Hydraulic Systems

Hydraulic Systems Chapter 11

Material taken from Goering, 2003, Off-Road Vehicle Engineering Principles

Basic Principles “ Hydraulic

fluids are virtually incompressible at the pressures used in hydraulic systems. “ Liquids transmit pressure equally in all directions. “ The next slide is an illustration of a hydraulic jack.

Basic Principles con’t “ “

By applying a downward force on the small piston, the jack can be made to lift a heavy mass on the large piston. The liquid fills the entire volume between the two pistons, and because it is incompressible, some liquid must move into the large piston chamber when the small piston is pushed downward.

Goering, 2003, Off-Road Vehicle Engineering Principles

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Basic Principles con’t “ The

pressure beneath the small piston is equal to F1 / A1. “ The same pressure acts on the large piston because the liquid transmits pressure equally in all directions. “ Because A2 > A1, the force on the large piston is much larger than the force on the small piston.

Pump Systems “

Open loop: provides cooling medium

“

Closed Loop: temperature build up Vickers, Mobile Hydraulics Manual

Lab-Volt, Hydraulic Trainer Manual

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Standard Symbols “ Hydraulic

circuit symbols were standardized by the NFPT and by the International Standards Organization (ISO). “ The small arrow on the pump symbol indicates pressurized oil is being forced into the motor. “ Drawing an arrow through a component indicates the component is variable rather than fixed.

Standard Symbols con’t “ Although

most hydraulic systems have only one reservoir, the symbol can be used at more than one place on a drawing of a hydraulic circuit to eliminate the need to draw numerous return lines. “ This is analogous to the drawing of electrical circuits, where the electrical ground symbol can be used at many places in a circuit drawing.

Electric – Hydraulic Analogy

Vickers, Mobile Hydraulics Manual

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Vickers, Mobile Hydraulics Manual

Vickers, Mobile Hydraulics Manual

Hydraulic Pumps “ The

3 basic types of hydraulic pumps are gear pumps, vane pumps, and piston pumps. “ The pump converts mechanical power at the pump shaft into hydraulic power at the pump outlet.

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Gear and Vane Pumps “ The

gear pump and the vane pump are examples of fixed-displacement pumps. “ The pump displacement is the theoretical volume of oil the pump can deliver per revolution of the pump shaft. “ In the gear pump, the displacement is equal to the volume of space between 2 gear teeth and the housing, multiplied by the total number of gear teeth on both gears.

Gear Pump

Kurtz, 1999, Machine Design for Mobile Applications

Gearotor Pump

Kurtz, 1999, Machine Design for Mobile Applications

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Gear and Vane Pumps con’t “ In

a vane pump, the rotor turns counterclockwise (ccw); centrifugal force keeps the vanes in the rotor slots in contact with the housing and oil is carried from the inlet to the outlet in the spaces between the moving vanes. “ The displacement of both the gear and the vane pumps is fixed when the pumps are manufactured and can not be changed.

Vane Pump

Kurtz, 1999, Machine Design for Mobile Applications

Vane Pump

Kurtz, 1999, Machine Design for Mobile Applications

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Axial Piston Pumps There are two types of piston pumps: axial and radial piston pumps. “ A radial piston pump has pistons that move perpendicular to the pump axis. “ Axial piston pumps received their name because the pistons move parallel to the axis of rotation. “ The pistons are carried in a rotating cylinder barrel. As the piston shoes slide along the cam plate, the angularity between the drive shaft and the barrel forces the pistons to reciprocate in their bores. “

Fixed-Displacement Axial Piston Pump

Material taken from Goering, 2003, Off-Road Vehicle Engineering Principles

Axial Piston Pumps con’t “ This

pump has fixed displacement because the angle between the drive shaft and barrel is fixed. “ The displacement is equal to the area of each piston times the piston stroke times the number of pistons. “ Similar pumps are available in which the angularity can be controlled to vary the pump displacement. These pumps are called variable-displacement pumps.

