# What joint can I use for two thin parallel shafts to transmit both rotational and and linear power (in the axis of the shaft)?

I am here regarding a personal project but I am not a mech engineer. The shaft is a watch stem that is displaced from the crown stem. The two shafts are positioned like in the photo below but are 0.9mm in diameter and about 2mm to 4mm apart (it can vary). The shafts can be threaded too. One of the shafts rotates and drives the other and it also can be pushed and pulled along it's axis a distance of 1mm and the other shaft needs to follow that.

I have considered a universal joint with a third shaft/rod between the two, but they cannot be sourced at this size and are hard to manufacture and I am not not certain that the linear force will transmit well with a steep angle.

The other option is use an ordinary gear on each shaft for the rotation. And then couple the linear motion of the shafts with a metal bracket sitting around both shafts that is allows free rotation of the shafts but secures the linear motion of the shafts.

Another similar option would be to use 4 bevel gears. They would transmit the rotational power between shafts easily and the angled surfaces would restrict both shafts to the same linear movement.

Could someone help confirm this for me, I have no knowledge if this will work.

• bevel gears would not work in this way, as the outer (faster spinning) diameter of one wheel would be in contact with the inner (slower spinning) diameter of the other wheel and vice versa. I am also not aware of a geometry that would even allow them to mesh properly (at least not an efficient one). Perhaps something like an Oldham coupling or a similar coupling would suffice. It really depends on the speed, forces and required accuracy. Commented Apr 25, 2021 at 23:11

How about an arrangement like this?

It might be easier to make than herringbone gears depending on you equipment.

• I will go with that Commented Apr 26, 2021 at 16:39

You can use Herringbone gears. Depending on your geometric restrictions there are several configurations that may do the job.

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• Yes, this would be the most ideal and elegant solution but with a 1mm bore diameter I cannot source this and is hard to manufacture myself Commented Apr 26, 2021 at 16:39