Lecture 6. Systems review exercise To be posted this weekend Due next Friday (3/6)

Transcription

1 150 Systems review exercise To be posted this weekend Due next Friday (3/6) Lecture 6 Coming week: Lab 13: Hydraulic Power Steering Lab 14: Integrated Lab (Hydraulic test bench) Topics today: Pumps and motors (Hydraulic Hybrids) 1

2 151 Pumps Source of hydraulic power Converts mechanical energy to hydraulic energy prime movers - engines, electrical motors, manual power Two main types: positive displacement pumps non-positive displacement pumps 2

3 Pump - Introduction 152 3

4 153 Positive displacement pumps Displacement is the volume of fluid displaced cycle of pump motion unit = cc or in 3 Positive displacement pumps displace (nearly) a fixed amount of fluid per cycle of pump motion, (more of less) independent of pressure leak can decrease the actual volume displaced as pressure increases Therefore, flow rate Q gpm = D (gallons) * frequency (rpm) E.g. pump displacement = 0.1 litre Q = 10 lpm if pump speed is 100 rpm Q = 20 lpm if pump speed is 200 rpm 4

5 Non positive displacement pumps 154 Centrifugal Pump Impeller Pump 5

6 155 Non-positive displacement pump Flow does not depend on kinematics only - pressure important Also called hydro-dynamic pump (pressure dependent) Smooth flow Examples: centrifugal (impeller) pump, axial (propeller) pump Does not have positive internal seal against leakage If outlet blocks, Q = 0 while shaft can still turn Volumetric efficiency = actual flow / flow estimated from shaft speed = 0% 6

7 156 Positive vs. non-positive displacement pumps Positive displacement pumps most hydraulic pumps are positive displacement high pressure (10,000psi+) high volumetric efficiency (leakage is small) large ranges of pressure and speed available can be stalled! Non-positive displacement pumps many pneumatic pumps are non-positive displacement used for transporting fluid rather than transmitting power low pressure (<300psi), high volume flow blood pump (less mechanical damage to cells) 7

8 157 Types of positive displacement pumps Gear pump (fixed displacement) internal gear (gerotor) external gear Vane pump fixed or variable displacement pressure compensated Piston pump axial design radial design 8

9 158 External gear pump Driving gear and driven gear Inlet fluid flow is trapped between the rotating gear teeth and the housing The fluid is carried around the outside of the gears to the outlet side of the pump As the fluid can not seep back along the path it came nor between the engaged gear teeth (they create a seal,) it must exit the outlet port. 9

10 Gerotor pump 159 Inlet port Outlet port Inner gerotor is slightly offset from external gear Gerotor has 1 fewer teeth than outer gear Gerotor rotates slightly faster than outer gear Displacement = (roughly) volume of missing tooth Pockets increase and decrease in volume corresponding to filling and pumping Lower pressure application: < 2000psi Displacements (determined by length): 0.1 in 3 to 11.5 in 3 10

11 160 Vane Pump Vanes are in slots As rotor rotates, vanes are pushed out, touching cam ring Vane pushes fluid from one end to another Eccentricity of rotor from center of cam ring determines displacement Quiet Less than 4000psi 11

12 Pressure Compensated Vane Pump

13 PC Vane Pump (Cont d) 162 Eccentricity (hence displacement) is varied by shifting the cam ring Cam ring is spring loaded against pump outlet pressure As pressure increases, eccentricity decreases, reducing flow rate Spring constants determines how the P-Q curve drops: small stiffness (sharp decrease in Q as P increases) large stiffness (gentle decreases in Q as P increases) Preload on spring determines pressure at which flow starts cutting off 13

14 163 Each piston has a pumping cycle Axial Piston Pump Interlacing pumping cycles produce nearly uniform flow (with some ripples) Displacement is determined by the swash plate angle Generally can be altered manually or via (electro-) hydraulic actuator. Displacement can be varied by varying swashplate angle 14

15 164 Thrust-plate rotates with shaft Bent-Axis Piston Pump Piston-rods connected to swash plate Piston barrel rotates and is connected to thrust plate via a U-joint More efficient than axial piston pump (less friction) 15

16 165 Radial Piston Pump Similar to axial piston pump, pistons move in and out as pump rotates. Displacement is determined by cam profile (i.e. eccentricity) Displacement variation can be achieved by moving the cam (possible, but not common though) High pressure capable, and efficient Pancake profile 16

