|Title of Invention||
"A PROCESS FOR PRODUCING THRUST BEARING OF POLYTETRAFLUOROETHYLENE (PTFE) BASED COMPOSITE PADS".
|Abstract||This invention relates to a process for producing thrust bearing of polytetrafluoroethylene (PTFE) based composite pads comprising bronze/graphite filled polytetrafluoroethylene material as bearing lining characterized by combined adhesive and mechanical bonding under heat and pressure under compression moulding to steel backing having dovetail grooves.|
|Full Text||FIELD OF INVENTION
This invention relates to a process for producing thrust bearing of polytetrafluoroethylene (PTFE) based composite pads. The pads of the present invention may be example be employed for large thrust bearings of hydrogenerators in hydroelectric power plant.
BACKGROUND OF INVENTION
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. These bearings are specially designed for specific application, depending on the operating requirement of the machine. These bearings are custom-made for individual machine and are designed for trouble free operation without wear or loss of efficiency.
Presently the material used for trust bearings is tin based white metal. The white metal is tin based alloy universally used for the bearing lining because of its excellent anti-seizure property and good thermal conductivity. Also the processes of manufacturing and repair are very simple and well established.
These are disadvantages associated with the present system of using tin based white metal alloy as bearing lining. The main disadvantage is that white metal is of poor mechanical strength and its softening temperature is low.
Another disadvantage associated with the present system of using white metal is that the loading pressure is usually restricted to 3.5 MPa and temperature to 90°C.
Still another disadvantage associated with the present system of using white metal is that coefficient of friction of white metal in boundary lubrication regime is 0.30.
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' and 'stop' operations, resulting in operation under boundary and mixed lubrication 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 may damage the bearing. The wear of the metal surface will also be higher in the start-up period to overcome this problem, the bearing is hydrostatically lubricated during these transient operations.
Further disadvantage with the present thrust bearing material is when the hydrogenerator is stopped. During this stop phase, when the water flow in the turbine runner is also stopped the rotor continues to rotate due to inertia. The operational speed reduces slowly because the friction is low due to the presence of the oil film between the bearing and runner of the rotor. The rotor is permitted to 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 resulting in high friction and wear. Hence the machine is allowed to cool down for sufficiently long period before starting it again. Thus the machine cannot be restarted and used for a long period, typical value being 10 hours.
Thrust bearing pad with elastic metal plastic coating (EMPC) were developed in Russia to improve service performance and reliability of hydrogenerator. This pad consists of an upper surface plate made of fluorine plastic sheet (Teflon) and a wire bronze spiral is soldered to the steel backing. It is claimed that the pads with EMPC have a number of advantages in comparison with the white metal coating. This material is expected to have low thermal conductivity. Also from fundamental information available pure PTFE will have high wear rate in comparison to PTFE composites.
Improved bearing material has been described in patent entitled "POLYMER BASED PADS FOR LARGE THRUST BEARINGS" (892/Del/99) was filed in June 1999. In the aforesaid patent application the polymer material was adhesively bonded to steel backing. Bronze and graphite filled PTFE has been selected on the basis of friction and wear evaluation. A two millimeter thick layer of the polymer was then adhered to the steel backing. The adhesive used for bonding was selected based on a comparison of bond strength of 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 with white metal bearing clearly demonstrate that the polymer layers have friction which is 40-50% lower than that of the white metal layer. Though the adhesive bonding strength is sufficient for successful functioning of the thrust bearing, combined adhesive and mechanical bounding is considered to enhance further the bonding strength.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide further improvement in bonding of the PTFE based composite material lining to the steel backing for manufacture of the thrust bearing pads.
Another object of the present invention is to provide a PTFE based 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 existing white metal (babbit).
Yet another object of the present invention is to provide a PTFE based composite pads for large thrust bearing have better thermal conductivity as compared to EMPC pads referred to earlier.
Still another object of the present invention is to provide a PTFE based composite pads for large thrust bearing for higher load carrying capacity.
DESCRIPTION OF INVENTION
According to this invention there is provided a process for producing thrust bearing of polytetrafluoroethylene (PTFE) based composite pads comprising bronze/graphite filled polytetrafluoroethylene material as bearing lining characterized by combined adhesive and mechanical bonding under heat and pressure under compression moulding to steel backing having dovetail grooves. material was pressed with dovetail grooves steel backing and pressured for one hour.
