Modeling an open circle with a clasp

ross_levin

I'm relatively new to OnShape and have been doing the online training.  I have a model that needs a precise internal diameter that is essentially a ring that can be opened & closed.  My first pass is here: 

https://cad.onshape.com/documents/60cc8453899075311a488660/w/046ee2b9a298583ad79dcb82/e/27621cafa4850702cdb74bf3?renderMode=0&uiState=6449a56e4d176d06698853ac

I have it modeled and printed in its closed position which makes it easy to maintain the inner diameter.  I'm thinking that I need to make a more robust clasp design for this item.  In order to do so, I will need to model & print it in an open position so that I would be starting with an arc instead of a circle.  I suppose a construction circle will help as a guide to maintain the inner diameter but I don't yet know how I can make sure that when the printed ring is closed that the precise inner diameter will be maintained.

Would it be as simple as matching the circumference of the construction circle to the length of the arc?

I'd appreciate any tips. 

S1mon

The two ends of the clasp won't bend anywhere near as much as the band itself. You can derive the arc length from the closed position, split the closed model into two parts, and model the open position band based on the arc length, and then transform one end to the open location. Typically when bending materials the theory is that the neutral axis maintains its length. That's why I sketched an arc that's in the middle of the 1.5 mm thickness.

As a side note, you probably want to add some fillets to the sharp corners on this part to reduce stress concentrations, and depending on the material and its flexibility, it looks like the clasp will need to go through too much flexing to snap on (unless the catch works in/out of plane only).

The good news is you can print a bunch of variations on this design and test them out until you get something that works.

S1mon

Turns out my neutral axis is a bit simplified when looking at a ring. You may want to look at this: http://ecoursesonline.iasri.res.in/mod/page/view.php?id=1357#:~:text=Neutral%20axis%20of%20curved%20beams,in%20case%20of%20straight%20beams.

john_lopez363

From what I've inferred, this looks like a travel indicator ring for a Mountain Bike shock absorber!  If so, I'd think that "precise" circumference will not be necessary for the part to function as described.  If I'm mistaken on the usage, then please ignore my post. 



I Split the circles at the 90degree point.  Then replaced all the arc's you had (excluding the circles) with single Splines segments... this made it easier to measure the length of the spline segment (in the attempt to keep dimensions as close as possible) and will make it easier to adjust these dimension if the size is off.

 https://cad.onshape.com/documents/a3a16211cd76e539cff58721/w/9b7d1419a0ce35317db0caca/e/beb4a1d7089c21ea11061e5c

ross_levin

Thanks so much for taking the time to answer my question.  These give me a good direction to continue on.

Yes, this is a travel indicator for bicycle suspension.  Original travel indicators often fail or are forgotten to be replaced when service is performed and require disassembly to install a new ring, hence the open ring with the clasp approach.

I started printing these with PLA & PETG but they seemed too rigid.  I then tried TPU and it works far better.

"Precise" is subjective but I don't want it too tight that it could grind debris into the stanchion, but tight enough to stay in place with all the vibration caused by riding.

For what I have so far, the 34 & 36mm rings work pretty well for for the forks.  However, the 27mm diameter for the rear shock came unclasped which motivated me to find a better way to prevent lateral shifting of the clasp ends.  If my new approach works, I'll apply it to the larger sizes as well.

These print quickly, so it is easy to iterate.

Thanks again for the help!

john_lopez363

@ross_levin
Was playing around with a couple of variations on a theme... Just some ideas to help with the clasp issues:

link: https://cad.onshape.com/documents/a3a16211cd76e539cff58721/w/9b7d1419a0ce35317db0caca/e/beb4a1d7089c21ea11061e5c

SethF

ross_levin said:

I started printing these with PLA & PETG but they seemed too rigid.  I then tried TPU and it works far better.

For less rigid, if you want to avoid TPU, You could try one of the various brands of PLA+. Generally less tensile strength than PLA, but with much higher toughness, it's a lot better suited for a lot of applications than its brittle sibling.

ross_levin

john_lopez363 said:

@ross_levin
Was playing around with a couple of variations on a theme... Just some ideas to help with the clasp issues:

link: https://cad.onshape.com/documents/a3a16211cd76e539cff58721/w/9b7d1419a0ce35317db0caca/e/beb4a1d7089c21ea11061e5c

Thanks - this is similar to where I was headed for preventing the lateral detachment.  Yours looks much more elegant that mine, however.

I'm also playing with modeling/printing it as a straight line so that the natural force of it trying to straighten itself out will add to the holding force of the clasp.  There must be a word describing this force that is escaping me.  

I appreciate your time, thanks!

ross_levin

SethF said:

For less rigid, if you want to avoid TPU, You could try one of the various brands of PLA+. Generally less tensile strength than PLA, but with much higher toughness, it's a lot better suited for a lot of applications than its brittle sibling.

I haven't considered PLA+ since it seems that PLA isn't for outdoor applications. I'll get some and experiment with it.  Thanks!