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• hydraulic energy transfer from hydrogenerator to hydromotor • lubrica-
tion 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 characterized by the following parame-
ters:
Kinematic viscosity

is defined as resistance against flowing through lines

and clearances. Higher viscosity means a thicker liquid. The viscosity unit
is m

.s-

(mm

.s-

is used in practice). Viscosity is dependent on temperatu-

re. 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).
Viscosity is also influenced by pressure. Viscosity increases with the pres-
sure increase.

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 qua-
lity. The foaming quality can be very substantially affected by the hydrau-
lic system design (tank design, air infiltration).

Requirements for Liquid Properties

The main requirements can be put as follows:
• small dependence of viscosity on temperature change • corresponding
viscosity guaranteeing good efficiency of system • good lubricating capa-
city and high mechanical oil film stability • chemical stability and neutra-
lity against materials used • operational stability (liquid resistance against
ageing) • low foaming quality • availability and reasonable price.

Specification of Hydraulic Liquids According to ISO 6743/4

HM and HV oil performance classes are particularly suitable for using in
hydraulic circuits.

- 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 mechanis-
ms with high mechanical heat stress and for machines operated in unpro-
tected environment throughout the year.

- 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 creation of permanent emul-
sion and especially with excellent viscosity temperature dependence. They
are intended for similar purposes like HM class oils, but with increased
requirement for a very low dependence of oil viscosity on temperature, par-
ticularly for h.-p. mechanisms of mobile machines operated within a wide
range of ambient temperatures.

Biologically Degradable Liquids

The ecological standpoint must be taken into account especially with hyd-
raulic mechanisms in plants, which come into contact with water outdoors,
with equipment and machines in agriculture and forestry, building machi-
ne drives and mobile technology on general. 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 pre-
sent. The using of the liquid must be tested. Nevertheless, practical experi-
ence shows clearly that carefully adjusted systems operated with environ-
mentally-friendly liquids run smoothly.

Safety at Work

If all the elements are appropriately designed, hydraulic circuits are reliab-
le and safe even at great pressures. However, a breakdown due to material
defect or fatigue can occur in them, which could endanger the safety of the
staff or contaminate the environment with oil. Examples of defects mani-
festing themselves outwardly:

• oil dripping,
• destruction of pressure-loaded circuit parts.

Oil dripping from an untight connection or another spot, which collects on
the floor, can cause slipping and falling of persons or floor damage and
penetrating in the environment, it can cause extensive pollution of large
quantities of water. The destruction of pressureloaded circuit parts does not
cause any explosion. If the crack is small, oil is dispersed to the environ-
ment in form of mist, if the crack is large, mostly the whole tank will be
discharged. The destruction of pressure hoses will manifest itself by oil lea-
king, but very frequently also by bursting of hose socket. In such an instant,
the hose can spring up and cause a serious injury. Both failures cause pol-
lution of the environment and a small spark can be the cause of a huge fire.
It follows that the following principles must be taken into account for
designing, installing and operating hydraulic circuits:

• do not use hydraulic elements for pressures lower than the circuit wor-

king pressure,

• keep the machine clean, sprinkle the poured out oil with wood chips or

VAPEX absorbate, sweep the floor, clean it with an appropriate solvent
and dry,

• while dismantling, prevent oil spilling by installing suitable containers,
• units and all hydraulic elements including hoses and pipelines must be

protected against external mechanical damage and heat resources,

• if the circuit is under pressure, do not come close to the pipelines and

hoses, particularly do not expose your face,

• should oil be spilled into eyes, flush eyes with Ophtal or any other sui-

table agent immediately,

• should a nonflammable liquid be spilled into the eyes (except for water

emulsion), consult an ophthalmologist.

• do not smoke or use open fire in the store and near the units and hydro-

motors,

• shut down the hydrogenerator drive whenever you make any changes or

repairs,

• electrical equipment must comply with the ESC regulations,
• containers with industrial petrol may only be used in reserved spaces and

must be covered with a cover or a closure at all times,

• hydromotors, which could start turning automatically in case that the dri-

ving hydrogenerator switches off (pressure drop), must be mechanically
or hydraulically locked against this possibility,

• all hydraulic systems and circuits must be protected by a pressure valve

against overloading, as well as hydromotors, which could become over-
loaded after hydraulic locking,

• a person responsible for maintenance and setting of the hydraulic unit

must be appointed.

The above-mentioned safety at work principles are not complete. There are
different conditions for each hydraulic circuit and different possibilities of
endangering the environment. These principles, however, should reduce
the accident rate and improve the working conditions of users.

Hydraulic Liquids

The correct function, life, operational reliability and economy of the hyd-
raulic equipment are substantially influenced by the usage of suitable hyd-
raulic liquids. Mineral oils, also called hydraulic oils, are the most fre-
quently used liquids.

Hydraulic liquids fulfill various tasks in the hyd-

raulic equipment. The most important of them are the following:

Kind of oil

Kinematic viscosity in m

-1

at the temperature

Setting

0°C

20°C

40°C

60°C

80°C