In all cyclic Planetary Transmissions the gears are in constant mesh. The different ratios are obtained in changing what item is held and/or driven. Clutch packs, Band units and/or Sprags (Mechanical Diodes or One Way Clutches) are used to achieve these different ratios.

During a gear change there is a period where one item being held is released and another unit applied. The timing of this release-grab is critical and affects the shift feel and its ability to hold engine torque.

 

Sprags

 

Sprags (as known as One Way Clutches and Mechanical Diodes) allow rotation in one direction only. In most cases they allow rotation in the direction of engine rotation.

When a Sprag is the holding or connection unit, the application of the next unit is straight forward. There is no need to release the Sprag as it will over run when the next unit has applied.

For example are the TH350 & TH400. The Intermediate Clutch Pack connects to the High/Reverse Drum via Sprag. On the 2-3 Shift there is no need to release the Intermediate Clutch Pack as once the High/Reverse Clutch is capable of holding the engine torque, the Sprag will over run.

 

Clutch Packs

Clutch Packs are applied by Hydraulic pressure and released by Return Springs. The weight of the these Return Springs, the volume of oil along with the size of the oil exhaust holes dictate at what speed the Clutch Pack will release. As a result Clutch Packs make poor Timing units when used without a connecting/supporting Sprag.

Late Model Subaru’s and Skyline Transmissions use this set up and can have issues with a flare on the 2-3 shift when cold.

Band Units

 

Band units are applied and released by means of a Servo Piston. This Piston will usually have an Apply and Release surface area.

Where these two areas are the same size then the use of a heavy Piston Return Spring is utilised. As the rate of the Spring is fixed and the Transmission Line pressure can be variable, the final piston release pressure is also variable. This allows for a smoother transition on gear changes at light engine loads.

 

Equal Area Servo Piston - 6 Square Inches

 

 80psi 480pds 200pds 47psi 58%

100psi 600pds 200pds 67psi 67%

120psi 720pds 200pds 87psi 72.5%

140psi 840pds 200pds 107psi 76%

160psi 960pds 200pds 127psi 79%

As the above table shows the ratio of Release Pressure vs Line Pressure is increasing as Line Pressure increases.

 

Now lets look at a Servo Piston where the Apply and Release areas are different sizes.

Line Pressure Apply Pressure Release Pressure Final Release Pressure % Difference

80psi 480pds 640pds 60psi 75%

100psi 600pds 800pds 75psi 75%

120psi 720pds 960pds 90psi 75%

140psi 840pds 1120pds 105psi 75%

160psi 960pds 1280pds 120psi 75%

As illustrated the Release Pressure is always fixed at 75% of Line Pressure with a Servo Piston Set of different Apply and Release areas.

 

Why Is Release Pressure So Important

 

It is this Pressure that applies the next unit (usually a Clutch Pack). At this release Pressure the applied unit must be able to hold all the engine torque until full Line Pressure has been reached within this Hydraulic circuit.

 

So Bigger Is Better

Sometimes yes, sometimes no.

The larger apply surface area will provide more clamping pressure however the Valve Body needs to be able fill this larger volume.

More importantly is the extra volume/flow of oil required when filling the release area and applying the next Clutch Pack. An example is the Ford C 4 series that have very restrictive Valve Bodies. By fitting an extremely Large Area Piston, the Transmission Line Pressure can momentarily drop as is struggles to fill the void.