Nobody is talking about cutting a new thread on this M8 bolt, because you can get it in the correct length as Jan requires,
Well, that’s not the compete story. When I opened this thread I was hoping for someone who could/would machine M8x115 (plus thread) bolts in at least 8.8 quality. But that idea has been abandoned now, unless the factory can offer them, but that seems unlikely – they did not fit such bolts originally.
So I have given in to using over-length bolts, with washers as required to get the required shaft length.
I believe you definitely had a standard bolt shortened to 115mm shaft plus thread. I cannot tell in which length the threaded part of M 8 bolts comes. It may be 20mm or 25mm. So an over all length of 140mm could fit your prop. And hey, this is a 100 hp at ~ 5500rpm Rotax that was quite happy with this bolt till the thread was f…d up a bit. I still have not seen the old bolt and its finish but myself, I would not mind to fit any 8.8 bolt to it. Shortening an overlength bolt is trivial with a decent hacksaw and no problem for the strength of the bolt, plus it was done like that before with Jan´s prop.The shaft surface finish does not matter much with steel alloys unlike aluminium, that definitely has a limited useful life when highly stressed – cylinder head cracks Lycosaurus . And no, Peter, 1 percent load on common bolts, not really! What is a correct preload with a torqued bolt: I´d say at least 70 % , up to 80 % of its strenght, for undefined time with steel. Dynamic loads do not matter as long as these remain below the preload of the bolt. I´d worry a lot more about the hub of the prop in case this was made of aluminium. I remember a series of cracked hubs but can´t tell where I read that.
VicYes – I agree the preload on a torqued up bolt will take it to a lot more than 1%. But it can’t be anywhere near as close to its yield strength as 70-80%. My guess would be 25%. I know this is not the same thing but prop blades are designed to run at 25% of their yield strength at max rated RPM.
Bolts are torqued to a very high fraction of yield stress, sometimes over yield stress for a single installation ‘cycle’. The fatigue situation with a torqued bolt then depends on what it is clamping and the applied load. Stiffer clamped assemblies (like solid aluminum prop hubs) generate a lower bolt stress variation for a given magnitude of cyclical load in the bolt tensile direction. Preload minimizes or eliminates stress reversals which are typically a fatigue issue.
Aluminum has a finite fatigue life at any stress, conversely steel has a cyclical stress below which fatigue life is infinite, termed the endurance limit.
Jan, I’m sure if you need to shorten bolts you should be able to find somebody with a lathe to make a nice job of it.
Pre-load in a bolt can vary. A structural bolt should typically be preloaded to 70-90%. AN bolts are pre-loaded to around 20-30% from what I can figure. I think this is done because a typical AN bolt shall be able to take both shear and tension plus some safety for bending perhaps, misalignment etc.
In other situations bolted connections are designed so the bolt shall only take direct shear. Then no preload should be applied.
A propeller/hub fit is typically a friction joint. The propeller is clamped to the hub by bolts. If the propeller is wood or composite, than I would imagine the preload to be enough to prevent relative motion, but not more than the wood/composite can handle before being crushed. The pre load could be much lower than the typical 70-90%. But the soft and plastic propeller material would also mean that most of the varying loads are taken directly by the bolts.
I wouldn’t cut threads either, only shorten the bolt if required making sure there is a decent lead in so the nuts aren’t damaged.
With a soft wooden or composite prop the torque is often taken by friction at the prop flange, depending on the crankshaft design. Recent crankshafts are typically spigotted into the propeller, old prop hubs are not. In either case the bolts cannot be torqued to a high level with a wood etc prop, IIRC it’s around 25 lb-ft or ~35 nm for a 3/8 inch bolt. Yes, the bolt torque needs to be checked occasionally, in particular with a wood prop to account for seasonal dimension change. If they loosen enough with an old style hub the prop can move on the flange and get hot enough to blacken the wood.
as I see it you may get you 8.8 bolts 140mm long somewhere and look what you have got. The threaded part can be 20 or 25mm. So either the 140mm type will be spot on to your ideal size or you could add spacers of ca. 5mm under the hex head. I´d happily provide you with nice shiny alu spacers with big chamfer on for a tenner plus shipping if you like, no prob . Cutting the bolt with a hacksaw is really not much different to mild steel, so don´t worry.
VicBought M8x150 today, in 8.8 quality, both the threaded and the non-threaded part were slightly too long. M8x145 would have the non-threaded shaft of perfectly adequate length but is not a common stock item, though it is mentioned in DIN931. The non-threaded part would be virtually reduced by adding some washers under the head – no shame there, the factory did nothing else – the threaded part needed harsher measures*. Tried the hacksaw first, as recommended, but gave up as it was rather uncomfortable. No bench vise was available so I could only place the hacksaw upside down between my belly and a hard point in the hangar construction, then run the bolts up and down the blade. This worked quicker than I had feared, but the bolts soon got too hot to hold. So when each was cut to perhaps 40% I gave up and did the rest with the Dremel tool with a 1,5" grinder disc – much quicker and far more comfortable. Repaired the thread with a simple flat file, though I’d have preferred to grind them on a rotating stone.
Went to install them, only then to find I had not brought the quintessential M8 washers – get back to the aerodrome tomorrow and if the weather holds out we’ll be back in the skies by noon. Remember to drive around the Werchter festival, though, this time – cost me close on an hour extra this evening, in the evening busy hour, and a detour of at least 20 kms.
NB what torque** would people recommend for these bolts? Please do not refer to the factory documentation – if I had that available I wouldn’t ask. And surely I’ll torque them, then run it for a couple of minutes, retorque, then fly some 10 or 15 minutes, then retorque again? Using that kind of nut with a nylon collar inside, yes yes, shiny new ones.