17 167 Piston Pump - flow ripples 1 piston Pumping Filling 2 piston Total flow Each cylinder has a pumping cycle Total flow = flow of each cylinder More cylinders, less ripple Frequency: Even # cylinders n*rpm Odd # cylinders (2n)*rpm Can be problematic for manual operator (ergonomic issue) Noise Displacement = # Cylinders x Stroke x Bore Area 17

18 # of Pistons Effect on Flow Ripples n=2 n=3 n=4 n= Flow Angle - rad 18

19 169 Pumping theory Create a partial vacuum (i.e. reduced pressure) Atmospheric / tank pressure forces fluid into pump usually tank check valve opens outlet check valve closes Power stroke expels fluid to outlet outlet check valve opens tank check valve closes Power demand for prime mover (ideal calculation) (piston pump) Power = Force*velocity = Pressure*area*piston speed = Pressure * Flow rate If power required > power available => Pumps stall or decrease speed 19

20 170 Aeration and Cavitation Disastrous events - cause rapid erosion Aeration air bubbles enter pump at low pressure side bubbles expand in partial vacuum when fluid+air travel to high pressure side, bubbles collapse micro-jets are formed which cause rapid erosion Cavitation fluid evaporates (boils) in partial vacuum to form bubbles bubbles expands then collapse as bubbles collapse, micro-jets formed, causing rapid erosion 20

21 171 Causes of cavitation and aeration For positive displacement pumps, the filling rate is determined by pump speed; (Q-demand) = D * freq) Filling pressure = tank pressure - inlet pressure Q-actual = f(filling pressure, viscosity, orifice size, dirt) If Q-actual < Q-demand, inlet pressure decreases significantly This causes air to enter (via leakage) or to evaporation (cavitates) To prevent cavitation/aeration increase tank pressure low viscosity, large orifice lower speed (hence lower Q-demand) 21

22 Aeration and Cavitation

23 173 Hydraulic Motor / Actuator Hydraulic motors / actuators are basically pumps run in reverse Input = hydraulic power Output = mechanical power For motor: Frequency (rpm) = Q (gallons per min) / D (gallons) * efficiency Torque (lb-in) = Pressure (psi) * D (inch^3) * efficiency efficiency about 90% Note: units 23

24 Models for Pumps and Motors

25 Non-ideal Pump/Motor Efficiencies Ideal torque = torque required/generated for the ideal pump/motor Ideal flow = flow generated/required for the ideal pump/motor Torque loss (friction) Flow loss (leakage) Signs different for pumping and motoring mode 175 Q act ual Pump volumetric eff: Friction Q i deal Pump mechanical eff: T in/out T ideal Q l oss leakage (Reverse if motor case!! ) Total efficiency: vol Ideal pump Functions of speed, pressure and displacements 25

26 176 Hydro-static Transmission A combination of a pump and a motor Either pump or motor can have variable displacement Replaces mechanical transmission By varying displacements of pump/motor, transmission ratio is changed Various topologies: single pump / multi-motors multi (pump-motor) Open / closed circuit Open / closed loop control Integrated package / split implementation 26

27 Hydrostatic Transmission

28 178 General Consideration - Hydrostats Advantages: Wide range of operating speeds/torque Infinite gear ratios - continuous variable transmission (CVT) High power, low inertia (relative to mechanical transmission) Dynamic braking via relief valve Engine does not stall No interruption to power when shifting gear Disadvantage: Lower energy efficiency (85% versus 92%+ for mechanical transmission) Leaks! 28

29 179 Closed Circuit Hydrostat Circuit Notes: Charge pump circuit (pump + shuttle valve) Bi-directional relief Circuit above closed circuit because fluid re-circulates. Open circuit systems draw and return flow to a reservoir 29

30 180 Hydrostatic Transmission Let pump and motor displacements be D1 and D2, with one or both being variable. Let the torque (Nm) and speeds (rad/s) of the pump and motor be (T1, S1) and (T2,S2) Assuming ideal pumps and motors: Transmission ratio Variable by varying D1 or D2 Infinite and negative ratios possible if pump can go over-center 30

31 181 Hydraulic Transformer Used to change pressure in a power conservative way Pressure boost or buck is accompanied by proportionate flow decrease and increase Note: Hydrostatic transmission can be thought of as a mechanical transformer (torque boost/buck) Q 1 Q 2 D 1 D 2 Research opportunity! 31

32 Hydraulic Hybrid Vehicles 32