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grs-pms(at)comcast.net
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 Posted: Mon Sep 25, 2006 2:31 pm Post subject: Replacing rivets with bolts |
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Bob:
This question came up a few months ago, and I posted some comments at that time. You are right, the bolt is substantially stronger than the AD rivet of the same diameter. Your wing won't be any stronger though, since most of the fasteners will still be rivets which will fail before the bolt can really do its share.
The issue to consider is the fatigue resistance of the spar. A driven or squeezed rivet expands to fill its hole and puts the material surrounding the hole in compression, which tends to keep fatigue cracks from starting. An AN bolt in a drilled hole doesn't have the same effect. Under normal operating stresses the material surrounding the bolt feels as though there is nothing in the hole, and it experiences fluctuating tensile stress. Fluctuating tensile stress tends to start fatigue cracks.
To keep the fatigue quality equivalent to a rivet, drill the hole slightly undersize and ream it to the actual (measured) diameter of the bolt. Bolts in the standard AN bolt series (AN3, AN4 etc) have a fairly wide tolerance on diameter, so the nominal diameter is misleading. What you want is a fit which is close (or tight) enough that the bolt has to be tapped into place, not just pushed in with the fingers. Reaming also makes sure that the hole is really round, so the bolt is in contact with all the material around the hole.
For the few bolts involved it might be a good move to invest in close tolerance bolts in the AN173 series. They have the same strength as AN3 etc, but the diameter is closely controlled.
I have taken my own advice on this issue, using close tolerance bolts in reamed holes on my CH601 HD spars, where the factory-built spars needed additional fasteners.
George
zenith-list(at)matronics.com (zenith-list(at)matronics.com)
[quote] Sent: Monday, September 25, 2006 12:17 PM
Subject: Places hard to rivet
Members,
Has anyone ever thought of just using a small (Same shaft diameter as the rivet) bolt, two washers and a lock nut in place of the rivet. I don't think there are very many places where this problem will crop up therefore not much extra weight will be added and the nut and bolt will be just as strong if not stronger than the rivet.
Bob Stone, Harker Heights, Tx
ZodiacXL DO NOT ARCHIVE
Date: 9/22/2006
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J2j3h4(at)aol.com
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| Posted: Mon Sep 25, 2006 3:48 pm Post subject: Replacing rivets with bolts |
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How about drilling and tapping the hole using the recommended tap drill size? When you tighten up the nut on the steel bolt it will deform the threads in the aluminum to give you a very tight fit. Radical, I know, so feel free to disagree. Also, don't call me if your wings fall off.
Jim Hasper
In a message dated 9/25/2006 5:33:52 P.M. Central Daylight Time, grs-pms(at)comcast.net writes:
| Quote: | Bob:
This question came up a few months ago, and I posted some comments at that time. You are right, the bolt is substantially stronger than the AD rivet of the same diameter. Your wing won't be any stronger though, since most of the fasteners will still be rivets which will fail before the bolt can really do its share.
The issue to consider is the fatigue resistance of the spar. A driven or squeezed rivet expands to fill its hole and puts the material surrounding the hole in compression, which tends to keep fatigue cracks from starting. An AN bolt in a drilled hole doesn't have the same effect. Under normal operating stresses the material surrounding the bolt feels as though there is nothing in the hole, and it experiences fluctuating tensile stress. Fluctuating tensile stress tends to start fatigue cracks.
To keep the fatigue quality equivalent to a rivet, drill the hole slightly undersize and ream it to the actual (measured) diameter of the bolt. Bolts in the standard AN bolt series (AN3, AN4 etc) have a fairly wide tolerance on diameter, so the nominal diameter is misleading. What you want is a fit which is close (or tight) enough that the bolt has to be tapped into place, not just pushed in with the fingers. Reaming also makes sure that the hole is really round, so the bolt is in contact with all the material around the hole.
For the few bolts involved it might be a good move to invest in close tolerance bolts in the AN173 series. They have the same strength as AN3 etc, but the diameter is closely controlled.
I have taken my own advice on this issue, using close tolerance bolts in reamed holes on my CH601 HD spars, where the factory-built spars needed additional fasteners.
George
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naumuk(at)alltel.net
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| Posted: Mon Sep 25, 2006 4:29 pm Post subject: Replacing rivets with bolts |
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Jim-
If you don't have the clearance room to rivet, how in the heck are you going to tap a straight hole?
Bill Naumuk
HDS Fuselage
Townville, Pa
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