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HomeAircraftHelicopterHelicopter Main Discussion › How strong is a 4mm bolt?
04-05-2010 01:56 PM  8 years agoPost 1
Bruce Naylor

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Swindon, Wiltshire, UK

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Just noticed that a newly released .90 machine uses a 4mm bolt to attach the bearing stack to the feathering spindle in the rotorhead.

I wasn't personally happy with 4mm bolts on a .50 - but on a .90???

Is there any data out there that says a 4mm bolt (assume HT) has enough margin for this appication.

I have in the past seen a 5mm bolthead sheer off in flight on an F3C machine, hense my concern.

An additional concern is the thread fit - the 4mm bolt in question seemed "undersized" - ie wobbled a lot when the majority of thread had been inserted.

I'm not out to bash the use of a 4mm bolt since I have no data to say otherwise, but I am interested in what margins we have here.

Bruce.

"If you know what you're doing, you're not learning much"

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04-05-2010 02:24 PM  8 years agoPost 2
flyboy0413

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Brookfield, CT

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tensile strength of the din 12.9 variety is 174,000 pounds/in^2. So, assuming a 4mm bolt had a minor diameter of 3.1mm, the maxiumum load that it can carry is roughly 2000 lbs. A 5mm bolt, w/ a 4mm minor diameter can hold up to 3390 lbs.

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04-05-2010 09:13 PM  8 years agoPost 3
Bruce Naylor

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Swindon, Wiltshire, UK

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Thanks for that info flyboy0413.

2000lb, about 900kg. A Google calculator states worst case with a 200g blade at 2000rpm at about 600kg root pull, so the maths suggest it should stay together assuming no other forces are acting on the 4mm bolt.

However, I've had further thoughts on the issue.

I'm sorta happy with my current setup - a 4mm bolt holding the blade on, and a 5mm bolt holding the blade grip to the feathering spindle in a .90 machine.

Using cross section area, and assuming a blade bolt has to fail in 2 places to let the blade go, a 5mm bolt roughly represents 2x4mm bolts (or 2 places on the same bolt), so both bolts are in the same failure zone so to speak.

The new questing is that since 4mm roughly represents a 2x3mm bolt cross sectionally, and assuming cross section relates to strength, should I be happy bolting on 700mm blades on my .90 machine with a 3mm bolt? If not, why am I happy with a 4mm bolt holding the bearing stack on the feathering spindle?

Time for bed - enough pontificating for one day :>

Bruce.

"If you know what you're doing, you're not learning much"

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04-06-2010 12:31 AM  8 years agoPost 4
bigdad390

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East. Liverpool, Ohio

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So, even though mathmatics show the bolt is strong enough, does the formula take into account stress over a period of time due to vibration, flight loads, lateral loads from pitch changes, etc?

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04-06-2010 01:38 AM  8 years agoPost 5
oldfart

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Vancouver, Canada

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So, even though mathmatics show the bolt is strong enough, does the formula take into account stress over a period of time due to vibration, flight loads, lateral loads from pitch changes, etc?
Good point. As there is a lot more to the forces in a rotor head then just the obvious, more static type of loads. Just ask anyone who has a lot of experience in designing rotor blades.

I would bet those loads would increase substantially under even a slight and brief flutter condition (e.g.when you hear a blade "fart" on rapid pull outs etc.)

I have seen 4mm blade bolts bend in smaller 600mm bladed helis.

Fortunately Bruce, with a tight machine, flying top class rotor blades, and doing the 2010 FAI/F3C maneuvers (they present rotor loads that are far less then do the more extreme type of 3D being flown by some today) you should be fine.

Phil

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04-06-2010 02:00 AM  8 years agoPost 6
flyboy0413

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Brookfield, CT

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Personally, I think that it is silly for any heli designer to use a 4mm bolt on a feathering spindle instead of a 5mm one on a 90 size machine. The only possible advantage I can think of is to save a few grams, but to do this in one of the most highly stressed areas of the heli is kind of dumb, especially since failure of this fastener results in a destroyed heli at best and a serious injury at worst.

Which heli is this done on?

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04-06-2010 02:06 AM  8 years agoPost 7
heli-cuzz

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Pittston, Pa. USA

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How strong is a 4mm bolt?
In laymen's terms, very very strong.
So, even though mathmatics show the bolt is strong enough, does the formula take into account stress over a period of time due to vibration, flight loads, lateral loads from pitch changes, etc?
I've flown many a 4mm spindle bolts on many cases of fuel with not one failure. Even in a crash the spindle bends and not the bolt, and yes, I do reuse the 4mm spindle bolts with no problems.
I have in the past seen a 5mm bolthead sheer off in flight on an F3C machine, hense my concern.
That's just plain $hit happens bad luck.

Fury 55 NIB Furion6 CGY750 fbl helicopter-Frenzy CGY750 fbl nitro-Frenzy fbl NOBAR90

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04-06-2010 03:56 AM  8 years agoPost 8
Rob43

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Midland, MI USA

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As a mechanical engineer, I believe the key to the bolts holding up relates to the strength reducing factors such as stress risers, and cyclical stresses. From the past discussions of this topic, I believe that the quality of fastener may relate to premature failure if weakened somehow, but there is a substantial reduction in strength as a result of shear stress, or bending stress. Both are apt to happen if the fastener becomes loose enough to lose it's preload. They have to be tight, and stay tight under the loads. Thread locker helps, but any problems with assembly, such as, the thrust assembly not properly seated, or a missing spacer shim, or what have you could account for loss of preload, and result in failure.

