This section will provide a short introduction to pump types operation and
hydraulic oil
the functions of hydraulic oil are to transmit power lubricate components
remove heat from the system seal clearances inhibit rust and corrosion
act as a break when required the properties of hydraulic oil are
viscosity lubrication film strength pore point flashpoint demon sibility
resistance to foaming chemical stability anti wear properties compressibility
viscosity is one of the most important criteria in the selection of a hydraulic
fluid there are two elements of hydraulic efficiency volumetric and
mechanical efficiency both of these properties are to a large degree in
viscosity dependent click on the hot spots the internal friction of the fluid
will increase this will result in increased resistance to flow component
and fluid temperature will increase operation will be sluggish pressure
drops throughout the system will increase internal and external leakage
will increase pump slippage will increase and reduce pump efficiency and
increase oil temperature increased friction between the moving parts will
result in increased wear and more heat the system will operate at lower
pressure there will be a loss of precise control one of the most common problems
with hydraulic systems is contaminated oil cleanliness of the hydraulic fluid
and system is essential to maintain the longest possible life of the fluid and
to ensure trouble-free operation sources of contamination built in dirt
produced during repairs ingress of rust paint etc from tank or reservoir
generated in pump valves etc ingress from cylinder oil seals ingress from new
oil and refill of oil the importance of regular checks and replacement of line
filters cannot be stressed often enough there are two basic types of pumps non
positive displacement pumps and positive displacement pumps the positive
displacement pump is most commonly used in hydraulic systems a positive
displacement pump delivers a specific amount of fluid per stroke revolution or
cycle this type of pump can be classified as fixed or variable
displacement displacement of pumps is normally given in litres per minute or
cubic centimeters per revolution click on the hotspots for information about
the principles of a positive displacement pump these principles are
the same for all positive displacement pumps during the suction stroke there
will be an increasing volume inside the pump this will create a negative
pressure at the inlet port which enables atmospheric pressure to force fluid from
the reservoir and into the pump during the delivery stroke the volume will
decrease inside the pump this will force the fluid out of the pump and into the
outlet pipe a gear pump develops flow by carrying fluid between the teeth of two
meshed gears gear pumps are referred to as unbalanced because high pressure at
the pump outlet imposes an unbalanced load on the gears and the bearings
normally they are used in hydraulic systems with pressure up to 20 mega
Pascal the two gears inside an external gear pump are placed side by side a
partial negative pressure is created at the inlet as the gear teeth unmess
drawing fluid into the chambers form between the teeth the chambers carry the
fluid around the outside of the gears is forced out as the teeth mesh again at
the outlet during operation the outlet pressure of the pump increases and the
gears are forced diagonally outward and against the pump Inlet side this
pressure force imbalance increases the bearing load on the gears thus
increasing wear and may cause leakage one of the most common types of internal
gear pump is the dorota pump the inner gear is connected to the drive shaft and
has one less tooth than the outer gear this results in the outer gear rotating
slower than the inner gear the volumes between the rotating teeth increased
during the first half of the revolution taking fluid in in the second half of
the revolution the fluid is forced into the discharge port the pumping chambers
are formed between the veins and are closed by the rotor ring and two side
plates normally vane pumps are used in hydraulic systems with pressure up to 20
mega Pascal the slotted rotor runs eccentric to the casing moveable vanes
are fitted into the slots and during rotation centrifugal force while the
pump outlet pressure outlet pressure can be directed to the back side of the vane
moves the veins outwards each pair of vanes forms a pumping chamber that there
is in volume as the pump rotates as the chamber passes the inlet port it
enlarges due to the eccentricity of the cam ring and rotor this creates a
negative pressure that allows fluid to be pushed into the cavity by the
atmospheric pressure in the inlet line as the rotation continues the volume of
the cavity is reduced and as the pumping chamber reaches the outlet port the
fluid is pushed out of the pump and into the system balanced vane pumps are
designed to reduce the unbalanced bearing load it has two inlet and two
outlet ports the rotor runs within an elliptical casing with the two
high-pressure ports being directly opposite each other the resultant radial
forces are equal and opposite which virtually eliminates the bearing
where due to loading the advantage of the pressure compensated variable vein
pump over the fixed displacement type is energy conservation where no or very low
flow is required in the system the pump automatically adjusts accordingly
maximum displacement occurs when the greatest eccentricity exists between the
rotor and the cam ring displacement of the vane pump can be very manually by an
external control such as a hand wheel or automatically by a pressure compensator
the pressure compensator senses the pressure in the system and adjusts the
pump displacement to maintain a preset pressure level the two basic designs of
piston pumps are radial and axial both are available as fixed or variable
displacement pumps axial pumps are used on systems with pressure up to 35 mega
Pascal's radial pumps are used on systems with pressure up to 70 mega
Pascal in axial piston pumps a number of Pistons reciprocate within a cylinder
block parallel to the axis of rotation the best-known of the axial piston pumps
is the swash plate pump the cylinder block is driven by the drive shaft in a
fixed position the Pistons which are fitted into cylinders in the drive block
are connected by piston shoes to the angled swash plate as the block rotates
the shoes follow the course of the swash plate causing the Pistons to reciprocate
within the cylinders as the Pistons move outwards a suction port is formed and
oil is drawn into the cylinders as the Pistons continue to rotate they move
inwards due to the angle of the swash plate thus discharging oil the discharge
in Inlet ports are located in the valve plate the displacement of the pump
varies according to the swash plate angle the size and number of Pistons and
the speed of the pump
the control consists of a yoke return spring which returns the yolk and the
swashplate to the full delivery position this spring acts against an oil pressure
operated compensator piston which is extended by the system pressure and is
in direct opposition to the return spring startup
as there is no system pressure the spring pushes the yolk and swashplate to
the maximum angle thus ensuring maximum flow on startup during operation when
the pressure increases within the system it overcomes the spring pressure and
moves the yolk and swashplate back towards the minimum flow position until
equilibrium is reached if the pressure decreases the spring overcomes the oil
piston pressure causing the yolk and swash plate to move to a greater angle
in this way the compensator adjusts the pump output to whatever level is
required to develop and maintain the preset operating pressure avoiding
relief operation at full pump volume during holding or clamping thus
preventing power loss the principle of this pump is the same
as that of the swashplate pump in this pump the cylinder block rotates at an
offset angle to the drive shaft cylinder block is attached to the drive shaft by
means of a universal joint to maintain constant alignment and to ensure that
they rotate at the same speed the universal joint does not transmit any
talk except for the acceleration and deceleration of the cylinder block the
pumping action is produced by the reciprocating Pistons the pump
displacement varies with the offset angle fixed displacement pumps are
usually available with 25 or 30 degree angles with variable displacement pumps
the offset angle can vary between 0 and 30 degrees the angle of the yoke is
controlled through an external linkage with some controls the yoke can be moved
over the center position to reverse the direction of flow from the pump in a
radial pump the cylinder block rotates on a stationary pintle inside a circular
reaction ring as the block rotates the Pistons will follow the inner surface of
the ring this ring is offset from the centerline of the cylinder block porting
in the pintle permits the piston to take in fluid as they move outwards and
discharge it as they move in