hylect7 (1)

8/10/2019 hylect7 (1)

1/20

Hydrostatic

Transmissions

Lecture 7

8/10/2019 hylect7 (1)

2/20

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

8/10/2019 hylect7 (1)

3/20

In-Line Configuration

The pump is directly connected to the motor

ll fluid is contained !ithin the pump"motorcombination

Usually uses a #ariable pump and a constantdisplacement motor

8/10/2019 hylect7 (1)

4/20

U-Shaped Configuration

Similar to the in-line e$cept that the motor isconnected under the pump

The motor shaft goes out the same direction

and the input shaft Used !hen the dri#e

a$le is under or behindthe prime mo#er

8/10/2019 hylect7 (1)

5/20

S-Shaped Configuration

Similar to the U-shaped configuration

The motor shaft goes out behind the primemo#er% but under it

Used !hen the dri#ea$le is under the le#elof the prime mo#er

8/10/2019 hylect7 (1)

6/20

Split Configuration

The motor and pump and motor are notphysically connected together

The motor can be located some distance

from the pump and across a barrier &ontains #ery high pressure hose that

connects the pump to the motor

8/10/2019 hylect7 (1)

7/20

Advantages of Hydrostatic

Transmissions

It offers the ability to operate o#er a !ide

range of speeds !ithout changing the prime

mo#er speed

It can change speeds rapidly because thereare no large parts !hich add inertia

It pro#ide dynamic bra'ing

There is no interruption of po!er to the!heels !hen shifting

8/10/2019 hylect7 (1)

8/20

Hydrostatic Circuits

Open circuit

ll fluid comes from the tan' and is pumped

to the motor (hen the fluid lea#es

the motor% it goes bac'to the tan'

)oes not re*uire acharge pump

8/10/2019 hylect7 (1)

9/20

Hydrostatic Circuits

Closed circuit

The fluid is pumped to the motor

s the fluid lea#es the motor% it is returned tothe pump inlet

+e*uires acharge pump

8/10/2019 hylect7 (1)

10/20

8/10/2019 hylect7 (1)

11/20

Hydrostatic Transmission Operation -

Pump

The cylinder is turned by the input shaft

The pistons are connected to a s!ash plate%

!hich #aries ho! much the pistons tra#el

The greater the angle of the s!ash plate% themore fluid is pumped

+e#ersing the angle of the s!ash plate

causes it to pump bac'!ards The more fluid is pumped per re#olution% the

more horsepo!er is needed

8/10/2019 hylect7 (1)

12/20

8/10/2019 hylect7 (1)

13/20

Hydrostatic Transmission Operation -

otor

The motor accepts the fluid from the pump

and turns a differential or !heel% depending

on the configuration

(hen the pump re#erses direction% the motorturns bac'!ards% gi#ing you re#erse

,ot all hydrostats are designed to pump

bac'!ards

Many motors use a shuttle #al#e to re#erse

the flo! of hydraulic fluid

8/10/2019 hylect7 (1)

14/20

8/10/2019 hylect7 (1)

15/20

Hydrostatic Transmission Components

ll circuit types re*uire a relief #al#e topre#ent o#erpressuriing during dynamicbra'ing

The pump and motor ha#e case drain lines to'eep fluid that lea's internally from buildingpressure behind the piston

&ase drains are connected to the tan'

&harge pumps must pro#ide enough fluid toreplace lea'age and cool the pump"motorassembly

8/10/2019 hylect7 (1)

16/20

8/10/2019 hylect7 (1)

17/20

Servo Activated Hydrostatic

Transmissions In larger hydrostats% the s!ash plate can be

hard to mo#e

In these systems% a small piston assembly is

attached to the s!ash plate and acti#ated bylo! pressure ./00 1SI2

These ser#os are controlled by a small

shuttle% ma'ing it easier to mo#e the s!ash

plate

8/10/2019 hylect7 (1)

18/20

8/10/2019 hylect7 (1)

19/20

Hydrostatic Transmission Testing

Use a flo! meter hoo'ed to the case drain

line to measure to output3 It should be belo!

the charge pump #olume

ll hydrostatic transmissions ha#e tighttolerances3 The filters should be change at

regular inter#als

Many systems ha#e a pressure sensor that

shuts do!n the prime mo#er if the charge

pressure falls belo! a certain le#el

8/10/2019 hylect7 (1)

20/20

Hydrostatic Transmission Testing

If the prime mo#er has a problem% it could fail

to supply the needed po!er to the hydrostat

l!ays rule out the prime mo#er first !hen

chec'ing for insufficient po!er problems &harge pressure #aries% but should be

bet!een 450 to /00 1SI