be very very careful with the bolt torques. In your case the hub is said to be carbon composite. There must be a lot more of those carbon hubs for the Rotax and I´d look at these to compare numbers. The hub in your photo looks hollow, are there distance tubes inside to prevent the bolts crushing the case ? And speaking of high tensile: I do hope the nyloc nuts are 8.8 as well ??? Myself I never fit nylocs but use Loctite in different grades, except where wire or tabs are specified.
VicWhat I meant was that if you torque the M8 bolts according to spec, this could be way too much for the propeller. You may brake it or worse, too much tension will be applied to the propeller and this will weaken it and it breaks in flight. Only the propeller manufacturer can tell you the exact torque to use.
I could also leave out the spacer, it is only there to keep the spinner away from the engine cowl, but that would open a whole new can of worms regarding weight and balance, I can’t be bothered now.
I would not advise leaving out the 40-mm propeller spacer or changing anything else that affects the geometry and mass distribution of the rotating assembly. Small changes can substantially change the vibration response of the system. Leaving out the spinner back plate is probably OK.
most propeller manufacturers have a web presence
This one most certainly hasn’t – if he had, nobody would be buying from Duc and the rest of them – his prices are 40-60% of general market…
to account for the additional friction of the Nylock nuts
Cheese us, that’s a point I hadn’t realised. And yes, I did have observed most propeller are secured by safety wire, only I hadn’t thought any further. Stupid me!
So yes, I will have to revert to the generic bolt torque table – thanks very much! Alternatively I’ll ask the airframe manufacturer, but communications aren’t always very smooth.
But perhaps the most important, which I ought to have mentioned before: the bolts that I am now struggling with do no more than fix the complete propeller to the Rotax engine flange – with a 40 mm spacer between, plus an alu disc that carries the spinner. As it is, the alu carrier disc has been found to be botched so it is left out for the time being. I could also leave out the spacer, it is only there to keep the spinner away from the engine cowl, but that would open a whole new can of worms regarding weight and balance, I can’t be bothered now.
The complete propeller (two half-shells and three blades) is assembled with another six bolts, one on each side of each blade, and these six have been carefully left as they were.
Illustration with the spinner carrier in place, it is the inner six bolts that are of concern now:
http://i.imgur.com/gMKJs8E.jpg
Jan, most propeller manufacturers have a web presence or some other way to get technical info and I would recommend looking for that data. Propeller installation data for a third party propeller on a Rotax does not have to come from the airframe manufacturer. That said, as a last resort the typical installation torque for an M8 x 1.0 or M8 x 1.25 bolt of class 8.8 is 25 n-m. Bolt Torque Table
If you want to account for the additional friction of the Nylock nuts, measure the torque needed to freely rotate the nut on the bolt, and add this to the installation torque specified by the table. Propeller bolts on certified aircraft are typically locked with safety wire, i.e. not with lock nuts. One reason is that thread locking masks the torque being applied to the assembly at installation. For the same reason unplated bolts and unlubricated bolts often have slightly different torque values in an installation torque table.
Make sure washers fit the bolt diameter properly. Also note that when dynamic balancing a propeller, the mass of a washer is enough to make a measurable difference in propeller balance, so each propeller bolt needs to be the same length and use the same number of washers.
NB what torque** would people recommend for these bolts? Please do not refer to the factory documentation
I’m afraid the only one who can tell you that is the propeller manufacturer. The bolts will most probably overtighten a carbon propeller (I would assume, without ever having torqued a carbon propeller). A assume the bolts are used to hold the propeller.
Bought 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.
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.
VicI 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.
