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EPeters_PE1-Visitor
1-Visitor
June 11, 2019
Question

Cam motion at start of analysis that is in a dwell region / MDO

  • June 11, 2019
  • 2 replies
  • 15542 views

I have a CAM problem I have been working on.. one big issue... my cam has motion at the start of the analysis, when it should just be gliding along my lower dwell? 

 

Here is what I have been doing: 

 

I start my dynamic run at the center of my lower dwell / Create a snapshot called Start

 

Make sure I have my springs in the system pre-loaded as I would like. 

(Using spring to input rates I have found by running FEA on the parts, this is a heavily loaded Cam with a low displacement, .010" is the total cam rise and that is reduced through a lever arm.)

 

Set an Initial Condition, with my cam rotation at full speed, and my snapshot <Start> used.

Run a "Zero Time" dynamic to check that my loads are for the most part balanced.

 

Run a dynamic analysis for my cycle time, that is 

Lower Dwell (2nd half) | Rise | Upper Dwell | Fall | Lower Dwell (first half)  all at 500 Hz, or just .002 seconds.

 

My Cam roller seem to have motion, like it needs to "find it's contact" at the start of the run.  I've tried everything I can think of, what gives?

 

Why is flying around at the start of the run, the loads should all be just pushing the CAM and Follower together, and then staying in contact and motion less until it start to go up the ramp at .0002 second time shot.

 

Attached:

Cam Area.JPG, this shows the area of the concern, the cam on the right

Full Cycle.JPG, the motion of the center of the pin radially to the CL of the Cam through a whole cycle, notice at start the short time of bouncing around that is going on.

Short Time at Start.JPG from a run done on just the start of the motion... from 0 to .0002 seconds.

 

Is there something I'm missing?  

 

 

 

 

 

 

 

 

 

2 replies

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gfraulini17-Peridot
17-Peridot
June 12, 2019

Could you share the assembly with us?

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EPeters_PE1-VisitorAuthor
1-Visitor
June 12, 2019

Sorry can't not publicly... 

 

I may be able to build something similar.

 

 

C
ChrisKaswer16-Pearl
16-Pearl
June 12, 2019

I can suggest a few things to look at - not sure if you've done these, but if not, add them to your check list for cam-follower dynamic analyses.

  • Have all your bodies been assigned proper, accurate mass properties?
  • Are you using a "non lift-off" cam definition? (if not, try this)
  • Are your steps (time increments) set small enough?
  • Is your cam defined against rotation increments that are equal or smaller than your integration time steps? If not, this may cause issues with the follower's motion. I would regularly define my cams with 0.1 degree increments when looking for results every degree (or even 0.5 degree).
  • Try running a kinematic analysis first to make sure your motion is good before trying a dynamic study.

I hope these help.

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EPeters_PE1-VisitorAuthor
1-Visitor
June 12, 2019

All masses are assigned

I am looking for lift off, I can "glue it" and look for tensile forces that would be the same as seeing lift off.

Besides the run set up how can I control time increments.

Analysis Definition:

  • Duration .002 seconds
  • Frame Count 50
  • Frame Rate 24500 (49/.002)
  • Minimum interval 4.081632653061225e-05 (as output by MDO for 50 frames over the run, is there a good rule of thumb on number of frames and min interval?)
  • Cam design is a modified trapezoidal shape was defined with equation driven curves.
  • Kinematic will not work as there is a spring used to transfer motion from the follower to the rocker arm to model rocker arm deflection under load.

This setup takes about 20 minutes to solve w/o graphics on... any tricks to faster run times?

C
ChrisKaswer16-Pearl
16-Pearl
June 12, 2019

Your setup is the only way to control desired time increments, and the solver will bisect when/if necessary. Of course kinematic studies will not give you loading, BUT, they will provide a way to make sure your cam pair is operating properly with the given surfaces/curves you've assigned. You may discover issues with the assigned geometry by inspecting the results of a kinematic analysis. Always run a kinematic study first to make sure everything provides the expected output. You may see undulations in your kinematic follower lift for your cam pair, and if so, this can lead to potential dynamic issues - especially if your spring and/or masses are large. Given your equation to define cam shape, how did you use this to construct the actual geometry? Also try setting an initial velocity of your follower's joint axis to zero along with your defined cam rotation speed.

 

Edit - just noticed the solve time...very large for this type of system. There may be something funny about masses and/or springs with the given driving input. Seems like there is too much high frequency response "noise" getting into the result.

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