6.5.2. Commissioning
The hydraulic unit is fitted with two filling holes that are covered by red transport caps during the transport. The cap that is more
accessible to the operator has to be removed and replaced by a filling cap with a gauge, otherwise a risk of making the entire system
inoperable arises, along with the risk of pump damage. When transporting the hydraulic unit, always replace the filling cap with a
gauge with a transport cap, see figure. Start the hydraulic unit within short intervals. Check the piping for noise and leakage. Vent the
hydraulic circuit. Venting is performed usually by raising and descending the arm, clamping and unclamping the vice. Repeat several
times, until the system is ventilated (usually 5-10 times).
6.5.3. Hydraulic Liquids
The correct function, service life, operational reliability and economy of the hydraulic equipment are substantially influenced by the
usage of suitable hydraulic liquids. Mineral oils, also called hydraulic oils, are the most frequently used liquids.
Hydraulic liquids
fulfil various tasks in the hydraulic equipment. The most important of them are the following:
• hydraulic energy transfer from the hydrogenerator to the hydromotor • lubrication of moving parts of hydraulic elements • rust
protection • discharge of impurities • water drain • air vent, etc. • removal of loss heat developed by volume losses and friction.
The hydraulic liquid quality is specified by the following parameters:
Kinematic viscocity
is defined as a measure of internal friction of the fluid. Higher viscosity means a thicker liquid. The viscosity unit
is m2.s-1 (mm2.s-1 is used in practice). Viscosity is dependent on temperature. The amount of temperature influence, i.e. the change
of viscosity with temperature, is determined by the viscosity index. The bigger the viscosity index, the smaller the dependence of
viscosity on temperature (in standard mineral oils between 85 and 90, in better quality oils 100 and higher).
The compressibility
affects the rigidity of the system and thus also the precision of motion in high-pressure devices. The
compression value is relatively small. The compressibility of liquid increases with the content of air or other gases.
Foaming quality
is undesirable with regard to the function of liquid but also with regard to its ageing. Chemical additives reduce the
foaming quality. The foaming quality can be very substantially affected by the hydraulic system design (tank design, air suction).
Requirements for Liquid Properties
The main requirements can be put as follows:
• small dependence of viscosity on temperature change • corresponding viscosity ensuring good efficiency of the system • good
lubricating capacity and high mechanical oil film stability • chemical stability and neutrality against materials used • operational
stability (liquid resistance against ageing) • low foaming quality • availability and reasonable price.
Specification of Hydraulic Liquids
HM and HV oil performance classes are particularly suitable for the use in hydraulic circuits.
HM
- high-quality oils containing additives against oxidation, corrosion, foaming, for reducing wear and tear and viscosity modifier
improving also low-temperature properties. They are intended for hydrostatical mechanisms with high mechanical heat stress and
for machines operated in unprotected environment throughout the year.
HV
- are highly refined oils with additives against oxidation, corrosion, foaming, for reducing wear and tear and a viscosity modifier.
High-quality oils with outstanding oxidation stability and anti-abrasive properties, low foaming quality and good resistance against
formation of permanent emulsion and has excellent viscosity temperature dependence. They are intended for similar purposes as
HM class oils, but with increased requirement for a very low dependence of oil viscosity on temperature, particularly for HP
mechanisms of mobile machines operated within a wide range of ambient temperatures.
Biologically Degradable Liquids
Lubrication properties of environmentally-friendly liquids are as a rule identical to those of mineral oils. As regards the resistance
against ageing and tolerance of liquids to construction materials, no generally valid knowledge is available at present. Usage of the
liquid must be tested. Nevertheless, practical experience shows clearly that carefully adjusted systems operated with
environmentally-friendly liquids run smoothly.
Do not use any vegetable oils in the hydraulic unit. Risk of contamination of the whole system and irreversible damage to
the unit!
0°C
20°C
40°C
60°C
80°C
Oil type
Kinematic viscosity in mm2.s-1 at the temperature
Freezing point
in °C
MOGUL HM 32
MOGUL HM 46
MOGUL HV 32
MOGUL HV 46
220
400
180
350
100
170
67
110
32
46
32
46
15
18
17
25
8,5
11
11
14
-40
-30
-40
-36
