ZDZ 80 single prop bolts sheered off

[Peter Gordon]

After landing my 30% Lanier Edge 540 after it's 4th flight in total (2nd on the same prop), I heard a tinkling sound inside the spinner as I hand taxi'd it to the pits. Removing the spinner, I found that all 6 prop bolts had sheered off inside the prop hub. I had changed from a solid 24x10 to a laminated 26x10 after the first two flights. The 26x10 is thinner in the middle, but I don't think the bolts bottomed out while tightening (I would have noticed that). Anyway, sent my engine to RCS for repair on May 21st, they still haven't received it yet. Good thing I insured it!

Has anyone else experienced this? They say to check the bolts after every few flights if using a wood prop (due to the wood compressing). Is there anything else I should be looking for? When I start the engine I give the prop a pretty good whack by hand with a think leather glove.

Peter

[can773]

When you tighten a bolt there is a lot of friction between the head/washer and the surface you are tightening against. This friction can take a serious amount of shear loading (this is part of bolt preloading). If over time the bolts became lose (for any reason, like the wood compressing) then the cross section of the bolts will be required to take all the shear loading, which they may not be able to do.

I dont run big motors like this, but I can understand the reasoning for needing the bolts to be properly torqued up all the time. You might just have to keep a closer eye on them.

[Kevin McGrath]

The shearing problem with European six bolt hubs is fairly common and almost always associated with not checking bolt tightness often enough particularly with a wood prop,and specifically when the prop is new.
Its advisable to cross tighten the bolts to get an even torque on the hub,and to develop a gentle hand so as to not mash the prop.
"Whacking" the prop when starting isnt a real good idea,and accomplishes nothing in terms of getting a hotter spark with electronic ignitions that retard for starting,like your ZDZ.The whacking process comes from the need to get the flywheel moving fast enough past the coil for a good spark with magneto equipped fixed timing engines like Quadra and particularly Zenoah,and thats why many of them are equipped with spring starters.
A properly set up electronic ignition engine should be a very easy starting animal with a few flips with the choke on til it fires and stops ,and a few more flips at low throttle to get it running.
Set the prop to come up on compression about two oclock and swing through the arc and get your hand out of the way about 11 oclock.
Its worth noting that I had to replace the bolts with shorter ones on my ZDZ 60cc,and my Moki 44cc twin,since the standard bolts bottomed in the hub when using North American props like Zinger .

[Propworn]

A trick I like to use is to thread the bolts in without the prop and bottom them out, by hand only. Do not tighten just turn them in until they stop. Now measure the distance between the washer and the prop hub and note the distance. As long as the thickness of the prop is greater than this you should have no problem bottoming out the bolts. From this measurement you can also determine how deep the bolts are going into the hub. So when you remove them measure the depth of the hole. If the hole is deeper than the distance the bolt is going into the hub you can use a bottoming tap and easily tap them deeper.

Dennis

[Otis_flies]

Don't feel bad....just got my 8 twin back from RCShowcase with a new hub on it from the same thing. Didn't recheck the bolt tightness on a new prop. Won't make that mistake again.