Patent Application
20090038739
The present invention relates to a method and system for repairing a part having a damaged dry lubricant layer. More specifically, the present invention relates to using a high-temperature polymeric adhesive to replace a dry lubricant layer that is bonded to a composite or metal part used in a gas turbine engine.
Certain fabrics may be used to provide a dry lubrication system for two contacting surfaces, commonly metals, that may otherwise wear and/or have a high coefficient of friction. For example, a fabric containing polytetrafluoroethylene (PTFE) may commonly be attached to various metal parts used in gas turbine engines. An adhesive layer is used to bond the fabric to the metal surface. During service, the fabric may become worn and require replacement.
In some cases, the dry lubricant fabric may be used in an area of the engine, such as the high pressure compressor (HPC), which operates at high temperatures. Consequently, it may be important that the selected fabrics and adhesives be durable at an operating temperature of the part to which they are bonded. For areas of the engine that have lower stress, it is common to use a dry lubricant film to provide lubricity and protect against wear. The films, which may include PTFE and polyimide, are bondable to the surface of the part by the same techniques as the fabrics.
Most of the high-temperature adhesives historically used contain hazardous materials prior to cure and require extensive measures for safe usage. There is a need for an improved method and system of replacing the lubricant layer using less hazardous adhesives.
The present invention relates to a method and system for repairing a part of a gas turbine engine that has a damaged or worn lubricant layer bonded to a composite or metal surface of the part. The lubricant layer covers the surface of the part and provides lubricity to reduce wear and reduce the coefficient of friction between the part and a second surface that the part contacts. The method includes removing the damaged or worn lubricant layer. A polymeric adhesive is used to bond a replacement dry lubricant layer, which may be a fabric or a film, to the surface of the part. The polymeric adhesive is durable at the operating temperature of the part. The adhesive may be applied as a film or a paste, and in some embodiments, may include a non-metallic filler.
A method is described herein for repairing a part of a gas turbine engine that has a dry lubricant layer bonded to a surface of the part. The dry lubricant layer may be a fabric or a film, and is configured to provide lubricity between the part and a second surface that the part contacts. The part is formed from metal or a composite, and may commonly be located in an area of the engine that operates at high temperatures. Over time, the dry lubricant layer may become damaged or worn. As described herein, a less hazardous polymeric adhesive, which is stable at an operating temperature of the part, may be used to attach a replacement lubricant layer to the surface of the part. The polymeric adhesive may be a paste adhesive or a supported film adhesive.
High pressure compressor (HPC) 18 includes a plurality of stages. Each stage has a row of blades and a row of stator vanes (not shown in
Centralizing pad 34, as better shown in
During service, fabric layer 40 will become worn or damaged and require repair or replacement.
Damaged fabric layer 40 may originally be attached to bottom portion 38 using a high-temperature adhesive, due to the extreme operating temperatures within HPC 18. Historically, in some cases, adhesives that were used for bonding fabric layer 40 to bottom surface 38 may be hazardous. A method and system is disclosed herein for replacing damaged fabric layer 40 using less hazardous materials.
In the exemplary embodiment described herein, lubricant layer 40 is a fabric layer. It is recognized that a dry lubricant film may also be used, particularly for lower stress areas. The dry lubricant films are bondable to the surface of the part using the same techniques described herein for fabric layer 40 and provide a similar function. These films may commonly include integral fillers, which provide greater compressive stability. Examples include, but are not limited to, a PTFE film or a thermoplastic polyimide film, such as Kapton® polyimide film from Dupont or Upilex® polyimide film from UBE Industries.
Method 50 includes steps 52-68, and begins with removing the original, damaged or worn fabric layer (step 52) from the bottom of the centralizing pad (see
Step 56, which is applying a primer to the metal surface, is optional and is included in method 50 if a replacement fabric is not going to be bonded to the metal surface within approximately two hours after step 54. In step 56, suitable primers may include, but are not limited to, solvent reductions of the adhesive resin system. The primer may then be dried and cured. Next, in step 58, the replacement fabric is prepared, which may include cutting an appropriately sized piece of fabric to fit the pad. It is preferable to cut an oversize piece of fabric and later trim the fabric (step 66) after it is bonded to the metal surface of the pad. Prior to bonding the fabric to the metal surface (step 62), a mask is created around the bond area (step 60), using, for example, a Kapton®-backed flash break tape or other types of approved maskants.
