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18 AUGUST 2004 TRIBOLOGY & LUBRICATION TECHNOLOGY n previous articles, we have discussed at some length fluid flow and funda- mental hydraulic principles. With this as background, let’s peel back another layer of the onion and look more closely at hydraulic systems. At the heart of the sys- tem is the hydraulic pump. Hydraulic Pumps provide one and only one function: to provide flow of the hydraulic fluid. This is a question on the CLS ® exam! Many people think the pump provides pressure—wrong! Pressure comes from resistance to flow due to a variety of causes: restrictions in the lines, viscosity, load on the actuator, etc. The pump converts mechanical energy from the electric motor or a gas or diesel engine used to drive the pump to create fluid energy. The pump is of the type called a positive displacement pump. This means that the pump is designed with special seals and close tolerances such that one revolu- tion of the pump delivers a measured amount of fluid with little possibility for leakage under normal pressures. Thus, as pressure in the system increases primarily due to the load on the actuator, the motor or engine must work harder (draw Lubrication Fundamentals by Dr. Robert M. Gresham Contributing Editor Hydraulics: An inside look at pumps Hydraulics: An inside look at pumps I These hard-working mechanisms are at the heart of any hydraulic system

Hydraulics: An inside look at pumps...Hydraulics: An inside look at pumps I These hard-working mechanisms are at the heart of any hydraulic system 18-21 tlt fundamentals 8-04 7/8/04

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Page 1: Hydraulics: An inside look at pumps...Hydraulics: An inside look at pumps I These hard-working mechanisms are at the heart of any hydraulic system 18-21 tlt fundamentals 8-04 7/8/04

18 A U G U S T 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

n previous articles, we have discussedat some length fluid flow and funda-mental hydraulic principles. With this

as background, let’s peel back another layerof the onion and look more closely athydraulic systems. At the heart of the sys-tem is the hydraulic pump.

Hydraulic Pumps provide one and onlyone function: to provide flow of thehydraulic fluid. This is a question on theCLS® exam! Many people think the pumpprovides pressure—wrong! Pressure comesfrom resistance to flow due to a variety ofcauses: restrictions in the lines, viscosity,

load on the actuator, etc. The pump converts mechanical energy

from the electric motor or a gas or dieselengine used to drive the pump to createfluid energy. The pump is of the type calleda positive displacement pump. This meansthat the pump is designed with special sealsand close tolerances such that one revolu-tion of the pump delivers a measuredamount of fluid with little possibility forleakage under normal pressures.

Thus, as pressure in the system increasesprimarily due to the load on the actuator,the motor or engine must work harder (draw

Lubrication Fundamentals

by Dr. Robert M. Gresham Contributing Editor

Hydraulics: An insidelook at pumpsHydraulics: An insidelook at pumps

I

These hard-working mechanisms are at the heart of any hydraulic system

18-21 tlt fundamentals 8-04 7/8/04 1:49 PM Page 18

Page 2: Hydraulics: An inside look at pumps...Hydraulics: An inside look at pumps I These hard-working mechanisms are at the heart of any hydraulic system 18-21 tlt fundamentals 8-04 7/8/04

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y A U G U S T 2 0 0 4 19

more amperage or use more fuel) to deliverthe necessary flow. Indeed, the pump sys-tem will deliver this flow to the point of itsown destruction (stalled motor or engine,blown seals, hoses, etc.). For this reason,the system is always protected with a pres-sure relief valve. Thus, the pump providesfluid flow.

Brad Poeth of Eaton Corp. is one of theinstructors for STLE’s Hydraulic SystemsEducation course. Brad is an expert onpumps and has a wealth of practical experi-ence in design and operation of hydraulicpumping systems and troubleshooting. If youreally need an expert on pumps, contact Bradat [email protected], (952) 294-2017.

Hydraulic pumps are of several basictypes: gear, vane and piston pumps.

Gear pumps are gener-ally the least expensive,least sensitive to fluidtype, most tolerant of con-tamination and least effi-cient of pump designs.System pressures are typi-cally 1,500 to 5,000 psi.Gear pumps are mostoften used on mobileequipment where thesecharacteristics make the

most sense given the level of maintenance,contamination and needed performance.

The gears rotate in opposite directions,one driven by the motor and the other as anidler. The fluid enters the inlet chamber andis carried between the gear teeth and thehousing around the outside of the gears.Fluid cannot go through the center as thereis insufficient clearance. Then the twostreams recombine and push out the pres-sure side of the pump.

