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Re-engineering the Shift Spring


Scud
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...and me too! I'll buy two. I expect someone over here could use one, and I'll use the other.

For interest, I had two broken shift springs examined under a microscope by an expert. He said both springs broke at defects in the wire, one at a rust pit, the other at a gouge from something like pliers or clamping jaws. So a marginal original spring design, plus a defect in the wire, can equal a slow trip home.

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Real V11 Aficionados will have both a Chook-made/LookyPhil Shift Extender and (whatever-cleaver thing we will end up calling this spring) . . . :rasta::luigi:

 

I absolutely must have one, if even to wear with my dog-tags. :thumbsup:

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Progress report. Today, while watching (ahem,supervising) the new  pup Austin.. 37119073492_9266b49997_c.jpg2017-09-17_07-12-24 by Charles Stottlemyer, on FlickrI think he will be a keeper..  :rasta:  :)   I made the first version of  "spring winder tool 101 tooling."  Still don't have any music wire.

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Austin is one lucky dude hanging in The Shop with the likes of :notworthy:  Chuck.

 

As a lad, I was nearly as lucky rebuilding two very different engines with the guiding hand of a delightful neighbor, Hal Alred.

 

 I remember my Mom saying, "Mr. Alred has an airplane in his garage."

 

Snarky teen (me) replies, "Aw, Mom, you'll believe anything!"

 

Turns out, the fine man truly was an Airframe&Powerplant Mechanic dating from the Second World War. And, yep, he had a complete fuselage and wing he had built in our little residential neighborhood. And four or five Lycomings and Continentals. What an amazing guy. I feel so totally lucky that he was willing to show me so many great things in my youth.

 

Just as lucky as I feel, right now, "knowing" all of you fine fellows! :notworthy::thumbsup::mg:

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Ok, lets get started on this problem. Austin was late this morning, so I borrowed the CNC for a few minutes to make a wire guide for the lathe.

24598076618_bcf11586a7_c.jpg2017-11-16_04-13-12 by Charles Stottlemyer, on Flickr

This will get capped with a piece of 1/8" delrin and will fit in the lathe tool post.

I'd previously made this tool that fits in the lathe chuck. The music wire and Scud's preselector magically appeared on my porch last night, too, so it's time to see what we can do. 

38414365286_d54f3bbc57_c.jpgIMG_20171116_125939454 by Charles Stottlemyer, on Flickr

Scud had already done a really nice job of cleaning up the preselector, and had installed a new spring. Before I took anything apart, I pulled on the pawl with a fish scale and got 2 1/2 lbs to pull it to the pin. Notice in the center of the picture how the hooked end rides on the shifter thingy. (technical term) It would probably be good to put a matching radius on that sharp corner the hook rides on. 

It's a little hard to see, but right in the center of the picture.

37755467384_1a84a5c1b9_c.jpgIMG_20171116_132909160 by Charles Stottlemyer, on Flickr

Tim had checked the total travel of the spring using (ahem) a plastic protractor, and got 38 degrees. Using my really expensive machinist's stuff, I measured 39.  :)  Nice work, Scud.

Luigi apparently didn't look at his machinery handbook when he designed the spring. It has a safe travel of 30.48 degrees. (!) Previously in this thread, I had mentioned that the spring I took out of the Mighty Scura was bent 12 degrees from over travel. 

So. I installed the old spring, and it tests 2 lbs. on my fish scale.  :huh2:  It's what I have.. Everyone "knows" that the spring works until it doesn't, so I'm going to experiment with a spring with 10 degrees more of an initial bend and see how it works.

Time to make a proof of concept spring.

38414364356_8ac3907f13_c.jpgIMG_20171116_160325599 by Charles Stottlemyer, on Flickr

It's the one on the top. No attempt was made to make the legs the right length. I was just checking whether this tooling would work, and if the ID of the spring was correct. Right on the money. :thumbsup:  The Machinerys Handbook predicted the arbor diameter within .003". I love it when a plan comes together.. :mg:

I still need to make a wire bending jig before I can make springs, so I'll have to wait until Austin is done with the mill. Probably early next week.

 

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I ordinarily would not trust calculations hand-written on "notebook paper." but since about a third of the lines are melted off by *some unknown solvent*, I feel really good about this effort. :grin:

 

I presume there may be *special tool fluid* involved . . .

