Lycoming crankshaft idler gear backlash

[russnrenea]

Hey Everyone,

Hoping someone can shed some light or provide insight to a problem I think I may have.

I have a One Design DR-107 with a Lycoming O-360. Recently I noticed a "slop" in the rotation of the propeller when turning by hand, wasn't much but definitely more than other aircraft. This airplane is fairly new to me so I can't tell if it's been this way all along or not.

Further investigation shows that the slop I was feeling is backlash in what appears to be the crankshaft idler gear to camshaft gear. I'm seeing about 2 degrees of rotation on the crank before the camshaft starts to move, once under load and rotating there is no slop or backlash. Stop the rotation and rotate in the opposite direction again the crank will rotate about 2 degrees then the camshaft starts turning.

I checked the mag timing and the slop/backlash doesn't appear to be in those gears, I get the points to open and a very slight rotation (less than 1/2 degree) and the points close, go back the other way (1/2 degree) and the points open again, so I don't see an issue there. Can't pull a mag and look at those gears without removing the engine, they sit right up against the firewall.

I don't have a vacuum pump so I opened that access and can see the crankshaft gear. I don't see any major issues, some slight discoloration is some gears over others, but for a 16 year old 300 hour engine that had some sit time, it looks ok, I think.

I have only done two oil analysis since I have owned it. This first one came back with high iron 141ppm, however that was on oil that had been there for quite some time, and through a period of inactivity and some starts with months in-between flying, so I expected somewhat high levels of something. The second sample came back still high but much lower than the first at 68ppm. Although that is the first sample since installing an oil filter. I'll be cutting that open tomorrow and checking the media.

My question,

Does the amount of backlash in those gears seem excessive? I see in the Lycoming manual it gives backlash limits, however looks like I'll have to pull the accessory case to take those measurements. Any way without doing that?

My current line of thinking is to change the oil/filter, fly 10 to 15 hours and look at the backlash, oil analysis and filter again. However that will put me in the middle of aerobatic season, so if I have a problem I might like to tackle it now instead of during great flying months.

Any thoughts, tips, suggestions are appreciated.

Thanks,

Russell

 

[Tom-D]

My question,

Does the amount of backlash in those gears seem excessive? Russell

short answer = no.

you have 3 gears in that train each has an allowed backlash.

For the exact amounts follow the overhaul manual.

 

[Dan Thomas]

Are you sure you're not just feeling the rod bearing clearances with the pistons at TDC and BDC? In a four-cylinder engine, when any piston is at TDC, the one opposite will also be at TDC and the pair behind them are at BDC. The crank can move quite freely at that point since it isn't moving anything.

If that engine has ever had a propstrike, the crankshaft gear retaining bolt and alignment dowel could be loose. There's an AD on that:

http://rgl.faa.gov/Regulatory_and_G...9FA5E5F8683A0A4686256E9B004BC295?OpenDocument

Dan

 

[Tom-D]

Are you sure you're not just feeling the rod bearing clearances with the pistons at TDC and BDC? In a four-cylinder engine, when any piston is at TDC, the one opposite will also be at TDC and the pair behind them are at BDC. The crank can move quite freely at that point since it isn't moving anything.
Dan

Either way, it doesn't mean squat.

the AD is not in play. IAW post 1

 

[russnrenea]

Dan,

I had those same thoughts. However with the vacuum pump pad off I can physically see the camshaft doesn't move for those two degrees of rotation of the crank that I talked about in my post. This occurs at any point on the rotation.

3934V, I looked up those values for backlash, and understand the cumulative effect. Just trying to verify I don't have a problem and satisfy my concerns without pulling the accessory case, I really don't want to!!

Thanks for the responses.

Russell

 

[Tom-D]

Dan,

I had those same thoughts. However with the vacuum pump pad off I can physically see the camshaft doesn't move for those two degrees of rotation of the crank that I talked about in my post. This occurs at any point on the rotation.

3934V, I looked up those values for backlash, and understand the cumulative effect. Just trying to verify I don't have a problem and satisfy my concerns without pulling the accessory case, I really don't want to!!

Thanks for the responses.

Russell

If it ain't broke, don't fix it until it is.

replace that vac pump pad and go fly.

 

[Henning]

Put a degree wheel on your crankshaft and measure the timing on your cam in proper rotation. If split overlap occurs at spec (typically TDC) you're fine.

 

[Tom-D]

Put a degree wheel on your crankshaft and measure the timing on your cam in proper rotation. If split overlap occurs at spec (typically TDC) you're fine.

Why would you do that ?

 

[Henning]

Why would you do that ?

Well, if it degrees in properly, then one knows that the slack is normal backlash slack. If it degrees in 1-2° out, one knows that it's wear. 1-2° out on cam timing also has an effect on power output and CHT.

 

[Tom-D]

Well, if it degrees in properly, then one knows that the slack is normal backlash slack. If it degrees in 1-2° out, one knows that it's wear. 1-2° out on cam timing also has an effect on power output and CHT.

And where would you get the criteria to know if it was in or out ?

 

[Henning]

And where would you get the criteria to know if it was in or out ?

I'd imagine that the info is in the engine manual though split overlap typically is designed for TDC. I'm sure if you call the engine or cam manufacturer they'd give you the info for troubleshooting purposes.

