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What Is Hydramatic Transmission or Fully Automatic Transmission? How does It work?

What is Hydramatic Transmission?

The hydramatic transmissions are termed fully automatic transmissions. They have essentially three or four-speed and reverse epicyclic gearboxes with brakes and clutches operated by oil pressure. These are controlled by the joint action of a governor, whose speed is proportional to that of the car. Valve actuated transmission is currently in use.

  • Hydramatic drive
  • Controlled-coupling hydramatic transmission
  • Three-speed hydramatic transmission
hydramatic transmission diagram

The hydramatic drive does not have a clutch pedal to engage and disengage the transmission system from the engine. The motion of the vehicle is controlled entirely by the accelerator and brake. The hydramatic drive combines a four-speed forward and reverses automatic geared transmission with a fluid flywheel.

Read also: What Is Overdrive Transmission And How It Works? [Overdrive Mechanism]

Gear Transmission System

The geared transmission consists of three sets of constant-mesh helical planetary gears placed in series. Two planetary gear sets provide the four forward gear changes, and the third is used for reverse.

planetary gears placed in series

The fluid flywheel acts the cushion the impact of the automatic shift as well as reduce the torque reactions of the engine. A centrifugal governor in the transmission selects the proper gear for each speed and throttle position.

The change from one gear to another is accomplished through hydraulically operated pistons. In some cases assisted by springs, within the control planetary unit. The speed at which the various shifts occur is governed by the throttle position as well as by the centrifugal governor.

planetary unit

When the throttle is opened, the gear shifts at higher and higher speed is obtained. Driver control is provided by a lever and segment mounted on the steering column beneath the steering wheel.

This adjustment can be done to any one of the four positions-neutral, drive, low, and reverse. In the drive, four forward transmission shifts automatically for any one of the four forward speeds. The low position is for hill climbing or muddy road.

Read also: What is Fluid Drive? It’s Construction and Working Principle

Hydramatic Gear System

The hydramatic gear system is shown in fig. It consists of three sets of planetary gears to obtain four forward speeds and the reverse speed. In the front unit, the internal gear is driven by shaft linkage to a torus cover that is bolted to the engine flywheel.

Hydramatic Gear System

The fluid flywheel, which is a part of the gear train, is also enclosed in this cover. The sun gear is attached to a drum around which a brake band. It is placed for locking the drum and sum gear when the front unit is used for gear reduction.

This arrangement makes the planet cage the driven member of the front unit when the sun gear is held stationary. The planet cage is splined to a hollow intermediate shaft, whose front section connects with the driving member of the fluid flywheel, and whose rear section is splined to the clutch hob of the rear unit.

When the planet cage receives power from the internal gear of the front unit, it transmits it forward through the intermediate shaft to the driven member.

Rear unit

In the Rear unit, the internal gear is fastened to a drum and brake band assembly for locking it when the unit is in reduction. The sun gear is the driving member. It receives its power from the main shaft. The planet cage, splined to the output shaft is the driven member when the internal gear is held stationary.

Reverse Unit

The Reverse Unit consists of sun gear fastened to the internal gear of the rear unit. A planet cage splined to the output shaft and internal gear. The internal gear has external as well as internal teeth and is held stationary when the anchor pin engages in external teeth.

The front and gear unit contain hydramatically operated multi-disc clutches. These are used for locking various gears together when the units are in direct drive. In the front unit, the clutch can lock the planet cage and the sun gear. In the rear unit, the clutch connects the rear section of the intermediate shaft with the internal gear when it is applied.

First Gear

To obtain first gear, both the rear and front units are placed in a reduction by the application of brake bands to both drums to hold them stationary.

First Gear.

With the sun gear of the front units locked, the power is transmitted from the flywheel to the torus cover and to the planet cage, which revolves around the stationary sun gear at a reduced speed. The cage transmits power to the driving torus of the fluid flywheel through the front section of the intermediate shaft.

From the driven torus the power is transmitted by the main shaft to the sun gear of the rear unit. The sun gear revolves around the planet pinions which drive the output shaft. The combined gear ratio of the two sets of gears is the product of their gear ratios. The gear ratio in the internal gear and sun gear of the front unit has 67 and 30 teeth and those of the rear unit have 67 and 41 teeth respectively.

Second Gear

To obtain a second gear, the front unit is placed in a direct drive by releasing the front band and applying the front clutch. Thus, the power is transmitted from the flywheel to the torus cover to the locked front unit.

