Gear Head
Understanding operation of planetary
gearsets helps in noise diagnosis
 | ||
Well, I thought to myself that this must
mean it doesn’t take much to amuse me, as I found
understanding the operation of planetary gear sets to be
intriguing. I do admit that it doesn’t make for the best
bedtime reading, but I did see the importance of learning it.
Let me explain.
Our instructor began by teaching us that a
simple planetary gear set consists of 1) a center sun gear, 2)
a carrier with three or more pinion gears that are free to
rotate on their pins around the sun gear and 3) the internal
ring gear, which meshes with the pinion gears. Then he started
to say, “If you hold the internal gear stationary and
drive the sun gear, you will see the pinions drive the carrier
in the same direction as the sun gear but at a reduced speed
with increased power.” This is where some of my fellow
classmates started to get bored.
 | ||
He continued: “If we switch this
around and now the center sun gear is held and the turning
effort is applied to the internal gear, this drives the carrier
through the pinions in the same direction as the internal ring
gear but at a reduced speed of the internal ring gear in a
reduction not as low as the previous but still with increased
power.”
By this time I could hear some heavy
breathing somewhere in the classroom, the type you hear before
a good snore. The instructor continued to explain that
these are simple examples of a first-gear
and then a second-gear ratio. He then went on to say that if
both the sun gear and the internal ring gear were driven in the
same direction at the same speed, the whole planetary assembly
became locked in 1:1 ratio. Needless to say, most of the guys
were “sleep drooling” past their hands and into
their class materials, and so they never even heard how reverse
is achieved. That was the grand finale of the night!
 | ||
OK, so where am I going with this? Through
the years of being on the tech line diagnosing problems, I have
come to appreciate understanding planetary rotation when it
comes to diagnosing noise.
A great example of this is the Mercedes
722.6 transmission, which has three planetary sets. In first
gear, all three are in a reduction mode.
When the 1-2 shift takes place the front planet locks 1:1 while
the other two remain in reduction. If the noise disappeared on
the 1-2 upshift, I knew the noise was somewhere in the front
planetary. When a 2-3 shift takes place, the rear planetary
locks 1:1, so if the noise disappears then, the rear planet is
suspect. When a 3-4 shift takes place, the center planet locks
1:1, so if the noise disappears then, the center planet is
suspect. When a 4-5 shift takes place, the front and rear
planetaries work together to overdrive the center planetary,
which is connected to the output shaft.
 | ||
My point here is that when you take the
time to go through the planet system “once”
determining which and when planets are in reduction, overdriven
or locked
1:1, you rarely have to go back and think
it through again. With the 722.6, I would rarely have the need
to break my head thinking through what is turning and holding
because of already knowing what I have just explained.
 | ||
As an additional benefit from taking the
time to think this through, I also saw that the N3 Hall-effect
speed sensor in the conductor plate reads the rotation of the
K1 clutch drum, with which the front sun gear is integral. So I
know that the front sun gear is held in first and fifth, which
explains why one would not see an N3 speed-sensor signal in
first and fifth gears but there would be a reading in 2nd, 3rd
and 4th gears.
I recently had to put myself into the
AF23/33-5 (AW55-50) transmission used in the Saturn Vue and Ion
sedan as well as various Volvo vehicles. This transmission
caught my attention for two reasons: First, it is the only
passenger car transmission I know of in which the first-gear
ratio (4.68) is lower than reverse (3.18). What also fascinated
me is that this is truly a five-speed but fifth gear is 1:1.
I mentioned this at our San Francisco
seminar, and a gentleman came up afterward and said he saw this
as well, and he mentioned how the final drive had such tall
gears in it. In fact, the final drive has a 2.44 ratio. With
this configuration we can see that less torque and higher
engine speeds is what the engineers had in mind in this
powertrain package.
But getting back to the transmission, this
AF23/33-5 sports three planetary gear sets, as you can see in
figures 1 and 2. GM calls the planetary set shown in Figure 1
the front-carrier sun-gear assembly (gear set #24).
GM calls the middle and rear planet
assemblies, as I refer to them (see figure 2), the front
differential-drive-gear carrier assembly (gear set #259) and
the 1-2 reverse-carrier gear assembly (gear set #18). Figures 3
and 4 show these gear sets fully assembled with most of the
driving and holding elements identified. Figure 5 illustrates
how the whole gear train meshes, including the differential.
When I look at the assemblies in this
fashion and compare this with an application chart, I can
quickly identify what each planetary gear set is doing in each
gear. I then make myself a chart that tells me which planetary
set is in a low reduction, a lesser reduction, a locked 1:1
gear set or an overdriven gear set. From this chart I can
quickly identify possible noise locations. Figure 6 is a chart
I have made for this transmission.
The chart shows that in first gear all
three planetary sets are in a reduction. When a shift into
second occurs, only planetary gear set 24 changes ratios to a
lesser reduction. The other two remain the same. So if we had a
noise that changed pitch on the 1-2 upshift, we could be
looking at this planetary gear set as being the problem. When a
2-3 shift occurs, only gear set 18 changes ratios to a lesser
reduction. So if we had a noise that changed pitch on the 2-3
upshift, we could be looking at this planetary gear set as
being the problem. When a 3-4 shift occurs, both gear sets 18
and 259 lock 1:1 but gear set 24 remains in ratio. When a shift
into 5th gear takes place, this gear set locks 1:1, putting the
gearbox in direct drive. So if a noise changed pitch on the 1-2
upshift and disappeared with the shift into 5th, gear set 24 is
a sure bet.
As you can see, once you think through the
planetary gear sets and place them into a chart like the one I
did in Figure 6, it becomes easier to know which planetary set
could give you noise. It is a process of elimination.
Another benefit that comes when you
understand power flow in this fashion is that you will never
have to be concerned about the direction of any one-way holding
devices. You will know, because you will understand when such a
device holds a member in the planetary gear set or assists in
driving it. From that understanding, you can determine the lock
direction and freewheel direction of any one-way holding
device.
You also will begin to see how many units
hold a sun gear to achieve 2nd and 4th gears and drive the same
sun gear for reverse. This also helps you to diagnose. Take,
for example, a very old problem we experienced with a 604
transmission. The sun gear would break at the weld and there
would be no reverse. It would take off in first gear and shift
into neutral, as the sun gear couldn’t hold. Since
failsafe is second gear, you stopped moving until you cycled
the ignition and the whole process repeated itself.
Again, the benefits are there when you take
the time to think a unit through and make a chart so you can
use it as a quick means to diagnose a problem. And if you are a
true gear head, you will see this article as a good bedtime
read – if you haven’t fallen asleep already. 
©2005 Transmission Digest
