2.2 Gear Pumps


Figure 2.2. Sketch of section of a gear pump.

These pumps are also aptly named, since they consist of two intermeshing gears. One intriguing part of these pumps is that, for most people, the flow goes in a direction different from that first thought. Referring to the sketch on the diagram, we see two rotating gears, set in a Figure-Eight chamber, with the fluid being carried around the outside periphery of each gear before meeting again to be pumped away. As a matter of fact, the fluid is carried in discrete packets as it moves towards the exit. Each packet moves at a speed directly proportional to the rotational speed of each gear, so the total volumetric flowrate is proportional to the rotational speed of the pump.

Although the gear teeth are shown in the diagram as square-toothed, they can also be sinusoidal. If the gears are sinusoidal, then the output flowrate from one gear will vary sinusoidally, from zero to a maximum value. Corresponding output from the other gear will also be sinusoidal, but the phase difference will be exactly radians. This means that sum of the two outputs will be a steady, or uniform, flow because the two flows will exactly balance each other. While this is not exactly like destructive wave interference learned in physics in the study of light and sound waves, since we have a uniform maximum output remaining, it does appear similar.

In fiber spinning operations, the gears are normally square-toothed, with a relatively large number, say 40 or so, teeth per gear. The gears are usually designed to operate over a relatively narrow range of rotational speeds and many utilize a standard diameter, so additional capacity, or flowrate, is obtained by choosing gear units of varying thickness. This would be analogous to centrifugal pumps being designed for higher flowrates by using impellers of greater thickness. For other applications, such as pumps for heavy oils and lubricants, one normally finds fewer teeth per gear (perhaps eight or so) and a sinusoidal shape to the teeth.

With gear pumps, the clearance between gear teeth and barrel of gear housing is important, because, with an adverse pressure gradient, the liquid can leak back through the pump through this clearance. This is one of the reasons that gear pumps used in fiber spinning, which may be exposed to opposing pressures of a 1000 psi, are square-toothed. The flow between the teeth tips and the barrel is nearly Couette, which the student will recall as flow between parallel plates. One can calculate the amount of leakage by taking into account the difference between the drag flow, produced as one moving parallel produces a linear velocity profile and flow in the pumping direction, and the adverse pumping flow, which produces a parabolic type of flow in the opposite direction. This difference can be shown to depend strongly on this distance between the teeth tips and the barrel.