MAINTENANCE
45
Hydraulic Fluid Requirements
General Information
The ability of a hydrostatic transmission to maintain
desired performance for the expected life depends on
the quality of fluid being circulated in the units. Fluid
quality can be measured in three general categories:
lubricity, contaminants, and temperature. Obtaining
satisfactory values in these three areas depends on the
fluid and the circuit in which it is used.
Etnyre field service experience indicates that most
transmission failures result from a breakdown of fluid
quality. Primary causes are solid particle contamina-
tion and overheating. Water or oil in the fluids are also
significant contributors. These problems are usually the
result of poor maintenance. Sometimes fluid selection
can lessen the impact of unusual environmental fac-
tors.
Hydraulic fluid requirements
The catalog ratings and performance data are based
on operating with good quality petroleum based fluids.
Premium hydraulic fluids containing high quality rust,
oxidation and foam inhibitors are required. These in-
clude premium turbine oils, API CD engine oils per
SAE J183, M2C33F or G automatic transmission fluid
(Type F), power shift transmission fluids meeting Alli-
son C-3 or Caterpillar TO-2, and certain specialty agri-
cultural tractor fluids.
Although fluids containing anti-wear additives are
not necessary for satisfactory performance, when used,
the fluids must possess good thermal and hydrolytic
stability to prevent wear, erosion and corrosion of the
internal components.
Where water contamination is likely due to conden-
sation, operation in a damp environment or frequent
splashing, a fluid with outstanding hydrolytic stability
is recommended. Consult the Etnyre Service Depart-
ment for recommendations for these fluids. Avoiding
the use of Dexron II transmission fluids which do not
meet Allison C-3 specification is recommended.
Viscosity & Temperature Requirements
The fluid must have sufficient viscosity to maintain
an oil film at the bearing surfaces. This depends on fluid
selection and on operating temperature. Recommended
viscosity for best life and efficiency, and minimum for
intermittent and continuous operation is listed below.
Fluid Viscosity Limits
For Petroleum Based Fluids
Maximum Continuous SUS (cSt)
500 (110)
Optimum Viscosity SUS (cSt)
70 (13)
Minimum Continuous SUS (cSt)
55 (9.0)
Minimum Intermittent SUS (cSt)
47 (6.4)
Viscosity index improved (multi-viscosity) fluids
may noticeably shear down in service when used in
hydrostatic transmissions. This will lower the operat-
ing viscosity below the originally specified value. The
lowest expected viscosity must be used when selecting
fluids. Consult your fluid supplier for details on vis-
cosity sheardown.
Fluid temperature affects the viscosity of the fluid
and resulting lubricity and film thickness. High tem-
peratures can also limit seal life, as most nonmetallic
seals are adversely affected by use at elevated tempera-
tures. Also, fluids may break down or oxidize at high
temperatures, reducing their lubricity and the resulting
life of the unit. Cavitation is more likely at high tem-
perature. Recommended limits for operating tempera-
tures are tabulated below.
Fluid Temperature Limits
For Petroleum Based Fluids
Continuous Limit
180ºF.
Intermittent Limit
220ºF.
These temperature limits apply at the hottest point
in the transmission, which is normally the motor case
drain.
Heat problems sometimes occur because of prob-
lems with one of the circuit components. A partially
stuck relief valve will heat up the oil rapidly. Circuit
components that have developed high internal leakage
rates will contribute to heat generation.
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