AW55-50SN Bushing Failure - atraonline.comatraonline.com/gears/2009/2009-04/2009_4_22.pdf · 22...
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22 GEARS April 2009 O ne of the more common failures in the AW55-50SN series transaxle is bushing failure. A major contributor to this problem is low lube volume or pres- sure caused by a worn main regulator valve bore, which feeds the second- ary regulator valve. Lube and converter pressure originate from the main regulator valve. Wear in the secondary regula- tor valve bore will have the greatest effect on low lube pressure. You can check lube pressure at the pressure tap located on the rear cover (figure 1). Normal lube pressure can be as high as 30 PSI at temperatures below 0ºF, and as low by Mike Souza AW55-50SN Bushing Failure Figure 1 Figure 2 Figure 3
Transcript of AW55-50SN Bushing Failure - atraonline.comatraonline.com/gears/2009/2009-04/2009_4_22.pdf · 22...
22 GEARSApril2009
One of the more common failures in the AW55-50SN series transaxle is bushing
failure. A major contributor to this problem is low lube volume or pres-sure caused by a worn main regulator valve bore, which feeds the second-ary regulator valve.
Lube and converter pressure originate from the main regulator valve. Wear in the secondary regula-tor valve bore will have the greatest effect on low lube pressure.
You can check lube pressure at the pressure tap located on the rear cover (figure 1). Normal lube pressure can be as high as 30 PSI at temperatures below 0ºF, and as low
by Mike Souza
AW55-50SN Bushing Failure
Figure 1
Figure 2 Figure 3
GEARS April 2009 23
as 5 PSI in drive and 8 PSI in reverse with temperatures at or above 150ºF.
Lube pressure readings that start low and drop lower as the unit heats up can indicate a worn pressure regulator valve bore, worn bushings, or low pump out-put. Another cause that isn’t as easy to see is wear in the main case where the drive gear bearing outer race spline is fitted (figures 2 and 3).
The amount of clearance between the outer race and the case for a slip fit is only a few thousands of an inch, and allows for a certain amount of move-ment. The difference in the heat expansion rate between the alu-minum case and steel outer race can increase the movement by 0.005” or more.
The additional wear in the case is caused by constant force between the transfer gear assem-bly and the drive gear pushing away from each other in all ranges (figure 4). Excessive wear will place the entire drivetrain off cen-ter, causing the bushings to become side loaded. This side-loading effect will cause the planetary bushing to walk out of the bore, which can increase the loss of lube and cause drivetrain failure.
The outer race is held in by a snap ring and can be removed eas-ily for case wear inspection (figure 5). Omega Machine & Tool has a repair for this problem, which requires machining the case to fit a steel sleeve into the worn area.
(Special thanks to the Sonnax Tasc Force for providing the tech-nical information and wear pho-tos.)
Figure 5
Figure 4
The additional wear in the case is caused by constant force between the transfer gear assembly and the drive gear pushing
away from each other in all ranges (figure 4).
