Meccano Line Saver
The trolley of a blocksetter or a tower crane is a good example of wire being used to move a piece of machinery. There is no problem with the wire tension, because the wire runs straight out from the winding drum and back again.
But things would get a little complicated if the arm was jointed, such as would be the case if one were building a robot arm or an excavator. Because the wire would start to shorten as it went round the joint. When the joint is activated and bent, the wire starts to tighten alarmingly because it has to go round a pulley at the joint. Conversely, when the arm is straightened, it slackens, so much so that turns might loosen on the winding drum or capstan and cease to be effective. This is shown in the first diagram.
If however, the wire were actually taken through the axis of rotation of the joint, then there would be no effective change in its length as the joint were moved, thus neatly overcoming the problem. The second diagram shows how this can be achieved using a simple Meccano mechanism.
It is a happy coincidence that the groove of a half inch pulley mounted in a rod held in the centre transverse bore of a coupling almost neatly bisects the adjacent end transverse hole. The construction of the device thus becomes clear. A pulley or more than one pulley can be mounted on the rod between couplings, and threaded pins or similar can be attached in the end transverse bores of the couplings. It is the latter pins that actually form the joint.
But the device has a second advantage as one can see from the moving strips, between the 45 degree position and the 135 degree position. The couplings are allowed to float, and take up there own median position neatly dividing the joint arms at any angle. The result of course is that the control wires remain at the same tension at any degree of joint angulation. This device can be repeated over multiple joints along the course of a wire if necessary.
When you use a control line, then if it goes round a movable arm, it shortens on the movable arm if it is held still on the stationary one (I am not sure how to put this in a better way).
Now I have built a device that prevents the wire from shortening. A Meccano half inch pulley is mounted in the middle transverse hole of a coupling. It so happens that the groove lines up with the centre of the adjacent hole, and if this hole is also the axis of the moving arms, then the line always passes through that axis as well. As the coupling is floating, it always takes up the angular mid point between the two arms.
In my excavator, which relies on wire control lines, I had to invent this mechanism in order to overcome this problem at the boom base and the boom/stick joint.
Picture of the Linesaver in action in the excavator
Now here is your exercise for the day. The pulley groove diameter is 1.08 cm. If the arms move through 90 degrees, then the how much does the line shorten?