| Theory of operation
Dual Range Hydra-Matic
Last used in the 1955 Oldsmobile
Oldsmobile was given the task of developing transmissions for the
other upper car lines of General Motors. Chevrolet had their cast iron Power
Glides and Aluminum Turbo Glides and Buick the Dyna-Flow for their automatic
transmissions. I was just getting into the automatics around 1965 when the
Aluminum Power Glide had been out a couple years. I was taught the Jet-a-way
transmission at Dunwoody Institute in Minneapolis in 1963 but never worked
on them until many years later. The Jet-a-way was first used by Oldsmobile
in 1956 and I had a 1957 Olds with one in it. Earlier, I had a 1955 Oldsmobile
with the Dual Range and never gave a thought as to how it worked until recently.
The forgotten Dual Range
It was at least forgotten on my part as my lifes work kept me moving
onto the newer transmissions as they were introduced and immediately ended
up in my repair stall with problems. After a life time of rebuilding thousands
of various types, one day I reflected back as to what type of units were
they. Then it occurred to me that there was this Dual Range of which I know
nothing about. Between buying a service manual from 1955 and the internet,
I'm very impressed with the engineering and success of that unit. I now
refer to it as the ultimate four speed shifting machine.
About the drawings below.
I used to teach automatic transmissions in Phoenix in the mid seventies.
I found out that it was best to learn about a subject and then put it into
my own words and drawings for teaching.
The Dual Range had been perfected for twenty years before I
owned one. Lately, I have talked with the Oldsmobile car collectors that
have this trany, and their statement is they still work flawlessly. Of course
some will wear out or need rebuilding in time, but that is expected.
The first drawing is simply a layout of the parts that are enclosed
in a cast iron case. The transmission doesn't use a Torque Convertor. Torque
Convertors have a third member inbetween the two turbins called a stator.
This stator redirects the oil flow between the turbins for greater efficency.
The Hydra-Matic as G.M. called it, used a fluid coupling to act as the "clutch"
between the engine and transmission.
Automatic transmissions use planetary gears that are helical cut (teeth
cut at an angle). A gear set is in three units. A sun gear is in the middle
with three or four gears in mesh called planet pinions. The planet pinions
are mounted in a planet carrier and rotate on axles pressed into the carrier.
The planet pinions are surrounded in a ring gear or internal gear that looks
like a drum with teeth cut inside of it. Planetary gears are always in mesh
with each other so their id no grinding of gears possible like a manual
transmission where gears slide in and out of mesh with each other.
Reduction or direct drive
Reduction is obtained by driving one member, holding another member
to get reduction in the third member. Direct drive in accomplished when
two members are driven then the input and output are the same gear ratio.
In this transmission, holding a gear member is accomplished by bands in
the two front planetary sets or a cone clutch in the reverse gear set. Direct
drive is by applying a multiple disk clutch to drive a second planetary
Gears with external teeth in mesh with each other, rotate in opposite
directions. A gear with external teeth in mesh with an internal tooth gear,
they both turn in the same direction.
| When the engine is running, the flywheel turns the fluid coupling
which turns the front pump to keep the coupling full, supply the valve body
at regulation oil pressure and also provide a lubrication system for the
various shafts and gears.|
I thought it to be very odd how the fluid coupling worked.
The coupling housing is kept full of oil with the front or rear pump which
makes sense. The engine turns the outer shell ( coupling housing) just like
a Torque Converter on other cars, ok? The coupling housing turns a shaft
that operates the front pump like torque converters do. Here is what's odd.
The shaft that drives the pump, continues back and drives the input ring
gear of the front planetary. The sun gear is held by a band so the planet
pinions and carrier are turning clock wise and connected to a shaft that
goes forward and drives the drive coupling at reduction. The driven coupling
now wants to turn but won't as long as your holding the brake. The oil from
the drive coupling against the driven coupling is just churning and creating
turning resistance to the engine. This is why you feel the engine slow down
when you pull the shift lever from neutral to drive.
When you release the brake, the driven coupling begins to turn, it
turns a shaft that's it's connected to and drives the rear planetary sun
gear. The rear band is applied that holds the rear unit ring gear. The sun
gear drives the planet pinions counter clock wise that make them walk around
the held ring rear clock wise. The pinion gears are connected to the planet
carrier and they turn as a unit. The carrier is splinned to the output shaft
and car creeps forward in low gear with a combined reduction of 3.82 : 1
At this point, the transmission valve body, Governor, throttle position
valve is prepared and waiting for what you'll do next. Transmission oil
control circuits are way to complex for this simple narrative at this point.
| For second gear, the fluid coupling is going to change from being
in reduction to direct drive. That means the band that hold the front sun
gear has to let go and the clutch that drives the sun gear must be applied.
This shift could occur at five miles an hour. Low gear is really low. This
is why an Oldsmobile can really light up the tires at full throttle in low
gear with that 324 cubic inch Rocket V/8 up front.|
| Third gear is complicated. The front unit goes into reduction and
the rear unit goes into direct drive. That means, the front band now applies
and the clutch must release. The rear band releases and the rear clutch
applies. The transmission valve body works with governor pressure, throttle
position pressure, different mainline pressures, shift valve changes and
other factors under all speeds and loads without lock-up or engine run-a-ways.|
| Can you imagine accelerating a Super 88 at wide open throttle up
to 75 miles an hour and then feel the trany up shift into forth gear and
keep climbing the speedometer? That front band has to let go as the front
clutch applies and all you feel is a firm up shift that presses you into
the back of the seat. The sun gear has to go from standing still to a few
thousand revolutions a minute in a split second. That's what I call engineering
and workmanship by our Grandfathers. Oldsmobile had ten body plants, 8 assembly
plants, one transmission and engine plant in those days and every car was
expected to be a performer. GM dropped the Olds a couple years ago, but
in my memory it was some automobile.|
| The Dual Range had tremendous torque for backing up a car as two
planetary were in reduction with a 4.3 : 1 ratio. I can imagine some old
fart taking a wrong turn leading down a steep grade ending at a dead end
with no way to turn around. He did this with a mumbo Air Stream trailer
in tow. He could put the Rocket 98 in reverse and have all the power in
the world and torque to go with it and pull it off.|
The old transmission designs like this Dual Range all had the
problem of timing the adding and dumping clutches when up shifting and down
shifting. If a band needed to apply for an up shift, a clutch had to release
elsewhere to make the shift happen. If the band came on before the clutch
released, the trany would bind up for at least a split second. If the band
applied to slow after the clutch released, you had engine run-a-way (slipping).
Considerable engineering went into valve body design to prevent any of the
glitches. Then a fella by the name of "Simson" changed all that.
Simson gear flow
The Torque Flite was the first to use it. Two planetary units with
a common sun gear was all it took to get three speeds and reverse. The sun
gear was either held or driven to make this happen. The beauty of this design
was it added clutches to up shift and dumped clutches to down shift. Every
transmission for the last 45 years uses this power flow now. The draw backs
are, only three speed forward and reverse has a second gear ratio, not a
real low like the Dual Range.