Mates lock when slider mate created

Reece_Isaac

Appologies if this has been covered, I've tried some key word searches but haven't found a direct answer, yet.
This question comes from an untrained hobbyist so my errors might be basic.
 
 I have uploaded a model aeroengine (not my work, DLed from a share site along while ago) to try the mates and animate the piston, conrod, crankshaft assembly.
 
 I have fixed the crank case and attached the cylinder liner.  I have applied a Revolute to the crankshaft/crankcase, a Revolute to the crankpin/conrod big end and a Cylindrical to the piston gudgeon pin hole/conrod little end with limitations. There is no gudgeon pin at this time.
 I have these movements working as expected.

 When I apply a Slider mate to the piston/cylinder liner the whole mechanism locks.
I can get the piston moving OK in the cylinder but as soon a solve is pressed all Mates go red.
 
I have tried different types of mates here and selected different items first when creating the mates with no success.
What am I doing wrong in this instance?
Thanks for any help.

cody_armstrong

@Reece_Isaac I think this is caused by the geometry you are working with.  Take a look at the link below.  The revolute mates holding the crank shaft to the connecting rod and the connecting rod to the piston have no linear degrees of freedom.  Meaning the they would need to be perfectly on center with the piston bore to not violate another mate.  In your model it does not look like this is the case.  If you mate the other way, piston first, then connecting rod, then finally attach these to crankshaft you will see this.


Link

philip_thomas

@Reece_Isaac - it was very easy to fix
In any system of mates where there are two or more revolutions (are there are here), if all the components dont directly line up (as here - the cylinder sleeve is in the wrong longitudinal position), then it's impossible to satisfy the mates, and they turn orange (we thought about using red, but it freaked people out). The fix was to edit Revolute3 and make it a Cylindrical mate (adds back in the 1 additional degree of freedom needed to allow the connecting rod to move slightly along the crankshaft journal to account for the geometric errors in the model (not your fault). Also, there were some limits added unnecessarily to the same mate, turn them off.
Hey Presto - it works just fine
email me if you still can't get it to work - pthomas@onshape.com

Reece_Isaac

Thankyou both Cody and Philip for solving my problem.  I did try cylindrical mates variously one at a time and everywhere as I did consider the various axis' not being in line. What you showed with you solutions was the direction of the Z axis of the Mates. I have revisited the problem and applied my first solutions with the one major difference being the change of direction of the Z axis in the Cylindrical mate where it is applied. I only need apply one cylindrical mate in the right direction and the mechanism articulates as expected.
This has been a good learning example for me as I just wasn't understanding why the Cylindrical mate weren't work here as intended.
Now to set to work on my next problem to nut out. I've already got it here but I'll potter along and see how I go.
Thanks again.

nav

Hi @Reece_Isaac i suggest you share your model publicly and paste the url here so we can take a look.

andrew_troup

You sound as though you might be specifying the same relationship in two different ways.

 If the revolute mates are telling the piston where to be in relation to the endfloat axis of the crankshaft, then a slider mate of the piston in the liner may potentially "fight" with that constraint.

I would probably just edit the revolute mate of the big end to a cylindrical mate, and let the piston dictate where the con rod is along the endfloat axis.

Apologies for the confusion if I have misunderstood: nav's advice is a safer way to ask for clarification.

Reece_Isaac

Have tried various permutations to no success. I can get the piston, conrod and crankshaft to relate correctly together or I can get the piston to relate the the cylinder but I can't get everything going together.
https://cad.onshape.com/documents/7b3b9fa0d9ed4a22aacd7425/w/1027a1c3266847fe8e099acc/e/319c949adc3247bf8df2539b

cody_armstrong

@Reece_Isaac I think this is caused by the geometry you are working with.  Take a look at the link below.  The revolute mates holding the crank shaft to the connecting rod and the connecting rod to the piston have no linear degrees of freedom.  Meaning the they would need to be perfectly on center with the piston bore to not violate another mate.  In your model it does not look like this is the case.  If you mate the other way, piston first, then connecting rod, then finally attach these to crankshaft you will see this.


Link

philip_thomas

@Reece_Isaac - it was very easy to fix
In any system of mates where there are two or more revolutions (are there are here), if all the components dont directly line up (as here - the cylinder sleeve is in the wrong longitudinal position), then it's impossible to satisfy the mates, and they turn orange (we thought about using red, but it freaked people out). The fix was to edit Revolute3 and make it a Cylindrical mate (adds back in the 1 additional degree of freedom needed to allow the connecting rod to move slightly along the crankshaft journal to account for the geometric errors in the model (not your fault). Also, there were some limits added unnecessarily to the same mate, turn them off.
Hey Presto - it works just fine
email me if you still can't get it to work - pthomas@onshape.com

Reece_Isaac

Thankyou both Cody and Philip for solving my problem.  I did try cylindrical mates variously one at a time and everywhere as I did consider the various axis' not being in line. What you showed with you solutions was the direction of the Z axis of the Mates. I have revisited the problem and applied my first solutions with the one major difference being the change of direction of the Z axis in the Cylindrical mate where it is applied. I only need apply one cylindrical mate in the right direction and the mechanism articulates as expected.
This has been a good learning example for me as I just wasn't understanding why the Cylindrical mate weren't work here as intended.
Now to set to work on my next problem to nut out. I've already got it here but I'll potter along and see how I go.
Thanks again.