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  #1  
Old 06-06-2017, 03:41 PM
Jared_Solomon's Avatar
Jared_Solomon Jared_Solomon is offline
 
Join Date: Sep 2006
Location: Powder Springs, GA
Posts: 58
Default Engineers: Check my math

Hey All,

i've been plugging away at my RV-14 and I've run into a snag. After installing the main landing gear and performing the toe-in/toe-out measurements it appears that my left landing gear has a twist in it resulting in 2.8 degrees of toe-in. I decided to machine myself a shim to take the twist out (I have the equipment, so why not!). I installed the shim and it went on no problem. It got me thinking about the preload I might be putting on the bolts with the 2.8 degrees of deflection. I reached out to Van's. I spoke with Gus who in turn spoke to engineering. His response was engineering felt it should be fine, but he thought it was best to replace the gear. There shipping me out a new gear leg today. As a side hobby i've been trying to learn more about mechanical engineering. So I decided to try and run some calculations of my own to see how how much pre-load, particularly in shear I am putting on the bolts with the shim. I used an online beam stress calculator to run my calculations. Am I in the ballpark?

Beam Length (Bolt Length) = 2.25
Center of the deflection = 1.125 (half way point)
Area Moment Of Inertia = 0.00019
Young's Modulus = 30E+6 * .38 (reduced for shear) = 11400 KPSI


I also ran the calculation for the full Young's Modulus of 30E+6, which brought the shear force to 0.7 KPSI.







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Jared Solomon
Powder Springs, GA
2017 Dues Paid
RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

KPUJ (Silver Comet Field)
Youtube channel: http://www.youtube.com/jmsolomon2000

RV-14 Build Log: http://rv14.jaredsolomon.net

Last edited by Jared_Solomon : 06-06-2017 at 10:34 PM. Reason: Fixed images not working
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  #2  
Old 06-06-2017, 04:16 PM
salto salto is offline
 
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Location: Wee Waa Australia
Posts: 229
Default

Strewth Jared... You won't fly if you do to many of those. 😀
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  #3  
Old 06-06-2017, 06:56 PM
SgtZim SgtZim is offline
 
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Location: Crittenden, ky
Posts: 131
Default

Interesting issue. I'm not an engineer, but studied Aero E many many years ago. I would look at this more as tension vs shear. I'd guess your numbers are good - also consider the direction of the preload is horizontal since you're correcting for toe in/out. Any operational loads are going to be primarily vertical with landing unless you really hammer the brakes or hit something hard. Anyway - non issue since they are sending a new leg. Van's is very good about that sort of thing. I expect I will need about .75 degree correction on my left leg. Van's says they tend to do some warping with the heat treatment.

I had no luck at all with their "dangling bobs" method for lining things up. I found a little magnetic laser at Harbor Freight I used with the initial set up of the gear, and plan to use it again for the final correction when I get it up on the gear permanently.

Happy building!
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  #4  
Old 06-06-2017, 08:07 PM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
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Location: Cincinnati, OH
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adding a shim between flanges will create bolt bending when the axle is loaded. that is bad. bolts should be loaded in tension only unless it's an interference fit. I doubt this is an interference fit. adding a tapered shim between flanges is even worse because when you torque the bolt the tension itself creates bolt bending because the flanges will slide along the shim. basically, all you have to remember is do not bend bolts. but if you do have potential for bending bolts, like a carb lever attachment, put the shank in bending and not the threads. good call to replace the gear leg. ( I can't see the pictures to comment on the calculations but I wouldn't bother with this kind of calculation, just correct the issue. )
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Last edited by Steve Melton : 06-08-2017 at 09:19 PM.
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  #5  
Old 06-06-2017, 08:22 PM
mrhorspwr mrhorspwr is offline
 
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Location: Okc, Oklahoma
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I ran into that and the problem was the lower fitting was drilled wrong from vans
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  #6  
Old 06-06-2017, 08:43 PM
BillL BillL is offline
 
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Location: Central IL
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I can not see the pictures, but will assume the wedge is located between two flat pieces through which the bolt acts. If that is the case the 2.5 deg is not much for the bolt bending, but the head and nut is another matter. Either a wedge, or spherical, self aligning washer on each end would address that. Then you would have been fine if needed. The proper loading case of the bolt would be to assume no friction, look at the wedge angle and calculate the shear loading of the wedge trying to spit it self out using static forces resolved orthogonal or parallel to the surfaces. Reality is that with a .2 friction factor, you should have enough normal force to resist a slip, but this would get you the worst case shearing force. Then you can look up the acceptable shear load in the MIL-HDBK-5J tables. You might want to add this pdf to your collection. Mr. Google can find a copy for download.
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  #7  
Old 06-06-2017, 10:57 PM
Jared_Solomon's Avatar
Jared_Solomon Jared_Solomon is offline
 
