Quimby Screw Pump

Electrically driven pumps are now extensively used in connection with domestic water supplies to raise water to the house tank. A type of electrically driven pump extensively used is the Quimby Screw Pump, shown in Fig. 101. This type of pump is suited principally to forcing water and not to raise it by suction; hence to operate successfully it should be set at such a level that water will flow into it by gravity. When water does not flow to the pump by gravity, the suction pipe should be made short and straight as possible, and should be provided with a foot valve. The four screws that act as pistons in propelling the water are mounted in pairs on parallel shafts and are so arranged that in each pair the thread of one screw projects to the bottom of the space between the threads of the opposite screws. The pump cylinder fits the perimeters of the threads closely without actual contact, and the faces of the intermeshing threads make a close running fit without bearing on and wearing the face of the screws. There is no end thrust on the screws in their bearings, because the back pressure of the column of liquid is delivered to the middle of the cylinder and the endwise pressure upon the screws in one direction is exactly counterbalanced by a like pressure in the opposite direction. The suction opens into a chamber underneath the pump cylinder and the liquid passes through this chamber to the two ends of the cylinder, and is forced from the two ends towards the center by the action of the two intermeshing pair of threads, and thence out through the discharge port to the house tank. The power to drive the pump is applied to the main shaft, a, and part of it is transmitted to the auxiliary shaft, b, by the gears, c.

Pumps for house service are usually fitted up to work automatically. The manner of so connecting a Quimby Pump is shown in Fig. 102. The pump is operated by a direct connected electric motor that is controlled by a weighted float in the house tank. When water in the tank is low, the weighted float raises the chain and counterweight, a, until the disc, b, trips the switch lever, c, throwing the contact bar, d, over, as shown by dotted lines, to close the circuit and turn the electric current on to the motor. Then, as the tank fills with water, the float raises and the counterweight pulls down on the chain until the upper disc trips the lever, c, thus breaking the circuit and shutting off current from the motor. By adjusting the two discs the pump can be made to operate under the slightest loss of head in the tank, but it is better to so place the discs that they will close the switch when the tank is almost empty and open it when the tank is full. This avoids frequently starting and stopping the pump and insures a frequent change of water in the tank.

Fig. 102

Screw pumps run at speeds ranging from 900 to 1,400 revolutions per minute, according to their size and the service under which they operate. Direct current no, 220 or 500-volt motors of General Electric, Crocker-Wheeler or Sprague types, are found most satisfactory for this work. The size, capacities, etc., of Quimby Pumps can be found in the following table:

Table XXXVIII - Size And Capacity Of Quimby Pumps

Electrically driven pumps of the plunger type are also used for house service pumps. Pumps of this type, however, should be provided with a rheostat or starting box to turn the current on to the motor gradually. If the full current were turned on instantly the armature would probably be burned out; also the pounding due to suddenly starting in motion a large column of water might injure some of the more delicate working parts of the pump.

Hot Air Pumping Engines are extensively used for supplying water to country or suburban residences, and in tall apartment houses to pump water from the service pipe to the house tank on the roof. This type of pump can be operated by any kind of fuel and requires no skilled help to run it. In suburban localities, where a hydraulic ram or a windmill would not be practicable, a hot air pumping engine will prove the next least expensive to operate.