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Old 06-07-2017, 06:48 AM
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snopercod snopercod is offline
Join Date: Aug 2016
Location: Asheville, NC
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Default GIGO

Your math is right but the formula is wrong. What you have is not a beam with two point loads on it but more like a beam with a single point load in the center. When assembled and torqued, the midpoint of the bolt is going to be forced to deflect about .040 to conform to the 2.8 degree angle. My sense is that this won't be a problem, but I can't prove that mathematically. Since you're a machinist, if you were worried about the bending load on the bolt, you could always ream out the holes on one side. But since they're sending you a new gear leg this is all an intellectual exercise, isn't it?
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Old 06-07-2017, 07:40 AM
Boyd Birchler Boyd Birchler is offline
Join Date: Sep 2014
Location: IN
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In the certified world they have been wedge shimming Cessna's axles for 60 plus years (with much steeper angles than your 2,8 degrees). Although it does put a bending load on the head and nut, I have never seen it to be an issue.
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Old 06-07-2017, 07:41 AM
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AlexPeterson AlexPeterson is offline
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Location: Maple Grove, MN
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Default Some ME thoughts on the soon to be obsolete shim...

The loading scenario is essentially identical to the same system without a shim, assuming both sides of the shim are flat surfaces. The previous post is right in pointing out that your analysis model is not correct.

As others have said, the bolts will have some amount of bending strain in their center, depending on how much clearance the holes have. Additionally, one or both ends of the bolts (head and nut ends) could contain a bending moment. Whether or not these latter two exists depends on how much clearance the holes have. Without analysis and understanding the exact bolt geometry upon torquing, it is not possible to know how much margin is consumed by the angular deformations.

Additionally, understanding cyclic fatigue loading of bolts in systems like this is not simple, and requires understanding the elasticity of all members in the stack, among other variables.
Alex Peterson
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Old 06-07-2017, 09:20 AM
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Jared_Solomon Jared_Solomon is offline
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Location: Powder Springs, GA
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Talking Re-Ran the calculations

Thanks everyone for your replies. I re-ran the calculations with a different model based on your feedback. Which can be seen down below. The tolerances on the holes in both the axle flange and gear leg flange are very small, around -0.002. The contact points of both the bolt head and nut are perpendicular to the bolt axis(flat). Due to the small tolerances i'm thinking that the majority of the load (shear) is being applied uniformly along the bolt shank and the walls of the flange holes on both the axle and the gear leg. Therefore I'm thinking any moments on the bolt head and nut would be minimal.

The latest calculations show a 0.500 KPSI load on the center of the bolt, to deflect it 2.8 degrees (0.055in displacement) with a max shear force of 0.250 KPSI.

The NAS1304-29 bolts used in this joint have a min shear rating of 96KPSI. A 0.250KPSI pre-load seems negligible on the bolts, the flanges and the shim. The loads appear to be lower then I would have expected based on pure "intuition". I'm beginning to think that Van's engineering dept's position may be supported by the data. Unless my calculations are still way off

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Last edited by Jared_Solomon : 06-07-2017 at 09:29 AM. Reason: typo
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Old 06-07-2017, 11:02 AM
sblack sblack is offline
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Location: Montreal
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when is the last time you have seen a barn landed at 6 ft/sec? Buildings are built with generous factors of safety of 3 or 4, whereas airplanes are built generally with 1.5. You have to be much more careful.

I would put a tapered washer under the nut, assuming that there is enough clearance in the hole that the bolt doesn't bind when tightened. That would ensure that the bolt is in tension only.

Most shims that I have seen for gear axles have been for 1 deg or less. 2.8 seems like a lot. If VANS will provide a new one then that is the right answer.
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Old 06-07-2017, 12:13 PM
David Paule David Paule is offline
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Location: Boulder, CO
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The July 2016 issue of Kitplanes has an article, starting on page 46, about the things that a designer has to consider regarding bolted joints. It includes some decent references. It does not include actual analysis.

Caveat, I wrote the article.

Many of the comments in this thread are excellent, by the way.

How would I approach this joint? Simple - I'd contact Van's and ask them what to do, and then do that. Period.


Last edited by David Paule : 06-07-2017 at 12:17 PM. Reason: Added the comment about calling Support.
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Old 06-07-2017, 02:01 PM
Tom Martin Tom Martin is offline
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"when is the last time you have seen a barn landed at 6 ft/sec?"

Not to drag out my perhaps ill placed barn comments but you asked the question. A 1500 lb steer/cow/bull, at five mph, which they can easily do, would be 7.33 ft/sec.
Cattle do run into the side of buildings, it happens all the time when they are frightened or feeling frisky.
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Old 06-07-2017, 03:21 PM
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bk1bennett bk1bennett is offline
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It is not the load on the beam that would concern me but, rather, the loads on the joint. Joints are far more complex to analyze than a beam. The design loads are based on intimate contact between flanges and you cannot guarantee that your shims are going to give you anything approaching full contact. For such a critical joint I would make sure the assembly matches the design.

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