Hydraulic Gear Pump - External - Grinding Stack Squash/Clearance
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    Default Hydraulic Gear Pump - External - Grinding Stack Squash/Clearance

    Hi All,

    I am working on an 8GPM External Gear Pump having a 1/2" set of steel gears. The pump had overheated and scored its internal aluminum plates that seal the gears. We have ground down the plates in order to achieve a nice smooth and tight seal for the gears once more. But now we need to grind down the width of the outside aluminum housing to establish the internal squash/clearance.

    Does anyone have sound information regarding the clearance needed internally? As per this pump at the moment...... the internal gears and side plate stack is 1.830" and we have ground the outside housing to 1.840. I feel .010" is far too much clearance and feel something more like .002" or less is required.

    I suspect these pumps are built like this to accommodate the heating of the metals internally. A pump may run upward of 220+- degrees if I recall correctly. The difference between the internal stack and the external housing must have some sort of compensation. I am unable to Google anything worthy.

    Your thoughts.
    Thanks
    Pete

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    Well, you've nailed down the materials, their dims and the temp range.

    I'd start by using that info and some CTE data from matweb to plot your clearances through operating temp ranges. You may need to take in ambient temp of the pump's surrounding fluid as well. From there it seems like a judgement call based on the pump's service conditions.

    Curious to see what the pump pro's have to say.

    Sent from my SM-G930R4 using Tapatalk

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    Grinding is one thing. Lapping is another part of the process however. Where I worked at, the entire stack had to be lapped and IIRC final total clearance was around .001. So that means everything has to be sub thou ±. Call the engineers who designed the thing and ask them how it should be fixed. Also, the gears are not a normal gear tooth profile. If you examine them you will see that they mesh in a specific manner.

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    I was looking at thermal expansion rates for 6061 as I suspect that is what this pump is made from. It shows 13 10-6 per degree F.

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    The thermal coefficient of expansion should serve to increase the clearance if the housing expands more than the gears do. So you're ok that way.

    .001" clearance might be nice in a perfect world, but I'd be happy with .001" per side, and over the tips (next to the housing) of the teeth as well. I've already had my fun with rotating things that don't quite have enough room for a bit of grit.

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    Some gear pumps have a pretty neat trick of using a floating stack like your describing, using the generated pressure to load the pump gears axialy to keep the lube film thickness just right. Its generally the cheap pumps that rely on just a fixed clearance.

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    That was a good mention as I went and looked at this pump and its seals and sealing areas. I found the pump pressure DOES set on the ends of the stacks to apply pressure to them.


    Quote Originally Posted by adama View Post
    Some gear pumps have a pretty neat trick of using a floating stack like your describing, using the generated pressure to load the pump gears axialy to keep the lube film thickness just right. Its generally the cheap pumps that rely on just a fixed clearance.

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    If the housing is aluminum and the gears are steel the side clearance will increase as the operating temperature increases due to the different COE between steel and aluminum.

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    One other thing worth adding, especially if its your system, what have you done oil wise post failure? Simple drain and replace is IME never the best option with hydraulics, far better to send a sample off get a analysis ran and then run a filter cart set-up with the recommended filters and polish the tanks contents back into spec. If you can plumb the cart to flow through some of the circuit all the better. Like this you catch the crud and get it out of there that other wise will damage your repaired pump. Contamination in hydrulics tends to snow ball in a bad way, correct filtration is even more important post a part failing than it was before it failed as you now almost certainly have contaminants doing laps of the circuit!

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    This pump has an alum - steel gear - alum sandwich that is housed in an outer housing. What I seek is the difference between the axial length of the sandwich stack and the housing. I feel the housing should be .001 or .002 longer than the stack in order to accommodate the thermal growth of the sandwich.

    Quote Originally Posted by Illinoyance View Post
    If the housing is aluminum and the gears are steel the side clearance will increase as the operating temperature increases due to the different COE between steel and aluminum.

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    Agreed! Cleanliness is close to Godliness in hydraulics.

    Quote Originally Posted by adama View Post
    One other thing worth adding, especially if its your system, what have you done oil wise post failure? Simple drain and replace is IME never the best option with hydraulics, far better to send a sample off get a analysis ran and then run a filter cart set-up with the recommended filters and polish the tanks contents back into spec. If you can plumb the cart to flow through some of the circuit all the better. Like this you catch the crud and get it out of there that other wise will damage your repaired pump. Contamination in hydrulics tends to snow ball in a bad way, correct filtration is even more important post a part failing than it was before it failed as you now almost certainly have contaminants doing laps of the circuit!

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    I just found this article in MaintenanceWorld.com. The article in whole was talking in generalities in regards to gear pumps.

    "Maintaining the close tolerances between the housing and the cogs is critical to efficient operation. The clearance between the edges of the teeth and the housing and the ends of the cogs and the back and front walls of the housing are very small. Between the teeth and housing it is in the order of 0.1 mm (0.004”) while the clearances between the front and back faces of the gears and the ends of the housing are only 0.025 mm (0.001”). The fine clearances reduce liquid re-circulation back from the high-pressure discharge to the low-pressure suction side and make these pumps one of the most efficient available."

