Struggling with one way roller mechanism

[RJdoerr]

TL;DR

Working on designing a roller which contains a spool of material which can be pulled out by hand (Freespin) and then retracted back in by a motor.

I am working on designing a roller which contains a spool of material which can be pulled out by hand (Freespin) and then retracted back in by a motor. The first thought was to use a one way (Sprag) bearing but this will not work as the free spool direction is the same as the driven thus only achieving one or the other. It would be preferable to not use a clutch system but that is where my head is going next. Would love to hear any and all thoughts or ideas around this system. Thanks!

 

[Baluncore]

Welcome to PF.

I think we need a diagram that shows the arrangement of the cylinder, the spool and the motor. Relative size will be important.

What force is needed to pull the material out by hand ?
What force will the motor provide to retract the material ?
What decides when the retraction motor operates ?

 

[RJdoerr]

Thank you for the reply,

Here is a quick mock up to hopefully get a better understanding, I can provide a more detailed image in CAD at a later point if needed. Here the motor is depicted as direct drive, note this does not need to be the case. The retractive force for the motor is around 5 ftlb. Minimal force to pull out is desired.

Thank you again for the reply and let me know what else I can provide to gain a better understanding.

 

[Baluncore]

That seems to be arranged like a roller blind, which has a spring to retract it.

Is the material fixed in length on the roller, or is it consumed externally ?
You show the material being pulled upwards, against gravity ?
What supports the material while it is being retracted ?

What decides when the retraction motor will begin to operate ?
When does the motor stop retracting the material ?

 

[DaveC426913]

Trailer winches have a manual extension and geared retraction. A ratchet is used to switch between them.

I guess this is the clutch system you mentioned.

 

[RJdoerr]

Similar yes, keeping the pull out force minimal is why a rotator spring was not pursued. As well as this spool needing to roll out about 7’ of material. So yes, the material is a fixed length and not consumed.

The material is pulled up, and in reference to the image, to the right, so both vertical and horizontal components of force.

The material is supported by a structure which is is covering. Think of this device like a tarp covering a semi grain trailer, although those would lack the manual pull out operation as it is powered in both directions.

A person will be in control of pressing a switch to power the retract operation (minimal complexity here for now, opportunity for automation here later, but out of scope for now.)

Thank you!

 

[DaveC426913]

Similar yes, keeping the pull out force minimal is why a rotator spring was not pursued.

Yes. The ratchet winch I referenced in post 5 has no friction from the ratchet itself, which is toggled out of the way when in extension mode.

 

[RJdoerr]

This is correct, but the motor would need to be attached to the same shaft that the material is on, no? Then the motor would limit the free spooling ability.

The closest solution that I can find is a coaster brake hub on the bike, but only using the "neutral" state, and what would be the forward pedaling state, which is when the motor would be driving the shaft. In the neutral state, the hub in theory should be able to spin in what would be reverse if sitting on the bike, and this should allow the free spool ability.

See this great explanation

Although the video does not show the hub turning in reverse, I see nothing preventing the hub from doing so as long as the hub is backed out of the forward position.

Thoughts?

 

[jrmichler]

The retractive force for the motor is around 5 ftlb. Minimal force to pull out is desired.

As well as this spool needing to roll out about 7’ of material.

The roller blind suggested in Post #4 would do almost everything you want, except for minimal pull out force. It would be much simpler and lower cost than a motorized solution.

If you absolutely want a motorized solution, try searching powered conveyor roller. Look for a roller that uses a direct drive motor (no gear reducer). Electric motors spin freely when not energized, so you would not need any additional clutches or gadgets.

You can directly drive a roll with a motor. You would need to find a motor designed for the torque you want at the low speed that you want. If the motor bearings are appropriately rated, you can use the motor as the roller support at the drive end. If not, you would need to support the roller with bearings on both ends and connect the motor through a coupling.

 

[Averagesupernova]

Look into how a recoil start mechanism works on a small gas engine. In your case, the part that turns by pulling the cord would be driven by the motor instead. The crankshaft of the engine would be the part that the material you want to roll out manually is wound on.
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The recoil mechanism is similar to a sprag clutch but when released allows freewheeling in either direction.

 

[Flyboy]

That's... actually not a bad idea. gives you fine control of the rotational position of the roll.

 

[Baluncore]

If the motor is not geared, you could pull the material out against the free rotating motor without a clutch. If the motor is a geared motor, then it will need a clutch, or some control complexity.

Consider detecting tension in the material, by sensing the force on the roll support bearings. Detected tension can be used to automatically power the motor to spool out material. If you press the retract button, the tension detection is ignored, and the material is pulled in by the motor.

There are several possible "pull-detectors". We need to know the path taken by the material as it is pulled, in order to select the most reliable detector. An optical sensor could identify lifting of the material as it is pulled.

 

[Tom.G]

Another possible implementation is use a torque motor to rewind the roll. Set the torque low enough that the film can be pulled when needed.

Possible complication is that the motor is powered when extending the film. This is not a problem for low power torque motors. This could be avoided if the roll spindle is mounted in a slot such that, when the film is being extended, the spindle is pulled away from a switch that disconnects the motor.

Note: If you have used an open-reel audio tape recorder, the take-up reel is driven by a torque motor, keeping some tension on the tape.
Common electric motors are constant-speed devices where the torque varies as needed. A torque motor is a constant-torque device where, the speed varies as needed.

Note 2: A ordinary motor can act as a torque motor by limiting its current to a low value. Watch out for temperature rise though.

Cheers,
Tom