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Spalled Fuel Pump Eccentric

Doug Rohrer

Well Known Member
My nearly new (rebuilt) O-320-D2B ND engine started making metal around 100 hours, according to Blackstone. There seems to be a problem with a Lycoming cylinder which I am changing out, but I also found a serious problem inside the accessory case. The eccentric cam on the idler gear that drives the fuel pump push rod seems to show signs of overheating and spalling, which would, of course, put steel into the oil. Can anyone advise how this could happen? How is this part lubricated? I now have to remove the engine from the plane and remove the accessory case to get good access to this gear and the push rod. I don't want to put new parts in there if the same thing will happen again. BTW, the engine ran fine and the fuel pressures were normal for a carbed engine. See attached pic.
 

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Take a look at your gear teeth. Looks like pitting on the faces of the teeth, which could be from previous rust. Attached below is a vacuum pump gear which I bought from a guy on VAF, showing rust pitting. I advised him to inspect his accessory gear train. He did and found all of the gears with similar pitting. He replaced them all.

So, I'd guess your eccentric could also have rust pitting which is what led to the spalling.

I've reattached your photo with the suspect teeth circled. And these gears only get splash lubrication.

IMG_5724.JPG Spalled Fuel Pump Eccentric.JPG
 
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I got the engine off the plane and the accessory case removed. The eccentric gear was spalled and the gear teeth even had some pits. I will have an A&P look at the other gears. New idler on order. The strange thing is this gear looked pristine when I put it in 143 hours ago...
 
Gear

Did you pre oil the engine by motoring with starter with spark plugs removed?
What were the ambient temps for break in?
What was the oil pressure? High green oil pressure is believed to help on Lycomings and it certainly can't hurt.
What kind of assembly lube was used?
Were the gears, pump pushrod and gear shafts inspected by a reputable shop?
What kind of oil for break in?
 
Many folks will inspect a gear and think it is all right to reuse because the gear teeth appear to be smooth, not pitted, and shinny. This isn't the case. If you were to look at a gear from an on end point of view, the gear tooth surface from apex to the bottom of the v, are NOT straight surfaces, The gear tooth profile from apex to bottom of the V, kinda should have a bump in the surface. So the line from apex to the bottom of the v is actually curved and then kinda straight. so the line from apex to v is rounded not straight. This is so that when two teeth mesh, in operation, they roll against each other rather then the teeth being flat to flat against each other. I don't know if i am describing this well, but if you look at your new gear, from an on end point of view, you will see the non flat gear teeth profile as they should be. Even a couple of teeth, on one gear, that are worn straight will cause spalling, over time, when reused. The flatter the surface and the amount of teeth like that on one gear, the quicker the spalling will happen. When I was working and at the airshows. I had a couple of gears on the demo table with a tag on the saying "would you reuse these?" Folks would pick then up and examine them and after a while, ask if you needed a magnifying glass to see the defects or if they were cracked and it only showed with magna flux etc. Because, they couldn't see anything wrong visually. Then I would show them a proper gear tooth profile and point out the hump on the surface and tell them to re-examine. They would all be astonished that a gear tooth could look so good but be so worn. I remember, I actually had one guy tell me he was going to go home and take the rear case of his, self-overhauled engine, that wasn't installed yet, to reinspect all the gears. Never heard from him how he made out, but he was adamant that the 5 mins. we spent to together, was awakening. Anyway, that might explain your rapid wear on the teeth.
The problem at the gear cam...I don't know, It almost looks like the pin was hard against the cam all the time. Was the pin in upside down? the head should be toward the pump arm not the cam. Don't know if that would be an issue but maybe??? Is the pin super free to move in it's slot in the accessory housing? Never saw wear like that with signs on heat on the cam before. Maybe the pump was some how pushing the pin against the cam harder then ususal all the time. That could cause the heat signature as well as the wear. Or maybe it was just super pitted there but not anywhere else. One thought, while the engine was waiting to be run, was it a long time and did you constantly turn the engine over by hand, thinking you were re-oiling everything inside but in reality, you were wiping all the oil off things inside. If you did that alot, over a long period of time, maybe that allowed the corrosion? Just a few thoughts to consider.....
Good Luck,
Mahlon
 
