(quoted from post at 14:11:52 10/07/17)
(quoted from post at 08:21:50 10/07/17)
I don't think gear lash is an issue. The pumping action is created by oil trapped between the teeth and the housing wall being pulled around the OD of the gears. Tip clearance is the principal issue there yet we continue to see pumps fail to maintain prime even though the appear to have good tip clearance.
TOH
I respectfully disagree. Gear lash is every bit as important as the tip clearance. Look at this pump drawing and follow the oil.
As the gear teeth mesh in the center they form a seal. If there is more than a minimum of gear lash there, it makes an opening that oil under pressure can pass back thru, which results in the oil going in a circle instead of being pushed out the pressure port. The gear tips carry the oil around the sides and attempt to squeeze it out the pressure side, but that attempt fails if there is no seal between the gear teeth to stop it from going back to the suction side. Extra gear lash also makes an opening so that the pump can't "suck" the oil into the inlet. That suction also relies on a good seal between the gear teeth. Most loss off prime issues are from excess gear lash (including gear end play). There's probably another picture somewhere that shows the oil going back thru the center due to excessive lash, but I think you can see by looking at this one what the result of too much gear lash would do. Low or no pressure, low or no suction, loss of prime.
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My analysis is a bit different and here is how I would describe it.
Involute gears are made with a root clearance defined by any number of different tooth form standards - ANSI/AGMA being a major one. There is an intentional [b:1a3d1c68ba]clearance[/b:1a3d1c68ba] left between the tip of a tooth and the root of the meshing gear.
Gear lash is the measure of free play between the face of two meshed gears and is not a part of the tooth form standard. It is typically measured and/or specified at the pitch circle:
Backlash is a necessary component of almost all gear designs:
[b:1a3d1c68ba]Wikipedia[/b:1a3d1c68ba]
[i:1a3d1c68ba]Depending on the application, backlash may or may not be desirable. It is unavoidable for nearly all reversing mechanical couplings, although its effects can be negated or compensated for. In many applications, the theoretical ideal would be zero backlash, but in actual practice some backlash must be allowed to prevent jamming. Reasons for the presence of backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal expansion.[/i:1a3d1c68ba]
When designing gears and gear sets the easiest way to control backlash is with gear center spacing. The further you widen the center spacing on the gears the more you increase the lash. Moving the centers closer together reduces lash. But no matter how close you put them together, even to the point of binding, you cannot eliminate that root clearance which is there as part of the tooth form standard.
So what does all of that mean inside a gear pump? You measure gear lash by immobilizing one gear and measuring the free movement of the second one. When the gears are engaged and operating at RPM the driver tooth is fully pressed against the driven tooth and the gear lash (clearance) is all on the back side of the meshed teeth. As the gears rotate the meshed teeth are actually sliding against each other along the involute curvature of their faces which are in full contact for the entire time the teeth are engaged. In a gear pump oil is not going to bypass rearward through the engaged teeth of the operating gear set regardless of any (reasonable) amount of backlash they may have when not under load. Oil can and will easily bypass through excessive housing end or tip clearances.
TOH