear of the metal surface are higher. The very short period of such heat generation can cause damage to the bearing during immediate start. During this stage, friction torque is highest•To reduce this friction, a separate oil system is needed to provide hydrostatic support and contact between the mating surface is eliminated. Still further disadvantage associated with the present system of using white metal is that when the hydrogenerator is stopped, the water flow in the turbine runner is also stopped and the rotor continues rotating due to inertia. The operational speed reduces slowly because the friction is low in presence of the oil film between the bearing and runner of the rotor, The rotor is permitted for coast down till it reaches a sufficiently low speed at which the oil film still exists. At this speed, the brakes are applied to stop the rotor quickly. During this stage the asperity contact starts and large amount of heat is generated on the brake track and the bearing-runner surfaces. The large amount of friction and wear take place. Hence the machine is allowed to cool down for sufficiently large period before starting it again. Thus the machine cannot be restarted and used for the long period, typical value being 10 hours. In this stop phase also, Jacking is done before the brakes are applied when the speed of the rotor reaches sufficiently low to reduce the friction and wear of the bearing. Thrust bearing pad with elastic metal plastic coating (EMPC) developed by Russian to improve service performance and reliability of hydrogenerator. This pad consists of an upper surface and is a plate made of fluorine plastic (Teflon) with wire bronze spiral soldered to the steel backing. Russians claimed that the pads with EMPC have a number of advantages in composition with the white metal coating. The main disadvantage of this material is that it is poor in thermal conductivity. Also from fundamental information available, pure PTFE (Teflon) will have higher wear rate in comparison to to PTFE composition. Therefore, white metal has to be replaced by a material having lower coefficient of friction and better wear resistance. If this material is better in thermal conductivity, it is a further advantage. Therefore, the main object of the present invention is to provide a polymer composite pads for large thrust bearing with a new surface material which has low coefficient of friction and better wear resistance as compared to the white metal (Babbitt) thrust bearing. Another objective of the present invention is to provide a polymer pad for large thrust bearing which metal has higher load carrying capacity. Yet another objective of the present invention is to provide a polymer thrust bearing for hydrogenerator which is easier to manufacture, require less maintenance and does not require scraping of the surface. Further objective of the present invention to provide a polymer pads for large thrust bearing for hydrogenerator which will operate effectively under boundary lubrication regime encountered during start and stop operations of the hydrogenerator. Using these polymer pads for thrust bearing in hydrogenerator, during start and stop operation, the hydrostatic lubrication will not be needed and a separate high-pressure oil system to supply pressurized oil between the bearing and runner will not be required. According to the present invention there is provided polymer based pads for large thrust bearing of hydrogenerators in hydroelectric power plant, having improved friction and wear resistance comprising a steel backing consisting of plurality of thrust pads being adhesively bonded to said steel backing, the said pads having an inside diameter of 160 mm and outside diameter of 320 mm and is consisted of PTFE (polytetra fluoroethylene) composite having 40% PTFE, 55% Bronze and 5% graphite, the said pads resulting lower friction than the existing white metal pads during start and stop operation of the generator and eliminating need of any supporting lubrication system. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings. Figure 1 shows thrust bearing assembly. Figure 2 shows the segmental pad with babbit lining (existing material) as per prior art. Figure 3 shows the EMPC thrust pad of Russian development Figure 4 shows the polymer thrust pad as per present invention. According to the present invention the thurst bearing consists of polymer pads comprising of a selected bronze/graphite filled PTFE ( polytetrafluorothylene) material as bearing lining which is bonded by adhesive to steel backing. Bronze/graphite filled PTFE has been selected on the basis of friction and wear evaluation. A two millimeter thick layer of the polymer is then adhered to the steel backing. The adhesive used for bonding was selected based on bond strength for both adhesive-bonded polymer lining and white metal lining. Polymer lining pads have been tested in a simulated thrust bearing test rig. The comparisons of the coefficient of friction in low speed boundary regime clearly demonstrate that the polymer layers have friction white metal which is about 40 - 50% lower than that of the white metal(babbit) layers. The polymer pads for large thrust bearing of