IMG_2737.jpg

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Ahh, Dang! Caught me..  :whistle:

I screwed up a $10000 taillight reflex pin bundle when I was an apprentice *many* years ago, and don't do math in my head. Period. If I write it down, I'll see the error..  :huh2: even (or maybe especially) simple stuff.

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Ok, lets get started on this problem. Austin was late this morning, so I borrowed the CNC for a few minutes to make a wire guide for the lathe.

24598076618_bcf11586a7_c.jpg2017-11-16_04-13-12 by Charles Stottlemyer, on Flickr

This will get capped with a piece of 1/8" delrin and will fit in the lathe tool post.

I'd previously made this tool that fits in the lathe chuck. The music wire and Scud's preselector magically appeared on my porch last night, too, so it's time to see what we can do. 

38414365286_d54f3bbc57_c.jpgIMG_20171116_125939454 by Charles Stottlemyer, on Flickr

Scud had already done a really nice job of cleaning up the preselector, and had installed a new spring. Before I took anything apart, I pulled on the pawl with a fish scale and got 2 1/2 lbs to pull it to the pin. Notice in the center of the picture how the hooked end rides on the shifter thingy. (technical term) It would probably be good to put a matching radius on that sharp corner the hook rides on. 

It's a little hard to see, but right in the center of the picture.

37755467384_1a84a5c1b9_c.jpgIMG_20171116_132909160 by Charles Stottlemyer, on Flickr

Tim had checked the total travel of the spring using (ahem) a plastic protractor, and got 38 degrees. Using my really expensive machinist's stuff, I measured 39.  :)  Nice work, Scud.

Luigi apparently didn't look at his machinery handbook when he designed the spring. It has a safe travel of 30.48 degrees. (!) Previously in this thread, I had mentioned that the spring I took out of the Mighty Scura was bent 12 degrees from over travel. 

So. I installed the old spring, and it tests 2 lbs. on my fish scale.  :huh2:  It's what I have.. Everyone "knows" that the spring works until it doesn't, so I'm going to experiment with a spring with 10 degrees more of an initial bend and see how it works.

Time to make a proof of concept spring.

38414364356_8ac3907f13_c.jpgIMG_20171116_160325599 by Charles Stottlemyer, on Flickr

It's the one on the top. No attempt was made to make the legs the right length. I was just checking whether this tooling would work, and if the ID of the spring was correct. Right on the money. :thumbsup:  The Machinerys Handbook predicted the arbor diameter within .003". I love it when a plan comes together.. :mg:

I still need to make a wire bending jig before I can make springs, so I'll have to wait until Austin is done with the mill. Probably early next week.

 

Hey Chuck excellent work, Skud too of course.

Re the stress raiser on the selector stop assembly where the spring tang goes.I thought the same as you so rounded this edge off when I did my selector mod to make life easier on the spring. Heres a photo of mine, maybe I didnt cover this in the mod post.

 

Ciao

gearbox mod 014(0) - Copy.JPG

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Thanks for your guys’ hard work, and even better Chuck is taking us through his problem tackling technique.

So cool!

 

Thanks. I'll show how to make a spring when I get the tooling finished. 

 

Scud said..

 

I estimated travel for downshifts at 38 degrees. The extra coil gives us 48 degrees range - but carries the potential risk of a too-weak spring. What if we use half of the 10-degree excess available travel for preload?

  • 2.5 coils is 900 degrees.
  • Therefore, 895 degrees would be 2.48 coils. I reckon that would give 5 x 1.8 inch/lbs as pre-load to compensate for the weaker, thinner wire. Does that make sense - or is that too small of a difference to mean anything? The at-rest spring would appear a little more open - the opposite of the fatigued spring in your earlier picture, which is 12 degrees closed.

If I understand this correctly, these are the pressures applied by the two springs when installed:

  • Stock (1.5 coils):  2.9 in/lb x 22 degrees = 63.8 in/lb
  • Thinner (2.5 coils): 1.8 in/lb x 22 degrees = 39.6 in/lb
  • We could get to 48.6 in/lb with 895 degrees of coil (5 degrees extra preload), or to 57.6 in/lb with 890 degrees (10 degrees extra preload)

Gosh, this is more analysis than I signed up for... but I want a spring that won't break.

I'll also try the .071" dia. spring at 890 and 895 degrees. The brass piece has a groove for that wire on the other side.

 

I'm doing a little thinking about how to make a test fixture as we type. I have a low speed electric motor..

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