Label dispenser - ideas for braking

[Ziv Z.]

Hello,
I'm developing a desktop label dispenser with the main purpose of applying label A on top of label B accurately, and immediately dispense the double label for the user to pick up and use.

The dispenser consists of two motors (at the moment stepper motors), two optical fork sensors(for identifying the labels), and arduino controller.
In my previous revision of the device (which was mainly made for proof of concept), i identified that there is a lack of tension in the rolls, between the roll itself containing all the labels, and the stepper motor which is pulling and collecting the release liner, which made the device somewhat unreliable.

So now i'm developing an improved version, and one of the first things to tackle is a tensioning mechanism.

From what i researched until now, seeing all of the solutions are based on a rotating element with some element of friction introduced against it, whether it is a brake band on a spring, or something more stationary like a U shaped band around the rotator disc.
I thought about integrating motors on the free rotating axis' and use them for constant braking, but i'm not sure that it's a good idea and they might break quickly.

I'm looking for a smart and simple solution, perhaps more elegant than what I've found so far, for maximum reliability and consistent performance.
Does anyone have an idea that might help me?

 

[TheTick]

Rotational dampers may help

 

[Compositepro]

A strip of fabric draped over the roll of material being unwound. The strip is fixed at one end and a weight is attached to the other.

 

[Ziv Z.]

Which material is the fabric supposed to be?
Is there a specific name for that type of product?

Regarding the rotational dampers, won't they die pretty fast?
I saw that most of them have about 50K turn cycles, also won't they be too hard braking for the step motors to pull?
Because when you "yank" on them is when the resistance is strongest.

Is there some type of bearing that instead of being smooth is the opposite of that? some sort of friction bearing?

 

[Jboggs]

"Which material is the fabric supposed to be?
Is there a specific name for that type of product?"

I have seen this very low tech approach used successfully several places. One good thing about it is no moving parts. Also, the applied tension is easily adjustable by changing the weight. It should be a material that is somewhat immune to wear and will not damage or mark your labels. I have seen leather straps with weights on the end work very well, but the product material was steel ribbon.

 

[handleman]

Look at a packing tape gun.

 

[FACS]

I've designed several labeling machines.

First, consider a servo motor to pull the label. Steppers are not very good at this application. Everyone uses servos. Steppers do not hold position and you need accuracy for labels.

There is a drive directly attached to the motor output shaft, this roller turns and pulls the label. This drive motor also has a timing belt that runs to the rewind wheel. You have to put a friction clutch on this wheel. The wheel turns at an exact ratio as the drive roller so that the rewind keeps up. Now, the problem arises when the roll of scrap that you are rewinding gets larger. Then the diameter changes, and with that the feed rate changes. So you will be pulling on the label ribbon too hard after a few winds. That's where the clutch comes in, it will maintain tension but let the wheel slip to override the extra speed.

Some companies, use a simple leather strap with a adjustable spring for tension. It seems crude but it works.

Some (me) use a spring adjustable clutch. I have uploaded a picture of one of my designs but it is not loading the link. I'll check back later to see if it works.

https://files.engineering.com/getfi...71-93c2-c7cac2f6aab3&file=friction_clutch.JPG

Very simple.
Shaft collar on the shaft.
Shaft collar compresses a spring
Spring pushes on a large washer.
Large washer pushes on a ring of plastic or leather.
Plastic pushes on another large washer.

Adjust tension by moving the shaft collar on the spring.

You also need a dancer arm for the unwind spool otherwise the slack from this will tangle the ribbon.

 

[BUGGAR]

See "Dancing Rolls". Can you use gravity (or magnetics) like they do?

 

[Ron]

What about an idler roller between the paper roll and the stepper. Weight of the roller might be enough tension.

 

[Ziv Z.]

Thanks for the replies!
FACS i think i will try your solution, is there a specific kind of plastic i should use?
do you consider wear and schedule replacement of the plastic ring?

Also, do you think its a must to use servo motors?
I'm designing a fairly small dispenser (roughly the size of a small Nespresso machine), and up until this point the steppers worked fine.
BTW, is it possible to pick your brain on some other questions regarding the development of the dispenser?(i may have some more as the development continues)
If you can share an email or some other contact information if that's not a problem.

Regarding idler rolls and other solutions that touch the top of the labels, i can't use these solutions because i can't have them touching the top of the labels

 

[FACS]

Ziv,
We used blue UHMW (abrasion resistant glass filled) . It lasts quite long but does require periodic adjustments.

If steppers are working for you then just stay with that. Programming servos is different and since you say this is small it may be fine. Servos are faster and handle larger rolls so there is a cut off when steppers won't keep up. If you are running light loads at low speeds they will work. Going faster will be an issue, and stepper motors tend to reverse drive if they do not have a gear box. This will cause you to loose position thus messing up your release count. An absolute encoder may help.

Not touching the top of the labels will be difficult in maintain tension, I'd like to see how you managed that.

The label sensor is a big deal. Not all optic horse shoe sensors are created equal. Don't use a cheap one.

Also, the stripper plate needs to be machined with high tolerance. Any skew at all will result in your label walking off and ripping. I struggled with that a long time.

If you have other questions, I'd be happy to help. Posting the questions in here may get you more results as other people do have ideas, even if they never designed a dispenser.