From: MV (MV@dc9.tzo.com)
Date: Fri Dec 09 2005 - 08:28:00 PST
Sprags are really simple devices. They are used a lot in Automatic
transmissions. They look a lot like a roller bearing. There is an
outer shell an a inner shell, very similar to a roller bearing. The big
difference is that the "rollers", don't actually roll. They are shaped
in such a way that they allow movement between the inner and outer shell
freely in one direction, but when movement is attempted in the opposite
direction, they jam between the two shells. It is very much a wedging
action. Once they jam, the two shells (inner and outer) are locked
together. There is a some type of spring device that holds the
"rollers" in place and preload one side of the roller so that it is
ready to jam when the rotation in in the "jam" direction.
Here are some pictures of sprags on the Lenco website. Lenco
transmissions are very famous racing transmissions - They have been used
by a lot of top fuel dragster drivers. Extremely strong.
As you will see they look just like roller bearings, but notice that the
inner "race" has a spline it it.
http://www.lencoracing.com/Sprag%20info.htm
If you stop by an automatic transmission shop, I bet they would have
some broken ones laying around.
I first learned about sprags when I accidentally shifted my 70 camaro
into reverse while accelerating through 80 mph down an a long on-ramp in
Flint, MI. I was panicking when the throttle on my 350 Chevy engine got
stuck with the throttle wide open during a "test". The almost immediate
deccelleration due to the transmission being in reverse at 80+ mph
caused the engine to jolt, allowing the throttle to shut. The engine
dynoed at over 400 hp, so needless to say I needed to regain my
composure after that incident! Fortunately I was in the car by myself
and traffic was very light.
Needless to say the main sprag in the transmission cracked, and
eventually had to be replaced about 200 miles later! Those turbo 400
transmissions were seriously overbuilt! I replaced the spray and the
same transmission still works 25 years later.
Dave
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Dave Sofio wrote:
> I've been trying to find/read/understand a bit about sprags but have not
> been terribly successful in even finding the basic material.
> When I bought it, I'd been told my 5-ton would give me a big surprise if
> I didn't follow a magic ritual: after backing, for every X feet I'd just
> backed up, I should drive X feet or more forwards, and do this in first
> gear.
> I found out what the surprise was, but not under those circumstances,
> and it is a recurring thing: a huge kick in the ass. Typically it
> happens as I back slowly to approach my steep uphill temporary
> driveway; I sense a slight resistance that isn't just from the slope,
> and then BAM: feels like something busted--and bad--or someone ran into
> me. I also have had it happen as I drive away from having done some
> back-and-forth maneuvers to get out of a tight spot. Since it is such
> a violent thing, it does seem as if it should be avoided, but if there
> is a way to avoid or stop it, I haven't a clue. Is this "kick"
> damaging? Is it a sign of a problem? I don't have drive problems, that
> I've noticed, in either forward or reverse.
>
> Explanatory reading material on sprags would be appreciated!
>
> Mahalo,
>
> Dave
>
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