Generally speaking, a pressure compensated pump is a piston pump, usually with 9 pistons.

The pistons are housed in a chamber very much

like an old fashioned revolver where six bullets are stored in a rotating chamber. Each of the pump pistons has a “foot” that has a swivel
joint. The foot is forced against a circular plate (called a swashplate) that does not rotate but is capable of changing its angle in relation to the
pistons.The barrel that holds the pistons is attached to the input shaft on the pump so that when the prime mover rotates, the barrel with
pistons rotates as well. If the swashplate is perpendicular to the pistons, there is no movement of the pistons within their respective cylinder
cavities in the barrel. The result is that there is no displacement. If the swashplate is angled to its maximum away from perpendicular, the
pistons are forced in and out of their respective cavities within the barrel as each piston “foot” is forced to slide around the angular swashplate
causing maximum displacement.

The swashplate is typically spring offset to the maximum angle (maximum displacement) position. There is a piston opposing the spring and if
high-pressure fluid is allowed to enter the piston chamber, it causes the swashplate to move toward perpendicular or zero displacement.

In a pressure compensated pump, there is a small valve spool that normally vents the chamber behind the swashplate piston. There is an
adjustable spring at one end of this spool that is used to establish the desired pressure. The other end of the spool is exposed to the system
pressure. This is usually done internal to the pump so there is no external plumbing.
When system pressure rises to the level established by the adjustable spring, the spool moves and allows pressurized fluid to enter the swashplate piston
chamber. The fluid pushes against the swashplate spring and reduces the angle of the plate toward minimum.

If the fluid passing across an orifice causes the pressure at the pump, the pump will de-stroke (move toward minimum angle) until the pressure drops to the
setting of the adjustable spring on the spool. The spool now returns to its original position and stops flow to the swashplate piston. The spool will now continue
to modulate the flow to the swashplate piston to maintain a swashplate angle that will produce the desired pressure in the circuit.

When we add load sensing, we add an additional valve spool that responds to a differential pressure as opposed to some maximum pressure. One end of the
spool is exposed to the outlet pressure on the pump (just like the pressure compensated system). The other end of the spool is offset by a spring that sets the
load sensing pressure. This spring may require anywhere from 6 to 15 bar and may or may not be adjustable. A pilot line to some remote location that is
downstream from the control orifice connects the spring chamber of the load sensing spool. When the differential pressure through the plumbing and across the
control orifice reaches the spring setting of the load sensing spool, the spool shifts and allows pressurized fluid to enter the swashplate piston chamber. The
spool now modulates to maintain the differential pressure just as in the case of the pressure compensated system. If the resistance to flow increases to the level
of the pressure compensator, that system overrides the signal from the load sensing spool and pushes the pump to minimum displacement.
Variable displacement. In a variable displacement unit, if the vector normal to the cam plane (swash plate) is set parallel to the axis of rotation,
there is no movement of the pistons in their cylinders. Thus there is no output. Movement of the swash plate controls pump output from zero to maximum.
Pressure. In a typical pressure-compensated pump, the swash plate angle is adjusted through the action of a valve which uses pressure feedback so that the
instantaneous pump output flow is exactly enough to maintain a designated pressure. If the load flow increases, pressure will momentarily decrease but the
pressure-compensation valve will sense the decrease and then increase the swash plate angle to increase pump output flow so that the desired pressure is
restored. In reality most systems use pressure as a control for this type of pump. The operating pressure reaches, say, 200 bar (20 MPa or 2900 psi) and the
swash plate is driven towards zero angle (piston stroke nearly zero) and with the inherent leaks in the system allows the pump to stabilise at the delivery
volume that maintains the set pressure. As demand increases the swash plate is moved to a greater angle, piston stroke increases and the volume of fluid
increases; if the demand slackens the pressure will rise, and the pumped volume diminishes as the pressure rises. At maximum system pressure the output is
once again almost zero. If the fluid demand increases beyond the capacity of the pump to deliver, the system pressure will drop to near zero. The swash plate
angle will remain at the maximum allowed, and the pistons will operate at full stroke. This continues until system flow-demand eases and the pump's capacity is
greater than demand. As the pressure rises the swash-plate angle modulates to try to not exceed the maximum pressure while meeting the
flow demand
 Series 45 Housings
Housing
One piece
 No side cover or trunnions
Swashplate cradles machined
without holes in side of housing
resulting in:
 Increased stiffness
 Fewer leak paths
 Fewer parts
Large case drains ports
 Minimize case pressure
 Series 45 Housings
 SAE 2 bolt “C” Mount
 Frame J

 SAE 4 bolt “C” Mount

 Frame E, G, H and J

 SAE 2 bolt “B” Mount

 Frame H, J, K, and L

 Controls directly adjacent to

servo
 Direct communication
between control and servo
Valve Plate Design Details

Valve Plate pilots around the bearing cup in the end cap.

CCW CW
Load Sense Operation Theory
 Pump control maintains system pressure higher than the
load pressure by the margin pressure. The margin
pressure is the difference between system pressure
and the load sense signal pressure. The load sense
control monitors margin pressure to read system
demand. A drop in margin pressure means the system
needs more flow. A rise in margin pressure tells the LS
control to decrease flow.
 The pressure drop between the pump outlet and load
pressures is caused by valve’s variable orifice
 If the orifice is large, it takes more flow to create a pressure
drop equal to the margin setting
 If the orifice is small if takes less flow to create a pressure
drop equal to the margin setting.
Controls CONTROLS MENU

Basic Function – Pressure Compensated

 Operation: PC
• S45 pumps are biased to full Spool
stroke
• Control maintains system
pressure at or below PC setting
by varying output flow
• System pressure is ported to the
Servo which controls swashplate
angle (i.e. output flow)

Case
Pressure
Servo
Pressure
System
Pressure
Series 45 – Product Presentation
Controls CONTROLS MENU

Basic Function – Load Sensing/Pressure Comp.

