The background of a gear pumps
The background of gear pump
As shown in pic 12.18, when the outer diameter of both gears are partly surrounded by casings, and the upper gear turns clockwise while the lower gear turns counter-clockwise, the fluid between tooth grooves moves from left to right of the gear and the fluid is transferred, avoiding flowing backwards from right to left at the meshing part.
This is called a gear pump, whose simple structure works reliably and is widely used for reliably small capacity pumping. Some gear pumps use internal gears as in pic 12.19.
Pic 12.18 External gear pump (left)
Pic 12.19 Internal gear pump (right)
Various tooth profiles such as the involute tooth profile are used for gear pumps. While spur gearing is widely used, helical gears and multistage gears are also used to make the rotation smooth and reduce flow rate change associated with the change of meshing point. As for a normal gear which has two or more pair of gears meshing, the capacity between the two contact portions of meshing part changes, and unless suitable release grooving is arranged, high pressure is generated because of the compression of fluid in the capacity decreased section while bubbles are generated because of vacuum in capacity increased section. Therefore, the tooth profile (called continuous contact tooth profile) whose single contact point moves on the tooth flank continuously and the contact point traces out a figure-of-eight is sometimes used to eliminate the closed space as in pic 12.20.
Pic 12.20 Continuous contact type gear pump
However, this tooth profile has the transmission performance equivalent to only one half pitch when straight teeth are used. Therefore, you need to install another transmitting gear outside the casing, or set the helical pitch to more than one half pitch. Setting the helical pitch to an integral multiple of one half pitch has the advantage that the flow rate remain unchanged.
Pic 12.21 Root type gear pump
This type of tooth profile includes the one whose addendum flank is composed of a semicircular arc, cycloid tooth profile as shown in pic 12.21, and one whose tooth profile is a sine curve.
When helical gearing is used, an open space may be formed if the torsional amount is excessive. There are two types of open space : one is the case of a space alongside inner surface of the cylinder. The condition to prevent this is that the flat part (doesn’t have to be the full width) of at least one tooth’s tip contacts the inner surface of casing cylinder constantly throughout both sides of the gear.
Another type of open space occurs at gear meshing point. The condition to prevent this is at least one continuous contact line constantly goes throughout both sides of the gear. This condition is always satisfied in case of continuous contact tooth profile. As for discontinuous contact line, assuming the meshing ratio is n0, the condition is gained as shown below.
When you have equality, only one complete contact line exists and there is no closed space as long as backlash exists. If you have inequality, there are two or more complete contact lines and closed space becomes a problem.