At this point, the metal surface of the centralizing pad is ready for receiving a replacement fabric. The replacement fabric is bonded to the metal surface (step 62) using a polymeric adhesive, such as a film adhesive and/or a paste adhesive, as described in further detail below in reference to
After bonding the replacement fabric to the metal surface, a next step is to cure the adhesive (step 64). In step 66, excess fabric is trimmed off. Although step 66 is shown after curing (step 64), it is recognized that the excess fabric may be removed prior to curing. Finally, in step 68, the repaired centralizing pad is inspected for various criteria.
In addition to metal parts, method 50 also may be used for composite parts that have a dry lubricant layer bonded to a surface of the composite. The composite part may be formed of high temperature materials, which may include, for example, polyimide. In those cases in which the part is a composite, some of the steps in method 50 may be altered or omitted. For example, surface preparation (step 54) of the composite may be different than the process described above for a metal part as will be understood by those skilled in the art.
As stated above, as an alternative to a fabric, a PTFE film or a polyimide film may be used as the dry lubricant layer. In both cases (fabric or film), it is necessary to make the surface of the lubricant layer more bondable to the surface of the part (i.e. raised portion 38a of pad 34). In the case of replacement fabric 70, the fabric may include a more bondable material, such as glass or polyaramid (for example, Nomex® meta-aramid fibers from Dupont), as part of the fabric architecture. For the PTFE or thermoplastic polyimide films, the surface may be etched prior to attaching the film to raised portion 38a.
In order to bond fabric layer 70 to raised portion 38a, a polymeric adhesive is attached to raised portion 38a. The polymeric adhesive may be a film, as shown in
Film adhesive 72 is attached as a sheet to raised portion 38a and is used to bond replacement fabric 70 to raised portion 38a. In some embodiments, it may be preferred that film adhesive 72 has a minimum thickness. However, in applying a thin adhesive layer, it may be difficult to ensure a uniform thickness across raised portion 38a. An advantage of using film adhesive 72 is an ability to better control the cured thickness of the adhesive, particularly in those embodiments in which film adhesive 72 is a supported film adhesive.
Prior to placement of film adhesive 72 on surface 38a, film adhesive 72 is partially cured, such that flow of the adhesive at room temperature may be limited. Moreover, a film adhesive is premixed and mitigates mixing error, and is typically easier to apply; thus it can minimize voids or entrapped air between raised portion 38a and adhesive 72. For film adhesives, the processing requirements to meter out the material and calendar the film limit the minimum economical batch size, thus limiting the variations of filler material available.
In some embodiments, film adhesive 72 is a supported film adhesive, which aids in handling of the adhesive. In one embodiment, film adhesive 72 may include a scrim support that is made, for example, of fiberglass, which is also able to withstand high operating temperatures. The support aids in processing of the film and in controlling an adhesive thickness. Other examples of suitable support materials include, but are not limited to, carbon fiber, nylon, polyester, and other low-density or non-woven materials. The particular support material is chosen, in part, based on the type of film adhesive to be attached to the support, and the temperatures that the support will be exposed to.
The scrim side of film adhesive layer 72 contacts the surface of raised portion 38a. In some embodiments, at room temperature, film adhesive 72 may have a low enough viscosity such that when adhesive 72 is compressed by fabric 70, a portion of adhesive 72 may flow across the surface of portion 38a. As such, the thickness of adhesive 72 may not remain uniform across portion 38a. The scrim support may be used to control flow of the adhesive and ensure a minimum and uniform thickness of adhesive layer 72.
A manufacturing process for supported film adhesive 72 is similar to that of a pre-impregnated material (i.e. prepreg) in which resin is impregnated into a fiber matrix and then cured. For a supported film adhesive like adhesive 72, a liquid resin is applied to the scrim support to form a sheet of film adhesive. In some embodiments, the resin is then staged to remove some of the solvent from the resin. The supported film adhesive is commonly rolled up and stored at low temperatures. The supported film adhesive may also include a carrier layer, such as polyethylene, that is removed just prior to placement of adhesive film 72 on the surface of raised portion 38a.
Film adhesive 72 includes polymeric adhesives that are stable at an operating temperature of centralizing pad 34, which may operate at temperatures ranging from 300 to 650 degrees Fahrenheit. Polyimide adhesives are well-suited for centralizing pad 34, since they are able to resist wear and withstand the high temperatures in the HPC. Both addition-formed polyimides and condensation polyimides may be used for film adhesive 72. In some cases, the condensation polyimides may have a higher thermal oxidative stability, as compared to the addition polyimides.
