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Wednesday, July 8, 2015

A pull rod for position adjustment of a cam-driven mechanism

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kinematic diagram cam, lever and pull rod
Kinematic diagram of a cam-driven mechanism
A cam-driven mechanism is commonly used in most production machines since all motions and timings can be controlled. Not only the displacement is controlled, but also the velocity and acceleration as well as jerk can be controlled. Cam-driven mechanism allows overlapping motion between machine parts since the positions of all relevant parts can be determined from the timing diagram which is desirable for high speed application.

A simple cam-driven mechanism consist of the following parts as shown in the kinematic diagram:
  • Cam: for motion generation (displacement, velocity, acceleration and timing).
  • Cam follower: rolling part mounted on a lever.
  • Spring (not shown): to keep contact between cam surface and cam follower.
  • Lever: to transfer continuous cam rotation to swinging motion.
  • Pull rod: to transfer the motion from the lever to the slider.
  • Slider: end equipment (processing equipment)
In this post, we're going to focus on the pull rod (also known as push rod or tie rod) which is one of the common parts for most machines. The pull rod allows position adjustment of its connected parts since its length can be adjusted. Normally, the pull rod consists of the following parts:
  • Pull rod
  • Rod end bearing RH thread
  • Rod end bearing LH thread
  • Nut RH thread
  • Nut LH thread
    Hexgonal pull rod with both female rod end bearings
    Pull rod with both female rod end bearings
    The pull rod usually made of a hexagonal post. The mechanic can use a wrench on the hexagonal part to tighten or loosen the pull rod from the rod end bearings. Rod end bearings must have RH thread on one side and LH thread on the other side otherwise the distance between the rod ends will remain the same.
    example of real pull rod use
    Example of pull rods on the machine
    Adjustment of the pull rod length usually happens when both sides of the rod end bearings are already connected to other parts in the machine (in this example, it is connected to the lever and the slider already). To adjust the length, no need to disconnect the rod end bearing, first we have to loosen both RH and LH nuts so that the pull rod can be turned. Then turn the pull rod in either direction and its length will change. By doing this, we can then adjust the position of the connected parts which, in the case, is the slider. After the slider is at the desired position, tighten both nuts.

    The male rod end bearings version is also available. We can use the same nuts, but the hexagonal post will have threaded holes instead (see the following picture).
    Hexgonal pull rod with both male rod end bearings
    Pull rod with both male rod end bearings
    The increment of the pull rod length (distance between both rod end bearings) is determined by the pitch of the thread on the rod end bearings. For this example, the M10 thread has a pitch of 1.5 mm. One turn of the pull rod will change the distance of each rod end bearing by 1.5 mm. Therefore, the increment is 2 times the pitch (2 x 1.5 = 3 mm/turn).

    The pull rod length is increased or decreased according to the following directions.
    Turning direction and pull rod length
    Pull rod length extension and retraction according to the turning direction
    The following is the animated picture showing how the slider position can be adjusted by turning the pull rod.
    animated gif: how to adjust pull rod length
    Animated picture of pull rod length adjustment
    Watch the following video for how the cam driven-mechanism works and where the pull rod is used in the system. The simulation uses NX4 motion simulation module.


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