Ch. 15

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Author:
sep293
ID:
84204
Filename:
Ch. 15
Updated:
2011-05-10 00:07:33
Tags:
Mechanisms
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Description:
Review for Mechanisms Final
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  1. When is the inverse dynamics or kinetostaics approach used?
    Use to determine how much spring force is needed to keep the follower in contact with the cam at a chosen design speed.
  2. The system spring constant keff tends to be dominated by:
    • the ks of the physical return spring.
    • The design has a large degree of control over keff
  3. If the follower stiffness is in series with the return spring the effective spring constant will be dominated by:
    the softest spring
  4. Preload of a spring occurs when
    the spring is compressed (or extended if an extension spring) from its free length to its initial assemby length.
  5. Why is preload important for a force-closed cam-follower?
    • Preloading creates residual force on the follower even when the cam is at its lowest displacement.
    • This helps maintain good contact between the cam and follower at all times.
  6. Cam-Follower Design Choices
    • For most cam-follower systems the value of c can be estimated for a first approximation to be about 0.05 to 0.10 of the critical damping cc
    • cc = 2m(k/m)1/2 = 2m*omega_n
    • The spring constant and preload must be selected to guarantee a positive force at all points in the cycle.
  7. In a force closed system the cam can only _________ on the follower.
    push
  8. The follower spring is responsible for
    providing the force needed to keep the joint closed during the negative acceleration portions of the follower motion.
  9. If the force Fc goes negative at any time durinng the cycle what will happen to the follower and cam?
    • They will experience follower jump which is where the follower and cam will part company.
    • The follower jump is also called the valve float in automotive engine valve applications.
    • When they meet again it will be with large and potentially damaging impact forces.
    • The follower jump will occur near the point of maximum negative acceleration.

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