What is overspecifying my crankshaft revolute mate

iain_downs

I'm attempting to design a steam engine.  I've built up the parts and now I'm starting to combine them.  I have a lower cylinder cap, piston rod, con rod, crankshaft and main bearings (other bits, but these will do for now.  https://cad.onshape.com/documents/6f64bbef3b1f362c146611c6/w/7178a8e7f7e8d462ffd60d3d/e/571008cf32efee34068ebaa4

I had the crank revolving and driving the con rod and piston rod until I added a slider mate to the piston rod / lower cap.  I read in a post that I should do it backwards, so I unmated, did the cap, piston rod and con rod and that worked.  Now when I try to link the crank to the main bearing with a revolute mate it claims to be overdefined.

I'm a bit stuck for how to diagnose this or what could be wrong.

Any help much appreciated.

NeilCooke

Try using a cylindrical mate instead of revolute. Sometimes tolerance issues cause chains of mates like these to lock up. 

iain_downs

Thank you that worked.

Now to hope that adding the cylinder, eccentric, D Valve and so on don't also overdefine!

Is there any general diagnostics that can be used to work out what's going wrong in these situations?

Iain

srbh

@iain_downs
Have a look at mine.

https://cad.onshape.com/documents/5784d5f1e4b037e1fb8183ec/w/73fc45fed762929a5d50460c/e/3d9abc17eeb224c07413bb79

As an aside, you don't need to create each bit in a separate Part Studio. With a simple model, you may find it easier to create most of the connected bits together. (Although others will promptly tell us there are 17^3 ways of doing it better).

Christmas wishes

Tony_C_

That's cool.  Got a lot further than I did....

https://cad.onshape.com/documents/a289fac44669e296712d59ed/v/1aed405bd33b33527706c479/e/7ba8d22c1dd299a4b12616e6

You may want to add a planer limiting mate on the piston though to make it so it can't go too far down. It got stuck a few times I was rotating it around.

iain_downs

Thanks.

I'm keeping the parts separate (possibly wrongly) to make it easier to make drawings for manufacture.

I've also made it heavily parameterised, because I'm not entirely happy with some of the sizes.  The closest I have to existing drawings seem to be over-engineered.  For example, I've set the main and big end bearings to be 20mm as I have some build notes of a marine engine of similar size (slightly bigger and compound) which has 1 inch bearings.  Seems overkill to me and I want to be able to shrink the journals and all related items if I decide they should be 15mm.

I'm still interested in if there are any diagnostic tools for when mates are overspecified.  IS that a feature request?

Iain

Jake_Rosenfeld

Hi @iain_downs ,

I couldn't find an improvement request for what you are suggesting.  Please add one from the "Improvement Requests" section of this forum.

If you make all your parts in one Part Studio, you can still make individual drawings of the Parts!  You can right click on any part in the graphics area or Parts List and click "Create Drawing of Part <X>...".  This will also help your parameterized modeling, because if you decide to change the size of your bearings, and the surrounding parts are parametrically modeled around the bearings, the model could update automatically.

iain_downs

Thanks.  I've actually started to do that a little, it's good advice.

The way I'm mainly attacking the parameterisation is through extensive use of a feature studio which is shared by all the parts.

This probably says more about my background as a developer than anything else.

This is a VERY clunky way of doing things though with a continuous round of save, new version, updated linked documents everywhere.  And of course you can't easily see the properties without a good deal of mucking around.

Iain