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    Quote Originally Posted by HurleyByrd View Post
    I am unable to Google anything worthy.
    The dead-tree Clymer manual for 1959 Triumph TR3's Hoburn-Eaton oil pump had the entire rebuild procedure covered for a pump very close in size. Service was to be done at 100,000 mile intervals, sooner if pressure had dropped. Mind - it was a re-purposed Canadian Massey Ferguson farm tractor engine, so a tad more durable than an MG or the like.

    From memory, you have just done that same rebuild procedure.

    The pump in that instance had a Cast-Iron body, steel shaft and gears, steel end plate, and the slimmer clearances you suspect are wanted. Oil temp in the crankcase was a bit less - around 180-200 F IIRC, and perhaps more stable, cold-weather initial starts excepted.

    Your temps are higher. Your hydraulic fluid may have much lower viscosity than 30 wt motor oil of that era. Your operating pressure may be a significant multiple (60 psi or so) of the TR3.

    Not sure how much of those combined factors working against success your system can tolerate. Loss of pressure is likely to occur at a lesser clearance.

    I'd class use of Aluminium body and Aluminium seal plates with steel gears for a wide temp range as - no better word - DAFT!

    Daft enough, I'd go for actual contact - zero clearance, IOW - when assembling at room temperature, let the greater expansion of the Aluminium body set the hot clearance, (WTF else can one do?) put it back to work, and go order a properly Engineered pump to replace it whilst it was wearing itself out. Again.

    All the best with it!

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    Quote Originally Posted by HurleyByrd View Post
    I just found this article in MaintenanceWorld.com. The article in whole was talking in generalities in regards to gear pumps.

    "Maintaining the close tolerances between the housing and the cogs is critical to efficient operation. The clearance between the edges of the teeth and the housing and the ends of the cogs and the back and front walls of the housing are very small. Between the teeth and housing it is in the order of 0.1 mm (0.004”) while the clearances between the front and back faces of the gears and the ends of the housing are only 0.025 mm (0.001”). The fine clearances reduce liquid re-circulation back from the high-pressure discharge to the low-pressure suction side and make these pumps one of the most efficient available."
    Indeed they do. They ALSO make the design one of the most sensitive of all to loss of those tight clearances.

    TANSTSAAFL.
    Last edited by thermite; 04-19-2018 at 12:31 AM.

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    Thats exactly why his pump is not like that and has the floating plate and relies on its own generated pressure to set the clearance on the gears as a self generating function of fluid pressure and fluid viscosity. If he removes that head space the pump will no longer perform that self loading - self gaping design. His pump is what was developed post the basic gear pumps were head space does set gear clearance.

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    Leakage around the ends of gear teeth is a lot harder to controll,and if the housing or gear tips are noticeably scored,the pump will not hold high pressure.Some housings had U shaped inserts that gears run against,again higher first cost.That is why these types are basically throw away,new is often cheaper than a few spares.If you want a lasting pump,you must go to axial piston types.

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    Quote Originally Posted by adama View Post
    Thats exactly why his pump is not like that and has the floating plate and relies on its own generated pressure to set the clearance on the gears as a self generating function of fluid pressure and fluid viscosity. If he removes that head space the pump will no longer perform that self loading - self gaping design. His pump is what was developed post the basic gear pumps were head space does set gear clearance.
    UN. fortunately.. before your earlier post on that, he had already re-worked it off the older type's needs the rest of us have also been on about.

    Question on the table now is if the floating-plate, self-loading design philosophy can be restored to proper functionality.

    Small size of it, one wonders if it may be cheaper at this point to replace it rather than throw more time at it with less than 100% chance of success of getting that part back right.

    Salvageable? Not? Can't be the first time something like this has happened, can it?

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    Those floating end plate designs, do not, IMO, rely on any sort of differential pressure produced by the pump to control end clearance. The plate is immersed, both sides, in oil at whatever pressure is currently being produced by the pump in operation. The end plate pressure would be a function of the elastomeric gasket behind the end plate. Yes or no?

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    No because one side of the gears on the inside of the pump is at suction pressure, the other side is at pressure, the complete outer face is at pressure hence with some 50% greater area at pressure it tends to move towards the gears, but only if there is clearance for the oil to get behind it. Its a cleaver simple proven thing.

    Im not saying the OP can not possibly reduce that 40 thou, have never measured what the float is in there, just know theres a lot lot more than in a conventional style gear pump like found in say a car sump which does rely on a fixed clearance in the low single digit thou range and only ever wears to allow more seepage, the op's style ran correctly actually improve seal wise axially hence tend to live a lot lot longer.

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    Quote Originally Posted by adama View Post
    No because one side of the gears on the inside of the pump is at suction pressure, the other side is at pressure, the complete outer face is at pressure hence with some 50% greater area at pressure it tends to move towards the gears, but only if there is clearance for the oil to get behind it. Its a cleaver simple proven thing.
    ISTR we had a shaft bearing and/or seal in the same philosophy discussed "Right here on PM" that didn't seem to make proper sense at first glance, but was actually legendary for long-life, no?


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