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Axial Piston Pumps con’t “ The

swash plate on the variabledisplacement axial piston pump can be tilted to control the displacement. “ Because of the heavy forces on the swash plate, internal hydraulic cylinders are used to control the tilt.

Variable Displacement Piston Pump

Kurtz, 1999, Machine Design for Mobile Applications

Piston Pump Action

Kurtz, 1999, Machine Design for Mobile Applications

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Pump Delivery “ The

theoretical delivery from any hydraulic pump can be calculated by: Qpt = (DpNp) / 1000 “ Qpt =

theoretical pump delivery, L/min pump displacement, cm3/rev “ Np = pump speed, rpm “ Dp =

Pump Delivery con’t “ Pump

delivery is actually less than the theoretical amount because of internal leakage from the outlet port back to the internal port. “ So, pumps have a volumetric-efficiency: epv = Qpa / Qpt = (Qpt – Qpl) / Qpt “ epv =

pump volumetric efficiency, decimal actual pump delivery, L/min “ Qpl = pump internal leakage, L/min “ Qpa =

Pump Torque “ The

theoretical torque required to turn a pump shaft can be calculated by: = ∆pDp / 2π “ Where Tpt = theoretical “ Tpt “ ∆p

pump torque, N*m = pressure rise across the pump, MPa

“ Internal

friction causes the actual torque demand to be greater than theoretical Æ the pump has a torque efficiency: “ ept

= Tpt / Tpa = Tpt “ Where ept = pump

/ (Tpt + Tpf) torque efficiency, decimal “ Tpa = actual pump torque, N*m “ Tpf = pump friction torque, N*m

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Pump Power “ Hydraulic

power is the product of pressure and flow rate. Across a circuit with the same flow in as out, the power change: “ Ph

= Q∆p / 60 Ph = hydraulic power, kW “ Q = flow through device, L/min “ ∆p = pressure rise or drop across device or circuit, MPa “ Where

Pump Power con’t “ If

there is a pressure rise across the device, as in a pump, then the hydraulic power is positive. “ The

device will produce hydraulic power

“ If

there is a pressure drop across the device, as in a hydraulic motor or an orifice, then the hydraulic power is negative.

Pump Power con’t “ The

actual pump power is the power delivered to the pump shaft by the prime mover (usually an engine or an electric motor) and can be calculated using: “ Pin

= Tin*Nin *1.05*10-4 (kW)

“ The

actual power into the pump is greater than the theoretical (hydraulic) power output because of pump friction and internal leakage.

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Pump Power con’t “ The

pump power efficiency is:

“ epp

= Pph / Pps = epvept epp = pump power efficiency, decimal “ Pph = hydraulic power from pump, kW “ Pps = shaft power into pump, kW “ Where

Pump Efficiencies “ The

next figure shows pump efficiencies for a particular pump, but the curve shapes are typical. The efficiencies are plotted versus a dimensionless parameter, oil viscosity times pump speed divided by pressure rise. “ The volumetric efficiency of a pump declines with increasing internal leakage. The leakage is driven by the pressure difference between the outlet and inlet ports of the pump. If there were no pressure difference, the internal leakage would be zero and the volumetric efficiency would be 1.0 (100%).

Pump Efficiencies con’t “

Internal leakage increases and volumetric efficiency declines as the pressure differential increases. Goering, 2003, Off-road Vehicle Engineering Principles

•epp = pump power efficiency •ept = pump torque efficiency •epv = pump volumetric efficiency

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Pump Efficiencies con’t “ The

power efficiency is the product of the volumetric and torque efficiencies. “ The power efficiency must be zero at zero pump speed and high outlet pressure because the volumetric efficiency is zero. “ At high pump speed and low outlet pressure, the power efficiency also approaches zero because the torque efficiency approaches zero. “ There is always some combination of pump speed and outlet pressure that yields maximum power efficiency.

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