May 24, 2011

Woven Carbon Wear Characteristics

Lowell Caltrell


Customer was driving 55 MPH, saw an RPM flare and the lockup was no longer holding. There also was a growling noise when it tried to lockup. When the converter was cut open, there was fine, black, graphite-looking material on the stator assembly.

In our converter shop, a large number of 300mm converter pistons with woven carbon linings are coming in with the friction material worn down to the metal. The lining on these pistons acts much like a thrust washer: as it wears, the converter end clearance increases.

How many times has a converter with a woven clutch worn down to the metal come into your shop? It might be your competitor's rebuilt converter, an OE unit or one of your own units. On inspection, the unit looks pretty good. The O-ring, piston bore and turbine hub are fine. The unit seems mechanically sound, but the lining is gone or, in some cases, worn down to almost nothing Figure 1.

Does what you see in the converter match up with the customer complaint? In most cases you might reason that the transmission is at fault and that the wear is due to a lack of apply pressure. However, by examining the various stages of breakdown of this material, we can find a more likely cause of failure.

If you catch this problem before the lining is completely gone, you can find fine black particles in the oil because woven carbon material does not wear like paper linings. In fact, it can be compared to a brake pad, as the more it is applied the thinner it gets. We have seen this situation regardless of the manufacturer of the converter.

The wear pattern starts at the outside diameter because the piston is dished. On inspection of used piston linings, we have recorded the lining thickness as low as .005" on one side of the piston and .015" in other areas of the same piston. In the lower right quadrant of the piston shown in Figure 2, the lining is only .005" thick, although the lower center section is obviously thicker.

Figure 3 shows the next stage in the breakdown process. The piston was taken out of an OE unit that had never been rebuilt. Figure 4 ran 45,000 miles on a good used lining and shows the final stage of breakdown.

We can easily conclude that this wear pattern is par for the course for this material. This pattern has even been observed in units with on/off lockup apply, and especially in units where the lining takes up the end clearance, as it is almost always in contact with the cover.

General Motors allows a 20% wear factor in their remanufacturing process, which gives you a minimum thickness of .017". There also are other factors such as surface finish, cleaning process and the type of oil used. Since this lining starts out at .022" thickness, there is not a lot of room for error.

In our shop we have opted to change these linings 100% as we have a three-year, unlimited mileage warranty and it is a small price to pay for peace of mind. However, if you choose to reuse these linings, make sure you:

  • Use a non-water based cleaning process such as solvent
  • Maintain at least a finish of 20RA on the cover
  • Reuse only linings which have the least wear possible and check the thickness in several places, accepting only pistons linings with the minimum thickness of .017"

Lowell Caltrell is the general manager of the converter shop at Precision Transmission Parts in Bakersfield, Calif., with 20 years' experience in the converter industry.

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