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

Gravel

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

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?  

 

 

 

 

 

 

 

 

 

18 REPLIES 18

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

Could you share the assembly with us?

Highlighted

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

Sorry can't not publicly... 

 

I may be able to build something similar.

 

 

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

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.

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

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?

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

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.

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

Ran Kinematic..

first at a lower frame rate 50 frames, it did a "jump"

upped the frame rate to 500 and no issue, also can drag with/out seeing anything in the motion that is a problem.

 

YES there are a lot of very stiff springs coupled to masses you are not seeing, this model has ~30 DOF, you are only seeing the input device to a much larger problem.  Looking to induce controlled vibrations into a system. this is running FAST at low displacement against a stiff system.

 

My biggest issue is still at the start of this... look at the graph attached, notice that my cam position goes down / under the surface.  This imparts kinetic energy early in the run, which is being put on the main force vs time reactions in the springs like a carrier wave over the natural frequency, that I am attempting to stay out of harmonics on.

 

 

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

Is the plot you shared the position of the cam follower? What does this same measure look like for the kinematic result? The plot suggests high velocities and accels - how did you generate the cam surface? Did you synthesize the shape from your equation? You may need to dig deeper into the creation of the cam surface and define it more finely than it is now. Without knowing more details, it's tough to comment further. I've created and run very complicated dynamic cam-follower systems with 2-3 dozen DOF that would run pretty quickly (10-20 years ago in Pro/E). The input cam shape needs to have enough detail defined at small increments to feed high DOF systems. You may need to dampen some of your joints as well, where it makes sense.

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

Created the equations for the cam profile and broke into each segment of the total rise / or drop, cycle of BETA.

Below is the first piece relationship for the curve for a sinusoidal start of the rise using polar coordinates.

 

z=0

/* theta as a function of t = 0 to 1, => convert P_0# to degrees and multiply by fraction of travel for each phase
/* (-1) added to design a CCW cam, rise needs to go in the negative / CW direction
theta=(-1)*t*P_01*180/pi*RF_01
/*
r=D_BASE/2+RISE*(.38898448*t*RF_01-.03095444*sin(4*pi*t*RF_01*180/pi))

 

 

 

 

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

Kinematic is smooth, works as expected...