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Modified master cylinder attach hardware

mcencula

Well Known Member
I've recently been working on wrapping up the fitting of the brake pedals to the rudder pedals. In the interest of reducing free play and wear, I opted for the single, long bolt instead of two shorter bolts for the brake pedals. I also wasn't satisfied with the bolt in single shear with a stack of washers as a spacer for the master cylinder to brake pedal attachment. I decided to make a custom stud:
20200828-05-tn.jpg


A closer view. Note this is my prototype, so I hadn't bothered to thread the portion of the stud which extends through the master cylinder:
20200824-03-tn.jpg


The stud and spacer are silver soldered together. The stud end going through the brake pedal tab will be use a castle nut (installed loosely) and cotter pin. The stud end going through the master cylinder will use a fiber locknut (installed tight). The contour of the little spacer is there to prevent rotation between the stud and master cylinder.

I did a bit of analysis to check and ensure the result was OK from a stress and deflection perspective. I am not privy to the design loads Van's used for their analysis, so in the interest of picking a value, I chose a load matching to the burst pressure of the 3003-O brake line tubing. Since this is a linear static analysis, loads below this (rather arbitrary) load will be proportionally less. Also, note that as a a linear static analysis, any stresses shown above yield will not be accurately reflected in the model. Clipping in these two screenshots is at .5Su.

Van's design - significant bending stress, >.015 deflection.
20200823-01-tn.jpg


Modified design - reduced stresses, .0025 deflection.
20200823-02-tn.jpg


I installed one of the pedals per Van's instructions and measured free play at the top edge of the pedal: ~1/8".

I then disassembled and reassembled with the long pivot bolt and this little custom stud, and again measured free play at the top edge of the pedal: ~1/32".

This post isn't intended to be controversial or suggest that anything is wrong with Van's design...just trying to share what I did in case anyone finds this useful. If anyone decides to duplicate the effort, I created a drawing which, hopefully, is useful.
 
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Silver soldering heats the metals up a lot.

Have you given any thought to what may have happened to the strength of the material you used for the stud??

No criticism, just wondering.
 
Nice Mod! I wore out two sets of master cylinders due to side-loading. I was able to correct it with some shimming. I think you went a little overboard, but it is a nice design and set-up.
 
Silver soldering heats the metals up a lot.

Have you given any thought to what may have happened to the strength of the material you used for the stud??

No criticism, just wondering.

Good question. Yes, I did think about that, but not having any way to check it (such as a Rockwell hardness tester), I made an intentional decision to ignore that concern. The little spacer gets most of the heat, but you're absolutely correct that the bolt strength could be affected. If anyone has a Rockwell tester, I could send over my prototype to test.
 
alternate bushing design - master cylinder to pedal connection

Here is an alternate design inspired by mcencula's design.

If there were a demand I could look into getting them made.

Or maybe it's a solution looking for a problem.

I had a mockup printed in plastic for assembly fit check as shown in the photos. Flight parts would metal, I'm thinking 17-4 H1150 for example.

Latest dwgs are here.
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I'll take a set of you print some more out. The flat washer against the round cylinder bugged me a little, but I figured there's plenty of people using them to not be an issue. Something light and simple like that would be decent piece of mind.
 
printed plastic is for assy fit check - not airworthy

I'll take a set of you print some more out. The flat washer against the round cylinder bugged me a little, but I figured there's plenty of people using them to not be an issue. Something light and simple like that would be decent piece of mind.

Unfortunately, the printed plastic part is not airworthy. It's for assembly fit check only. Flight parts would be metal, I'm thinking 17-4 H1150 for example.
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revised design incorporating saddle wahser

Thanks Lenny for the heads up on availability of saddle washers. Incorporating a saddle washer removes a milling operation from the bushing.

I put the latest drawings here. Attached is a snip of the assy dwg. I put the bushing drawing in the same place.
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I created a dwg for the bushing

So where do we get the Custom bushing 17-4 H1150?????

Hi Fred,

I created a drawing for the bushing and added it to the filespace here.

I'm gonna use this as an excuse to buy a mini lathe and I can make some for you in a few weeks if you like. That could make you the beta tester.
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brake arm pivots on bushing, not bolt

Why open the pegasus saddle washer to 5/16? Shouldn't you open it to 3/16 so the bolt fits through?

Hi agent4573,

Thanks, you are correct! The saddle washer's ID should not be modified; it's sized for a #10 machine screw as purchased.

I updated the drawings in my google filespace.
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That makes more sense. I was looking at the drawing and thinking the bolt would be the inner diameter. Sorry, I'm a a little slow on the uptake sometimes.
 
That makes more sense. I was looking at the drawing and thinking the bolt would be the inner diameter. Sorry, I'm a a little slow on the uptake sometimes.

Hi agent4573,

Actually, you are correct! The saddle washer's ID should not be modified; it's sized for a #10 machine screw as purchased.

I corrected my previous posts.

Thanks!
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