How does a hydrostatic transmission work on a lawn mower?

Before going into details on how the lawn mower hydrostatic transmission works, let’s reconsider the reason why we need the transmission in the first place.

The combustion engine is great at extracting energy from fossil fuels, yet it has a drawback: its efficiency and power peak at a very narrow range, so it’s would be very insufficient to try regulating vehicle speed by altering the engine’s RPM alone.

That’s when a transmission comes to help. Its job is to trade engine torque for speed and vice versa, depending on what is in demand at the moment.

You can achieve maximum torque when you slow down the engine speed with the help of the “lower” gear (where the smaller gear is transmitting the power to the bigger gear), while the maximum speed is achieved with the help of the “higher” gear (when the bigger gear is transmitting the power to the smaller gear). Just remember how the gears work on the bicycle.

But they’re a plethora of ways you can change the gear ratio other than typical gears with teeth. For example, there are CVTs that utilize cones as means of varying the gear ratio, planetary.

What is a hydrostatic transmission and how it works?

Hydrostatic transmissions are a type of CVT (continuously variable transmission) that acts as a hydro-transformer (in the way). Hydro-transformer use liquid (typically oil) as a means of transmitting and varying the power.

However, when a traditional hydro-transformer in an automatic or hydro-glide gearbox use fans as means of transmitting, receiving, and altering power in an enclosed oil bath; hydrostatic transmissions utilize swash plates (variable and static) and oil pistons(4)(7) that are enclosed in the oil chamber as means of transmitting and altering the power.

Hydrostatic transmission has two sets of swash plates that are paired to accordion piston assemblies, of which there are also two pairs.

The swash plate–piston assembly that sits on the input shaft(5) and receives power from the engine is called a pump(1) as it converts the rotational energy to liquid flow energy.

The swash plate—piston assembly that sits on the output (drive) shaft(8) and receives power from pressurized oil flaw is called a motor(2) as it converts the liquid flaw energy to the rotational movement.

The pump(1) and motor(2) have pistons(4)(7) lined up in the circle as they ensure a constant contact patch with the swash plate and allow the dynamic swash plate angle to change for altering the oil pressure.

The gear ratio in this system as well as rotational direction is changed by altering the swash plates’ angle. Typically, only the pump (input shaft) swash plate(3) angle is altered by the travel lever or travel pedal, but there are also systems where the angle of both swash plates or only the motor swash plate angle is altered.

The difference in the gearing is achieved by the difference in the oil pressure within the pump and motor pistons. A higher pressure represents a higher or bigger physical gear.

The pressure is altered by the angle of the pump swash plate that presses the pistons in-words raising the oil pressure. The motor swash plate(6) angle can be altered too (in some systems), but pump pressure is always higher to insure the oil flaw.

The motor and pump are typically enclosed in one system, however, there are applications where they are connected via hydraulic lines.

The pros of hydrostatic transmission are: It’s easy and convenient to operate due to the continuously variable gearing ratio, they are cheap to produce due to the limited amount of components and repeating components, and they are also pretty compact.

These transmissions are typically used in tractors, some motorcycles, forklifts, and other industrial vehicles. 

Hydrostatic transmission application in lawnmowers

In lawn tractor applications you typically have only an input shaft/pump swash plate with an adjustable angle.

Lawn tractors utilize a one-transaxle solution that also bears the function of a rear axle.

They have a pair of drive shafts(8), a differential(9), a reduction gear set(10), a hydrostatic system(11) (which is called a center case in this application), and a brake disc(12) with brake shoos all enclosed in one case that consists of uppercase and lowercase cast aluminum halves.

In lawn tractor applications you typically have only an input shaft/pump swash plate with an adjustable angle.

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