Did you pre oil the engine by motoring with starter with spark plugs removed? Yes
What were the ambient temps for break in? 70*
What was the oil pressure? High green oil pressure is believed to help on Lycomings and it certainly can't hurt. 90 psi
What kind of assembly lube was used? 15% STP, 85% 100W mineral oil
Were the gears, pump pushrod and gear shafts inspected by a reputable shop? Yes
What kind of oil for break in? Shell 100W mineral oil

See my responses in italics.
 
Mahlon, I was hoping you would chime in. I have some experience with gear design as a mechanical engineer. I did some gear train design on a small turbine engine many years ago. I admit I did not look super close at my gears, as they were inspected by a pro shop, including mag particle inspection. The cam surface showed some use, but no pits or scratches. I guess it has to be perfect to work long term.
I do have an issue with your statement about the orientation of the fuel pump push rod. The rebuild and parts manuals clearly show the rod with the head up against the cam eccentric and the plain end down against the fuel pump arm. The head appears to have a slight crown which would impart a rotational force on the rod, which is common on cam lifter design. My push rod appears pristine with no wear or scratches on the head. It is obviously hardend.
The push rod does not bottom out; I checked.
I also had an A&P inspect the other gears in the case, and he said they were good to go.
I was careful to not rotate the engine much while waiting for installation on the plane.
I will check to make sure the oil galleys and oil feed holes in the shaft supports are clear and open. My new gear should be here in a few days, then reassembly starts.
 
Doug, I was typing and didn't type what I was thinking. I am very red faced! You are correct I typed wrong! The head goes up against the cam. Not the way I typed it. The head prevents the pin from falling into the engine with the pump removed as well.
If the gears were inspected by an engine shop then they should have been good to go. But then they would not have worn so quickly. I would suggest a close visual inspection of them all while you have it apart, just to be sure they are still serviceable. Do you think that the pump may be pushing too hard against the cam in the idle position and just pushing so hard all the time it caused the wear and heat signatures?
Sorry for the confusion,
Mahlon
 
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I would speculate that without specialized tools, the only real way to determine gear tooth wear in the field is to measure backlash in the gear train. Be advised that tooth wear is often not evenly distributed. When I overhauled the 540, I initially found the backlash on the idler gears within tolerances, but on visual examination, I saw a tooth that looked more worn than the others (the slight curvature of the tooth face was not there and appeared flatter). When I put this tooth in mesh with others, the backlash was no longer in tolerance. I ultimately found three teeth that did not produce acceptable backlash. I am told that the teeth that are engaged as the mag passes the magnetic resistance point will get more wear than the others, so be sure to be thorough in your examination. Not sure if this is an issue unique to the 6 cyl version or not. Also can't remember if it was the magnetic resistance point or possibly the point where the impulse arm catches the pin. Been too long. Either way, there are points in rotation that provide greater resistance and therefore greater wear.

I think this only applies to the left idler gear and in your case it is being replaced. However, it may be worth another look at the right idler gear. I am not sure that I would trust a visual inspection of the gear as I believe that without specialized tools, measuring backlash is the only way to test for tooth wear.

Larry
 
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Mahlon, thanks for the clarification about the push rod orientation. I was reluctant to dispute this issue with someone with your qualifications, so I had to triple check the Lycoming manuals. Moving on...
Larry, I will check gear train backlash before installing the cover. I have the required instruments. I am running two Pmags, so there is no impulse coupler or magneto, but these gears spent many years in a certified airplane with mags, so the wear could be there.
Thanks for all the advice.
 
Mahlon, thanks for the clarification about the push rod orientation. I was reluctant to dispute this issue with someone with your qualifications, so I had to triple check the Lycoming manuals.

Mahlon doesn't make mistakes very often...
 

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