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Idler shaft mount worn, help guys this really has me spooked.

terry.mortimore

Well Known Member
Hi guys, really need some input on this problem.

I have a IO-320 narrow deck engine with a EFII ignition and fuel injection system. I have had the idler gear shaft wear the mounting holes twice.

This engine has a O-320 case which was overhauled by Divco and put together with a new crankshaft and camshaft. I’m using a Sensenich ground adjustable propeller.

At the 191 hour mark the oil analysis showed elevated aluminum levels. Investigation discovered that the two 1/4” bolts that hold the idler shaft had broken and the bore that the idler shaft mounts into had worn.

The engine was dismantled and the case was sent to Divco who welded up the bore and re-machined it. The rest of the case was overhauled and the engine was reassembled with new bearings. I used aircraft grade bolts for the idler shaft that I sourced from aircraft specialties.

The engine now has 386 hours on it and the last oil analyst showed elevated aluminum levels again. The same problem again, this time the bolts held but their holes had worn the case. See the attached pictures.

So, I really need to figure out what is going on. The offending idler gear is only driving the camshaft.

The idler gear has been removed from the right side and the hole covered.

I’m using a 20 amp B&C alternator mounted on the vacuum pad.

Has anyone out there who is not running magnetos had similar problems?

I’d appreciate any input.

Thanks, Terry
 

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I would expect to see two of the idler shafts installed but if you are not using magnetos, then I guess you would not need the one.

What kind of load is the alternator on the vacuum pump drive putting on the gears / idler shaft?

IIRC, the idler shaft has a machined hole in the accessory case to center it and the two bolt holes just keep it from rotating.
 
A few thoughts on what could cause this:

The pitch diameter of one of the gears not concentric with the bore.
One of the gear shafts has runout.
Improper gear lash.
 
A few thoughts on what could cause this:

The pitch diameter of one of the gears not concentric with the bore.
One of the gear shafts has runout.
Improper gear lash.

I was thinking the same thing and checking backlash in a full rotation. The only other thing could be torsionals from high Cr or something really odd. Bent parts maybe. Lots of checking to be done for sure. Ideally, all the gears should be properly inspected for lead error, tooth my tooth by gear guys.

Hopefully, that boss picture was before it was welded up. Helicoils are a must do, like Bob said. Not those silly solid inserts. Personally, the plate where the tabs bend up has to be soft to allow the tabs to bend, that makes a poor contact area for the bolts and actually contributes to the loosening opportunity. Having said that, if it works for 99%, then it is adequate.

Bob, do you smack the head of that bolt with a hammer then retorque to ensure good seating? Do you use the tab plate or some other method to prevent self removal?
 
I don't think this is a common issue, so poor parts fit up or an out of spec part seems the most likely culprit. I am with Bill and Don that something in the gear train is improperly machined or the holes not drilled to spec. You would need to carefully measure gear clearance (teeth side clearance, as well as looking for pinching at the apex) between all of the 'in conact" teeth throughout the entire rotation. If it is off enough to break two posts off, you should be able to visually see and likely feel (assuming the pushrods are removed) the conflict. At some point in the rotation, the improperly meshed teeth are puttiing a sideways force on the idler gear.

That alternator shouldn't be adding enough load to cause this, especially if it is setup to only produce power if the main alternator drops out.

Larry
 
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Idler shaft mount shaft

Thanks for the feedback.

I put a dial indicator on the camshaft gear. It measures 0.0035 from one side to the other. I don’t see these limits in my overhaul manual, but this seems excessive to me. I checked the runout against the front face of the gear as well and it showed about a 0.007 difference.

With the wear in the case I have no way of measuring the gear backlash at this point.

I’m going to send the case to Divco for repairs again. I’ll have the work done to increase the size of the bolts to 5/16”. I’ll order new crankshaft shaft and idler gears. I’ll order a new camshaft as well.

I will check and double check the gear backlash when it goes back together but I worry that running without a mag installed has led to this problem. One suggestion made to me was that a mag may dampen any torsional vibration.

Thoughts?

Terry
 

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The concentricity testing method shown assumes the gear pitch diameter is concentric with the gear blank OD, this may not be the case, it should be checked by the pin method. https://www.engineersedge.com/gears/gear_size_inspection.htm

The top gear exhibits unusual wear on the tooth face indicating the gear faces are not running parallel, this would point to the pinion gear boss face not machined perpendicular to the camshaft axis or the pinion stub shaft not being perpendicular . Just a few thousands off can cause this type of problem.
 

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I will check and double check the gear backlash when it goes back together but I worry that running without a mag installed has led to this problem. One suggestion made to me was that a mag may dampen any torsional vibration.

Thoughts?

Terry

Highly unlikely. Due to the backlash, the mag likely absorbs 0 torsional vibration. Far too many engines out there without mags that haven't snapped off the bolt on the post to think that is the cause. Likely something is tight that is putting a side force on the idler gear. Further, I don't think that a vibration can cause the type of damage you are seeing.

