Free run tolerances on gear pump

[PBPouw]

Hello All,

I am trying to reverse engineer a hydraulic pump used on a 1944 warbird.

The pump we have has been damaged and only delivers around 500 psi were it should be 3000 psi.

The following data :

Hydraulic Pump - Prop Governor
Max RPM = 6800
Max Pressure = 3000 PSI
Fluid = MIL-H-5606G
Viscosity = 13.6 @ 40°C (ASTM D445)
Flow = Minimum of 250 Cubic Inch @ 2000 PSI

I made all the measurements and I analyzed the toothgear profile as :

Imperial Spur Gear
20dp - 14 Tooth
Pressure Angle 30°

The Bronze bearings get their oil supply from a recess in the center area were both gear mesh.

Now the questions :

Which are the required tolerances of the gear/housing and the tolerances for the gear shaft - glide bearings ?

Any help will be apreciated !

Pim
the Netherlands

 

[hydtools]

The clearance of gear tip to housing bore would be on the order of .0003 to .0008 inches. I suspect the bearing inserts are pressure loaded against the faces of the gear and take up any clearance. Shaft to bushing would be on the order of .0005 to .0015 inches on the diameter. Yes the gear tips may in some instances rub the bore of the housing.

Ted

 

[PBPouw]

Thanks Ted,

The tolerances you mention on the Shaft to Bushing are almost identical to the ones I measured from the existing pump.

The free run tolerance on the geartip - pump house seam to me a bid close. The problem here is that the housing is made from aluminum 2024, so any contact between the gear and the housing will result in damage to the housing.

I have tested this pump and it delivers around 500 Psi at 6800 Rpm, this is with a maximum clearance between the pumphouse and gearteeth of 0.008"

I am just curious how this pump failed because their is obvious damage to the pumphouse. The gear shaft are damaged, also the contact area between the flanks of the tooth and the bearing 0.004"has worn away.
What surprises me is that the bearing itself ( softer ? ) shows like it has been worn a bit but I not more then 0.0002" because I cannot measure a difference!

 

[MikeHalloran]

Gear tooth tip to housing clearance of .008" is HUGE.
Most of the gear pumps I've wrecked started with a few 'tenths' of clearance, and in the case of one popular aircraft pump, enough negative clearance so they had to be turned with a big wrench for a few revolutions after assembly.

I'm wondering if perhaps some pieces were omitted from a previous rebuild, e.g. some kind of radial shoe in the plane of the gears...



Mike Halloran
Pembroke Pines, FL, USA

 

[hydtools]

What are you calling obvious damage to the pump housing? The gears will wear into the housing and still maintain performance. Is there scoring on the housing or bearing carrier lands which separate outlet port from inlet port? Excess fluid will leak from pressure port to inlet port if scoring is severe enough.

The pump may have been rebuilt with new bushings which centralized the gears to original centers. The old bushings could have permitted the gears to wear into the gear housing.

Ted

 

[DRWeig]

Say Mike, hydtools said 0.0008 not 0.008 just in case you didn't make a typo.

I'm not much of a gearhead, but I, too, have bought such a pump that had to be torqued through a few revolutions by wrench at installation time after applying some grease.

Best to you,

Goober Dave

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[MikeHalloran]

Yeah, hydtools said .0008", and then PBPouw said .008".
Is there a typo? I have no way to know.

I doubt that the actual operating system pressure is intended to be 3000 psi; that's pretty damn high for a gear pump. ... especially one as simple as the model suggests. Above 100 psi, they tend to have extra parts to compensate for wear and to obstruct tiny gaps that might be acceptable at low pressures. ... or to be of completely different geometry, with longer leak paths.

Photos of the actual pump might help a bit.


Yeah, those negative clearance 'aerospace' pumps are mostly used for landing gear and flaps, so they run not more than a couple of minutes per flight. In continuous service, they wear out really fast.



Mike Halloran
Pembroke Pines, FL, USA

 

[PBPouw]

Thanks all for the comments.

Some more data,

The 0.008" freerun is the measured as a worse case scenario on the actual pump which is worn out, so yes this is way too big. Still this worn out pump during testing puts out around 500 Psi.
On the pumphouse the bore is measured ( undamaged area ) between 0.8000" and 0.8002". Unfortunately I cannot measure an undamaged diameter of the gears, they now measure 0.7982"

This pump comes from a Propeller governor and pumps hydraulic oil to change the blade angle. The pump rotates continuously during engine running / flight. A 3 way valve regulator controls the reducing / increasing of pitch. During neutral ( blade angle static ) operation there is no pressure in the pump, it only circulates the oil.

MikeHalloran, asked for more photo's, are the 2 top pictures not clear enough ?

Pim,
the Netherlands

 

[hydtools]

Gear pumps commonly run to 3000psi. For instances, Casappa or Barnes pumps. Aluminum housing. Steel gears.
Some may have pressure-balanced wear plates. Some have pressure-balanced inserts similar to those shown in the first post.

PBPouw, I would not expect the gears to be worn more than the housing. The gears should be hardened steel, whereas the housing is aluminum.

Ted

 

[keyepitts]

From the looks of it, you may be able to partially restore the original clearances by making new bronze bushings with a little more center-to-center shaft offset. Might be easier than building a complete new pump from scratch.