Service Manual

14
Hydrostatic Transmissions
Hydrostatic transmissions have become quite
popular in Turf equipment applications. Their in-
creased use has been due to their simplicity, low
maintenance requirements, compact design, opera-
tor convenience, and resistance to operator abuse.
Hydrostatic transmissions can be easily repaired
and maintained if you have a basic understanding
of the components and their function.
To begin to understand hydrostatic drive units, lets
start by looking at the various types and configura-
tions of hydrostatic transmissions.
The first type is a hydrostatic system which consists
of a hydrostatic pump with a remotely mounted mo-
tor. (Fig 1) In this type of hydrostatic system the
hydrostatic pump is mounted by, and driven by, the
units engine. The pump is connected to the drive
motor by hoses or steel lines. These motors can be
mounted directly to the wheels or to a drive axle.
Figure 1
A different type of hydrostatic drive system is an
inline pump and motor system. (Fig 2) In this sys-
tem the motor and pump are constructed as a sin-
gle unit, this eliminates the necessity of high pres-
sure drive lines between the pump and the motor.
This unit is normally mounted to a drive axle or
transaxle.
Figure 2
A similar version is the U-type transmission (Fig 3).
In this type of system the pump and motor are con-
structed as a common component with the pump
usually located above the motor.
Figure 3
All three systems work well in their designed appli-
cations. The remote motor design works well when
there is no transmission or transaxle, or when the
location of the engine and the drive system call for
such a configuration. The U type hydrostatic sys-
tem is more compact while the inline hydrostatic
system is usually easier to repair and maintain.
We will be using the inline hydrostatic pump and
motor system in this session for illustration pur-
poses.
A hydrostatic drive consists of a hydrostatic pump,
which pumps oil to a drive motor. The most signifi-
cant feature of a hydrostatic system is the pump.
The pump is a variable displacement pump. This
means that the output of the pump can be varied
and is not controlled only by the engine RPMs like
a fixed displacement pump. This requires that the
pump needs to be a piston pump.