 

[Tom-D]

I'd imagine that the info is in the engine manual though split overlap typically is designed for TDC. I'm sure if you call the engine or cam manufacturer they'd give you the info for troubleshooting purposes.

Valve timing degrees are not in the overhaul manual. the lead or lag of the cam is a designed feature, when the gears meet there dimensions and the backlash is with in book dimensions and the timing marks are set properly the cam will be where it should be.
The only degree wheel you need on a Lycoming is stamped on the rear of the fly wheel and used to time the mags. and you always rotate the engine forward to eliminate the backlash before you set the mags.

 

[Henning]

Valve timing degrees are not in the overhaul manual. the lead or lag of the cam is a designed feature, when the gears meet there dimensions and the backlash is with in book dimensions and the timing marks are set properly the cam will be where it should be.
The only degree wheel you need on a Lycoming is stamped on the rear of the fly wheel and used to time the mags. and you always rotate the engine forward to eliminate the backlash before you set the mags.

Problem is that the index drilling is not always correct and then you need to degree in the cam to see where it is, I've run into this on more than one cam. The spec is available, and most likely it will be split overlap at TDC. If I didn't have the spec, I would still measure and if it cam out other than TDC, I would be making some calls to find the spec. If no one else will say, Iskenderian will.

 

[kczeuch]

Problem is that the index drilling is not always correct and then you need to degree in the cam to see where it is, I've run into this on more than one cam. The spec is available, and most likely it will be split overlap at TDC. If I didn't have the spec, I would still measure and if it cam out other than TDC, I would be making some calls to find the spec. If no one else will say, Iskenderian will.

Too bad no resolution ever became of this old thread.

 

[kczeuch]

The Continental service manuals for both of my engines provide the cam specs, the actual valve opening and closing timings.
Lycoming doesn't include the information. I wanted to check my O-320-A1A.

I tried to get the valve lift specs, and the opening and closing of the intake and exhaust valve events.

Calling the Lycoming tech support folks is a real waste of time on the phone and on hold.

 

[kczeuch]

Lycoming thinks the valve timing info should be kept secret from Lycoming engine owners.

 

[kczeuch]

Let's say your thinking of replacing the cam in your O-360 or O-320.
You look up parts in the manual and learn they have a handful of supercedures for the camshaft part numbers.
It would be great to be able to compare the new specs to the obsolete specs.

 

[kczeuch]

Let's say you want to know if your good ol' Piper's cam is still providing full valve lift. There is no info published. So you call Lycoming. Tech support informs you that it's proprietary. A lot of help are they.

 

[kczeuch]

I saw that a member noticed what appeared to be excessive lash in the cam drive gearing.
Another member recommended comparing the factory specs.
Problem with that is that there are no factory specs - ain't none published.
Lycoming apparently thinks we Lycoming owners don't need to know.

 

[nrpetersen]

Q - Is there a different P/N for camshafts used with roller lifters, vs the cam used with mushroom lifters?

The resulting valve timing for that variation alone will be different for geometric reasons.

The attachment method used for the cam drive gear to the crankshaft end is certainly a mediocre design from a fatigue standpoint.

 

[Dan Thomas]

I saw that a member noticed what appeared to be excessive lash in the cam drive gearing.
Another member recommended comparing the factory specs.
Problem with that is that there are no factory specs - ain't none published.
Lycoming apparently thinks we Lycoming owners don't need to know.

Wrong. There are six pages of backlash specs in the Lycoming overhaul manual, from pages 1-28 to 1-33.
https://www.expaircraft.com/PDF/Lycoming-OH-Manual.pdf

 

[bnt83]

I saw that a member noticed what appeared to be excessive lash in the cam drive gearing.
Another member recommended comparing the factory specs.
Problem with that is that there are no factory specs - ain't none published.
Lycoming apparently thinks we Lycoming owners don't need to know.

Have you ever seen the Lycoming Direct Drive Overhaul Manual before? Plenty of various places have various revisions of it posted online in weird places.

Sure Lycoming doesn't have it published on their website but go to the pubs vender and BUY it, its not difficult. While you're at it download all the service bulletins and service instructions. It is easier than getting Chrysler Minivan OEM repair data from the mothership.

 

[Dan Thomas]

Q - Is there a different P/N for camshafts used with roller lifters, vs the cam used with mushroom lifters?

The resulting valve timing for that variation alone will be different for geometric reasons.

The attachment method used for the cam drive gear to the crankshaft end is certainly a mediocre design from a fatigue standpoint.

Yup. Totally different camshaft. Different cam profile.

The lifter bores in the case are also larger and have internal flat spots that mate with spots on the lifter body, to keep the roller aligned with the cam.

The only cam gear attach failures I have heard of involved dowel failures or retention bolt failures after a propstrike. Otherwise, those parts last way beyond TBO. Some engines are run to 4000+ hours.

 

[nrpetersen]

The only cam gear attach failures I have heard of involved dowel failures or retention bolt failures after a propstrike. Otherwise, those parts last way beyond TBO. Some engines are run to 4000+ hours.

Assuming the ground mating surfaces are only flat instead of one being very slightly conical, would make the dowel potentially absorb fatigue instead of just assembly alignment. Especially with the short bolt and funny metal tab to preload it. THX!