# Saturation Dive: ZF 8-Speed Automatic

by Timur Apakidze

For those who don’t share my fascination with diving, a saturation dive is conducted at deep depths for extended periods of time at depths of hundreds of feet. I have read with bemusement the various “deep dives” that have been conducted by various publications into the workings of modern automatic transmissions. These deep dives tend to be more akin to splashing about in the shallow end of a pool. The name Saturation Dive is an engineer’s attempt at humor, namely to convey that this series of articles is much deeper than the stereotypical deep dives.

This time around, we will be taking a detailed look at the ZF 8 speed RWD transmissions. The ZF 8 speed transmission family has been around for a while, so a lot more information tends to be available for it than for the 9 speed. First, a quick rehash of the basics of gears that were discussed in the last saturation dive, for details please refer to the article on the ZF 9 speed.

The simplest gear set consists of 2 parallel gears mounted on 2 parallel shafts. Shown in Fig. 1 is a gear set with a 20 tooth drive gear on the right and a 30 tooth driven gear on the left. For this gear set the speed of the driven gear is 1.5 times lower than the drive gear, and assuming no frictional losses anywhere, the torque on the driven gear is 1.5 times higher. This gear set has a ratio of 1.5:1. This type of a gear set is usually not favorable for packaging since it requires 2 parallel shafts, and there are large separating forces that push the 2 gears apart which means that the bearings supporting the shafts have significant radial loads on them, in addition to an axial load if the gears are helical.

A simple planetary arrangement is shown in Fig. 2. A simple planetary gear set has 3 members mounted on concentric shafts, the innermost gear is called a sun gear, the outermost gear is called the ring gear, and there are evenly spaced planetary pinions that mesh with both the sun gear and the ring gear. These pinions are free to spin around their own axes, and ride on the planetary carrier, which is the third concentric member. The radial forces in a planetary gear arrangement cancel out due to symmetry, and therefore the bearings supporting these shafts do not see much, if any radial loads. Since the 3 shafts are concentric, there are significant packaging advantages as well. The planetary gear set can also be scaled up to take higher loads by increasing the number of planetary pinions, packaging permitting. In engineering literature, a “stick diagram” is often used as short-hand to describe planetary gear sets, for the planetary shown in Fig. 2 the stick diagram is shown in Fig. 3.

So with that out of the way, let us take a look at the ZF 8hp. There are various CAD renders available from the ZF website, one of them is shown in Fig. 4. There are 4 simple planetary gear sets, and since package space is not at a premium, the gear sets are not nested like they are for the 9 speed. There are 5 shift elements, all of them conventional friction type – no dog clutches required because once again, there is space. 3 of the 5 shift elements are clutches, i.e. they couple rotating shafts together, and 2 are brakes, i.e. they lock the rotating shaft to ground.

As we can see from the CAD render the shift element function as follows

• Gear set 1 and gear set 2 have a common sun gear which can be grounded by brake A
• Brake B can ground the ring gear of gear set 1
• Clutch C connects the output of the torque converter to the sun gear of gear set 4
• Clutch D connects the planetary carrier of gear set 3 to the planetary carrier of gear set 4
• Clutch E connects the sun gear of gear set 3 to the ring gear of gear set 3

• Planetary carrier of gear set 1 is rigidly connected to the ring gear of gear set 4
• Ring gear of gear set 2 is rigidly connected to the sun gear of gear set 3
• Planetary carrier of gear set 2 is also connected the output of the torque converter
• Ring gear of gear set 3 is rigidly connected to the sun gear of gear set 4
• Planetary carrier of gear set 4 is always the output of the transmission

The stick diagram for the transmission system is shown in Fig. 3. The input is the output shaft of the torque converter, which is not shown in Fig. 5. The torque converter is obviously driven by the engine. The gear tooth counts for the Chrysler applications are as follows

1. Gear set 1: Sun gear S1 = 48 and Ring gear R1 = 96
2. Gear set 2: Sun gear S2 = 48 and Ring gear R2 = 96
3. Gear set 3: Sun gear S4 = 69 and Ring gear R3 = 111
4. Gear set 4: Sun gear S4 = 23 and Ring gear R4 = 85

For some of the European applications, ZF appears to be using slightly different tooth counts, and therefore slightly different ratios. Now, as usual the gory calculations.

