The difference between the two is the difference between a helical gear and a screw. A helical gear has teeth. A screw has a thread.
A helical gear pump works on the same principle as any internal or external gear pump. Gears rotate, fluid gets trapped between the teeth of gear, the decreasing cavity then forces the fluid into the discharge nodes. The helical profile helps in the smooth discharge of fluid and lesser noise as compared to the spur gear pumps. This is because of the gradual and smooth engagement of the teeth from a point to a line profile.
A screw pump utilizes one or more screws to pump fluids. As the screws rotate, fluid gets trapped between the threads of the screws. This fluid follows the profile of the thread and travels the length of the screw before being discharged.
Clearly, screw pumps discharge with lesser pulsations than gear pumps. Also, the inertia of screw pump mechanism is often lower than that of a gear pump. Hence this could provide greater rotating speed. Though such comparative advantages and disadvantages may not be the main point of difference between these two.
In a helical gear pump the fluid flows in a direction perpendicular to the drive shaft axis and it flows parallel with the drive shaft axis in the screw pump.
The helical gear pump typically can attain higher discharge pressure than the screw pump
One interesting observation for the two pumps is the trajectory of the fluid in both the cases. In a gear pump, a certain volume of fluid travels around an arc, like baskets on a merry-go-round. In a screw pump, a certain volume of fluid moves as per the locus of the thread, mostly helical. This is akin to a nut made of fluid screwing down the spindle.
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