A nodding donkey is a type of pump that is commonly used to extract oil if the oil is not carried to the surface naturally by internal pressure. The device has a component that bears a resemblance to a donkey or horse head, which moves up and down during operation. This vertical motion causes a long sucker rod to move through the well bore, which activates a pump at the bottom end. Oil, or an emulsion of water and oil, is then forced up to the surface where it can be collected. Nodding donkey pumps are commonly known as pump jacks, though other terms such as thirsty bird, horsehead pump and grasshopper pump are also used.
Many oil deposits require no pumping to extract the hydrocarbons. Since oil deposits tend to be pressurized, drilling a pipe into a supply is often all it takes to draw the oil up to the surface. This is not always the case, and even a well that start out pressurized can lose that pressure as the deposit is depleted. If the pressure at the bottom end of the pipe is not great enough to force the oil upwards, then some type of pump is typically required. Pump jacks were first designed around 1925 to address this issue, and similar designs are still in use.
Pump jacks are typically associated with wells that do not produce a large amount of oil, which are often referred to as stripper wells. Some of these wells simply lack large deposits of oil, while others have become depleted over time. Many of them produce 10 barrels or less of oil each day. About five to 40 liters (about 1.3 to 10.5 gallons) of liquid can be pumped for each stroke of a nodding donkey depending on the configuration.
The basic design of a nodding donkey consists of a beam mounted on a scaffolding so that it can tilt forward and backward. One end of the beam has a component that resembles the head of a donkey, and the other typically has a pitman arm connected to a counterweight. The end with the counterweight is also attached to a component that is capable of powering the device.
Early versions of the nodding donkey were powered by rods that connected to a device called the central power. This power source often actuated many pump jacks simultaneously, though modern versions often use individual electric motors. In either case the power source rotates the counterweight, which pushes the pitman arm up and down. This in turn causes the nodding donkey to perform the action it is named for. A pump at the bottom end of the well is then actuated by a sucker rod that is connected to the pump jack.
