Fluid Power Systems (ME353)
Fall 2012
Lecture 4
Source of Hydraulic Power Power Units and Pumps
Power Unit
The power The power unit: unit: – Provides energy for the operation of the hydraulic system – Moves fluid through the system – Provides a safe maximum system operating pressure – Assists in maintaining correct system operating temperature and fluid cleanliness
Power units are often supplied by manufacturers as a package
A basic power unit consists of: – Prime mover to power the system
– Pump to move fluid – Reservoir to store fluid – Relief valve or pump compensator compensator to control maximum system pressure
– Filter to clean the fluid – Plumbing to transport fluid to components
A prime mover may be an electric motor or Internal Combustion Engine
A coupler connects the prime mover to the pump
The power unit includes a pressure control valve to limit pressure
Filters and strainers are included in a power unit to remove debris from the fluid
Basic System Pump
The basic operating principle that moves fluid through a pump is similar in all pumps
– Enlarging the volume of a chamber allows fluid to enter the pump – Reducing the chamber volume moves fluid to the system – Inlet and discharge valves or ports control fluid movement through the pump
The maximum pressure developed pressure developed in a hydraulic system is determined by: – Resistance to fluid flow in the system (losses) – Force the prime mover can exert
The output flow rate of a hydraulic pump is determined by: – Volume of the pumping chamber (Displacement Volume)
– Operating speed of the prime mover (rpm)
Basic Pump Classifications
Hydraulic pumps can be classified according to: – Displacement (Positive & Non-positive) – Pumping motion (Rotary & Reciprocating) – Fluid delivery characteristics (Fixed & Variable)
Displacement relates to how the output of the Displacement relates pump reacts to system loads
Positive-displacement pumps produce pumps produce a constant output per cycle (no clearance for the fluid to escape)
Non-positive-displacement pumps Non-positive-displacement pumps produce produce flow flow variations due to internal slippage (has large internal clearances
– Allows fluid slippage in the pump – Results in varying flow output as system load varies
The basic pumpin basic pumping g motions motions used in hydraulic pumps are:
Rotary pump, the pumping action is produced by revolving components
Reciprocating pump, the rotating motion of the pump input shaft is changed to reciprocating motion, which then produces the pumping action
The delivery of the hydraulic pumps are classified as either fixed or variable delivery pumps have pumping chambers with a – Fixed-delivery pumps have volume that cannot be changed; the output is the same during each cycle designs, chamber geometry may be – In variable-delivery designs, changed to allow varying flow from the pump
Gear pumps are fixed-delivery pumps
Piston pumps may be designed as
When selecting a pump for a circuit, factors that must be considered are: – System operating pressure – Flow rate – Cycle rate – Expected length of service – Environmental conditions – Cost
Pump Types
Positive Displacement Pumps
a- Rotary Type 1- Gear Pump: Up to 30 m3/hr , 200 bar Gear pumps are commonly used
Gear pumps are positive-displacement, positive-displacement, fixed-delivery, rotary units
Gear pumps are produced with either external or internal gear teeth configurations
Pumping action of gear pumps results from unmeshing and meshing of the gears
– As the gears unmesh in the inlet area, low pressure causes fluid to enter the pump
– As the pump rotates, fluid is carried to the pump discharge area
– When the gears mesh in the discharge area, fluid is forced out of the pump into the system
The gerotor pump design is design is an internal-gear pump – Uses two rotating, gear-shaped elements that form sealed chambers – The chambers vary in volume as the elements rotate – Fluid comes into the chambers as they are enlarging and is forced out as they decrease in size
Normally the the inner gear is attached to a drive shaft shaft and the outer gear is turned by the inner gear at the point of contact
Balanced Gear Pump
This is a modification of a regular Gear Pump.
The tubing around the edges of this pump returns pressure -- not liquid – to to opposite sides of the shafts. Very little fluid is transferred anywhere by these extra tubes.
By equalizing pressure on each side of the shafts, friction and wear are both reduced in comparison to the standard gear pump.
Reversing Gear Pump
This type of rotary gear pump moves liquid in the same direction regardless of the direction the gears turn.
The valves (yellow) are forced open and closed by the difference in pressure on the input and output sides.
Fluid in the higher-pressure output side is shown in light blue while fluid in the lower pressure input side is dark blue.
Notice that when the green gear starts moving clockwise, the fluid in the outer circular tubes changes pressure but stops flowing, and fluid flows more or less straight up.
When the red gear is moving clockwise, fluid travels in a sideways 'S' shape through the outer circular tubes.
Lobe Pump
The lobe pump is a close relative of the gear pump
– Two three-lobed, gear-shaped units are often used to form the pumping element
– Output flow is larger than a gear pump of comparable physical size because of pumping chamber geometry
– Lower pressure rating than gear pumps
– Tend to have a pulsating output flow
Positive Displacement Pumps a- Rotary Type 2- Quimby Screw Pump: Up to 900 m3/hr , 75 bar
Screw pumps have pumps have pumping elements that consist of one, two, or three rotating screws
As the screws rotate, fluid is trapped and carried along to the discharge of the pump
The design of screw pumps allows them to operate at a very low noise level
Positive Displacement Pumps a- Rotary Type 3- Vane Pump: Up to 20 m3/hr , 130 bar
Vane pumps are positive-displacement, fixed or variable delivery, deli very, rotary units, balanced or unbalanced.
