1. Chapter 2 Hydraulic Pumps and Motors
Objectives:
The purpose of this chapter is to describe:
1. Operation and performance of hydraulic pumps and motors
2. Gear, vane, and piston pumps.
3. High-speed hydraulic motors and low-speed, high-torque hydraulic motors.
Upon completing this chapter, you should be able to:
Explain the operation of gear, vane, and piston pumps.
Evaluate the performance of pumps and motors by determining the volumetric, mechanical, and overall efficiencies.
Distinguish the different types of pumps and motors.

2. 2.1 Introduction Mechanic energy → Pump → Hydraulic energy
Hydraulic energy → Motor → Mechanic energy
The pump and motor is reversible in operation principle but slightly different in structure
2.1.1 Basic principles of hydraulic pumps and motors
1 Classification
Hydraulic pumps are broadly classified as positive displacement pumps or nonpositive displacement pumps.
Nonpositive displacement pumps are used in hydrodynamic systems to transport fluids from one location to another.
Positive displacement pumps are used in hydrostatic systems to overcome the pressure resulting from the mechanical loads on the system as well as the resistance to flow due to friction.
Positive displacement pumps are further classified as gear pumps, vane pumps and piston pumps according to the construction

3. 2.1 Introduction
2 Operation principle of the positive displacement pump
Cam 1 rotates → piston 2 reciprocates.
Piston 2 is pushed to the right →chamber 4 expands →a partial vacuum is generated →The atmospheric pressure pushes fluid from the reservoir into the pump.
Piston 2 is moved to the left → chamber 4 reduces → to push the fluid to flow into the system.

4. Check valve 5 and check valve 6 are logically reversible, so it is impossible for them to open at the same time.
They are called valve type oil-coordinating mechanism.
Besides valve type there are disk type and shaft type oil-coordinating mechanism
The volume during the discharge stroke is called the “displacement volume” of the pump.
The volume during the suction stroke is called the “suction volume” of the pump.

5. 2.1 Introduction
This kind of pump operates on the principle of volume variation of pumping chamber, hence they are called positive displacement pumps

6. 3 Structure characteristics of Positive displacement pumps:
(1) There are sealed variable volumes;
(2) volumes(pump chambers) can be cyclically increased and decreased;
(3) High-pressure chamber and low-pressure chamber must be separated each other
(4) There is a oil-coordinating mechanism.

7. 2.1 Introduction 2.1.2 Performance parameters
1 Operating pressure and rated pressure
a. Operating pressure
The actual operating pressure of a hydraulic pump depends on its load
The maximum operating pressure depends on its pressure relief valve.
b. Performance index
rated pressure and maximum pressure
2 Displacement and flow rate
Displacement (m3/r) is the fluid volume discharged (sucked) by a hydraulic pump (motor) per revolution.
Flow rate (m3/s or L/min) is the fluid volume discharged (sucked) by a hydraulic pump (motor) per unit time.
The actuator velocity depend on the oil-supplied flow rate. v=f(q)

8. 2.1 Introduction Theoretical flow rate qt: Actual flow rate q:
3 Power, torque and efficiency
theoretical power
theoretical input (output) torque
volumetric efficiency

9. 2.1 Introduction mechanical efficiency. overall efficiency to a pump
to a motor

10. 2.1 Introduction

11. 2.2 Gear Pumps

12. 2.2 Gear Pumps Advantages and disadvantages
Gear pumps are simple in structure, convenient in manufacture, cheap in cost, compact in size, light in weight, reliable in operation, and insensitive in contamination.
Gear pumps produce large pulsation of flow and pressure, large noise level, and are unable to vary displacement.
2.2.1 Operation of the external gear pump
1 the external view of the external gear pump
Side wear plate
Pump housing
outlet
Driving shaft

13. 2.2 Gear Pumps
2 main components

14. 2.2 Gear Pumps

15. 2.2 Gear Pumps 3 operation principle
The gears come out of mesh at the right suction chamber → the volume of the sealed suction chamber expands → a partial vacuum is created →the oil is pushed into the chamber by atmospheric pressure → furthermore transported to the left chamber.
In the left discharge chamber, the teeth go into mesh →the volume decreases → the oil is forced out the chamber into the hydraulic system.

16. 2.2 Gear Pumps 2.2.2 Flow rate and pulsating rate of gear pumps
The displacement of an external gear pump
The output flow rate
In practice, because the displacement is a cycle function of rotating angle, the displacement pulsation and instantaneous flow rate pulsation exist.
The flow rate pulsation may affect the system smoothness, cause pressure pulsation, and produce vibration and noise.
Flow pulsation rate
Flow pulsation rate is one important index of positive displacement pumps.
The less the number of teeth, the bigger the pulsation rate.

