Mechanism problem

[dbaronio]

Hi All

im having a problem creating a mechanism. it consists of a worm driven gear mounted on a shaft, that turns a screw which should force a collar to travel linearly. i have the worm gear drive set up fine as a mechanism, however im not sure how to get the screw and collar to behave the way i want to. the collar is mounted to the screw with a cylindrical and slot connection, and i can drag the components manually to acheive the movement i want, but i cant automate it. i have attached a video with the components being dragged to show the movement im after

Video Link : 7264

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[Kevin]

‌You need a constraint that also keeps the cylinder from rotating, at least I'm assuming that's what is happening in the video. I'm assuming the slot on the tube has something that rides in it that keeps this from happening and this what you need to make a connection for. It would also be helpful to understand what the slot connection is that you have defined. I'm assuming you created a slot connection for the screw definition.

[dbaronio]

Yes that is what im aiming for, sorry about the rubbish quality, it was the best way i could record it at the time. There is a key at the end of the tube that will ride along a channel in a housing, i was hoping i could constrain the tube to linear motion before i tried assembling the housing. The slot connection is defined between the screw thread on the shaft and a corresponding screw thread in the tube, so when i drag the tube the shaft and spur gear turn, which then turns the worm via a gear connection. However when i turn the worm or apply a servo motor to it, the tube turns with shaft and doesnt translate linearly. i tried to change the cylinder constraint i have on the tube to a sliding constraint, referencing the tube and shaft axis, as well as a reference between one of the assemblies fixed planes and one of the tubes planes for rotation, but the mechanism fails

[Kevin]

It's been my experience you want to keep the references within the constraint sets elements to be to the same components or assemblies. Assuming the shaft axis is on the assembly fixed planes, I would try adding an assembly axis so that it is through what I'm assuming would be one of the coordinate systems primary axes. You would then define the slider elements so that they relate to the tube and assembly (axis, rotation, and translation would have references to the tube and main assembly).

[TomD.inPDX]

There are a number of ways to do this.  I tend to make linkages between elements when I can but when I cannot, I drive additional servos.

In this case, just make a generic gear connection between either the input (worm) or the receiver (spur) and the shaft.  If the shaft has a linear mechanism constraint, the connection will be rotation to linear (rack and pinion).  Unless you have the mechanism option, you are limited in motor or connection options.