INTRODUCTION OBJECTIVES - Plant Services

Chapter 4 — Hydraulic Pumps 107Displacement. Displacement is the volumeof hydraulic fluid moved during eachrevolution of a pump’s shaft. Displacementis rated in cubic inches per revolution ofthe pump shaft and is calculated by applyingthe following formula:D = rpm × P rwhereD = displacement (in cu in. shaftrevolution)rpm = revolutions per minute of primemoverP r= pump shaft revolutions (in cu in.)Example: What is the displacement of apump with revolutions of 1.94 cu in. thathas a prime mover with revolutions of1120 rpm?Example: What is the gpm of a pump thathas a displacement of 4 cu in. that operatesat 1725 rpm?TERMSDisplacement is thevolume of hydraulic fluidmoved during each revolution,stroke, of a hydraulicpump shaft.A prime mover is a devicethat supplies rotating mechanicalenergy to a fluidpower system.D = rpm × P rD = 1120 × 1.94D = 2173 cu in./minA prime mover is a device that suppliesrotating mechanical energy to a fluid powersystem. The two main types of prime moversused in fluid power systems are electricmotors and internal combustion engines.See Figure 4-2.Gallons Per Minute. When used in fluidpower systems, gallons per minute is thenumber of gallons of fluid that a pumpcan force into the system every minute.Gallons per minute (gpm) is a measureof fluid flow that is used to measure smallvolumes of intermittently flowing fluidssuch as pump discharges. See Figure 4-3.Gallons per minute in a pump is typicallymeasured with a flow meter attached to thehydraulic system, but can be calculatedif certain variables are known. Gpm in apump is calculated by applying the followingformula:displacement × rpmgpm =231wheregpm = gallons per minutedisplacement = cubic inches of flowper revolutionrpm = revolutions per minute231 = constant (cu in. in one gal)Figure 4-2. The two main types of prime movers used in fluid power systemsare electric motors or internal combustion engines.Figure 4-3. Gallons per minute is a measure of fluid flow that is used to measuresmall volumes of intermittently flowing fluids such as pump discharges.