GEAR PUMPS :
Definition : When a pair of meshing gears are used to increase pressure of oil, it is known as gear pump.
Principle of gear pump:
It works on the principle of increasing the pressure of oil by using meshing of gear teeth one after another by squeezing oil between meshing teeth and forcing oil in forward direction.
Types of Gear Pumps
(a) External gear pump
(b) Internal gear pump
(c) Ge-rotor pump
● External Gear Pump :
When driver gear mesh with driven gear externally and used for increasing pressure of oil, it is known external gear pump.
Important features of external gear pump:
(1) It uses spur/helical gears.
(2) It can operate for maximum pressure of 300 bar.
(3) It can give discharge of 300 lit/min.
(4) It has volumetric efficiency about 65-85%
(5) It has low contamination sensitivity.
(6) It has low weight, low cost
Construction :
The basic gear pump consists of:
(a) Gear pair :
◆ Two meshed gears usually spur gears or helical gears are used.
◆ Each gear is mounted on a shaft which is supported on bearings in the end covers.
◆ One of these shafts, the drive shaft is coupled to the prime mover (electric motor).
(b) Housing :
◆ It consists of two cover plates that enclosed the ends of the gears. The gear pair is mounted in bearings and mounted precisely with minimum clearance in the housing.
(c) Suction and delivery port:
◆ For suction of oil in the pump, suction or inlet port is provided while delivery port is provided to discharge high pressure oil.
◆ These two ports are located on opposite sides of the gear case and it is essential that these ports should open directly on opposite sides of the mesh point of the gears.
Working:
◆ When motor rotates, it will rotate the driver gear and the driven gear. As the meshed gears start rotating one tooth space after another is evacuated and in the resulting space a vaccum is created.
◆ Atmospheric pressure in the tank forces the oil into the tooth space from the port (suction port) located at that point.
◆ This tooth space full of oil is carried around the periphery of the gear until the teeth again mesh and the oil is forced out of the space by the meshing tooth and flows out of the delivery port located at that side of the mesh point.
◆ The meshing of the teeth prevents the oil flowing back from the pressure to the suction side of the pump.
◆ The pressure is developed tooth by tooth from the suction side to the delivery side linearly.
◆ The pressure at the pressure side is determined by the resistance in the system. The most important resistance is the load on the hydraulic cylinder or hydraulic motor.
◆ In order to avoid cavitation, the suction pressure of the pump should not below 0.1 to 0.2 bar, below atmospheric pressure.
Multi Gear Pair
Definition: When there are 3 or 4 gears are mounted in a casing of gear pump, it is known as multi gear pump.
Construction and working :
◆ It consists of 3 or 4 gears placed in one casing. The centre gear is connected to the motor shaft.
◆ The two outside gears are driven and hence rotate in the direction opposite to each other.
◆ The advantage of this pump is that it has two independent outputs and they may be connected together as one.
◆ The main disadvantage is the short sealing range of the centre gear thus limiting the system pressure.
(b) Internal gear pump:
Principle :
◆ When internal meshing gear pair is used along with Crescent shaped separator to increase pressure of oil. it is known as internal gear pump it utilizes the same principle of external gear pump.
Construction :-
It consists of following main parts:
(1) Internal tooth gear : The internal spur gear is connected to the motor shaft.
(2) External tooth gear: It is mounted eccentrically with respect to internal tooth gear. It is driven by internal tooth gear.
(3) Crescent shaped separator : Between the above two gears, on one side there is a crescent shaped spacer is provided. It is stationary part of the housing around which oil is carried.
(4) Inlet and outlet ports: The inlet and outlet ports are located in the casing for suction and delivery of oil.
Working :
◆ When motor shaft rotates, it will rotate the internal gear and it drives the external gear.
◆ It affects a fluid fight seal at the meshing point of the teeth. The teeth which are un-meshing at the inlet port creates partial vacuum and suction of oil occurs.
◆ Oil is get trapped between internal and external gear teeth and both side of the crescent shaped spacer.
◆ Meshing of gear teeth reduces the volume in the high pressure cavity near the outlet port and discharge high pressure oil from the outlet port.
◆ The internal gear pumps are very efficient and produce less noise. The volumetric efficiency is about 95% and overall efficiency 85 - 90%.
◆ It can produce high pressure in the range of 350 - 400 bars,
Principle :
It works on the principle of internal gear pump by using gear shaped rotOr elements meshing internally. The word gerotor is derived from "Generated rotor".
Construction:
◆ It's construction is similar to internal gear pump but separator or crescent is not provided.
◆ It consists of following parts:
a) Outer ring : The outer ring is a casing in which ge-rotor pair is mounted.
(2) Outer ge-rotor : Outer generator in idler and it has always one tooth more than inner ge-rotor.
(3) Inner ge-rotor : The inner ge-rotor is the driver. The number of teeth of the outer and inner generator varies. The axis of inner ge-rotor is offset from the axis of the outer generator.
(4) Suction and discharge port: The suction and discharge ports are provided to suck and deliver oil respectively.
Working:
◆ When inner ge-rotor rotates with the help of motor, it creates partial vacuum near the suction port when mesh with the outer ge-rotor. The oil enters through the suction part.
◆ If the numbers of teeth for the inner and outer generator are 4 and 5 respectively, then while the inner generator makes one revolution, the outer one makes only 4/5 or 0.8 revolution.
◆ Due to offset movement between inner and outer ge-rotor, the volume near the delivery port decreases and oil is forced out through delivery part.
◆ The care should be taken as ge-rotors are to be manufactured with high precision otherwise internal leakage becomes excessive.
◆ They are more compact than external gear pumps.
◆ It can operate at the pressure of 125 bar, maximum speed of 2000 - 3600 rpm with maximum discharge of 200 lit/min. .
Advantages of ge-rotor pump:
1. High speed
2. Constant and even discharge
3. Operates well in either direction
4. Quiet operation.
Advantages and Disadvantages of gear pump.
● Advantages :
(1) Few moving parts :Gear pump has only two moving parts hence minimum wear and maintenance.
(2) Simple design and construction: The construction is simple and design is easy.
(3) Less cost : The gear pumps are cheaper than screw pump, vane pump.
(4) Easy maintenance : Gear pumps are simple for repair and maintenance.
(5) It can allow oil with small amount of impurities.
● Disadvantages :
(1) Noisy operation : Gear pumps make noise during its operation.
(2) Leakage of oil : Internal oil leakage occur in some type of gear pumps e.g. ge-rotor pump.
(3) Low pressure : It is used for applications where low pressure in the range of 300 to 350 bar is required.
