jrice45516
Heliman
Location: SoCal
| I hate to rain on your parade but......the shim does not solve the problem of premature clutch failure. In fact it can make matters worse.
TT's response to the clutch breakage problem by providing the shim is admirable. In my neighborhood, however, where the R30 is by far the predominant heli, we have seen premature clutch failure both with & without the shim. And when I say "premature," I mean in as few as 2 or 3 hovering flights on a brand new clutch, shimmed or not. Invariably, clutcharms break at, or cracks start at, the holes where the wall thickness is thinnest. And they start from the fanhub-side of the clutch.
It is true that, without the shim, the rubbing of the clutcharms on the fan hub due to in/out movement of the arm can be seen as demarcations on the hub. Premature cracking & eventual breakage, however, is not due to in/out movement of the arm.
Rather, the premature cracking is caused by the arm flexing up & down and this is why the shim can only make matters worse. Without the shim, at least when the arm tries to flex toward the hub, it runs up against the hub and can go no further. This is why the cracks start on the hub-side of the clutch. [The only time I have seen a crack start from the opposite side was when I was running a shim & pancaked my R20 hard into the ground; the impact bent the arm down toward the hub.] With the shim, however, the arm is allowed to flex even further in the direction of the hub, which only accelerates cracking & eventual breakage.
Why do clutcharms flex up/down? This is due to misalignment between the engine crankshaft & the startshaft assembly. The small bearing in the clutchbell also takes alot of punishment due to this misalignment. It is hit or miss whether you will have a problematic amount of misalignment in any given bird or after any given rebuild of your bird. That is why some pilots experience premature clutch failure & others don't.
The following is what I am doing to check & correct misalignment. It seems to be increasing clutch lifetime. I know it's a hassle but have found no other way.
I assemble the engine without fan (crankshaft is bare), engine mount, mainshaft & startshaft bearings, & sideframes. The startshaft assembly is left out. I then slip a straight tube (with I.D. as close to crankshaft O.D. as I can find) down through the startshaft bearings onto the crankshaft. [Another way is to add in the fan & clutch; then just lower a longreach 6mm drill bit down into the clutch bearing instead of slipping a tube over the crankshaft.] With the glowplug & engine backplate removed (I told you it was a hassle), I can turn the engine over with my finger while watching the clearance between the tubing and the top startshaft bearing's inner race. The tube or drillbit is not a perfect fit & so it wobbles a bit as I turn the engine but that's OK. I'm just looking for the smallest clearance between the leaning tube & the bearing race to remain the constant as the engine rotates. Of course, if there is misalignment, it will not stay the same. The size & direction misalignment will vary depending on what combination of parts you are using. Now, to try to minimize the misalignment, I loosen the engine in its mount & put shim stock between the engine & the mount until I get the best alignment I can. You can order stainless steel shim stock in sheets from SmallParts.com.
Many will say that misalignment starts with runnout of the fan on the crankshaft & will recommend going to a metal fan. I have tried that with 2 different brands of aftermarket metal fans, one from Ron Lund & one from HeliProz, & found that the stock plastic fan has far less runnout than either. When those metal fan thread onto the crank, whatever runnout exists is unchangeable becaused the threaded hole is not aligned. Tapered collets do absolutely nothing to change this. A metal fan with less than 0.5thousandths runnout at the hub would be nice, but this can only be accomplished if the fan is machined while on a crankshaft & probably while on the actual crank with which it will be used.
I tried flying the metal fan from HeliProz, disregarding the horrible runnout at the hub of ~3thou. The rigidly unforgiving out-of-true fan caused the lower startshaft bearing to spin in its sideframe bearing seat & the heat of friction melted the seat area. Can you say "frustrating"? Those sideframes were brand spanking new. ;-) Threw them away & permanently retired the useless metal fans. Hey, R&D has its price. The moral of that little story is NEVER use a metal fan whose runnout is not as good as the runnout of the crankshaft.
By the way, for all of the above, I am running the upgrade aluminum sideframe joiners & carbon fiber sideframe stiffener. This significantly stiffins up the airframe & no doubt makes the plastic sideframe bearing block areas even less forgiving of misalignment. A word of advice to any who install the metal joiners; be sure to use plenty of thread release on the bolts. Permatex makes some they call "anti-seize lubricant"; try an autoparts store.
If you are not having, or have not had, premature clutch failure, you are fortunate. Enjoy! |
But really, most of my problems are operator related 
