3.4.1 Gear pump

Gear pump

Gear pump is a pump where two gears, one gear being driven by the other, transport fluid in the gear pockets. A common feature of all gear pumps is that they have two gears, the driven gear usually runs in plain bearings. The bearings and shaft journal are located in the pump casing and surrounded by the pumped fluid. These bearings are thus dependent upon the lubricating qualities of the pumped fluid. Gear pumps are not therefore suitable for handling non-lubricating fluids. For reasonably acceptable pump life, gear pumps should not used for such dry liquids as water or petrol. Paraffin and diesel oil are examples of liquids whose lubricating properties, whilst not being high, are perfectly acceptable for gear pumps.

The simplest gear pump has two external gears, figure 3.41a. This type is used for relatively free-flowing fluids and can produce quite high pressure increases.

Figure 3.41a Gear pump with external gears.

A far greater range of application is possible with gear pumps having a small rotor mounted eccentrically in respect to a larger external gear. A crescent shaped partition separates the two gears figure 3.41b. When the rotor is driven, the gear also rotates. The difference between the diameters of the rotor and gear and the eccentric location means that the gear teeth engage only at one point. The tooth of the gear successively rotates out of the gear pocket in the rotor during the first half of rotation. This induces a vacuum and the gear pocket is filled with fluid from the suction line. The fluid is forced out into the delivery line by the gear teeth during the second half of rotation.

The number of teeth is kept to a minimum in order to make maximum use of material to obtain deep pockets between the teeth and achieve large displacement. By using sophisticated tooth forms the number of teeth for external
gear pumps can be as low as 10-15. Internal gear pumps can utilize the advantages of more favorable gear tooth engagement and therefore have even less teeth.

The pressure of fluid causes side forces on the gear which are taken up by the bearings. The axial force is usually relatively small, although the variations in side clearance and resulting pressure variations can cause wear. Wear which causes increased gear clearance does not affect the internal leakage (slip) as much as wear at the periphery and sides.

It is important to know the temperature range within which a gear pump is going to operate. Working components must take up greater clearances than normal if the pump is to operate with fluids at high temperatures. Some manufacturers supply pumps suitable for operating at temperatures of up to 300°C.

Gear pumps should not be used for fluids containing solid particles and should only be used with care for abrasive fluids. Wear can be reduced, when handling abrasive fluids, by selecting a pump having a somewhat larger capacity enabling it to operate at lower speeds. Gear pumps are not suitable for use within the foodstuffs industry where hygiene requirements must be complied with. They should not be used for handling shear-sensitive fluids.

The respective uses and applications for external and internal gear pumps varies somewhat. Greater pressure increases and increased flow is obtained with external gears, whereas internal gears are more suitable for high viscosity fluids and have better suction capabilities. To obtain a good capacity loss factor (filling efficiency of the gear pocket) it is always necessary to adjust pump speed to suit the viscosity of the fluid. A high viscosity requires a lower speed.

Gear pumps are made of grey cast iron, bronze and acid resistant steel, although more exclusive materials may be used. Shaft seals consists of stuffing boxes or mechanical seals. To increase pump life when handling non lubricating fluids, special precautions such as hard-faced shaft journals, self-lubricating plain bearings and special gear tooth surface treatment may be necessary.

Gear pumps are self-priming with a suction capacity of 4-8 metres. Pressure increases of 0.3-3 MPa (3-30bar / 43-435psi) are normal and up to 10 MPa in extreme cases. The variations depend upon the construction of the bearings and seals. Pumps should not be run at maximum pressure for fluids having poor lubricating qualities. Sometimes only 15% of the pump’s maximum pressure increase can be utilized.

There are a great number of models suitable for small flows designed for laboratory use, whilst larger industrial pumps have maximum flows of up to 300 m³/h. Flow is not pulsation free but varies with number of teeth and rotational speed. Two gear pumps where one is equipped with larger number of small teeth and slow speed, will have less pulsations in flow than a similar pump (same pitch diameter)  with large teeth (fewer) and high speed.

Figure 3.41b Internal gear pump showing the 3 working phases, filling, transfer and delivery.