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REDUDANCIES IN MOTION ANALYSIS

Question asked by Apostolos Lilias on Dec 10, 2010
Latest reply on Dec 10, 2010 by John Sutherland

HELLO

THE ATTACHED MODEL IS A CRANE ARM IN WHICH I AM TRYING TO FIND THE REACTION FORCES AT THE JOINTS. ALL THE PARTS ARE FIXED HORIZONTALLY BECAUSE IN THIS WAY I CAN BRIEFLY CALCULATE BY HAND THE FORCES SO I CAN HAVE AN IDEA IF THE RESULTS ARE CORRECT OR NOT. AT THE END OF PART BOOM-2 8800L THERE ARE THE APPLIED FORCES. DURING SIMULATION I AM FACING THE FOLLOWING PROBLEMS:

  1. I CANNOT WHY THERE IS REDUNDANCY ON CONCENTRIC7 MATE. HOW ELSE I CAN SIMULATE THE MOVE OF THE ROD OF A HYDRAULIC CYLINDER.IF I WILL USE FLEXIBLE JOINT (BUSHING) THEN THE ROD,DURING MOTION, GOES FAR FROM THE CYLINDER LIKE IT IS BROKEN.
  2. IN THE FIRST TIME I HAD 13 REDUNDANCIES (CONCENTRIC 6,7,8,11,13 AND COINCIDENT 1,2,8). SO I MAKE SOME OF THESE MATES FLEXIBLE (CONCENTRIC 6,8,11,13).BUT BECAUSE OF THIS MODIFICATION THE CYLINDER IS NOT RIGIDLY CONNECTED TO THE ARM SO WHEN I AM STARTING THE ANALYSIS DUE TO GRAVITY THE BOOM-2 IS FALLING DOWN AND THE CYLINDER AND THE ROD GOES APART FAR FROM THE ARM.SO I DECIDED TO ADD ON ROTARY MOTOR WITH FIXED DISTANCE TO KEEP BOOM 2 IN ITS HORIZONTAL POSITION AND ALL THE PARTS ARE STAYING TOGETHER. BUT STILL I AM HAVING 7 REDUNDANCIES (CONCENTRIC 7,COINCIDENT 1,2,8). I TRIED TO MAKE THEM FLEXIBLE AND THE REDUNDANCIES GET 0.ALSO IN ORDER TO PREVENT THE DISASSEMBLY OF THE MODEL I MADE A VERY BIG INCREMENT FOR THE VALUES OF STIFFNESS AND DAMPING OF THE BUSHINGS.BUT STILL THERE WAS A DISASSEMBLY SO I WAS OBLIGED TO UNSUPPRESS THE ROTARY MOTOR. CAN YOU TELL HOW I WILL AVOID THIS PROBLEM?
  3. THEN I CREATED A PLOT FOR CONCENTRIC 5 AND 8 AND I GET THE RESULT 1.2e6 WHICH IS COMPLETELY WRONG BECAUSE WITH AN APPROXIMATE HAND CALCULATION THE VALUES ARE 5.29e4 AND -5.63eE4.CAN YOU GIVE ME AN ADVICE WHY THIS HAPPENING?
  4. ALSO I CANNOT UNDERSTAND WHY WHEN A JOINT IS FLEXIBLE THE FORCE IS NOT PASSING BETWEEN THE PARTS OF THE JOINT COMPLETELY BUT ONLY A PART OF IT. FOR EXAMPLE FOR CONCENTRIC 8 THE FORCE IS 1.2e6 WHICH MUST BE EQUAL PRACTICALLY WITH THE FORCE IN CONCENTRIC 9. BUT INSTEAD I HAD 7E4 ON CONCENTRIC 7. GENERALLY WITH BUSHINGS THERE IS UNEVEN FORCE DISTRIBUTION BETWEEN THE PARTS OF THE MATE

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