New hydro pump flange installed - hope to solve cracked flange problems....

GyroRon

Former Gyro know it all
Joined
Oct 29, 2003
Messages
16,916
Location
Fort Mill South Carolina
Aircraft
Vans RV4 / Dominator 582 Ultrawhite
Total Flight Time
ALOT
My gyro now has just under 83 hours total time, and I have gone through 4 hydro pump flanges in that time. I can not explain why they are breaking so fast, and I know not every Dominator has had this problem. My Old Red Dominator had over 100 hours on it the last time I saw it and it had never had a flange break....

Anyway I bought a new 4 bolt pattern flange at the hydro parts supply shop the last time I was there to buy a replacement 2 bolt flange. Both style flanges cost around 20-25 bucks a piece. I was glad to be able to just be able to buy the flange and not the whole pump!

So the new flange bolts right up to the pump, but it is too small for the mounting plate RFD - Ernie - provides.... the centering hole in the pump mounting plate is too big with the 4 bolt flange. I took a old cheek plate off a gyro I used to own over to Tim Moses house yesterday and he cut out a piece from the cheek plate big enough to make a new pump mounting plate. He then machined in the adjustment slots and the smaller hole needed for the new flange - and he even drilled the holes to bolt the flange to the plate and he gave me 4 new AN bolts to hold it on!

Today I went out to the airport and installed everything. It looks better, stronger and hopefully will last for a while longer than the two bolt flange.

Tim is a engineer and he said the two bolt flange is not the best way to mount the pump due to the way the loads are placed on it. His opinion was this new 4 bolt one should hold up, we will see!

The part that was breaking on the 2 bolt flanges was the flange would crack between the bolt holding it on and the pump body and after a while the crack would go all the way across, then the other side would begin to crack. I changed them out before they both would break because if that happened I think the whole pump would fall off and the belt would try to engauge the pre rotator, bad news either way.
 

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Ron, that looks a lot more wholesome than the 2-bolt version.

After finding mine cracked, I put in a new 2-bolt flange and did some hand-fitting. It seems to be holding up OK after lots of short demo flights and prerotation cycles over 3 years. The hand-fitting involved a very careful carving-out of the AN 970 fender washer under each bolt head so it would clear the radius where flange meets pump body, and filing the bottom surface of the flange really flat. As supplied, this flange had a little "dihedral" in it. You would put bending loads on it when tightening the bolts.
 
Doug, I wonder why two-bolt flanges are used? Is there any preference for them? It's apparent that 4-bolt flanges are one of three standard ones for these pumps:
 

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Tim Moses looked at the 2 bolt set up as set up from Ernie and said that if the 2 bolts were placed vertically instead of horizontally the flange would probably hold up alot better. But the 4 bolt version seems the way to go IMHO. I am sure Ernie has seen this thread - if not Mike Boyette will probably point it out to him - and maybe Ernie will consider switching future hydro prerotator set ups to include the 4 bolt flange as standard. The cost to do so shouldn't cost more than a extra few bucks.
 
Looks good Ron.

Robust. Definitely will last for ever.
I bet this was the only hiccup on the Dominator.

Excellent piece of work.

Best wishes
Rehan Janjua
Air Command. Pakistan
 
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