The forward sprag assembly is usually taken for granted by most transmission builders. If it appears there is no or maybe just a little wear it is generally reinstalled. In this guide we will take a look at the importance of this little part in high performance applications of the 700R4 and 4L60E transmissions.
The sprag is a one way clutch, which by definition, is a mechanical device that lets a unit rotate in one direction, but prevents it from rotating in the opposite direction. This unique type of clutch is desirable because it releases as soon as torque applied to it is interrupted, thus eliminating the timing overlap problems associated with releasing one member, and applying another.
The sprag sometimes is confused with the roller clutch. The sprag differs with the roller clutch in that the roller clutch incorporates a set of spring loaded steel rollers between an inner and outer race. One of these races has ramps machined into it. When one of the races rotates one way the rollers stay on the low part of the ramp and the assembly is free to rotate. When one of the races rotates in the opposite direction, the springs force the rollers up their ramps, wedging them solidly between the inner and outer races, preventing rotation.
The sprag design is preferred by many for high performance applications. Like the roller clutch, the sprag uses inner and outer races. The major difference is the sprag's inner and outer races are smooth, and it uses dog bone shaped sprag elements instead of rollers. The sprag’s have an irregular shape that makes them longer in one direction than the other. When one of the races rotates so that the sprag’s tilt to the short side or "lay down", there is clearance between the sprag’s and races, allowing the assembly to freewheel. But, when one of the races is rotated in the opposite direction, the sprag’s rotate toward the long side wedging between the races and preventing the assembly from rotating.
In the 700r4 and 4L60E transmissions the forward sprag transfers engine torque from the forward clutch to the input sun gear during acceleration in first, second and third gears in overdrive range. When the throttle is released in these gear ranges the sprag clutch is overrun to allow the vehicle to coast freely. During acceleration the sprag clutch is overrun only in fourth gear. In other words, in overdrive first gear range this little sprag and the forward clutch are taking the full responsibility of moving the vehicle forward. Think about this, the forward sprag is a major link between your high torque engine and your low geared rear end.
In 1987 GM updated the sprag assembly with two major changes. First, more lube holes were added to the overrun clutch hub of the sprag assembly. Second, the sprag element was changed from a 26 element to a 28 element. It has been determined; if you add more sprag elements you will increase holding power. In 1995, General Motors again updated the sprag assembly. The sprag element was changed to a Raybestos "SKF" 29 element single cage unit and the overrun clutch hub was changed eliminating the raised tabs on the teeth and the back side of the hub is now flat, to keep the end bearing inside diameter fully on the inner race.
In high performance applications any sprag race assembly manufactured after 1987 can be used. If your 700R4 or early 4L60E has the 1987 updated design inspect the races, hub and splines for wear. Make sure to run your finger inside the race and feel for small indentions. I have felt wear I could not see. I always upgrade the sprag clutch with the 29 element units. This 1987 updated assembly can be retrofitted to all previous models and is highly recommended. The 1995 unit has the tabs that hold the sun gear in the sprag assembly. these can also retro fit back to the earlier transmissions. The factory installed Raybestos "SKF" with 29 sprag elements can only be used in 1987 and newer units. The same goes for the Borg Warner 29 element sprag.
Recently Raybestos upgraded the SKF sprag with individual springs and now uses maraging steel in thier sprags. These newer sprags are much stronger than the earlier units. Individual springs on each element gives faster response time and better torque transfer. Higher quality maraging steel for higher fatigue limit than stainless
Maraging steel gives you an elevated level of strength, hardness, and ductility. These steels are created through an aging process that results in the development of a hard, brittle crystalline called martensite. The term "maraging" is, in fact, a simple combination of martensite and aging.
The construction of maraging steel allows it to withstand atmospheres that would quite simply destroy most standard steel. The aging process instills maraging steel with the ability to withstand sudden changes in speed and temperature, even at extreme levels. This quality has made maraging steel alloys an important component of many of the air and spacecraft used today.
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