the present invention consists of a new developed low friction polymer PTFE based composite (55% bronze + 5% graphite + 40% PTFE) adhesively bonded to steel backing. This PTFE composite material for large thrust bearing lining is provided to avoid the problems during start and stop operations. Such as a separate oil system to provide hydrostatic support is not necessary during these transient operations. The present system of pads for large thrust bearings is shown in Figure 1 and 2. Figure 1 shows assembly of thrust bearing with 8 segmental pads (5) of equally spaced gap between the pads (5). Figure 2 shows a white metal (4) (existing material) lined thrust pad (4) with inner radius (Ri) of 80 mm and outer radius (R0) of 160 mm. Figure 2 shows presently used material for the thrust pad which is white metal. This white metal is tin based alloy and grade-84 is used for the thrust bearing. The coefficient of friction of this white metal in boundary lubrication regime is 0.3. Such pads (4) for thrust bearings present problems during starting and stopping due to high friction of white metal and need oil system to provide hydrostatic support. Figure 3 shows the thrust bearing pad with elastic-metal-plastic coating (EMPC) (3) developed by Russian. This pad consists of an supper surface which is a plate made of fluorine plastic sheet and a wire bronze spiral (2) is soldered to steel (1) backing. In view of the above shortcomings, a new PTFE composite lined pad (7) for large thrust hearing with the following improvement is proposed in this invention as shown in Figure 4. The new polymer pad (7) consists of PTFE composite (40% PTFE + 55% bronze + 5% graphite) layer adhesively bonded (6) to steel (1) backing as shown in Fig. 4. Both white metal lined thrust bearing and new developed PTFE composite lined thrust bearing of identical dimension of identical were made and tested in a simulated thrust bearing test rig T under slow speed boundary lubrication regime, the friction coefficient, wear and temperature rise of the bearing materials were evaluated at different operating conditions like loads and speeds when the results were at different operating conditions like loads and speeds, when the results were compared for both type of bearings under similar operating condition, tt clearly demostrate that the PTFE composite lined bearings have a coefficient of friction which is about 40-50% lower than that of white metal lined bearing It was also observed on the basis of surface roughness and measurement of scratch depth before and after the test that the wear is lower with the PTFE based composite bearing. The proposed polymer pads (7) for large thrust bearing can replace the white metal pads (5) for large thrust bearing. It is considered that the proposed invention will result in lower friction during start and stop, and eliminate need for a hydrostatic lubrication system. Other important characteristic of the polymer pads made of PTFE composite are: i) low coefficient of friction within the limits of 0.12-0.16 with dry friction against steel and 40-50% lower than white metal pads under slow speed boundary lubrication. ii) 3-4 times higher wear resistance than that of white metal pads under similar ON-load period, iii) PTFE composite pads allow a specific pressure of about two times than that of white metal pads having a specific pressure of 3.5 MPa. iv)Due to low friction and better wear resistance of this new PTFE composite pads the thrust bearing will work satisfactorily during start and stop operations of the machine and eliminate the need for use of the hydrostatic support in such transient operations. The invention described hereinabove is in relation to a non-limiting embodiments and as defined by the accompanying claims. We Claim; 1. Polymer based pads for large thrust bearing of hydrogenerators in hydroelectric power plant, having improved friction and wear resistance comprising a steel backing (1) consisting of plurality of thrust pads (7) being adhesively bonded (6) to said steel backing, the said pads having an inside diameter of 160 mm and outside diameter of 320 mm and is consisted of FTFE (polytetra fluoroethylne) composite having 40% PTFE, 55% Bronze and 5% graphite, the said pads resulting lower friction than the existing white metal pads (5) during start and stop operation of the generator and eliminating need of any supporting lubrication system. 2. Polymer based pads as claimed in claim 1 wherein said pads of PTFE composite has low coefficient of friction of 0.12 to 0.16 with dry friction against steel which is about 40-50% less of present white metal pads and the said pads have better wear resistance of 3 to 4 times over white metal pads. 3. Polymer based pads for large thrust bearing as herein described and illustrated.

Documents:

892-del-1999-abstract.pdf

892-del-1999-claims.pdf

892-del-1999-correspondence-others.pdf

892-del-1999-correspondence-po.pdf

892-del-1999-description (complete).pdf

892-del-1999-drawings.pdf

892-del-1999-form-1.pdf

892-del-1999-form-19.pdf

892-del-1999-form-2.pdf

892-del-1999-form-3.pdf

892-del-1999-gpa.pdf

892-del-1999-petition-138.pdf

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