In this invention further improvement in bonding of the PTFE based composite to the steel backing has been done. After machining the dovetail grooves in the steel backing, the surface was cleaned and then adhesive, binder (Viton, LP-12) was applied with its hardener. After dry out the binder, the steel backing was assembled in a die and then the powder of the bronze and graphite filled PTFE (40% PTFE + 55% Bronze + 5% graphite) with particle size range of 30-50 micron was pressed under 350°C and 375Kg/cm2 pressure under compression moulding. During this compression moulding process the heating was gradually done under the applied pressure of 375 Kg/ cm2 to reach 350°C in the duration of four hours. This temperature was maintained under the same pressure for one hour. After that the heating was reduced to obtain 100°C and maintained at this temperature under similar pressure for one hour. The pressure was then released. The pad was then taken out from the die and it cooled to room temperature for machining. Similar process was followed for bonding all the PTFE based composite thrust bearing pads. After bonding these pads by the above methodology, the thrust pads were machined to the required surface finish. Also the test specimens prepared by this combined adhesive and mechanical bonding were tested for their bonding strength under tension and compression load,
Tests were carried out in the laboratory for evaluating the performance of PTFE based composite thrust bearing developed by adopting the combined adhesive and mechanical bonding methodology. These bearing pads were tested on the thrust bearing test rig under slow speed boundary lubrication and under loads upto 35Kg/ cm2. The starting friction and the temperature rise in the pads under different loads and speeds were found to be by 40-50% in comparison to white metal pads. The results of the laboratory tests on these thrust bearing were found satisfactory.
Also a prototype of PTFE based composite thrust bearing was developed by adopting the above bonding methodology. This indigenously developed prototype PTFE based composite thrust bearing was installed and tested in actual machine of hydrogenerator of 11.25MW at Ganwi Hydro Electric Power Station, Himachal Pradesh. The results are encouraging and the performance of this PTFE based composite thrust bearing is found better than that of the babbit (white metal) bearing.
The nature of the invention and its objects will be apparent from the following description made with reference to non limiting exemplary embodiments of the invention represented in the accompanying drawings:
Fig. 1 shows thrust bearing assembly.
Fig.2 shows the segmental pad with babbit lining (existing material) as per prior art.
Fig.3 shows the EMPC thrust pad of Russian development.
Fig.4 shows the PTFE based composite thrust pads with combined adhesive and
mechanical bonding based on present invention.
In accordance with the present invention, the PTFE based composite thrust pads for large
thrust bearing consist of a low friction polymer of PTFE based composite
(55% bronze+5% graphite+40% PTFE) which is bonded adhesively and mechanically to
the dovetail grooved steel backing. This PTFE
composite material for large thrust bearing is provided to avoid the hydrostatic support needed during start and stop operations when these pads are used. Also the wear and friction of these pads is lower than the white metal.
The present system of pads for large thrust bearing is shown is 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 its geometry showing inner radius (Ri) and outer radius (Ro). This white metal is tin based alloy, grade-84 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 in Russia. This pad consists of an upper surface plate made of fluorine plastic sheet and a wire bronze spiral (2) soldered to steel backing (1).
Figure 4 shows the polymer thrust pads with adhesive bonding described in the aforesaid patent application. This pad consists of a plate of a PTFE based composite material (7) bonded by adhesive (6) to the steel backing (1).
A new PTFE based composite lined pad for large thrust bearing with combined adhesive and mechanical bonding is proposed in this present invention as shown in Figure 5. The PTFE based composite (55% bronze + 5% graphite + 40% PTFE) powder with particle size range of 30-50 micron, was pressed with the dovetail grooved steel backing under the heat of 350°C and pressure of 375 Kg/cm2 for one hour. After cooling to atmospheric temperature, the pads were machined to their required surface finish. The
proposed PTFE based composite thrust pad consists of PTFE composite (40% PTFE + 55% bronze + 5% graphite) (7) with combined adhesive (6) and mechanical bonding to the steel backing (1) having dovetail grooves (8).
The proposed PTFE based composite pad for large thrust bearing with combined adhesive and mechanical bonding can replace the white metal pads 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.
The invention described herein above is in relation to a non-limiting embodiments and as defined by the accompanying claims.
1. A process for producing thrust bearing of polytetrafluoroethylene (PTFE) based composite pads comprising bronze/graphite filled polytetrafluoroethylene material as bearing lining characterized by combined adhesive and mechanical bonding under heat and pressure under compression moulding to steel backing having dovetail grooves.
2. A process as claimed in claim 1, wherein the PTFE based composite pad comprises PTFE, bronze and graphite with particle size of 30-50 micron.
3. A process as claimed in claims 1 to 2, wherein the PTFE composite pad is compressed onto the steel backing wherein the heating was gradually done under a pressure of 375 Kg/cm2 to reach 350°C in four hour, the said temperature is maintained for one hour.
4. A process as claimed in claim 3, wherein the heating is reduced to obtain 100°C temperature.
|Indian Patent Application Number||1161/DEL/2003|
|PG Journal Number||32/2011|
|Date of Filing||17-Sep-2003|
|Name of Patentee||BHARAT HEAVY ELECTRICALS LTD.|
|Applicant Address||SIRI FORT, NEW DELHI-110049, INDIA|
|PCT International Classification Number||F16C 33/00|
|PCT International Application Number||N/A|
|PCT International Filing date|