17. 2.2 Gear Pumps 2.2.3 Structure character of gear pumps
Oil trapped phenomenon and eliminating method
1 Oil trapped cause
In order to make gear pumps operate smoothly, the overlap coefficient must be more than one, this means more than one pair of teeth meshing simultaneously. As a result, a quantity of oil is trapped in the space between the two pairs of simultaneous meshing teeth. no oil passage connects either inlet or outlet port.
The oil trapped volume changes as the gears move. At fist the volume decreases gradually then increases gradually.

18. 2.2 Gear Pumps

19. 2.2 Gear Pumps 2 Oil trapped harm
When the volume decreases, compressing the trapped oil forces the oil out of the clearance, leading to the instantaneous high pressure and high oil temperature, and producing additional shock load on the components such as bearings.
When the volume increases, a partial vacuum generates due to no auxiliary oil entering, hence, the air dissolved in oil escapes, and cavitations exist.
3 Eliminating method
The method of eliminating oil trapped phenomenon is to machine unloading grooves on the two side plates.

20. 2.2 Gear Pumps
2.2.3 Leakage passage and automatic compensation of end face clearance
1 Clearance seal concept:
In hydraulic pumps, the seal between moving components is achieved by the micro-clearance.
The smaller the clearance is, the less the leakage is, but greater the friction force is.
The optimum clearance is 0.02~0.05mm.
2 The leakage paths from the discharge chamber to the suction chamber in gear pumps.
(1) through the contact line of gear mesh;
(2) through radial clearance(between housing and tooth top);
(3) through the end face clearances(between gears and side wear plates).
Among the three kinds of clearance, the slippage through end face clearance is maximum( about 75%)

21. 2.2 Gear Pumps 3 Automatically compensating the end face clearance.
The floating bush bearings are used to automatically compensate the end face clearances.

22. 2.3 Vane Pumps

23. 2.3 Vane Pumps classification single-acting (hydraulically unbalanced)
double-acting (hydraulically balanced)
advantages and disadvantages
Vane pumps can produce uniform flow, low pulsation, and low noise level
Vane pumps are sensitive to contamination and slightly complex in structure.
2.3.1 Single-acting vane pumps
Operation of a single-acting vane pump

24. 2.3 Vane Pumps
1 Composition

25. 2.3 Vane Pumps
（1）rotor and vane

26. 2.3 Vane Pumps
（2）stator

27. 2.3 Vane Pumps
（3）port plate

28. 2.3 Vane Pumps
（4）pump housing

29. 2.3 Vane Pumps （5）assembling relation, inlet port and outlet port
Oil entering：
Inlet port→axial groove →stator end face → kidney shaped port→suction chamber
Oil displacing：
Displacement chamber→kidney-shaped port→ stator end face → axial groove →outlet port
Stator center O Rotor center O1
Eccentric distance e

30. 2 Operation of a single-acting vane pump
When the pump is rotating , by the action of centrifugal force, all vanes are in contact with the internal surface of the stator.
（1）Seal volumes： Vleft and Vright

31. 2.3 Vane Pumps （2）Variation of seal volumes：
Vleft decreases and Vright increases

32. 2.3 Vane Pumps
In order to make displacement and suction chamber separated, the sealing angle between the intake port and discharge port in the end plates must be slightly bigger than the angle between two adjacent vanes.
When the rotor is in the position shown in the figure, volume A and B are oil-trapped volumes.
The volumes vary as the pump rotor rotates. Consequently,oil trapped phenomenon occurs.
3 Oil trapped phenomenon and eliminating method

33. Eliminating method : to machine triangle relief grooves
2.3 Vane Pumps
Eliminating method : to machine triangle relief grooves

34. 2.3 Vane Pumps 4、Characteristics of single-acting vane pumps:
(1) Vanes extend by the action of centrifugal force.
(2) Vanes slope back in the rotational direction .
(3) Varying the eccentric distance may vary the displacement.
(4) Operation pressure is lower because of hydraulically unbalanced force.