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Location: Powder Springs, GA
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Default Shims are standard practice

Quote:
Originally Posted by Steve Melton View Post
adding a shim between flanges will create bolt bending when the axel is loaded. that is bad. bolts should be loaded in tension only unless it's an interference fit. I doubt this is an interference fit. adding a tapered shim between flanges is even worse because when you torque the bolt the tension itself creates bolt bending because the flanges will slide along the shim. basically, all you have to remember is do not bend bolts. but if you do bend bolts, like a carb lever attachment, put the shank in bending and not the threads. good call to replace the gear leg. ( I can't see the pictures to comment on the calculations but I wouldn't bother with this kind of calculation, just correct the issue. )
Hey Steve,

Using tapered shims on the RV-14, RV-8 and RV-12 is standard practice. I fixed the pictures so you should be able to get a better idea of what I was describing. Van's sells shims in 0.25, 0.50 and 0.75 degree increments to fix this problem. They suggest stacking them as needed. I don't think they had something as large as 2.8 degrees in mind. But 1 - 1.5 degrees is not uncommon for these airplanes. My solution was to machine a shim to the correct angle instead of stacking a bunch of shims. The question is whether 2.8 degrees is "just too much". Van's took the conservative route and simply offered to replace the gear leg, even though the engineers at Van's said it should be fine. I thought it would be educational to run the calculations though and see if the engineering department's position that it should be fine could be demonstrated via calculations.
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Powder Springs, GA
2017 Dues Paid
RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

KPUJ (Silver Comet Field)
Youtube channel: http://www.youtube.com/jmsolomon2000

RV-14 Build Log: http://rv14.jaredsolomon.net
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  #8  
Old 06-06-2017, 11:05 PM
Jared_Solomon's Avatar
Jared_Solomon Jared_Solomon is offline
 
Join Date: Sep 2006
Location: Powder Springs, GA
Posts: 58
Default Thanks!

Quote:
Originally Posted by BillL View Post
I can not see the pictures, but will assume the wedge is located between two flat pieces through which the bolt acts. If that is the case the 2.5 deg is not much for the bolt bending, but the head and nut is another matter. Either a wedge, or spherical, self aligning washer on each end would address that. Then you would have been fine if needed. The proper loading case of the bolt would be to assume no friction, look at the wedge angle and calculate the shear loading of the wedge trying to spit it self out using static forces resolved orthogonal or parallel to the surfaces. Reality is that with a .2 friction factor, you should have enough normal force to resist a slip, but this would get you the worst case shearing force. Then you can look up the acceptable shear load in the MIL-HDBK-5J tables. You might want to add this pdf to your collection. Mr. Google can find a copy for download.
Hi Bill,

Thanks for the tip about MIL-HDBK-5J. I downloaded a PDF copy. Looks like it's got some great info in it.I fixed the pictures so the calculations can be seen. Would the head and nut experience more load due to the wedged shim? The nut is torqued to the correct spec (85 in/lb), so I would expect the clamping pressure at both the nut and bolt head to be the same as if the joint didn't have the wedge. Am I missing something?
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Jared Solomon
Powder Springs, GA
2017 Dues Paid
RV-14A (N214WJ) - Flying since Sept 2017
RV-7A (N211WJ) Flying since 2009 (SOLD)
RV-7A (N132RD) bought flying 2014 (SOLD)

KPUJ (Silver Comet Field)
Youtube channel: http://www.youtube.com/jmsolomon2000

RV-14 Build Log: http://rv14.jaredsolomon.net
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  #9  
Old 06-07-2017, 05:58 AM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 1,349
Default

Quote:
Originally Posted by Jared_Solomon View Post
Hey Steve,

Using tapered shims on the RV-14, RV-8 and RV-12 is standard practice. I fixed the pictures so you should be able to get a better idea of what I was describing. Van's sells shims in 0.25, 0.50 and 0.75 degree increments to fix this problem. They suggest stacking them as needed. I don't think they had something as large as 2.8 degrees in mind. But 1 - 1.5 degrees is not uncommon for these airplanes. My solution was to machine a shim to the correct angle instead of stacking a bunch of shims. The question is whether 2.8 degrees is "just too much". Van's took the conservative route and simply offered to replace the gear leg, even though the engineers at Van's said it should be fine. I thought it would be educational to run the calculations though and see if the engineering department's position that it should be fine could be demonstrated via calculations.
well, a tapered shim between bolted flanges may be OK for building a barn or something like that.
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Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 600+ for all

Simplicity is the art in design.
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  #10  
Old 06-07-2017, 06:04 AM
Tom Martin Tom Martin is offline
 
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Location: Ontario, Canada
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There are tens of thousands of 150 year old barns that were built without engineers and are held together with tapered wooden pegs.
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