Finding the torque required for telescopic cable mechanism

[Kajan thana]

Summary:: I am designing a linear telescopic system that gets elevated with a step motor, but in order to find the compatible motor, I need to work out the torque that is required for the elevation. I have uploaded my CAD model to this thread with how cables are connected. Could someone please teach me how do I go about calculating the torque that is required? it is even more difficult with a pulley that is also moving up in the system. Thank you in advance...

 

[berkeman]

The horizontal offset of the 3 stages seems like it would introduce a lot of extra stress and friction. What kind of bearings are you using between the stages? Also, the cable routing from the 2nd stage to the 3rd stage looks to be wrong to me.

Is this for a schoolwork project?

 

[Kajan thana]

Yh it is a schoolwork project Berkeman, those three stages are linear sliders. I got this idea from this video .

 

[berkeman]

Okay, thread moved to the schoolwork forums. Please take care to always post schoolwork (even projects) in the schoolwork forums. Thanks.

And in the video, it's important to notice that they are trying to keep the vertical telescoping mechanism co-axial to minimize bending stress friction. And note how the cable is routed -- it is different from what you show in your sketch...

 

[Kajan thana]

Thank you for all your responses. I used this sketch to design my mechanism.

Furthemore, would I be able to use this to find the torque.

 

[berkeman]

Is that sketch in the video? I only watched the very beginning up to where I took the screenshot.

The connections of the cable to the 3rd piece look wrong to me. You need a pulley at the top of the 2nd piece with the cable going over it and down to the bottom of the 3rd piece, much like in the screenshot that I posted. The way the cable is shown in your first post and in the sketch you just posted won't work, IMO.

Also on your torque question, do you see my point about the mechanical problem with having such an offset design for the lifting segments? That will create a lot of friction, and at least double the required motor torque. Can you consider re-designing the mechanism to be more coaxial to minimize the bearing friction? It's probably important to fix the basic design before spending much time on trying to calculate the motor torque.

 

[Kajan thana]

Is that sketch in the video? I only watched the very beginning up to where I took the screenshot.

The connections of the cable to the 3rd piece look wrong to me. You need a pulley at the top of the 2nd piece with the cable going over it and down to the bottom of the 3rd piece, much like in the screenshot that I posted. The way the cable is shown in your first post and in the sketch you just posted won't work, IMO.

Also on your torque question, do you see my point about the mechanical problem with having such an offset design for the lifting segments? That will create a lot of friction, and at least double the required motor torque. Can you consider re-designing the mechanism to be more coaxial to minimize the bearing friction? It's probably important to fix the basic design before spending much time on trying to calculate the motor torque.

This is the sketch from the video, later in the video he replaces the pulley with a screw eye hook, I think that's is why he didn't have another hook in stage two.

 

[Kajan thana]

Do you think this cable arrangment will cause the elevation. I still have not considered about the co-axial design to decrease the bearing friction.

 

[berkeman]

Do you think this cable arrangment will cause the elevation.

Yes, when the cable is fully taut, the bottom of each stage is aligned with the top of the previous stage.

 

[Kajan thana]

Thank you so much for your help.

 

[Lnewqban]

As very well explained by @berkeman above, the friction of the sliders and the off center load can create significant friction resistance as to increase the needed torque.
Friction resistance can become height dependent as the normal forces will increase as overlapping sliders length gets reduced as load is elevated.
Besides, the geometry of the wire-rope and pulleys changes as the sliders move respect to each other, increasing the tension much for certain positions.

 

[Kajan thana]

Hi Berkeman and @Lnewqban,
I have taken all your advice into account and slightly modified the design to minimise the bearing friction, I have increased the diameter of the second stage pulley and also introduced another pulley in the first stage to decrease the friction. I am learning a lot from your feedback, would you be able to evaluate this design again, please?

 

[Lnewqban]

I like this one better.

 

[berkeman]

slightly modified the design to minimise the bearing friction,

TBH, I'm not seeing any changes that would reduce bearing friction. Have you considered tilting the whole thing to the left some, so that the weight of each stage is carried over the CoM of the lower stage? It still won't be coaxial, but that may decrease the torque-induced bearing friction at the sliding surfaces...

 

[Lnewqban]

Following the following links, you can study the details of real life good telescopic lifts for an educated judgement of power requirements:
https://www.genielift.com/en/material-handling-products/material-lifts

https://www.genielift.com/docs/defa...49a4957a0c0bac974ac87cd.pdf?sfvrsn=7ade4409_0

https://www.genielift.com/docs/defa...bdd4a4b9cbae5c7afc5289c.pdf?sfvrsn=e00be8a0_0

See the exploded view detail in this manual:
http://manuals.gogenielift.com/Parts And Service Manuals/data/Parts/Material and Small Personnel Lifts/IO/23550.pdf

Note that some of these designs have little wheels or rollers inside the slider shapes in order to reduce friction and required power (look for part #44 in diagram of page 14).
You can also see that the shapes of the sliders is wide and flat, and that the pulleys are placed sideways, trying to reduce the friction-increasing off-center effect explained by @berkeman above.

 

[Lnewqban]