Gear Pump leakage - High engine RPM

[Emmett Clarke]

Hi All, I'm new to this Forum, all the way from Ireland.

Hoping someone can help - we're designing a mobile stacking conveyor (of which there are many brands already in existence) but as a primarily fabrication company we thought we'd spread our wings a little and become an OEM, at least in part.

Anyway we've built our first machine and it's largely running ok except for one issue - our pump assembly keeps leaking when our engine speed reaches its max 2200rpm. We have a 25/25/8cc triple pump fixed gear arrangement on an open loop system and we're on our 3rd pump assembly on this machine. The leak is small - a dribble that wouldn't stop a machine working but if it occurred at a hose fitting, you would make a point of stopping it. It has appeared on each of the three pumps at this speed, with no load whatsoever (except for whatever small amount of resistance there is in the manifold when returning to tank), and within 15mins of fitting the new pump. There is no signs of elevated oil or pump temperatures, and suction hoses are 1-1/4 inch, and max. 1m length for the 25cc pumps with suction strainers capable of 126litres/min on each line. Pressures on the suction line are virtually zero.

My only thinking at the moment is some sort of resonance or vibration when the engine hits that speed, inducing excess movement between the pump shaft and shaft seal, allowing oil to bypass.

Pump supplier is stumped and has asked manufacturer for some help on this too....

Many thanks in advance

 

[Tyler d4c]

Is the pressure beyond that the pump is reated for at that rpm

 

[Emmett Clarke]

Pump is rated for 280bar working (310bar peak) at this speed, and leaking at 2200rpm when its seeing max. 10bar when running through the control manifold before going back to tank.

 

[Vetech63]

Where is the leak? Between the pump sections or at the input shaft?

 

[Tyler d4c]

Is the pump properly centered off of whatever is driving it
All straight true and proper
Is there a torsional dampener of some kind

 

[Joe H]

Mark the pump sections with a line crossing the sections.

Run a strobe light, old style timing light and see if the sections are moving against each other.

Joe H

 

[Emmett Clarke]

Leak is between the pump assembly (shaft end obviously) and the crankshaft mounting plate (JCB430 engine). A small dribble running down the plate, but increases with increased pressure.

The attached images show the parts - the black plate is bolted on to the inner diameter holes on the engine, this is what the pump engages with via the cross shaped coupling. The red plate is bolted to the outer ring (housing) and the pump is bolted to this via the 4 holes in the middle.

 

[Vetech63]

That set up should work fine providing everything is aligned properly. What diameter is the pump shaft itself? I will say that all the similar drive couplers I've seen like that have always been designed for splined pump/motor shafts....not keyed shafts. You would be surprised at how much vibration a small key like that can cause at high speed.

 

[Emmett Clarke]

Our shaft is tapered. Just realising we haven't got the loose piece fitted, but I don't think it should make a difference....think its just a blanking plate to cover the hole

 

[Vetech63]

Is there not a key on the tapered shaft set up? There usually is from what I've seen.

 

[digger doug]

That tapered fit will vary axially with a very small change in dia. that either
the shaft or socket is machined to.

Does the coupling allow much axial movement ?

You might be at one end of the movement range even if the parts are in spec.
being at one end or another on the tolerances.

 

[Emmett Clarke]

there is, just not shown on that drawing. If my thinking is right, if we had fitted this plate it would have prevented a majority of the leaking oil from showing between the pump and plate, it would instead have flowed down into the crankcase of the engine. May explain why supplier hasn't 'seen' it before.

 

[Emmett Clarke]

yeah could well be, but to be fair the supplier has agreed that it shouldn't be happening and it's an issue that needs resolved, they just don't know what's causing it.

 

[Emmett Clarke]

That tapered fit will vary axially with a very small change in dia. that either
the shaft or socket is machined to.

Does the coupling allow much axial movement ?

You might be at one end of the movement range even if the parts are in spec.
being at one end or another on the tolerances.

Coupling is a slightly loose fit so I think it should allow enough for a bit of misalignment

 

[Vetech63]

Yeah, it shouldn't be leaking at all.

 

[SterlingR]

Does it have a case drain line?