Gear pumps can operate in both direc-tions, which may be of some use in certainsystem designs. Further, because the bear-ings generally see pressure from one direc-tion, the pressure side, the pump is said tobe unbalanced. Thus, gear pumps tend towear disproportionately on one side. Gearpumps can be of the more common external(Figure 1), internal or gerotor design.

Vane pumps are higher-performingpumps and are widely used, but they

require a fluid with excellent antiwear prop-erties. Vane pumps have a number of sur-faces that are subject to wear. Wear areasinclude the vane tip, rotor to side plate andthe vane slot in the rotor.

One advantage of vane pumps is thevane tip remains in contact with the camsurface as it wears, as long as the wear isuniform. This increases efficiency. Addition-ally, vane pumps, when designed with twoinlets and two outlets located on oppositesides, are balanced and, thus, there is uni-form and lowered stress on the bearings. Bymechanically varying the shape of thechamber, vane pumps can be made as vari-able displacement pumps increasing their

By mechanically varying the volume of the inlet and outlet, the pump can be madeinto a variable displacement pump.

Figure 2.

Because the bearingsgenerally see pres-sure from one direc-tion, gear pumps tendto wear dispropor-tionately on one side.

Figure 1.

Gear pumps aregenerally the least

expensive, leastsensitive to fluidtype, most toler-

ant of contamina-tion and least effi-

cient of pumpdesigns.

EXTERNAL GEAR PUMP

PUMPING CHAMBERS

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Page 3: Hydraulics: An inside look at pumps...Hydraulics: An inside look at pumps I These hard-working mechanisms are at the heart of any hydraulic system 18-21 tlt fundamentals 8-04 7/8/04

efficiency—and initial cost (Figure 2). Vane pumps are less tolerant of contami-

nation than gear pumps as particulate con-tamination tends to cause uneven wear ofthe vanes. Vane pumps can operate in therange of 1,000 to 3,000 psi system pressures.

Piston pumps are the Cadillacs of hy-draulic pumps. They are the most efficient,most expensive and highest performing.The pumps are capable of operating up topressures of 10,000 psi in radial pistonpumps.

Piston pumps are of either a radial oraxial design, where the pistons radiate fromthe axis of a circular cylinder block likewheel spokes, or where the pistons andcylinders are parallel to each other and theaxis of rotation (Figure 3). Piston pumps aremade with very fine tolerances and, there-fore, are very sensitive to abrasive wearfrom particulate contamination.

Piston pumps can be made of fixed orvariable displacement designs. Variabledisplacement designs allow for compensa-tion for system pressure changes and,therefore, are the most efficient on theorder of 92%-97%. Pump efficiency is asubject for yet another article, but in theinterim give Brad a call if you have furtherquestions.

Regardless of the type of pump used inour system, newly assembled hydraulic sys-tems must be thoroughly cleaned andflushed before startup. All sources of con-tamination must be aggressively mini-mized. And new or top-up fluid must be fil-tered as it is added, as a properly operatingwell-filtered hydraulic system will likely becleaner than the new fluid. In addition tocleanliness, type of fluid, temperaturerange/viscosity of fluid, fluid condition (oxi-dation, water contamination, etc.), systemloading (pressure), aeration and cavitationcan all affect pump and pump life.

In future articles we will take a closerlook at other system components and bestpractices, but for now: What does ahydraulic pump provide?

(If you don’t know, go back to ParagraphTwo, reread the first sentence, write it down,then rewrite it 500 times.) <<

Bob Gresham is STLE’s director of professionaldevelopment. You can reach him at [email protected].

Photo Illustrations provided by Brad Poeth —Eaton Corp.

20 A U G U S T 2 0 0 4 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

Figure 3.

AXIAL BENT-AXIS PISTON PUMP

Case Drain

Rotation

Outlet

Inlet

Swash Plate

Cylinder Block

PistonPre-Load Spring

Spherical Washer

Shoe Retractor Plate

Figure 15.32 An In-Line Piston PumpCOPYRIGHT (1999) VICKERS, INCORPORATEDC

Piston pumps are of either a radial or axial design, where the pistonsradiate from the axis of a circular cylinder block like wheel spokes, orwhere the pistons and cylinders are parallel to each other and the axisof rotation .

Hydraulic Pumpsprovide one and

only one function:to provide flow of

the hydraulic fluid.

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