# Forcing a part into "home position"

I have a design with two parts (at the moment):

• a fixed 10cm ring with vertical tracks
• a freely rotating block with a cut that may freely slide along that tracks:

I'm trying to find a low profile way to ensure that whenever the cut in the moving block faces upward vertically, the block is pushed up (almost) touching the inner face of the ring. That is, it returns to its "home position" whenever the cut faces up:

The block is just driven by a crank. Indeed, the crank can freely shift vertically. However, i'm hoping the block (along with the crank) to return to the home position whenever the cut points upwards.

The only way I could think of is to have a part like a leg sticking out from the bottom of the block so that it pushes against the ring or an external track (near the bottom of the ring) whenever the cut nears facing upward. The issue is that this leg will require quite a bit of extra space, e.g., when the cut faces down but the block is at the top, the leg will point upward. The method should not constrain the block's movement along the tracks or rotation.

Does anyone know of a better way?

• For clarity, you wish the solution to be entirely mechanical? For instance, are the use of servos and sensors an acceptable solution? Commented Apr 11, 2016 at 10:49
• @tillmas Yes, I'm hoping to find an entirely mechanical solution. Indeed, an electronic solution could be straightforward: a motor and a microswitch. Perhaps a mechanical one may be much less so. Commented Apr 11, 2016 at 11:03
• What does the cut signify in your overall design? As it stands, you've got a homogenous square of material with no "reason" for it to care what its vertical position is, let alone how to sense where the cut is. Do you completely not care where the block is (vertically) when the cut is not at 0 degrees, and to what precision -e.g. does it need to be pushed up when it's at 0deg0min0.1seconds? I strongly suspect if you can tell us what you want to do, not how to do it, we can change your entire structure. Commented Apr 11, 2016 at 13:21
• Do you need this solution to force the block along the slot when provided with torque from the crank? or would it be suitable to simply not allow rotation past the cut being vertical unless the block was at the top of the slot?
– Eph
Commented Jun 14, 2016 at 12:26
• @DeerSpotter Interesting. But do you mean this? This looks like a simple mechanism though.
– Kar
Commented Jun 22, 2018 at 15:09

One approach could be to use a spring mechanism to keep the block in the home position. For example, a spring could be mounted on the inside of the ring near the bottom and connect to the bottom of the moving block. When the block rotates such that the cut faces upward, the spring will push the block up and maintain it in the home position. The spring could be designed such that it does not interfere with the movement of the block along the tracks or its rotation. Additionally, a damping mechanism could be added to the spring to control the rate at which the block returns to the home position and reduce oscillations.

Modify the crankshaft into a camshaft.

I can't upload a picture, but I can describe it.

Take a Zero, or the letter 'O' and attach it to the camshaft of the second picture, the side facing you - pin the top of the Zero to the center of the shaft. The size of the zero be about the radius of the tube.

This is fixed to the shaft. Below it, in the remaining space within the tube, place a semicircle attached to the tube wall which just touches the Zero's bottom point.