How does a sprag work

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Robj

Robj

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Something I've pondered. I know there has to be a transmission person out there who can explain this in terms the layman can understand.

From an online HOT ROD article:
"Transmission Drivetrain Breakage
Don't Let Your Sprag Sag"

"....There are many ways to cause an overrunning clutch failure, but drivetrain breakage is the most common. When a drive line component behind the transmission fails under power, the engine revs freely and so does the transmission. The overrunning clutch is designed with spring loaded rollers which help slow and stabilize the transmission's rotating assembly. Since the energy of the transmission and engine are absorbed in the overrunning clutch, and then transferred to the housing, or sprag, which is pressed into the aluminum case, the sprag will actually try to spin itself out of the transmission case, losing its pressed fit and causing irreparable damage..."

When spun by hand it will spin freely in the sprag one direction but locks up the other way. I know this sounds stupid but I'm not a transmission guy. How does it slow the transmission from free spinning? It seems to only allows free spin or none at all like a ratchet. I don't see any braking action to slow or stabilize.

Thanks. Always looking for an opportunity to learn every day.
 
It has rollers that roll into a notch at one direction allowing it to turn. When run the opposite direction there are springs behind them that lock them into the top of the groove pushing them into like a wedge between the sprocket and the sprag housing to stop it from turning in that direction.
 
Yes, like a ratchet, allows rotation in one direction but not the other. Is a weak point behind H/P motors. For performance engines, a replacement "bolt-in" sprang is used.
 
I think your ratchet analogy is correct. I think what the write up is saying that it is pressed on and if it spins too fast it will grow enough to cause mechanical contact damage.

But I am no trans specialist
 
Yeah, I've physically seen one and played with it but I don't understand how it will slow the transmission if you break a driveline component and the engine high revs taking the transmission along for the high revving ride. It spins very freely and there's no friction or braking action the one direction and it if spins the other it seems it would abruptly lock up the transmission and if that's what it does, why would the free spinning transmission change rotation direction to lock up? I know there's an explanation to the magic within the components surrounding the sprag.
 
It cannot handle a extreme high rate of speed, suddenly. The springs compress solid, then the rollers lock, then bad things happen.
 
Here's the bad things! Inner portion of the sprag still stuck to drive drum.
20180314_172027.jpg
 
Demonic said:
It cannot handle a extreme high rate of speed, suddenly. The springs compress solid, then the rollers lock, then bad things happen.
Click to expand...
so how does bolting it in stop bad things from happening?
 
In the 727 series of trans the outer spag race (or cam part) is only pressed into the case with knurling to hold it from turning,
Under heavy loads or a complete sudden release of load the race will blow out of the case causing extreme damage And geartrain failure.
Up dated sprag kits are bolted through the rear of the case preventing the race from blowing out.
The sprag can still fail but the distruction will be at a minimum
Lon

When the sprag is released or blown out under throttle the internal gear train can reach speeds in exess of twice engine RPM blowing the direct drum into a massive grenade. ( Can you say oh sh;: )
 
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