What's new

A little help on constraints and lack of motion in the Crank/Pulley tutorial assembly

TCB

Member
Hi, First, I'm definitely an amateur at this. I have a little experience with Solid Works and F360. Since both have changed their license terms recently, and Alibre offered a nice deal to upgrade as I was a purchaser (Design Expert) a long time ago, I'm trying to convert my brain back to Alibre. I went through the basic design and assembly tutorial for the Crank/Pulley with an universal joint. I almost had the assembly working but there were some clearance issues which I modified some of the part lengths. When I place the crank/pulley/universal assembly in the base per the tutorial, it will not rotate. If I suppress the concentric constraint on either of the shafts and the base I am able to move the crank/pulley/universal assembly. Thanks for helping the neophyte.
 

Attachments

  • Tutorial Assybly.AD_PKG
    219.9 KB · Views: 4

HaroldL

Alibre Super User
I went through you model by following the tutorial and found some discrepancies in the dimensions given in the tutorial and your model parts.
Here are a few of the issues I found:
The Link: make sure both extrusions are the same length at 12.7 cm.
The Crank: The Fillet should be 1.905, you can't cheat on this because the hole is concentric to the fillet.
Make sure the yoke face is 12.7 cm in height and the shaft is centered on the yoke face.

I noticed that on a couple of the parts they switched from cm to inches and there were some places where the tutorial had you following and snapping to a grid. There are a couple of places where the snapping is off. I think it is a good idea to apply some dimensions to the sketches after you're done snapping to the grid. It will tell if you counted the right number of grid squares.

I suggest going back through the tutorial and rechecking every sketch and feature size and apply dimensions to all the sketches to verify their size and location.

After I rechecked everything I got the assembly to rotate as it should.

Post back if you have any more questions or run into any issues.
 

TCB

Member
Thanks, I'll go back through the parts and check everything. I had modified the link and crank owing to clearance issues.
Thanks again and I'll keep all you posted.
 

HaroldL

Alibre Super User
If the parts are created with the dimensions given in the tutorial there should be no clearance issues.

Also, as part of good design practice, it is important to fully constrain all sketches. I think the tutorial should be changed to add dimensions to every sketch especially the ones created by using the grid to size them. Then the dimensions will verify the sizes and fully constrain them.
 

TCB

Member
Well I went back resized all the parts and dimensioned, (size and location) everything (I think) and changed the default units to centimeters in the assembly (didn't realize that it would not use the units from the parts.
As before, the assembly flops around nicely until I constrain the last shaft to the support base.
I receive a "Constraint not consistent" or "Over constrained" error.

I'll try to post the package that fails tomorrow. And I'm going back and start the tutorial from scratch
 

TCB

Member
Here's the package - probably simple answer that I'm overlooking!!
 

Attachments

  • New Assembly 2.AD_PKG
    246.9 KB · Views: 5

albie0803

Alibre Super User
Hmmm, I initially got the result as you but then I dragged the crank arm closer to the desired position and then tried again and it worked.
I have no idea why unless it had something to do with the initial orientation that it wanted to pick. Maybe moving the crank to a closer position made the correct orientation the initial choice, or something. Maybe the Alibre guys might have a better idea.
I can't get it to rotate though.

.... A little time later......

Remove the 2 coincident restraints.
Coaxially constrain the crank to the pillar.
Put an offset constraint between the pillar and the handle.
The arm should now rotate. (Mine does)
 

HaroldL

Alibre Super User
I just completed the tutorial and found a few areas in it that may be confusing. However, I was able to create the assembly and turn the crank with the constrains called for.

ROTATING ASSEMBLY.gif


One thing that is not clear is the setting of centimeters for the units. When creating the Link the user is instructed to select Centimeters, but considering that all the parts will be created in centimeters, it is not mentioned to check the box "Set current settings as the default settings for parts, sheet metal and assemblies." Without this setting all subsequent parts will be in inches or whatever the system was set to before starting the tutorial. Or you need to set the units to centimeters for each part and the assembly, which is not indicated in the tutorial.
 

TCB

Member
Thanks for you all looking at this. Like I said, I'm going back and start from scratch on the tutorial later next week when I'm back in town. However, on the current parts/assembly, even after fixing everything (dimension congruity and fully constraining everything) it fails, and I would really like to know why. I would think that if I screwed something up in the future, I could unwind the problem by correcting the error and not having to restart every thing.
Thanks again, and I'll keep you updated on my ignorance.
 

TCB

Member
Well, I went back through the tutorial, making sure everything was "Fully Defined" as I went, and I was successful with having the assembly function properly.
I went back and reviewed all the original parts and found several that were not fully defined and corrected this. However, on attempting to assemble the parts
per the tutorial, the last constraint generated "Constraints Over Defined". I removed all the assembly constraints and went back and reassembled it in a different
sequence with only one coincident constraint and the rest coaxial and it worked.

Thanks to everyone that provided input and suggestions. It is much appreciated.
 
Top