35. 2.3 Vane Pumps
2.3.2 Double-acting vane
pumps

36. 1 Characteristics of double-acting vane pumps
(1) The internal face consists of 4 arcs and 4 transition curves.
(2) Vanes extend by the action of both centrifugal force and pressurized force.
(3) the vane slots are usually tilted forward a setting angle in the rational direction. .
(4) Operation pressure is higher because of hydraulically balanced force.
(5) A double-acting vane pump is not able to vary the displacement

37. 2.3 Vane Pumps 2 Port plate of a double-acting vane pump
Because the oil-trapped volume does not vary (arc), theoretically oil trapping phenomenon does not exist. In view of manufacture error, unloading grooves are machined on the port plate.

38. 2.3 Vane Pumps (1) Structure characteristic
Because a motor must be able to reverse, vanes should be arranged radially.
3 Double-acting vane motors
(2) Force moment
Because the hydraulic force acting on vane 3 is greater than that acting on vane 1, a force moment to the rotating shaft is developed.
(3) Starting problem
There are two methods to start a vane motor: one is to arrange springs in the bottom of slots, the other is first to provide pressurized oil for the bottom of slots then to provide pressurized oil for the operating chamber.

39. 2.4 Piston Pumps classification advantages and disadvantages
The axial piston pump is high in operation pressure, high in efficiency, and able to vary displacement.
The axial piston pump is complex in structure and high in initial cost, additionally produce a little large pulsation of flow and pressure.
2.4.1 Swash plate axial piston pumps

40. 1 Operation principle of swash plate axial piston pumps
Figure 2.18 Operation of the swash plate axial piston pump
1-swash plate; 2-piston; 3-cylinder block; 4-valve plate; 5-drive shaft; 6-inlet port; 7-outlet port
The swash plate and valve plate are stationary.
The cylinder block 3 and pistons 2 rotate. While pistons reciprocate.

41. A swash plate pump has a rotating cylinder containing pistons.
A spring pushes the piston against a stationary swash plate, which sits at an angle to the cylinder.
The pistons suck in fluid during half a revolution and push fluid out during the other half.
The valve plate contains two semi-circular ports.

42. 2.4 Piston Pumps

43. When a piston rotates from down to up and extends simultaneously, the sealed volume in the cylinder bore increases, hence a partial vacuum generates, and oil is drawn in through port 6 .
When a piston rotates from up to down and retracts simultaneously, the sealed volume in the cylinder bore decreases, and oil is discharged out through port 7 in the valve plate 4.

44. The angleγ of the swash plate determines the stroke of the pistons and therefore the amount of displacement of the pump.
By varying the angle of the swash plate, variable displacement can be achieved.
Problems:
(1) The fatigue of springs
(2) The friction and wear between the swash plate and pistons

45. 2 Structure characteristics of swash plate axial piston pumps

46. 2.4 Piston Pumps （1）cylinder block
There are 7 bores, the central bore is used to connect drive shaft, the right end face is in contact with the valve plate, and the external cylindrical face is used to arrange the cylindrical bearing.

47. 2.4 Piston Pumps （2）piston and shoe
The hollow structure of the piston is used to reduce weight and inertia, the leakage is used for lubrication and hydrostatic support, and the relief grooves are used to prevent the piston from sticking。

48. 2.4 Piston Pumps
（3）Swash plate
The swash plate may swing around its shaft, a pin inside the tail groove is used to fix the off angle of the swash plate, the hollow structure of the swash plate is used to reduce weight and inertia, and the leakage via central bore is used to lubricate the pin.

49. 2.4 Piston Pumps （4）Retracting ring
The retracting ring is used to pull out pistons by the acting of the spring force ( via a steel ball ) .

50. 2.4 Piston Pumps (5)valve plate Oil trapped cause：
In order to separate the suction chamber with the displacement chamber, the distance a of transient region must be larger than the port diameter b of the cylindrical bore.
when the port of the cylindrical bore is in the transient region, the operation volume become the oil trapped volume, furthermore, the oil trapped volume changes as the pump rotates. Consequently, oil trapped phenomenon occurs.
Eliminating method：to machine triangle relief groove or orifice.

51. 2.4 Piston Pumps

52. 2.4 Piston Pumps
（6）spring
Along axis center line, the spring which does not fatigue is used to make the shoes be in contact with the swash plate.
（7）pump housing
There are 4 ports, one is inlet port, another is outlet port, the others are used for leakages.
It is possible for two leakage ports to cool or preheat the pump.

53. 3 Variable displacement mechanism
2.4 Piston Pumps
3 Variable displacement mechanism
Manual variable displacement mechanism
Manual servo variable displacement mechanism
Constant horsepower variable displacement mechanism
Overall horsepower variable displacement mechanism

54. Manual servo variable displacement mechanism
The mechanism consists of a manual servo valve and a differential piston.