Larry
 
Terry, if I understand correctly, the case has been overhauled twice. Overhaul often includes a skim cut on the mating surfaces, reducing the bore center dimension below. Do you know the gearset met the minimum backlash spec at previous assembly?
.
 

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I did not think about the existing gear faces - with the idler banging around if the cont surfaces of the cam and crank gears are damaged, then they should both be replaced as well or just have the failure regenerated again.

Good thought on the centerline of a milled and rebored case, Dan. .010" definitely makes a difference.
 
Idler shaft mount worn.

Terry, if I understand correctly, the case has been overhauled twice. Overhaul often includes a skim cut on the mating surfaces, reducing the bore center dimension below. Do you know the gearset met the minimum backlash spec at previous assembly?
.

Hi Dan. I don’t know the history prior to buying the engine, but I have had it overhauled twice. The first time when I put the engine together and the second time after the idler gear failed the first time. I did measure the backlash when I was assembling the engine, but I have lost those records.

Divco did the work both times, I assumed they would have taken this into account when the did the work.

Terry
 
Idler shaft mount worn.

Thanks for the feedback.

I spoke to Divco today and explained the issue, they told me it’s a result of having the 1/4” bolts holding the idler shaft. They said my problem will be solved when I go to the 5/16” bolts. I hope it’s that simple, but I sure will be checking things when it’s going back together.

The gears are marked up and show signs of FOD going through them (probably pieces from the locking plate tabs). I will replace them and buy a new Camshaft with it’s attached gear.


Terry
 
My understanding of welding aluminum castings (typically alloy 356) is that yes, they are weldable, but unless re-heat treated, the weld is not as strong as the original heat treated casting. The idler attachment has already failed a couple of times, I'd suggest having a discussion with Divco regarding the weld alloy and strength of same. I didn't notice any threads left in the casting in your original pictures. Are they still there?

Personally I'd prefer a ring locked insert rather that welding again.
https://catalog.howmetfasteners.com/category/ring-locked-inserts
 
A-65 case, no idlers, mags drive directly from the crank gear. They welded a crack above one jug, then flycut the case mating surfaces so much the accessory case cover would barely slip over the studs. Worse, they sent it back with parentheses-shaped crank bores.

Years ago, perhaps not fair to relate now. Whatever...the moral is "Assume nothing, measure everything".
 
Case

The 5/16 bolts have been the standard for many years. The question is why didn't Divco configure the case for the 5/16 bolts the first time they overhauled it.
Looks to me like Divco owes you a new case.
 
I've run into loose bolts on these towers but not loose enough to cause damage. The fix was 5/16" helicoils and red loctite on the bolts.

A friend of mine had an off-field landing due to one of these coming loose.
 
A-65 case, no idlers, mags drive directly from the crank gear. They welded a crack above one jug, then flycut the case mating surfaces so much the accessory case cover would barely slip over the studs. Worse, they sent it back with parentheses-shaped crank bores.

Years ago, perhaps not fair to relate now. Whatever...the moral is "Assume nothing, measure everything".

Unfinished work - line boring is necessary before use.

Here is a short article on welding 356. Background information.
 
My understanding of welding aluminum castings (typically alloy 356) is that yes, they are weldable, but unless re-heat treated, the weld is not as strong as the original heat treated casting. The idler attachment has already failed a couple of times, I'd suggest having a discussion with Divco regarding the weld alloy and strength of same. I didn't notice any threads left in the casting in your original pictures. Are they still there?

Personally I'd prefer a ring locked insert rather that welding again.
https://catalog.howmetfasteners.com/category/ring-locked-inserts
I just finished an overhaul with the same root cause. Rep at Divco said they see this occasionally. I'd never heard of it. I had the idler attachments upgraded to 5/16", crankcase overhauled. Camshaft, left idler, left and right idler shafts with 5/16" variant all replaced.

There is a cyclic loading on the gear as it drives the mechanical fuel pump. I assume this caused a failure of the casting, or loosening of the fasteners. Both fasteners were well worn, but still present. Pieces of the locking plate was still present and look almost identical to yours. Backlash as reinstalled was acceptable.

When I found it, the left idler was essentially floating. Engine ran fine, but it could have failed in the next 10 minutes. Made lots of metal. So check your oil filters. That's how I found this. Although I did notice more than "normal" clicking when the engine was turned over, after finding the metal in the filter.
 
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Not wishing to hijack the original topic, but mine is somewhat related and I'm similarly mystified.

In my case the idler mount stayed securely attached, but the gear failed. This resulted in loss of drive to several accessories, but most importantly the camshaft, so the aircraft ended up in a paddock following the ensuing forced landing.

The second photo is a little misleading, as the separated outer section should lie the other way up. The witness marks are the result of touching the mount bolts after the gear came away from its hub, and before it fell out of mesh.

Clearly there has been a major side load on the gear, as it tried to chew its way into the base of the bearing on one side. Like the OP, I'd welcome any theories as to how this could have developed.

Glenn
 

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