Stepper motor as a brake – Failed

I have started building this prototype to automate carbon fiber tube winding process. Currently at the end of the task I have to apply shrink-tape by manually holding the roll in hand and guide that along the mandrel. Significant improvement would be applying same tape using machine gantry which is feeding the tape on mandrel with constant force. For constant feeding force I would need drag torque on shrink-tape roll axis.

Unfortunately my attempt described in this post failed and I will have to look for another option, anyway it was interesting to fail and now it is more clearer.

Shrink-tape braking test unit

Before I was starting building prototype I did calculations to estimate needed torque to hold required tension force. Although stepper used for this prototype does not hold needed torque, my intention here was to test the idea. Another benefit that I expected to have with this variant was some degree of control when variable resistor is connected to motor wounds and when resistance is changed torque changes. A day before I have experimentally found needed resistor range which was from 0 ohms to 500 ohms.

All calculation values were captured and a graph was drafted where on Y-axis you can see calculated torque value. When tube is wound shrink-tape roll diameter changes what gives torque increase and different color lines represent different tension forces for different diameters when tape is unwinding.

Braking torque calculations

Afterwards CAD model was created with Fusion 360 and parts produced using some plastic material laying somewhere in workshop. Here below is fabricated one of the pulleys.

Milled part from plastic

When all was assembled I could give it a try, after light pull I understood that idea has failed. Braking unit was not holding torque, as I was pulling the tape, I was feeling some resistance, but as I increased speed torque disappeared and it started to slip. I had a slight doubt that this could happen, but I was hopeful. It worked for small speeds, but was not working for my application.

Concept was tested and showed not to be working for this application, I have feedback now, I can readjust and attempt another option. Now I am thinking of making a torque motor with large diameter with opposite rotation direction. Have to do some calculations, some search and thinking. Till later!

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