One example of a suitable polyimide film adhesive is FM 680 from Cytec Industries, which is stable at temperatures up to approximately 650 degrees Fahrenheit (343 degrees Celsius). FM 680 is derived from the Avimid-N family from Cytec Industries and is a condensation polyimide with high thermal oxidative stability. Another suitable polyimide film adhesive is FM 57 from Cytec Industries, which is stable at operating temperatures up to approximately 450 to 550 degrees Fahrenheit (232 to 288 degrees Celsius). FM 57 is a condensation polyimide and is derived from the Avimid-R family from Cytec Industries. The particular polyimide film adhesive used may vary depending, in part, on the particular repair application and the range of the operating temperature of the part. Other polymeric film adhesives with a thermal oxidative stability comparable to polyimide may also be used for film adhesive 72.
In other embodiments, film adhesive 72 may include bismaleimide (BMI) film adhesives and cyanate ester film adhesives. Bismaleimide (BMI) is stable at temperatures up to approximately 425 degrees Fahrenheit (218 degrees Celsius). Examples of BMI film adhesives are FM 2550 from Cytec Industries and HP655 from Hexcel. Cyanate ester is stable at temperatures up to approximately 500 degrees Fahrenheit (260 degrees Celisuis). An example of a cyanate ester film adhesive is FM 2555 from Cytec Industries. Cyanate ester films are also available from YLA Inc.
Similar to film adhesive 72, paste adhesive layer 174 is a polymeric adhesive that is stable at an operating temperature of centralizing pad 34. As compared to film adhesives, a batch size for paste adhesives may be much smaller, thus resulting in a greater range of available commercial products with metallic and non-metallic fillers. Additionally, an advantage of paste adhesives is that they usually have a longer storage life, compared to film adhesives which are premixed and then frozen to retard the reaction rate. A second advantage of using paste adhesives in repair methods is that corrosion pits and tool marks in the substrate may be filled concurrent with bonding.
Paste adhesive layer 174 may be formed from polyimide, which is stable at high operating temperatures (up to approximately 650 degrees Fahrenheit or approximately 343 degrees Celsius). Suitable polyimides for paste adhesive 174 include, but are not limited to MVK-19, AFR-PE-4 and BIM from Maverick Corporation. In other embodiments in which centralizing pad 34 operates at a lower temperature, other suitable polyimide adhesives include, but are not limited to, bismaleimide (BMI) and cyanate ester. As stated above, bismaleimide (BMI) is stable at temperatures up to approximately 425 degrees Fahrenheit (218 degrees Celsius), and cyanate ester is stable at temperatures up to approximately 500 degrees Fahrenheit (260 degrees Celsius). Moreover, all of the disclosed paste adhesives may be used for the repair of other parts in the engine, and the particular adhesive selected is based on the operating temperature of the part.
Paste adhesive layer 174 uses a non-metallic filler. By using a non-metallic filler, polymeric adhesive 174 avoids any galvanic interaction between metal in the paste and the metal surface of raised portion 38a. Using a non-metallic filled adhesive, corrosion of a metallic-filler at the exposed surface between the fabric tows may be avoided. Suitable fillers that may be used in paste adhesive 174 include, but are not limited to, silica, thermoplastics and other commercially available products that thicken and limit flow of the paste. In some embodiments, film adhesive 72 of
In order to bond fabric layer 170 to surface 38a, polymeric paste adhesive 174 is applied to a fiberglass-rich side 170a of fabric layer 170. A thin layer of paste adhesive 174 may also be applied to surface 38a. Once applied, a thickness of paste adhesive layer 174 may be between approximately 10 mils and approximately 20 mils. A thickness of layer 174 may be minimized, in some embodiments, to prevent cohesive failure.
As described above in reference to method 50, after bonding fabric layers 70 and 170 to metal surface 38a (step 62), a subsequent step is to cure the polymeric adhesive.
The replacement of a fabric or film layer using a polymeric adhesive is described herein in the context of a centralizing pad, which is designed to keep a synchronization ring of the high pressure compressor centered concentrically to the casing of the turbine engine. The fabric or film layer provides a lubrication surface between the contacting surfaces of the centralizing pad and the casing. The disclosed polymeric adhesives are less hazardous than other materials that may be used, yet are well-suited for withstanding high temperatures within the high pressure compressor.
It is recognized that the method and system described herein for using polymeric adhesives to attach a dry lubricant layer to a metal surface may apply to other parts of the turbine engine that have contacting surfaces that may require or benefit from a dry lubrication layer located between them. As one example, a fabric layer, similar to those described above, may be used to provide lubricity between a bushing and a vane stem extending from the shroud to the outer casing of the engine. Another includes the rub surfaces of articulating linkage arms/bars. In addition to polyimide, other non-hazardous polymeric adhesives are disclosed herein, which may be used in other areas of the gas turbine engine for bonding a lubricant layer to a metal or composite part.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
