Heavy crush plate question

titanhank · #1 04-28-2016, 12:03 PM

Hi guys, i just installed a new 3 blade catto on my rv6 with a 20lb sabre crush plate. The cg is perfct, but it shakes like crazy above 2200 rpms. I removed the crush plate and the prop is laser smooth at 2700rpm. Can someone give me a good technique for centering the sabre heavy crush plate? I have held it every possible while having a friend torque the bolts and it still seems off center and creates vibration.

Thanks
Hank

alpinelakespilot2000 · #2 04-28-2016, 12:22 PM

From my experience and understanding, the positioning of the crush plate should NOT have an impact on the balance unless it is torqued insufficiently. In addition, a heavy crush plate should generally smooth things out when combined with a light prop, not create more vibration. Although pretty unusual, it might be that your crush plate is unbalanced. Might be worth exploring that possibility with Saber. Good luck.

kblack · #3 10-17-2016, 03:32 PM

Sam at Saber supplied me with a 20# steel crush plate for my RV-7 with a 180hp engine and Catto 3-blade prop. The standard manufacture technique is to over-drill the bolt holes for good clearance by 1/64" as I recall. However, this allows for a LOT of play when installing the crush plate. I found it impossible to get the thing adequately centered. Vibration was noticable, even after prop balancing, and I'm sure I would have to rebalance after every prop R&R.

At my request, Sam drilled a new set of holes between the original holes to nominal bolt size. The AN prop bolts are actually 3 to 4 thousand'ths under nominal size. We both thought I might have to do a little reaming and/or dressing the bolts to fit through the really thick crush-plate, but I did not. Bolts with a little wax just slid right in with essentially no play. Everything else about the Sabre spacer is made to such exacting tolerance, all bolts engaged threads by hand with very little drag while spinning in to tight.

Prop has not yet been rebalanced, but it runs noticeable smoother. And, I'm comforatable that I will not need to rebalance after prop R&R, since there seems to be zero play in the crush plate with the bolts engaged.

Kudos to Sam at Sabre for quick turn-arounds and working with me at no additional charge to get this done.

BTW. To anyone building a 7 with an IO-360 and lightweight prop. Just get the 20# steel crush plate right out of the box. You're gonna need it. And if possible, get Catto to fit the carbon fiber spinner front bulkhead for the much thicker than standard crush plate. Modifying and centering the spinner with the new crush plate was no fun!

titanhank · #4 10-17-2016, 04:05 PM

What a great idea. I just called sam and he is going to redrill my crush plate.. I hope i can finally get this thing to run smooth.

Thanks
Hank

lr172 · #5 10-20-2016, 09:32 AM

As mentioned, there is a decent amount of slop in the interface and 20#'s of gravity insures that you will be at one extreme end of the clearance. THe trick is to lightly snug the bolts and hold the spacer up while tightening a bolt or two (snug is enough, not fully tigntened). You apply just enough upward pressure to center the spacer in the slop. Too light and you are low. Too hard and you are high. You can usually tell when you are in the middle when the 4-5 remaining bolts turn easily with finger tips.

It is a trial and error affair, but doesn't take much time. My 20# has very little difference from the the vibration/feel without it.

Larry

lr172 · #6 10-20-2016, 09:40 AM

Quote:

Originally Posted by alpinelakespilot2000

From my experience and understanding, the positioning of the crush plate should NOT have an impact on the balance unless it is torqued insufficiently. In addition, a heavy crush plate should generally smooth things out when combined with a light prop, not create more vibration. Although pretty unusual, it might be that your crush plate is unbalanced. Might be worth exploring that possibility with Saber. Good luck.

Experience shows that it does cause vibration issues if not centered. Remember the scale here. The crush plate is almost twice the weight of the prop at a very small fraction of the diameter. With this very dense mass, small fractions of an inch make a big difference.

Larry

sblack · #7 10-20-2016, 10:22 AM

A dial indicator might be helpful here. There is a German guy with a machining channel on youtube who uses a 3 jaw chuck exclusively. They never center perfectly. Each time he installs a workpiece he loosens the bolts on the backplate slightly so they are snug but not tight, then he rotates the spindle by hand while watching a dial indicator and taps the chuck body with a copper or lead drift, very gently, until the dial indicator shows no runout, then he tightens the bolts. It takes him about 1 minute to do. This would work just fine on a crush plate, providing that the spinner bulkhead doesn't get in the way. Rotate and tap until running true.

Most people use a 4 jaw chuck when they want work to run perfectly true so this is a very unique technique. But the guy is a professional machinist, and he's German, so he must know something about precision. Stephane Goetswinter (I might have mangled the spelling) is the name if anyone is interested. Great channel if you like that sort of thing.