What is a Hydraulic Transmission?

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  • Written By: Lori Kilchermann
  • Edited By: Jacob Harkins
  • Last Modified Date: 07 November 2019
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A hydraulic transmission uses a hydraulic pump to push fluid through sets of pistons and wash plates that vary the amount of fluid pushed past them. The faster the fluid is pushed past the pistons, the faster the vehicle will go. This functions much like an electric motor where the more electricity is pushed through the brushes, the faster the motor will go. The transmission uses the engine to power its pump and thereby push the hydraulic fluid through the transmission.

When fluid is pushed past the pistons in a hydraulic transmission, it causes the pistons to spin. Since the pistons are attached to a splined shaft, they propel the vehicle forward as the pistons spin. The faster the hydraulic transmission allows the pistons to spin, the greater the speed generated by the transmission. As the pistons reduce the flow of the fluid, the hydraulic transmission slows the vehicle much like the water flowing over the paddles of a stream powered waterwheel.

When the wash plates are positioned perpendicular to the pistons, hydraulic fluid can not flow past the pistons. This halts the hydraulic transmission in place and the vehicle is effectively parked. By placing the wash plates past the perpendicular position of the pistons, the hydraulic transmission operates with an effective overdrive function. This helps the vehicle to travel faster while expending less energy.


In order for the fluid to drive the hydraulic transmission, the pistons must be driven into motion by the pressure from the pump. The pump is driven by an engine which can be electric- or petroleum-based energized. Once the fluid has pushed the pistons downward on their power stroke, the fluid is pushed back out of the transmission on the up stroke. This cycle of pushing down and emptying up is the power cycle of the hydraulic transmission.

Hydraulic transmissions have found a welcome home in off-road vehicles such as all terrain vehicles (ATVs). This type of smooth operating drive system is perfect for unbroken power application in soft or uneven terrain. The level of response is such that a novice rider can operate such a vehicle in less than perfect conditions with ease. This drive system has proven itself in all types of conditions and without serious maintenance issues. A change of filter at the proper interval is basically as easy as it gets. In both two- and four-wheel drive applications, the hydraulic transmission offers top-notch dependability.


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