Case Shuffle

[Flieger]

Interesting, I thought there had to be sliding for fretting to happen.

[chris_seven]

Max,

You need to define sliding in this case. I agree that there needs to relative motion but sliding can imply low levels of friction.

Fretting is much more of a stick/slip problem and is likely to occur at the peak of a 'friction hill' rather than when sliding is really occurring and friction levels have fallen.

I believe soft metals fret due to adhesive wear and this can occur when movement is very limited.

It would be interesting to have some idea of the amplitude of case vibrations to try to determine what is happening and maybe it is possible to install an accelerometer and look at the g levels.

Next time we run an engine on a dyno I will see what we can learn.

[Walt Fricke]

I favor the bushing approach shown in Craig's photos.

You can do this by clamping a case together, and boring down through the throughbolt holes to, and then a certain distance beyond, the parting line. This insures that the bushing holes are perfectly aligned. Then you can clearance the near side a little based on case to bushing fit, press the bushing into the other side so it butts up to the end of the drilling (all the while being careful of oil squirter passages), and you are nearly set. The trick for this is then to press narrowing inserts into the outside of the throughbolt holes where you went in to bring their ID back to standard.

Much easier than jigs, and I think more precise than standard pinning.

Whether or not this really prevents some harm I don't know. I have had this done on all my cases, mag or aluminum, just because.

I've got no idea what was done to factory racing cases so the througbolt bores lined up well enough at the parting line to use the flat sided bulge as a parting line stiffener. Are there instructions for assembly of these motors which specify an orientation for the center part?

I've wondered about the need for those self-aligning washers, but if a set fell into my hands I'd use them for the cool factor.

Pumping 500 or more HP from air cooled VW 4 cylinder cases might be where shuffle pinning got its start? Or their 356 progeny? With the 911, didn't Ferry Porsche say that if they thought the original case could be used to produce the 300 HP of the 2.8 RSR, or just the 210 of the 2.7RS, they would have made a lighter (and less expensive?) case.

I know this doesn't answer Chris' question. And it is always good to hear from someone who has helicopter transmission engineers for buddies, and shows that lubrication, rather than increased clamping, may be better to prevent fretting. And explains stick/slip by analogy to the difference between sliding and static friction, which most of us have heard of even if we can't quantify it.

[chris_seven]

Walt,

I agree with most of your comments and all of your sentiments. I am not opposed to trying to pin cases but I am not confident in some of the methods used.

The problem that I have is when faced with detailed questions the machine shops that I have spoken to just tell me that they can do the job and not to worry.

Unhappily I do like to worry and have always believed that unless you either leave a clearance around one set of pins you would have to line bore and I repeatedly told that the job is quite straightforward.

That's where I start to come unstuck - none of the machine shops I have spoken to can provide any information on 'fits and clearances'. None of them can show any drawings or provide either inspection plans or inspection reports on cases that have been pinned.

I don't expect them to hand over copies for me to keep but just sight of the relevant details would help.

The money isn't the issue but from an engineering perspective why carry out a process that doesn't do much and suffer the attendant risk.

The appeal of the case bolts with the 'lump' is the fact that they can be installed with the case bolted up and we have made a centreless ground bar to line up the bearing housings before carrying out the mods.

Opening up one side of the case and then reaming the oil holes is not too difficult.

The bolts we have now are marked with 'FLAT' laser etched onto the head so they are aligned in a specific position.

We are planning to install them with an m6 interference which should deal adequately with expansion.

I would agree that the hollow dowels used by Craig and installed in a similar manner are a much better idea than trying to line up two separately machined halves.

I have looked at Spirol Hollow Dowels and their design data states that fits and clearances should be identical to those used for solid dowels.

I am not sure of the force needed to press fit a single dowel with a typical m6 interference but is you multiply this by the number of dowels the load required is not insignificant.

There is good data available for the clearance for a dowel to be a 'sliding fit' and for a 10mm dowel this would mean 0.002" on diameter or 37.5 microns in a single direction. More than enough for fretting to still occur.

if after pinning the two cases do they just slide together? If so then maybe they will still fret.

If you have to press them together, how do they come apart?

The fitted bolt concept which is a design feature we used commonly to align the high speed titanium couplings we used to manufacture for Military Helicopter Engine Test Rigs - Typically for T-700, Makila and RTM 322 engines- and they do work well. These engine run at around 21000rpm and will not suffer any significant vibration on the shaft bearings so balance and smooth running is essential.

It is quite easy to maker a puller to install the bolts and as they are fitted one at a time it may give the case an easier time.

We are just doing a layout to convert the bolt to a 'Tie Bar' so it can more easily fitted and removed.

We have drawn it up with 2 x flats to give the oil circulation even more chance and then use self-aligning washers on both ends.

With regard to lubrication on the surfaces I have a telephone conversation booked later today and should find more information.

We need a compound that will 'adsorb' onto the surface of the magnesium so it won't be squeezed out.

We are also looking at repairing badly fretted cases without having to narrow the engine using a 'Cold-Spray' that is already being used for helicopter gearbox repair.