Prop bolts - Info required

[Guest Baphomet]

I have a 3 bladed 'Brolga' hub and prop. I've had to remove it from the aircraft, and when re-fitting, realised I don't have any detail re bolts and torque. The bolts appear to be mild steel 8mm (6 of them) so I guess the nominal torque will be of the order of 10 - 11 ft/lb. If anybody out there has manufacturers data, can you let me know what the number is? On a side issue, the bolts seem to be cadnium plated, and the local supplier tells me you can't get that finish anymore, something to do with the coating being carcenogenic.

 

 

[eastmeg2]

Baphomet,

 

I'm certain you will find that they are "AN" series aircraft quality bolts and you won't find them in a suburban hardware store.

 

There was a good article in a recent RAAus magazine, Oct 09 I think, about AN bolts and why they are used.

 

Most CFI's have a healthy stash of them as they buy them in bulk and will usually on-sell them in small quantities to pilots like you and me.

 

Cheers,

 

Glen

 

 

[Guest Maj Millard]

Unfortunatily if it's a Rotax engine you have to use a metric threaded bolt, as that is what the bosses in the prop flange have. AN bolts although preferable, don't come with metric threads. In some installations you may have the room to use AN bolts right through, with a Nylock nut on the back. The bolts should also have drilled heads, so you can lockwire them.

 

You need to find a good bolt supply place (not Hardware store !) and purchase the correct length, high grade metric bolts. The ones I have used for some time are black, and have allen key socket heads, but the normal head is fine. If the bolts you currently have, are the correct length, take them along. You need to be correct with your length as you should only use two washers at most, on each bolt.

 

You can not mess around with prop bolts. I put the threaded bolts through the threaded bosses, then I put a metric nylock nut on the remaining threads, and I drill and lockwire the heads. Torque for a brolga from memory should be around 12-16 FT lbs. Certainly no more. I am relying on my memory so hopefully somebody will help us out on this forum, with the actual figures...............................................................................................

 

 

[Guest Baphomet]

Thanks Major, you are correct. They most certainly are metric (8 mm) NOT "AN" they appear to be a grade '5' and are not high tensile (grade12). The replacements I have been able to aquire thus far, have come from a specialist bolt supplier, and not a hardware store. I'm pretty sure the black variety you refer to are a grade '8' (probably what I should use)

 

 

[Guest Maj Millard]

Baphomet, I have been told by some that 'Grade 8' are only as strong as grade 5 anyway. Can't say I have realy researched metric bolts, but I do have many sets that have done the job now for years with pretty hard usage, so I do know they are up to it. They also come with about 25ml of thread on them, which goes through the threaded boss then leaves enough thread hanging out for you to put on a Nylock nut, if you've got room............

 

 

[Guest Baphomet]

Found this on the web "The lowest grade is 4.6, commonly known as commercial grade. Next comes grade 8.8, known as structural grade, and finally, the highest grade is 12.9, known as high tensile bolts. The first number represents the ultimate tensile strength of the bolt, 400 Mpa, 800 Mpa or 1200 Mpa respectively. The second number represents the point at which the bolt will permanently stretch. (Officially, the 0.2% proof load stress). A 4.6 bolt permanently stretches at 60% of its ultimate, an 8.8 at 80% and a 12.9 at 90 %.

 

I'm more interested in how much torque to apply, as it's a synthetic hub.

 

Baphomet

 

 

[Guest Maj Millard]

Baphomet, Thanks for that info. Generally most people tend to want to over-tension prop bolts, which is a mistake, and it often causes problems such as bolt failures in use, and crush damage to hubs etc. Propellor bolts are what are referred to as 'working' bolts, in as much as they are in tension, and shear during operation. If a bolt is overtorqued, the additional 'working' loads may well take them beyond their ultimate failure strengths. Generally 12-16 ft Lbs is sufficient, and use of a quality torque wrench is required to achieve equal tension/load on all bolts. Reference to the prop manufacturers torque specs is highly recommended..............................................................................................

 

 

[eastmeg2]

I can say from recent experience when I assembled a new Bolly prop that it would be very easy to over tension the bolts as I was achieving the required torque on my torque wrench and I barely felt like I'd even started getting some tension on. The setting was very close to the start of the scale on my wrench.

 

Cheers,

 

Glen

 

 

[Shane]

Retensioning a Brolga Propeller

 

 

 

This video is a 4 blade Brolga on a Drifter

 

But the tensions are the same

 

Shane

 

 

[Guest Baphomet]

Hi Glen, yes thats the problem. This has all come about because I was progressively retensioning the bolts and one of them failed at about 11 ft/lbs, so I've decided to replace them all.

 

Shane, thanks I've seen that video and he mentions a figure of 168 inch/lbs which seems on the high side, and is above engineering table recommendations for that diameter bolt. Thats why I was hoping someone would have the Brolga manual.

