Hydramatic Transmission – Advantages, Working, Diagram

Learn all about What is Hydramatic Transmission?, their features, and how they work. Become an expert with our informative guide.

What Is Hydramatic Transmission?

Hydramatic Transmission is classified as fully automatic transmissions. The epicyclic gearboxes contain three or four speeds, a reverse gear, and oil-operated brakes and clutches. A governor controls the transmission by adjusting the oil pressure in response to the car’s speed.

Hydramatic drive doesn’t need a clutch pedal to engage/disengage from engine, unlike other transmission systems. The accelerator and brake pedals solely control the motion of the vehicle. The system combines a four-speed automatic geared transmission with a fluid flywheel, resulting in smoother gear shifts and improved performance.

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Hydramatic Transmission Diagram

What Is Hydramatic Transmission Diagram
Hydramatic Transmission

What Is Gear Transmission System

This geared transmission uses three constant-mesh helical planetary gear sets in series, with the first two gear sets providing four forward gear options. Additionally, the third gear set is solely designed for reverse gear, thereby ensuring seamless transition in both forward and backward movement.

The fluid flywheel plays a significant role in reducing the impact of automatic shifts. It also helps to minimize torque reactions of the engine. This results in a smoother driving experience. Additionally, the centrifugal governor in the transmission system determines the ideal gear for each speed and throttle position. This ensures optimal performance and fuel efficiency.

Gear Transmission System
Gear Transmission System

The transmission system employs hydraulic pistons and, at times, springs in the control planetary unit to shift gears, thereby enabling smooth and efficient gear shifting operations. Both the throttle position and the centrifugal governor determine the speed of the gear shifts.

As the throttle is opened, the vehicle shifts into higher gears at progressively higher speeds. To provide the driver with better control, a lever and segment are mounted beneath the steering wheel on the steering column, thereby enabling the driver to shift gears seamlessly and efficiently. Additionally, this feature ensures a smoother driving experience, even at high speeds.

The transmission system can adjust to any of its four positions, which include neutral, drive, low, and reverse. In the drive position, the transmission automatically shifts through four forward gears for any of the four available speeds. The low position is intended for challenging situations such as hill climbing or traversing muddy terrain.

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What Is Hydramatic Gear System?

Gear Transmission System
Gear Transmission System

This cover encloses the fluid flywheel, which is part of the gear train. When the front unit is used for gear reduction, a brake band is wrapped around the drum to which the sun gear is attached. The brake band locks the drum and sun gear in place.

If the sun gear fix in place, the planet cage becomes the driven component of the front unit. The planet cage is connected to a hollow intermediate shaft. The front part of the shaft links to the driving member of the fluid flywheel. The rear section of the shaft splines to the clutch hob of the rear unit.

Receiving power from the front unit’s internal gear, the planet cage is an essential component of the transmission system, ensuring the smooth and efficient transfer of power from the engine to the wheels. Subsequently, the planet cage transfers this power to the driven member via the intermediate shaft.

Rear Unit

The Rear unit is composed of multiple parts. These include an internal gear, drum, brake band assembly, sun gear, main shaft, planet cage, and output shaft. The drum and brake band assembly have the function of maintaining the internal gear in reduction mode. The main shaft, on the other hand, propels the sun gear and serves as the primary driving member. When the internal gear is stationary, the planet cage serves as the driven member. The planet cage is splined to the output shaft.

Reverse Unit

The Reverse Unit comprises three main parts: sun gear, planet cage, and internal gear with external and internal teeth. The planet cage, on the other hand, splines to both the output shaft and the internal gear. To keep the internal gear stationary, the anchor pin engages with the external teeth. Meanwhile, the sun gear and planet cage work together to reverse the output shaft’s direction.

Both the front and rear gear units utilize multi-disc clutches to lock different gears together during direct drive. The front unit clutch has the capability of locking both the planet cage and the sun gear. On the other hand, the rear unit clutch connects the rear section of the intermediate shaft with the internal gear when applied.

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First Gear | Hydramatic Transmission

First Gear
First Gear

“Locking the sun gear of the front unit allows for the transfer of power from the flywheel to the torus cover. From there, the power transfers to the planet cage. Afterward, the cage rotates around the stationary sun gear at a reduced speed. It then transfers power to the driving of the fluid flywheel via the front section of the intermediate shaft.

The power transfer process begins with the main shaft, which transfers power from the driven torus to the sun gear of the rear unit. From there, the sun gear rotates around the planet pinions that drive the output shaft. The combined gear ratio of the two gear sets is the product of their gear ratios. Specifically, the gear ratio in the internal gear and sun gear of the front unit is 67:30, while that of the rear unit is 67:41.

Second Gear

Second Gear
Second Gear

To shift to second gear, release the front band and engage the front clutch to put the front unit in direct drive. As a result, power is transmit from the flywheel to the torus cover, and then to the locked front unit.

