Metal Mesh Foil Bearings for Oil-Free Turbomachinery

MAJOR APPLICATION: Cost effective gas foil bearing technology for high speed oil-free turbomachinery

Sponsor: Turbomachinery Research Consortium (07-08) , hardware donated by Honeywell Turbocharging Systems

Objective: To evaluate performance of metal-mesh foil bearings for automotive turbochargers.

Significance:

High temperature, high speed oil-free rotating machinery needs of a proven low friction bearing technology to give adequate load support (stiffness) and with enough damping to limit rotor synchronous responses and avoidance of rotordynamic instability. Gas foil bearings are a reliable and proven technology, albeit too costly. Each foil bearing is a custom piece of hardware, with resulting variability even in identical units, and limited scalability. The present research aims to replace the foil structures with a cost effective, metal mesh structure for use in automotive turbochargers. Prior experiments conducted at Turbomachinery Laboratory demonstrate metal meshes have a superior damping performance at high temperatures and in an oil free environment in comparison with squeeze film dampers.

A thin metal mesh donut easily (and cheaply) replaces the elastic underspring structure (bump strips or leafs) in a gas foil bearing, as shown in Figure 1 extracted from a recent patent. This type of bearing consists of a top circular foil (2d), an insert of porous material (2c), i.e. metal mesh installed underneath, and an external circular foil (2b) for seamless installation within a bearing cartridge (2a). The three elements (2b-2d) are pinned at (2h). In operation, a gas film (2g) separates the rotor (2f) from the topfoil. Most importantly, the metal mesh structure provides the needed stiffness and structural damping for control of rotor vibrations.

Figure 1 Metal mesh-foil bearing [*]

* Air Foil Bearing Having a Porous Foil, Y-B lee et al., International Patent # WO 2006/043736 A1.

Test facilitY

Honeywell Turbo Technologies donated two ball bearing turbochargers (TC), series T25, for this TRC funded project. The TC unit does not have a compressor but a stub shaft, 5 mm in diameter and 35 mm in length, to hold a miniature test journal. Small metal mesh-top foil gas bearings 25-30 mm in diameter, fabricated in-house, will be installed on the journal for measurement of the bearing load capacity, liftoff and touch down speeds, drag power, and dynamic force coefficients during high speed operations at room temperature.

Figure 2 shows current positioning of TC test rig. An external compressor facility, 18.2 bar (250 psig) and 42.48 m3/min (1,500 SCFM) will provide pressurized air and a pressure regulator, 9.30 bar (120 psig) max., will adjust the inlet pressure into the TC driver. A ball valve located directly upstream from the turbine inlet throttles the incoming air. Exhaust air flows through a safety structure before exiting to the outside environment. A thick plate structure installed for safety will prevent escape of the turbine wheel incase of catastrophic failure. This structure also serves as a mount location for an infrared tachometer to measure shaft speed.

Figure 3 illustrates a simple layout of the test bearing section and instrumentation. The TC shaft free end holds a press fitted journal, and a bearing cartridge contains a test metal mesh bearing. A very soft support structure, not shown, supports the bearing cartridge, and a torque arm restrains the bearing rotation. An optical tachometer measures the journal rotational speed, and two eddy current sensors positioned orthogonally record the journal motion amplitudes A torque arm and a strain gauge type load cell measure the bearing torque upon start up, while accelerating after lift-off, and while decelerating to shuts down. A mechanism holding dead weights will impose a static load on the test bearing. A simple dynamic test using an impact hammer will aid to identify the bearing dynamic forced coefficients, stiffness and damping, at various journal speeds and statically applied loads.

Figure 2. Current condition of turbocharger test rig with machined test journal

Figure 3. Planned layout of turbocharger test rig and instrumentation

TASKS in progress : Construction of (a) cylindrical metal mesh gas foil bearing and (b) Test Rig for prototype demonstrations. Conduct static and dynamic performance testing on Metal mesh gas foil bearings.

Status: Static load deflection tests of metal meshes display load versus displacement characteristics similar to bump type elastic structure used in gas foil bearings.

2008 work: Test Rig facility is under construction at Turbomachinery laboratory. The drive system of the test rig (see Figure 1) is fully operational with an expected maximum rotational speed of 120Krpm. Copper Metal meshes of two different dimensions are procured from a manufacturer. Static and dynamic testing on metal mesh donut is to be conducted to assess its stiffness and damping properties and their dependencies on various parameters such as mesh density, amplitude of vibration, radial and axial compression, frequency of operation etc. The additional components which are either under construction or need to be procured are included in the 3D CAD model shown below.

TEST RIG 3D CAD MODEL:

List of Components in figures:

1. Air supply 2. Exhaust 3. Turbine outlet safety structure 4. Turbine 5. Steel table 6. Turbine support structure 7. XY positioning table 8. Squirrel Cage 9. Squirrel Cage base plate 10. Squirrel Cage vertical support plate 11. Grub screw 12. Spring 13. Position sensing probe

14. Probe support plate 15. Force transducer 16. Test Journal 17. Test Journal end plate 18. Metal mesh housing 19. Torque socket 20. Eye bolt 21. TC support structure block 22. Load cell support 23. Weights 24. Torque Arm 25. Loading string 26. Pulley

METAL MESHES USED FOR TESTING