 

 

[Guest Maj Millard]

168" lbs is only 14 ftlbs, so that could be correct. I did have a four blade on my Drifter and I seem to recall 16 ftlbs.

 

 

[Guest Andys@coffs]

Under no circumstances over torque. The Brolga prop hub is a composite hub that has a metal plate in the center. On an Airborne Trike I once made the mistake of overtorquing (and it was only very marginally over) and couldnt work out why more degrees of turn didnt change the torque. It was because the metal was delaminating from the rest of the composite material.

 

The Airborne website has its microlight POH's listed and within the 582 classic manual identifies that the correct torque is 14ft lbs. details here Airborne Microlight Aircraft, microlights, trikes, ultralights, ultralight trikes and hang gliders, this manual http://airborne.com.au/images/manuals/x_series_manual.pdf on page 66

 

Hope this helps

 

Andy

 

 

[eastmeg2]

If I recall correctly, the shop whose link is given in post #12 can provide new hubs and bolts for Brolga props, but 68 inch Brolga prop blades can no longer be bought - anywhere.

 

 

[Guest Maj Millard]

The big problem with spinners on tractor props is if the come loose and ultimatlely 'off', they will almost always do damage to the propellor, or the rest of the aeroplane. You cannot have any' play' or movement in a prop spinner. It will always get worse not better, and it could be the start of a spinner saying 'se ya!'

 

In my years as an aircraft maintenance eng I alway made a point of placing my hand on the point of spinners, and checking for any play. You would be surprised what I found at tmes, even on standard GA line rental aircraft. I have also seen some results of spinners coming loose and departing.

 

Consiquently with the above in mind I choose not to have a spinner on my beautifull Powerfin prop on the Lightwing, even though it would look great with one. Additionally because the propellor is the highest stress component on an aircraft with up to 8 different forces constantly going on, (crankshaft No 2) I do like to pyhsically look at my prop bolts often. Doesn't mean I'm constantly disturbing them and retorquing them, I just like to look at them during preflight.....Keep your spinners movement free and running straight. ...........................................................................................

 

 

[Guest terry]

prop bolts

 

Baphomet, I have a broga 68" on my thruster so I have the good oil, you can take it as gospel.

 

The bolts for fitting the blades to the hub are torqued to 132" lbs which equates to 11 ft lbs

 

and the bolts to the gearbox is 168" lbs which equates to 14 ft lbs. You need a quality tension

 

wrench to get it right not a $50.00 job from supa cheap. Hope this gives you the answer your looking for.

 

Terry

 

 

[eastmeg2]

The Bolly web site indicates they make the 68" Brolga left & right hand with Dura-Tuff edge (& it's tough!) $380.00 eachBrolga

Yes, but ask about actually ordering it and it's a different story . . .

 

 

[Guest Baphomet]

Thanks for the response guys I'll go with 14 ft/lbs. I may not know the answers, but at least I know when I have to ask questions :-)

 

 

[Kev]

Baphomet,

 

I used to have a Brolga prop, although it was a 4-blade, not 3-blade. the torque values from memory were: 10ft/lb for the clamp bolts and 15ft/lb for the mounting bolts.

 

Beware of substituting grade 8 bolts for grade 5. If the manufacturer specifies grade 5, then that's what you use. Grade 8 are harder (for this read more brittle) and will fail by shearing whereas a grade 5 will bend.

 

One or two trike flyers in US have found this out to their cost, when they replaced their hang bolts with grade 8's supplied by their local hardware store.

 

Safe flying

 

Kev

 

 

[Guest Baphomet]

Thanks for the reply Kev, one of the problems is that I don't have the original prop info, so I don't know what the specification is.

 

Baph

 

 

[Guest terry]

prop bolts

 

I do have the original sheet of specs that came with the prop so my previous post is gospel. It pays to keep every thing.

 

Terry

 

 

[Guest Baphomet]

Thanks Terry, for your (and anybody else thats interested) info, I've confirmed the Brolga prop hub was (and still is) manufactured in the USA by Ultra-prop. It was fitted with blades made by Aerofibre here in Aus. The tech data is on the Ultra-Prop site

 

Brolga Ultra-Prop | Composite Propellers for Ultralight Aircraft

 

According to the site you can use grade 5 bolts or better (i.e. 8.8) and torque them to a max of 170 inch/lbs (hub)

 

Cheers

 

Baph

 

 

[Kev]

Baphomet,

 

Have just checked on the airborne website:Airborne Microlight Aircraft, microlights, trikes, ultralights, ultralight trikes and hang gliders

 

Torque values are: blade 11lb/ft and mounting bolt 14lb/ft. Retorque every 20hr. Dismantle & inspect every 200hr.

 

Safe Flying

 

Kev