SITIS Archives - Topic Details
Program:  SBIR
Topic Num:  AF071-172 (AirForce)
Title:  Improved Bearing Compartment Sealing for Gas Turbine Engines
Research & Technical Areas:  Air Platform, Ground/Sea Vehicles

  Objective:  Develop and demonstrate innovative concepts and solutions for improving gas turbine engine oil wetted bearing compartment sealing quality and performance capability.
  Description:  Since bearing compartment seals are located in the heart of gas turbine engines, low leakage rates over the engine inspection/overhaul intervals are of paramount importance to consistent overall engine thermal efficiency and fuel consumption. Bearing compartment seals must also be durable and meet full engine life at elevated seal operating temperatures, varying seal surface speeds, varying seal differential pressures, and also be compatible with various generations of gas turbine engine lubricating oils, all in the presence of axial shaft motion. Degradation of these seals over time can have a deleterious effect on the life, and, therefore, increase the cost of ownership of turbine engines. Typical bearing compartment seals are carbon based, and carbon is an inherently brittle material, which results in added maintenance cost when seals are broken during assembly or handling. Due to the operating environment and current design methodology, these seals are also subject to housing fretting wear. Nonetheless, carbon seals remain the primary choice because of their lightweight and resistance to wear. This SBIR topic seeks new improved sealing concepts (new materials, coatings, and design configurations). Application should result in new improved shaft-to-ground seals and possibly counter rotating intershaft seals for advanced fighter engines. The ideal outcome after a Phase II effort in executing this topic would be a new validated seal design demonstrating technology capable of being incorporated into advanced fighter engines for bearing compartment sealing. The seal demonstration and validation should incorporate some or all of the following: 1) new configuration concepts, 2) new materials, and/or 3) new surface treatments and or coatings. These should be applied as required to both the seals (currently carbon) and the runners and housings. The housings are titanium and should remain so, although treatments and or inserts may be incorporated. The new seal designs should demonstrate as many of the performance characteristics and improvements listed below as possible. [1] Robust resistance to handling and installation damage. [2] 5,400-hour operational life or greater with minimal wear or fretting of any part of the sealing system. Any validation should include consideration of operation at low humidity altitude conditions. [3] Improved allowable radial clearance motion with a goal of 0.100 inch. [4] No degradation in sealing performance after 6 months of storage on a shelf or installed in a stored engine. Understanding of exudation behavior of any carbon material used should be demonstrated. [5] Low air and oil leakage at delta pressure conditions from 2 to 85 psid (outside pressure high), especially at low delta pressures. [6] 450 feet per second rubbing velocity capability for grounded seals, and 1,200 feet per second for intershaft seals, if proposed. [7] Air temperatures outside of the compartment up to 900 °F. [8] Allowable shaft axial motion up to 0.250 inch. [9] Compatibility with oils and preservative fluids used in the engine (e.g., MIL-PRF-7808L Grade 3, MIL-PRF-23699F C/I). The validation should include computer simulation, lab and bench top testing, rig testing, and, if possible, engine testing.

  PHASE I: Demonstrate the feasibility of various concepts (new materials, coatings, and/or design configurations) that address the performance and durability issues cited above. Provide a plan for practical deployment of the proposed seal design.
  
  PHASE II: Develop and validate a sealing demonstration of sealing solution technologies that could be incorporated into advanced fighter engines to improve the performance characteristics as outlined in the description above.

  DUAL USE COMMERCIALIZATION: Military application: Successful sealing concepts when incorporated into military gas turbine engines would improve performance and lower costs. Commercial application: Successful seal designs for military engines will be applicable to commercial gas turbine engines as well both for commercial propulsion and for ground power generation applications.

  References:  1. Shaughnessy, Dennis and Dobek, Lou, "High Misalignment Carbon Seals For The Fan Drive Gear System Technologies," http://www.grc.nasa.gov/WWW/TurbineSeal/papers/2004/04Shaughnessy.pdf. 2. Turbomachine Sealing and Secondary Flows Part 1-Review of Sealing Performance, Customer, Engine Designer, and Research Issues http://gltrs.grc.nasa.gov/citations/all/tm-2004-211991-part1.html.

Keywords:  carbon seals, air-oil seals, gas turbine engine, bearing compartment seals, bearing housings, fretting, coatings

Questions and Answers:
Q: 1. The engine operating mission profile affects seal life. For example, the inlet & outlet pressures, shaft speed, air temperatures, and time at each condition (idle, cruise, slram, and slto) are required to estimate the required life of 5400 hours. Please supply mission profiles for the shaft-to-ground seals and intershaft seals.

2. Please supply approximate diameter of the seals.

3. Does the inter shafts are inside each other (overlapping)or have an axial gap between them?

4. Do we have space (seal cavity) limitations to accomodate the seals? If so what are the limitations?
A: 1. The 5400 hour life requirement/goal is an engine manufacturer goal for all seals in the engine. This topic is not for a particular seal position although a proposal may target a particular seal position. The proposal should demonstrate life capability of the proposed solution in comparison to current practice. An absolute life estimate to a particular seal position is not required.

2. The approximate range of seal diameters depending on position is 2.5 inch to 9.5 inch.

3. Inter-shaft seals seal between two concentric shafts rotating in opposite directions. Radial and face sealing approaches have been used.

4. Each seal position would have its own envelope requirements.
Q: 1. The engine operating mission profile affects seal life. For example, the inlet & outlet pressures, shaft speed, air temperatures, and time at each condition (idle, cruise, slram, and slto) are required to estimate the required life of 5400 hours. Please supply mission profiles for the shaft-to-ground seals and intershaft seals.

2. Please supply approximate diameter of the seals.

3. Does the inter shafts are inside each other (overlapping)or have an axial gap between them?

4. Do we have space (seal cavity) limitations to accomodate the seals? If so what are the limitations?
A: 1. The 5400 hour life requirement/goal is an engine manufacturer goal for all seals in the engine. This topic is not for a particular seal position although a proposal may target a particular seal position. The proposal should demonstrate life capability of the proposed solution in comparison to current practice. An absolute life estimate to a particular seal position is not required.

2. The approximate range of seal diameters depending on position is 2.5 inch to 9.5 inch.

3. Inter-shaft seals seal between two concentric shafts rotating in opposite directions. Radial and face sealing approaches have been used.

4. Each seal position would have its own envelope requirements.

Record: of