(a) Gear Type Pump
A gear pump is a positive displacement rotary pump. It commonly has two meshing spur gears inside a close-fitting casing. One gear is driven by the shaft and the other is an idler gear.
Operation
As the gears rotate, the teeth unmesh at the inlet side. This creates a low-pressure area, so lubricating oil enters the pump casing.
The oil is trapped in the spaces between the gear teeth and casing. It is carried around the outside of the gears from inlet to outlet.
At the outlet side, the gear teeth mesh again. This reduces the space available and forces the oil out through the discharge port.
Oil does not pass through the centre between the gears because the meshing teeth form a seal. Since a fixed volume is delivered each revolution, the gear pump is a positive displacement pump. A relief valve is therefore required to prevent excessive pressure if the discharge is blocked.
(b) Materials for Gear Pump Components
- Casing/body: Cast iron, cast steel, bronze, or aluminium alloy for small pumps.
- Gears: Hardened steel, alloy steel, stainless steel, bronze, or cast iron.
- Shafts: Carbon steel, alloy steel, or stainless steel.
- Bearings/bushes: Bronze, white metal, phosphor bronze, or ball/roller bearings.
- Seals: Mechanical seal, gland packing, nitrile/Viton oil seals.
- Relief valve parts: Steel or stainless steel spring and valve components.
For lubricating oil pumps, cast iron casing with hardened steel gears and steel shafts is common.
(c) Suitable Applications of Gear Pumps
- Lubricating oil transfer and circulation
Gear pumps are suitable because lubricating oil is clean, viscous, and has good lubricating properties. The pump gives steady positive flow.
- Fuel oil transfer and booster service
They are used for diesel oil and heavy fuel oil transfer because they handle viscous liquids well and can produce moderate to high pressure.
- Hydraulic oil systems
Gear pumps are used in hydraulic power packs and control systems because they give positive delivery and compact construction.
Other suitable uses include:
- Sludge oil transfer
- Bilge oily water transfer, where liquid is not too contaminated
- Boiler fuel oil supply
- Steering gear auxiliary hydraulic systems
- Cargo oil stripping for suitable viscous liquids
Gear pumps are not suitable for liquids containing hard abrasive solids because close clearances between gears and casing can wear quickly.