On Mon, 21 Aug 1995 JMardy@aol.com wrote:
> My TR250 hated to back up. It felt like the parking brake was on.
> The manual says that "internal damage" can result from trying to back
up while the OD is engaged.
John,
Yes, here's what happens. When going forward, the planetary gear
set in the overdrive has the option of "overdriving" the output shaft of
the OD unit. This will occur when you engage the solenoid and the sun
gear ceases to turn. Then the engine drives the planetary gears, and the
annulus then turns faster than the planetary gears driving it. At this
point you're overdriving.
Now, as a safety backup mechanism, to guarantee that the car will
always be in 1:1 (direct) or overdrive when going forward, the annulus is
connected to the sun gear via a sprag (one way) clutch behind the
planetary gear set. This basically acts like a ratchet. When overdrive
is engaged, the output shaft spins faster than the input shaft,
and the clutch does nothing. When OD is disengaged, the driving force
from the input shaft 'locks' the sprag clutch, and the output shaft is
forced to turn AT LEAST as fast as the input shaft.
When reverse is selected, it's important that the overdrive does
not try to engage. Doing so would make the the annulus try to overrun
the sprag clutch, but this time in the reverse direction. No can do.
When the planetary gear set and the sprag clutch fight, the sprag clutch
wins. So the cone clutch which controls the planetary gear set is forced
to spin against the OD case, which is of course stationary. The faster
you go in reverse, the harder the hydraulic pressure forces the spinning
cone clutch against the stationary OD case.
As the cone clutch wears, OD engagement becomes less forceful.
When the cone clutch is worn out, the OD will not engage. So, should you
worry? No not really. If your OD works, be happy and think about other
things. If it doesn't engage with quite the fortitude of days past,
you'll know why, and next time TRF has a special on A-type cone clutches,
you'll say to yourself, "Hey, there's something I need". Yes you can put
it in yourself, and you can remove the OD from the car without taking the
transmission out. It's a short weekend job.
Greg
Greg Meboe meboe@wsunix.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University, Pullman, Wa.
'85 XJ-12 H.E. (daily) '67 Spit-6 '74 TR-6
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