Rob

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04-06-2010 12:38 PM  8 years agoPost 9
bopshi

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greenport ny

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When I worked on real airplanes I used to remove engines that were attached to engine mount with 4 five sixteenths bolts. on a Mooney I believe. So if the bolts r made of high quaility metal they can be really strong. So that little 90 size Heli should be ok with those bolts. Also I try to keep my helis above head height as simple safety rule for just in case senario. Dave

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04-06-2010 02:17 PM  8 years agoPost 10
LaurenceGough

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UK

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My own personal view is that 4mm feathering shaft bolts are good up to ~600mm blades and that is it, anything more than that needs 5mm bolts.

That being said this is just my view and probably is wrong, just what I feel comfortable with.

These bolts go through a large amount of stress.

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04-06-2010 02:50 PM  8 years agoPost 11
Taipan

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Sydney, Australia

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I believe T-Rex 700's use 6mm bolts on their spindles. Would that be overkill?

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04-06-2010 05:20 PM  8 years agoPost 12
smallplanes

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S.C. ,SSA

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Well it is stronger than a 3mm bolt Just kidding


Spectra G 26cc
Jewel
Bat 27cc
Trex 700<br

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04-06-2010 08:01 PM  8 years agoPost 13
Rob43

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Midland, MI USA

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6mm won't be overkill if it can work loose, even if it doesn't come out...the bending stress when the blades lift and the rotor blade grip is bending on the bolt will likely fail it. I've read ofnumerous T700 feathering spindle bolt failures on here and HF. It's not a good day when that happens.

Rob

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04-06-2010 08:15 PM  8 years agoPost 14
oldfart

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Vancouver, Canada

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As others have mentioned before, there are many different steels from which bolts are cut. They all have different qualities for different applications and also different pricing.

If a manufacturer chooses to save money by using a lower priced steel bolt to save costs, the type of steel from which that bolt is made from may well not be the best type or even an appropriate type for the application. The result will always be a higher failure rate.

This rate may be acceptable to some consumers who may have lower standards then others.

Heck I would bet that a proper steel bolt of 4mm diameter would work better then a 6mm bolt of an improper type of steel. SPecially if one is installed improperly and the other is not.

Some countries steel industries are substantially more advanced then others. This also can be a factor in some of our products.

Phil

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04-08-2010 01:46 PM  8 years agoPost 15
Bruce Naylor

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Swindon, Wiltshire, UK

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Does anyone know how easy it is to over-torque a 4mm bolt with, say, a regular allen key? Can I assume that over-torquing issues subside as the bolt diameter increases? How does over-torquing affect strength? Basically put, how tight should we be fitting head bolts?

Questions, questions, questions ...

"If you know what you're doing, you're not learning much"

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04-08-2010 02:47 PM  8 years agoPost 16
MattJen

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UK

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also
it is not so much how big the cap screws are but how much thread they have, it is no good having a strong M4 bolt when it only goes into the featheirng shaft a little.

i think this was the problem on the early trex's there was not enough thread cut.this was certianly a problem on some of the smaller electric ones they did, cos i re threaded my own after a club member lost his blades when the bolt let go.

I am swinging on my Predator turbine using 810 blades at 1450rpm i pulling 380 kilos, I spoke to Bud at quick uk as i am using QUK M5 cap bolts with a good thread, he said they were rated to 2 tonnes so i well within my limits.

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04-08-2010 09:42 PM  8 years agoPost 17
Justin Stuart (RIP)

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Plano, Texas

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What brand of .90 is using this 4mm bolt?

Avant RC
Scorpion Power Systems
Thunder Power RC
Kontronik Drives

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04-09-2010 02:54 AM  8 years agoPost 18
GMPheli

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W. Bridgewater, MA USA

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"Does anyone know how easy it is to over-torque a 4mm bolt with, say, a regular allen key? Can I assume that over-torquing issues subside as the bolt diameter increases? How does over-torquing affect strength? Basically put, how tight should we be fitting head bolts?

Questions, questions, questions ..."

I don't believe feathering shaft bolts should be tightened too much. These forces are addative, right?

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04-09-2010 06:46 AM  8 years agoPost 19
Bruce Naylor

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Swindon, Wiltshire, UK

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I purchased a feathering spindle from Hobbyking for their new .90 to evalulate it for use in a Robbe head block, and it came machined for 4mm bolts. Had it been 5mm, then a quick touch in a lathe would have produced a very cheap replacement for the Robbe part.

"If you know what you're doing, you're not learning much"

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04-09-2010 06:48 AM  8 years agoPost 20
Bruce Naylor

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Swindon, Wiltshire, UK

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http://www.hobbyking.com/hobbycity/...eathering_Shaft

"If you know what you're doing, you're not learning much"

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