 Operation: LS LS PC
• S45 pumps are biased to full Signal Spool Spool
stroke Pressu
• Control adjusts swashplate angle re
to maintain constant delta
pressure (margin) across an
orifice (external control valve)
• System pressure is ported to
Servo which controls swashplate
angle (i.e. output flow)

Case
Pressure
Servo
Pressure
System
Pressure
Series 45 – Product Presentation
E, G, F, and J Frame Two Spool
Load Sense Control
LS Signal

LS

PC

System Servo Case

Typical pump-valve system schematic/flow path. Unit at neutral condition
Typical pump-valve system schematic/flow path. Unit activated to extend-raise
 J Frame
Bias Spring

Unique Features:
 Tapered servo piston
 Bias spring location
 Swashplate design
 Control location
depends upon
axial/radial porting

Axial Ports
J Frame

 Mounting Flanges:
 2 Bolt SAE “B” Flange
 2 Bolt SAE “C” Flange
 4 Bolt SAE “C” Flange

45c 60c
310 c c
51c 65cc
260 c
75cc
210

• J Frame will have 2 kits with 5 angles

 Series 45 Rotating Kit
 Cylinder Blocks
 Nine piston design with
brass slipper
 Ductile iron cylinder block
 J Frame
Bias Spring

Unique Features:
 Tapered servo piston
 Bias spring location
 Swashplate design
 Control location depends
upon axial/radial porting

Axial Ports
J Frame

Control location
with radial ports

Radial Ports
J Frame
Displacement Piston Bias Spring Piston

Tapered
Roller
Bearings

Tapered
Roller
Bearing

Shaft
Valve Plate
Seal
Cylinder Block Kit
J Frame Adjustable Displacement
Limiter
Series 45 E Frame
 E Frame – 100, 130, and 147cc
One rotating kit and three angles
100c 130c
310 c c
147cc
260
210

Rated Speed @ 1 bar abs. inlet

• 100cc 2450 rpm
• 130cc 2200 rpm
• 147cc 2100 rpm
 E Frame Adjustable
Displacement Limiter
Order code for
adjustable displacement
limiters is AAA.

Rate of change is
approximately:
6cc/turn
 Series 45 Shafts
Shaft Bearings
 Bearing Cup slip fit into housing
and end cap.
 Bearing Cone slip fit onto shaft front
and rear.
 No shims
 Timken tapered roller bearings.
Series 45 F Frame
 F Frame – 74 & 90cc
Two rotating kits and two angles
74cc
310
90cc
260
210

Rated Speed @ 1 bar abs. inlet

• 74cc 2400 rpm
• 90cc 2200 rpm
 F Frame Cross-Sectional View
Bias Spring

Servo Piston

Tapered Roller
Piston Bearing

Tapered Roller
Bearing Shaft Seal

Swashplate

Valve plate Bias Piston

Cylinder Block
Failure Mode: Over Pressure
Over Pressure

 Wear
 Failure most likely to be high cycle, non-
catastrophic leading to degradation of pump
performance (loss of efficiency, slower control
response times, etc.)

 Most likely components to fail due to wear are

the swash bearings, servo piston/swash
interface, servo piston/servo guide interface
 Failure Mode: Over Pressure

K Frame swash
bearing from
prototype duty
cycle
Failure Mode: Over Pressure
K Frame Servo Piston/Swash Interface
Over Speed
 Running Open Circuit pumps over speed can cause failure due to
either excessive cavitation or block tipping/piston stick

 Excessive cavitation
 Rated speed is determined by cavitation limits
 Failure due to excessive cavitation is highly dependent on duty
cycle
 Excessive pressure drop through end cap and cylinder block
causes air in the oil to come out of solution and impinge on
valve plate
 Harsh transients accelerate cavitation effects
 Speed can be increased by charging the pump inlet or
decreasing pump max angle
Failure Mode: Over Speed
Series 45 E Frame field failure
The wrong shaft was used.
The correct shaft should have
had the undercut at the end of
the shaft spline.

There was a thrust load on

the end of the shaft.
Over Speed
 Block Tipping/Piston Stick

 Maximum Speed is determined by block

tipping/piston stick limits

 Block lift testing done at minimum spring force, max

angle, and minimum outlet pressure. Max speed is
set 5% below lowest lift/stick speed seen in the lab.

 Maximum speed can be increased by de-stroking

pump
 Over Speed

E frame
failure
Piston Stick
Low Inlet Pressure

 Minimum Inlet Pressure

 Continuous operation at 0.8 bar abs
○ Reduced life must be acceptable
 Intermittent operation at 0.5 bar abs
○ i.e. cold start

 Low inlet pressure causes the same type of

cavitation failure as running over rated speed
 Series 45 Low Inlet Pressure

Low Inlet Pressure

High Case Pressure
 Maximum case to inlet delta pressure
 Continuous: 0.5 bar
 Intermittent: 2.0 bar (i.e. cold start)

 Failure most likely to be a low hour catastrophic

failure

 Failure characterized by damaged slippers

(slippers either lift from swashplate, or pull from
pistons and damage to the slipper retainer)
High Case Pressure

J Frame Cylinder Block

High Case Pressure

K Frame Cylinder Block

High Case Pressure
High Internal Temperature

High temperatures
/low oil viscosity

J Frame Pump
Series 45 E Frame field failure
The wrong shaft was used.
The correct shaft should have
had the undercut at the end of
the shaft spline.

There was a thrust load on

the end of the shaft.
The End….