Sunday, April 19, 2009

Design trick about selection of ball joints for fine adjustment of pull rod length

Normally we can find a lot of ball joints with pull rod at every machine. It is used to transmit power from a lever to another link in cam or even pneumatic driven mechanism. It's the part of the system that we can adjust its length due to imperfection in manufacturing of machine parts. For example, we may require high accuracy of mechanism at one position but we cannot achieve it with the fixed length of rod, therefore the use of adjusting rod like this is required.

This adjusting rod consists of right-handed ball joint at one side and left-handed ball joint at another side. This is because we need to adjust the distance between center of both ball joints by turning the pull rod then the screw threads will increase or decrease the distance depending on the turning direction of the pull rod. Of course, if we use right-handed ball joints at both sides, there will be no length changes from turning of the pull rod, but sliding of the pull rod to one side!

Normally, the coarse threaded ball joints will be used in most application because it's normal standard thread. But we then have the limitation of accuracy by this pitch of the thread.

Let's see the example. From the above picture, we use two M8 ball joint (LH & RH threads). The standard pitch of M8 screw thread is 1.25 mm. That means, by turning 1 revolution of the pull rod, the center distance between both ball joints (or the length of the adjusting rod) changes by 2 x 1.25 = 2.5 mm

How can we get more accuracy in each turn? Shall we change to smaller screw thread to reduce the pitch e.g. change to use M6 which has pitch of 1 mm?? Or shall we use fine thread??

We can do anything, but the trick in this post is to put additional M10 coarse thread in between those 2 ball joints and separate the pull rod into 2 pieces.

The pitch of M10 coarse thread is 1.5 mm. Now we can see that we turn "pull rod 1" by 1 revolution, it will pull ball joint M8 (RH) inside by 1.25 mm (= pitch of M8 coarse thread). At the same time, it will push "pull rod 2" to the same direction by 1.5 mm (= pitch of M10 coarse thread). Therefore, totally, the change in length of the adjusting rod is 1.5 - 1.25 = 0.25 mm!!
Of course, there should be a nut to lock the final position (not present in the picture)

With this simple technique, we can see that we can now get higher accuracy of length adjustment. Ten turns of this new rod change the length of adjusting rod the same as 1 turn of normal adjusting rod.

I hope this can be useful for you :)

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