Vane pump consists of a slotted rotor, fitted with moveable vanes, that rotates within a cam ring in the pump housing
– Rotor is off center in the ring, which creates pumping chambers that vary in volume as the pump rotates
– As chamber volume increases, pressure decreases, decreases, bringing fluid into the pump
– As volume decreases, fluid is forced out into the system
Parts of a typical vane pump
Positive Displacement Pumps a- Rotary Type 4- Piston Pumps:
Piston pumps are positive-displacement, fixed- or variabledelivery, reciprocating units
– Several variations efficiency (90%), high – Many provide high volumetric efficiency (90%), operating pressure (600 bar or higher), and high-speed operation
Piston pump classification is based on the relationship between the axes of the power input shaft and piston motion
– Radial – Axial – Reciprocating
Positive Displacement Pumps a- Rotary Type 4-1 Radial (Rotary) Piston Pump: Up to 20 m3/hr , up to 700 bar Radial piston pumps have the highest continuous operating pressure capability of any of the pumps regularly used in hydraulic systems
Two variations of radial piston pumps: – Stationary-cylinder design uses springs to hold pistons against a cam that rotates with with the main shaft of the pump
– Rotating-cylinder design uses centrifugal force to hold pistons against a reaction ring
Positive Displacement Pumps a- Rotary Type 4-2 Axial Piston (Parallel Cylinder) Pump:
Axial piston pumps use pumps use two design variations:
– Inline – Bent axis
Inline has the cylinder block and pistons located on the same axis as the pump input shaft
– Pistons reciprocate against a swash plate – Very popular design used in many applications
Bent axis has axis has the cylinder block and pistons set at an angle to the input shaft
– Geometry of the axis angle creates piston movement – Considered a more rugged pump than inline – Manufactured in high flow rates and maximum operating pressures
An inline swash plate & Bent axis axial-piston pumps
Positive Displacement Pumps b- Reciprocating Type 1- Piston / Plunger Pump:
Large, reciprocating-plunger pump designs were widely used when factories had a central hydraulic power source Today, plunger pumps are typically found in special applications requiring high-pressure performance
Positive Displacement Pumps b- Reciprocating Type 2- Diaphragm Pump: Relatively low flow rate and pressure
Cars often use a Diaphragm Pump to move gasoline from the gas tank to the carburettor or fuel injection plugs.
Non-Positive Displacement Pumps
Centrifugal pumps are non-positive-displacement units to move fluid – Use centrifugal force generated by a rotating impeller to – Large clearances between the impeller and the pump housing allow internal pump slippage when resistance to fluid flow is encountered in the system
– Typically used in hydraulic systems as auxiliary fluid transfer pumps
For high flow rate
For high flow rate
Centrifugal Pump Impeller
Casing
Impellers are Impellers are classified according to the to the major direction of flow Centrifugal pumps may have: 1. Radial-flow impellers 2. Axial-flow impellers 3. Mixed-flow impellers, which combine radial- and axial-flow principles
radial single-suction closed impeller
Open mixed-flow impeller mixed-flow impeller
Axial-Flow impeller
Axial Flow Pump
Jet Pump
Jet pumps are non-positive-displacem non-positive-displacement ent pumps
– Sometimes used to transfer fluid within hydraulic systems – Jet pump creates flow by pumping fluid through a nozzle concentrically located within a venturi
Performance Curve H-Q Curve:
•
•
Positive Displacement Pumps The flow rate is (constant) not depending on the head (pressure). H and Q depends on r.p.m r.p.m but not on each other.
Non-Positive displacement Pumps (Centrifugal) •
•
The flow rate is depending on the head (pressure). H and Q depends on each other and also on the r.p.m.
Additional Design Features Features of Pumps
Some basic pump designs can be modified to allow the unit to produce variable flow outputs at constant operating speeds
This is achieved by including pump elements that allow the volume of the pumping chamber to be changed, either manually or by pressuresensing devices
Unbalanced-vane pumps can be designed to produce variable flow outputs
– A cam ring moves in relation to the center of the rotor – No flow output is produced when the centers of the ring and rotor are concentric
– Flow increases as the centers move apart
Axial- and radial-piston pumps can pumps can include variable flow output
– Variable flow is created by varying the length of the piston stroke – Inline design varies the angle of the swash plate – Bent-axis design varies the angle between the cylinder barrel and pump input shaft
– Radial-piston design uses a moveable reaction ring
Pressure Compensation Feature
Pressure compensation allows a pump to sense system pressure and vary pumping chamber volume – Provides only sufficient fluid to maintain the desired system operating pressure
– When system resistance is low, the pump produces maximum flow
Dual pumps are pumps are two pumps housed in a single case and driven by a single prime mover mover
– Some units include integral valving to provide relief and unloading capabilities
– Typical application is a high-low circuit where a large cylinder quickly extends followed by high-pressure high -pressure clamping
