The Rotary Gear Pump
The rotary gear pump is perhaps the simplest form of pump used for fire service, according to the American-LaFrance Bulletin. It consists of a pump case, two pump heads and two gears, the latter mounted on shafts running on ball bearings. There is no contact between the moving gears and the pump case or heads, but the clearance is reduced to a point where practically no slippage occurs. The pump contains no valves because, due to its construction, water is traveling in the same direction all the time and at a oniform speed. It is perhaps this absence of valves which gives an air of mystery to its operation for those who are not familiar with its construction. In the action of the rotary gear pump there are two gears having very coarse teeth which mesh with one another. The pump casj surrounds the two gears, there being just enough clearance between gears and casing to permit of rotation of the former. There are in addition the two heads mentioned which not only enclose the gears, but serve as bearings for the shafts of the gears. Now it is evident that if the two gears are rotated as described, the little spaces will be filled with air, which will be carried around to the top. These little “chunks” of air cannot be carried back into the bottom for the reason that as the teeth of the two gears come into mesh the air is crowded out and its only chance of exit is at the discharge. If the suction end were connected to a source of water supply the water would gradually enter the pump to replace the air which has been carried out by the spaces. After all the air has been removed the water will be raised and carried to the top as just described. It will be noted that the teeth on these gears are large and therefore the space between the gears will hold quite a considerable amount of water. If for instance a space held one pint of water and there are 16 spaces on the two gears, one complete revolution of the gears would place 16 pints or two gallons of water in the upper chamber. If the .pump made 500 revolutions per minute this would be 1,000 gallons per minute that it would deliver.