A typical example of a hand pump is the bicycle pump. This common air pump is used to inflate bicycle tires, sports balls and rafts. The body of the pump is a narrow cylindrical compression chamber made of metal that stands upright on the ground. The tube is closed at both ends with a hole in the top cap for the plunger rod and one or two foot rests at the bottom. There is a wooden or plastic handle at the top attached to a metal plunger rod with a rubber piston at the bottom. A metal compression spring is often attached to the plunger just above the piston. The piston is only 1/2 to 3/4 of an inch tall, with a tapered shape that retreats slightly from the inner compression chamber at the top, and then spreads to the diameter of the tube at the bottom. A rubber outlet hose is attached to the bottom of the tube with a free-spinning metal fitting that has a threaded inner sleeve to fit around the valve stem of a tire with a central block for contacting the valve pin. Adaptors are available for attachment to the inner threads with needles for sports balls or special couplers to fit various valves on inflatable rafts. Many pumps also have air pressure gauges. The average bicycle pump stands two feet tall, but some models are designed smaller to clip against the frame of a bicycle for easy transport. These pumps are small enough to work between the hands of an operator.
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The sleeve of the hose is twisted onto a valve stem by hand. The operator then lifts the handle to prepare the pump for filling the tire. This puts the piston at the top of the compression chamber and raises the handle for the operator to force downward. Air enters the tube through the hole in the top cap around the plunger rod. The piston allows air to flow past its tapered flair and into the compression chamber as it lifts inside the cylinder. Air trapped under the piston is ready to be forced into the tire under pressure.
When the operator forces the pump handle downward, the piston at the end of the plunger rod is pushed to the bottom of the compression chamber. Air flows into the rubber hose and pushes the valve open. After passing through the valve, the air flows through the tire stem and into the tube. The valve only allows air to flow in the direction of the inflatable. The pressure on the valve drops after the compressed air is forced into the tire and the valve closes.
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