First gear

To achieved first gear, both brakes A and B are locked, and the input shaft is connected to the sun gear of gear set 4 by applying clutch C. Since brake B is connected to the ring gear of gear set 1 and brake A is connected to the sun gear of gear set 1, both of these members are connected to ground. This means that the planetary carrier for gear set 1 is stationary as well, and since this carrier is rigidly connected to the ring gear of gear set 4, the ring gear of gear set 4 is therefore grounded. The sun gear of gear set 4 is connected to the input, therefore the gear ratio of first gear is

1st =S4+R4S4 =10823 = 4.696 (1)

This ratio is almost identical to the first gear ratio of the 9 speed transmission, but these ratios are achieved through completely different means.

Second gear

The transmission upshifts to second gear by opening clutch C and applying clutch E. The ring gear of gear set 4 continues to be fixed, but the sun gear of gear set 4 is now overdriven with respect to the input shaft because gear set 2 acts as an overdrive (sun grounded, carrier input, ring gear output), while gear set 1 continues to act as an underdrive. The ratio is therefore

2nd =⎛

R2S2+R2⎞

⎠⎛

S4+R4S4⎞

⎠=⎛

⎝96144⎞

⎠⎛

⎝10823⎞

⎠= 3.130 (2)Third gear

To achieve third gear brake A is released and clutch C is applied, while brake B and clutch E remain closed. By applying both clutches C and E at the same time, the ring gear and planetary carrier of gear set 2 are spinning at the same speed, which means that the sun gear of gear set 2 also spins at the same speed as the input. Since brake B is grounded and the sun gears for gear set 1 and 2 are rigidly linked together, the carrier of gear set 1 is now underdriven. Since the planetary carrier of gear set 1 is rigidly linked to the ring gear of gear set 4, the kinematic state of gear set 4 is as follows

• The sun gear is rotating at input speed
• The ring gear is underdriven with respect to the input speed
• Therefore gear set 4 acts as a mixer module

After some tedious algebra, the ratio is

3rd =(S1+R1)(S4+R4)S4R1+S1(S4+R4) =144×1087392 = 2.104 (3)Fourth gear

Upshift to fourth gear is accomplished by released clutch C and applying clutch D. Since clutch D ties the planetary carriers of gear sets 3 and 4 together, the planetary carrier of gear set 3 is now in effect the output. By closing clutches D and E, all 3 elements of gear sets 3 and 4 now spin as a unit at the speed of the output shaft, which means that the ring gear of gear set 2 and the planetary carrier for gear set 1 also spin at the same speed as the output. The input is the planetary carrier of gear set 2, while the ground is the ring gear of gear set 1. If I have lost the B&B while explaining the operation of fourth gear, all I can say for reassurance is that even if I were explaining this at a transmission conference I would have lost a vast majority of my audience. The ratio is

4th = 1 +S2R1(R2+S2)S1 = 1+96144 = 1.667 (4)Fifth gear

If fourth gear calculations were complicated, the fifth gear is (pardon my French) the Piece de resistance of these calculations. The upshift to fifth is accomplished by releasing clutch E and closing clutch C while leaving brake B and clutch D engaged. That sounds simple enough, but now all 4 gear sets are in the mix.

• Since clutch C is closed, the sun gear of gear set 4, the ring gear of gear set 3 and the planetary carrier of gear set 2 spin at the speed of the input shaft.
• Since clutch D is applied, the planetary carriers for gear sets 3 and 4 are spinning at the same speed as the output.
• Ring gear of gear set 1 is grounded through brake B
• The first constraint is that the sun gears of gear sets 1 and 2 are rigidly linked together
• The second constraint is that the sun gear of gear set 3 is rigidly linked to the ring gear of gear set 2
• The third constraint is that the planetary carrier of gear set 1 is rigidly linked to the ring gear of gear set 4

A picture is worth a 1000 words

The sun gear for gear sets 1 and 2 is whipping around at 2.15 times the input speed due to the gear constraints. The ring gear of gear set 1 is obligated to spin at 0.72 times the input speed, which sets up an underdrive ratio of

5th =S1S3(R2+S2)(R4+S4) + S1R2(R4R3−S4S3) + R1S2S3(S4+R4)S1S3(R2+S2)(R4+S4) + S1R2(R4R3−S4S3) + R1S2S3S4 =88266871 = 1.285 (5)

I started this calculation at 10 am, and did not finish it till 4 pm. If there is any interest, I can do a separate write up on the details of this calculation.