Once the front unit is in direct drive, power flows directly from the front section of the intermediate shaft to the fluid flywheel. Finally, the rear unit carries the power to the output shaft, resulting in the vehicle moving in second gear. It is worth noting that the gear ratio for the second gear is 2.63:1, thereby allowing the vehicle to achieve an optimal balance between speed and torque.

Third Gear

Third Gear
Third Gear

To shift to third gear, release the clutch and apply the front band to shift into reduction. To shift back, release the front band and engage the rear clutch. This locks the front unit while connecting the rear unit in direct drive. Power flows from the flywheel to the front unit, then to the rear unit, and finally to the output shaft. The gear ratio for the third speed is 1:45:1.

Fourth Gear

Fourth Gear
Fourth Gear

To obtain the 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. The flywheel powers the locked front unit, intermediate shaft, rear unit, and output shaft.

Reverse Gear

The reverse unit does not transmit power while in any of the forward speeds. To engage reverse, shift the selector lever to the reverse position, which will shift the front unit into reduction. Then release the rear band and clutch and secure the gear by engaging the anchor pin in the external teeth of the reverse internal gear.

In the first gear, the power flows from the flywheel to the torus cover, front internal gear, planet cage, intermediate shaft, and the fluid flywheel. The main shaft then transmits it to the rear sun gear. Locking the reverse internal gear holds the rear planet cage to the output shaft, thereby preventing the reverse planet cage from rotating freely.

The rear unit planet cage functions as an idler, causing the rear unit internal gear to rotate in reverse. In reverse, the rear unit internal gear drives the reverse sun gear in the opposite direction, causing the reverse planet cage to move and rotate the output shaft in reverse. The gear ratio in reverse is 4:30:1.

Hydramatic Transmission Parts

Controlled Coupling Hydramatic Transmission
Controlled Coupling Hydramatic Transmission

This type of transmission retains many features of the earlier drive, with some major changes. The torus cover drives the front planetary unit that is positioned between it and the fluid coupling. The sun gear is linked to an overrunning clutch and a sprag (one-way) clutch, which is immobilized by a band and servo.

Another change in the front unit is the use of a liquid coupling in place of an operated friction clutch for providing direct drive. Either a rear sprag clutch or a servo-operated overrunning band can hold the rear unit internal gear to shift it into reduction.

The hydraulic outer race of the rear sprag clutch and the transmission case shift the rear unit. Upon application, the outer race enables the rear sprag clutch to hold the rear internal gear. Releasing it shifts the rear unit into neutral. The gear system is similar to that used in the drive. However, the methods used to shift the gears and the locations of the 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.

The transmission initiates in first gear in the drive right range and withholds upshifting beyond the second unit until attaining a speed of 70-82 m/h. 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. The torque converter is a fluid coupling with a drive torus, a driven torus, and a torque multiplier.
  2. The front planetary unit has a sun gear, internal gear, planet gear, and carrier from the clutch and reverse cone clutch.
  3. The rear planetary unit includes a rear sun gear, planet pinions, internal gear, planet carrier, output shaft, neutral clutch, sprag clutch, and overrunning band.

Hydramatic Transmission Advantages

  1. This driving system enables straightforward control of the vehicle.
  2. This technology reduces fuel consumption.
  3. The use of planetary gearing results in less wear and tear on the vehicle.
  4. This driving system helps to minimize driver fatigue.
  5. The absence of a gear lever or clutch pedal makes this system particularly user-friendly.
  6. This driving system provides a smooth ride without any sudden jolts or bumps.
  7. The gear shifting mechanism operates quietly and without any noticeable noise.
  8. Various design features and improvements have extended the longevity of the system.

FAQ’s On Hydramatic Transmission

What is a Hydramatic transmission?

A Hydramatic transmission is a type of automatic transmission that was first introduced by General Motors in the 1940s. It uses a hydraulic torque converter to shift gears automatically, making it a popular choice for many different types of vehicles.

How does a Hydramatic transmission work?

A Hydramatic transmission uses a series of hydraulic pistons and gears to shift gears automatically. The system is controlled by a valve body that responds to changes in pressure and temperature, allowing the transmission to shift smoothly and efficiently.

What are the advantages of a Hydramatic transmission?

One of the main advantages of a Hydramatic transmission is its ability to shift gears automatically, which can be especially useful for drivers who want a smooth, hassle-free driving experience. It can also improve fuel efficiency and reduce wear and tear on the vehicle’s engine.

Are there any drawbacks to using a Hydramatic transmission?

Like any type of transmission, there are some potential drawbacks to using a Hydramatic transmission. For example, they can be more expensive to repair than manual transmissions, and they may not be as responsive or efficient as newer types of automatic transmissions.

What types of vehicles typically use a Hydramatic transmission?

Hydramatic transmissions were originally developed for use in military vehicles, but they are now commonly used in passenger cars and trucks. They can be found in a variety of makes and models, including some high-end luxury vehicles.

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