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Hydrostatic
Transmissions
Rajeev kumar

Hydrostatic Operation
 Hydrostatic transmissions are a pump and
motor connected in a circuit together
 Most are constructed using piston pumps and
piston motors
 Four basic configurations:
 In-line
 U-shaped
 S-shaped
 Split

In-Line Configuration
 The pump is directly connected to the motor
 All fluid is contained within the pump/motor
combination
 Usually uses a variable pump and a constant
displacement motor

U-Shaped Configuration
 Similar to the in-line except that the motor is
connected under the pump
 The motor shaft goes out the same direction
and the input shaft
 Used when the drive
axle is under or behind
the prime mover

S-Shaped Configuration
 Similar to the U-shaped configuration
 The motor shaft goes out behind the prime
mover, but under it
 Used when the drive
axle is under the level
of the prime mover

Split Configuration
 The motor and pump and motor are not
physically connected together
 The motor can be located some distance
from the pump and across a barrier
 Contains very high pressure hose that
connects the pump to the motor

Advantages of Hydrostatic
Transmissions
 It offers the ability to operate over a wide
range of speeds without changing the prime
mover speed
 It can change speeds rapidly because there
are no large parts which add inertia
 It provide dynamic braking
 There is no interruption of power to the
wheels when shifting

Hydrostatic Circuits
 Open circuit
 All fluid comes from the tank and is pumped
to the motor
 When the fluid leaves
the motor, it goes back
to the tank
 Does not require a
charge pump

Hydrostatic Circuits
 Closed circuit
 The fluid is pumped to the motor
 As the fluid leaves the motor, it is returned to
the pump inlet
 Requires a
charge pump

Hydrostatic Transmission Operation -
Pump
 The cylinder is turned by the input shaft
 The pistons are connected to a swash plate,
which varies how much the pistons travel
 The greater the angle of the swash plate, the
more fluid is pumped
 Reversing the angle of the swash plate
causes it to pump backwards
 The more fluid is pumped per revolution, the
more horsepower is needed

Hydrostatic Transmission Operation -
Motor
 The motor accepts the fluid from the pump
and turns a differential or wheel, depending
on the configuration
 When the pump reverses direction, the motor
turns backwards, giving you reverse
 Not all hydrostats are designed to pump
backwards
 Many motors use a shuttle valve to reverse
the flow of hydraulic fluid

Hydrostatic Transmission Components
 All circuit types require a relief valve to
prevent overpressurizing during dynamic
braking
 The pump and motor have case drain lines to
keep fluid that leaks internally from building
pressure behind the piston
 Case drains are connected to the tank
 Charge pumps must provide enough fluid to
replace leakage and cool the pump/motor
assembly

Servo Activated Hydrostatic
Transmissions
 In larger hydrostats, the swash plate can be
hard to move
 In these systems, a small piston assembly is
attached to the swash plate and activated by
low pressure (300 PSI)
 These servos are controlled by a small
shuttle, making it easier to move the swash
plate

Hydrostatic Transmission Testing
 Use a flow meter hooked to the case drain
line to measure to output. It should be below
the charge pump volume
 All hydrostatic transmissions have tight
tolerances. The filters should be change at
regular intervals
 Many systems have a pressure sensor that
shuts down the prime mover if the charge
pressure falls below a certain level

Hydrostatic Transmission Testing
 If the prime mover has a problem, it could fail
to supply the needed power to the hydrostat
 Always rule out the prime mover first when
checking for insufficient power problems
 Charge pressure varies, but should be
between 160 to 300 PSI

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Hydrostatic transmissions

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Hydrostatic transmissions