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Gear Pumps

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*5/23/2012

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Notes

“A gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. Gear pumps are also widely used in chemical installations to pump fluid with a certain viscosity. There are two main variations; external gear pumps which use two external spur gears, and internal gear pumps which use an external and an internal spur gear. Gear pumps are positive displacement (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. Some gear pumps are designed to function as either a motor or a pump.” (Wikipedia, Gear Pumps, 5/23/2012)

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 “Pumping apparatus that pumps molten polymer (polymer) and pressurizes that polymer can contain a pair of opposed shafts, each shaft carrying teeth that force viscous polymer from the inlet of the pump to its outlet. The pressure under which the polymer exists at the outlet of the pump is substantially elevated above the pressure existing at the inlet of the pump. For example, with high density polyethylene (HDPE), the inlet pressure can be from about 30 to about 40 psig at from about 3500 to about 5500 Fahrenheit (F), whereas the outlet pressure can be from about 2,000 to about 3,000 psig at from about 375.degree. to about 575.degree. F.

The polymer fills the space between the teeth at the inlet side and is conveyed to the outlet side of the pump, after which the teeth are brought to their point of closest approach, i.e., cyclically into meshing engagement with one another, the engagement serving to exclude the polymer and generate pressure. The design of the teeth is such that the clearance between adjacent surfaces is minimized in part to prevent back flow of polymer from the high pressure outlet side of the pump back into the lower pressure inlet side. The greater this back flow of polymer, the less efficient the operation of the pump, causing the pump's turning speed to be increased to compensate, and wasting energy in the operation of the pump.

Accordingly, to prevent this undesired back flow of polymer, the registry of the pump teeth relative to one another when in meshing engagement must be timed to be very close, but without any actual physical contact of the meshed teeth. If the teeth contact one another when meshed, premature and undesired wear of the teeth occur thereby not only allowing back flow of polymer, but also requiring shutdown of the pump and an expensive, premature reworking of the worn teeth. Since each shaft of such a pump can cost as much as $100,000, it is desirable to maintain the non-touching registry of the teeth on these opposed shafts for as long as possible. For example, when the desired non-touching teeth registry is maintained, the operating life of such a pump can extend for up to 5 years, whereas if touching during pumping occurs, this life span can be reduced to 2 years at the very best.”

[Moldt and Everitt, US Patent 8,177,535 (5/15/2012)]

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Roger D. Corneliussen
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Copyright 2012 by Roger D. Corneliussen.
No part of this transmission is to be duplicated in any manner or forwarded by electronic mail without the express written permission of Roger D. Corneliussen
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* Date of latest addition; date of first entry is 5/23/2012.