Second Gear

There it is transmitted directly by the front section of an intermediate shaft to the fluid flywheel. It carries back to the sun gear of the rear unit through the main shaft. The rear unit carries the power to the output shaft. The gear ratio for the second gear is 2:63:1.

Third Gear

To obtain third gear, release the clutch and apply the band in the front unit, place it back in reduction.

Third Gear

Release the band and apply the clutch in the rear unit to convert it to direct drive. The power flow is from the flywheel to the front unit, rear unit, and finally to the output shaft. The gear ratio for the third speed is 1:45:1.

Fourth Gear

To obtain fourth gear, lock the clutch and release the band in the front unit-shifting it into direct drive and bearing the rear unit into direct drive.

Fourth Gear

The power flow is from the flywheel to the locked front unit, intermediate shaft, rear unit, and finally to the output shaft.

Reverse Gear

In all the forward speeds, the reverse unit turns freely and transmits no power. To obtain reverse speed, place the selector lever in the reverse position, shifting the front unit into reduction. Release the rear band and clutch, and engage the anchor pin in the external teeth of the reverse internal gear, so as to hold the gear stationary.

The power flows from the flywheel to the torus cover, front internal gear, planet cage, intermediate shaft, and to the fluid flywheel, as in the first gear. From there, it is transmitted to the rear sun gear by the main shaft. When the reverse internal gear is locked, the reverse planet cage cannot turn freely, but hold the rear planer cage to the output shaft.

The rear unit planet cage acts here as an idler which causes the rear unit internal gear to turn in a reverse direction. The rear unit internal gear drives the reverse sun gear in a reverse direction, causing the reverse planet cage to move the output shaft in reverse. The gear ratio in reverse is 4:30:1.

Read also: 6 Most Common Gearbox Problems [That may lead to unnecessary repair]

Controlled-Coupling Hydramatic Transmission

This type of hydramatic transmission retains many features of the earlier hydramatic drive, with some major changes. In it, the front planetary unit occupies the space between the torus cover and the fluid coupling but is still driven by the torus cover. The sun gear is linked to a sprag (one-way) clutch and overrunning clutch, instead of with a band and servo, to hold it stationary.

Controlled-Coupling Hydramatic Transmission

Another change in the front unit is the use of a liquid coupling in place of a hydraulically operated friction clutch for providing direct drive. The rear unit internal gear can be held by a rear sprag clutch or by a servo-operated overrunning band to shift the unit into reduction.

The rear unit is shifted to direct drive by means of a hydraulically outer race of the rear sprag clutch and the transmission case. When it is applied, it allows the rear sprag clutch to hold the rear internal gear. When it is released, it shifts the rear unit into neutral. The gear system is similar to that used in hydramatic drive. However, the methods used to shift the gears and locations of gears differ.

Three-Speed Hydramatic Transmission

This type of hydramatic transmission offers three selective forward drive ranges-knowns as drive left, drive right, and low. Reverse and park are the other two positions. The drive left the range, the transmission starts in first and shifts automatically to second and third.

In the drive right range, the transmission starts in first but will not shift above the second unit until a speed of 70-82 m/h is reached. In the low range, the transmission remains in first gear regardless of the throttle opening or vehicle speed.

The three-speed hydramatic transmission consists of three major units.

  1. Torque converter as fluid coupling. It is composed of the drive torus, the driven torus and the torque multiplier.
  2. Front planetary unit. It consists of the sun gear, internal gear, planet gear and carrier from the clutch and reverse cone clutch.
  3. Rear planetary unit. It consists of a rear sun gear, planet pinions, internal gear, planet carrier, output shaft, a neutral clutch, sprag clutch and overrunning band.

Advantage of Automatic Transmission

  1. Simple driving control.
  2. Improved acceleration and hill-climbing.
  3. Reduced fuel consumption.
  4. Less wear and tear due to planetary gearing.
  5. Less fatigue to the driver.
  6. No clutch pedal or gear lever.
  7. Smooth running under all conditions due to hydraulic coupling and automatic gear change.
  8. No shocks or jerky driving.
  9. Noiseless gear shifting.
  10. Longer life.

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About Saif M

Saif M. is a Mechanical Engineer by profession. He completed his engineering studies in 2014 and is currently working in a large firm as Mechanical Engineer. He is also an author and editor at www.theengineerspost.com

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