Sixth gear

Sixth gear is achieved by locking the 3 clutches C, D, and E together while opening both brakes A and B up. This means all the gears in the transmission spin as a unit, and the ratio is quite simply

6th = 1.000 (6)Seventh gear

Seventh gear is accomplished by releasing clutch E and engaging brake A. Therefore gear sets 1 and 4 are essentially along for the ride, and gear sets 2 and 3 decide the ratio. The planetary carrier of gear set 2 and the ring gear of gear set 3 are connected to the input shat while the carrier for gear set 3 is connected to the output shaft. Gear set 2 acts as an overdrive in this configuration, with the ring gear spinning at 1.5 times the input speed. Gear set 3 acts as a mixer module since the sun gear is spinning at 1.5 times the input speed but the ring gear is turning at the same speed as the input shaft. The ratio is therefore

7th =11+⎛

S2R2⎞

⎠⎛

S3S3+R3⎞

⎠ =120143 = 0.839 (7)Eighth gear

Eighth gear is achieved by releasing clutch C and engaging clutch E. This means that gear sets 1 and 4 are still just along for the ride, and gear set 2 acts as an overdrive, but since clutch E is closed, all 3 members of gear set 3 spins as a unit. The ratio in this case is completely dictated by the ratio of gear set 2

8th =R2S2+R2 =96144 = 0.667 (8)Reverse gear

Reverse is achieved by locking both brakes A and B, and engaging clutch D. This means that the ring gear of gear set 2 is overdriven just like the seventh and eighth gear operation, but since the ring gear for gear set 4 is grounded and the ring gear of gear set 3 is linked to the sun gear of gear set 4, the carriers for gear set 3 and 4 spin backwards due to gear constraints. The ratio is

Rev =⎛

⎝1−R3R4S3S4⎞

⎠⎛

R2S2+R2⎞

⎠= −1744529 = −3.297 (9)Shift feel

All sequential upshifts and downshifts with this design involve just releasing one shift element and engaging another. In engineering literature, this is referred to as a “clutch to clutch” shift. Many skip shifts are also possible in the same way, e.g. a shift from Eighth to Fourth involves releasing brake A and engaging brake B while leaving clutches D and E engaged. This transmission therefore shifts very quickly. If the torque converter is replaced by a launch clutch, this transmission would be equivalent to a dual clutch transmission.

A shift from Reverse to 1st and vice versa is also a clutch to clutch shift. So when you are trying to execute than 7 point turn, this transmission will behave much better than the ZF 9 speed transmission. The ratio spacing is pretty much perfect as well.

Conclusions

A brilliant design, wish I had come up with it. Enough said.

##### Join the conversation
• Marc_aus on Jul 07, 2014

Hi TA Thanks so much for this in depth review. In some vehicles fitted with this transmission a Launch Control feature is available. I have been debating on another forum how this is achieved (technically) given the presence of a tourque convertor, and what is different during a LC proceedure than just mashing both pedals... This article got me thinking that maybe one or more of the clutches may be utilised to perform a clutch-drop type launch as in a DCT/DSG auto trans? Otherwise i cant understand how the LC proceedure allows higher revs pre-launch and not overstress the TC? Thanks in advance.

• See 1 previous
• Marc_aus on Jul 07, 2014

@Timur Apakidze Thanks for your informed reply, this answers a few other questions also. Also comforting to know that the TC is fairly safe. I always felt the internet consensus that the TC was at any real risk doing a TC/brake 2-pedal launch was unfounded anyway; with the TCU communicating with the ECU in all modern cars, the engine power would likely be cut prior to any damage to TC/trans... I had hoped there might be something more going on technically during LC - to put the TC/automatic-naysayers in their place but alas. I still defend the term 'slush-box' cannot be fairly used when speaking of the ZF8 (or it's peers) even when mated with a TC drive, given the priority to lock-up... but the vast 'a real car has three pedals' contingent are quite staunch in their view!

• Melonan on Mar 24, 2016

Great in-depth review for an amazing gear box design. Can some one post the details of the calculation of the 3rd, 4th and 5th gears.

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