How to Lock a Shaft at Infinitesimal Increments

[geometrixeng]

Hello Forum,

I am trying to prevent shaft from rotating in BOTH CW and CCW directions when the shaft is subjected to about 2.5 N-m of torque (about 22 in-lbs). The default state of the shaft should be "locked". Then with the push of a button, I want to release the locking mechanism, allow the shaft to rotate to a new angular position, and then release the button so that the shaft remains locked at that new angular position.

The shaft is about .500" in diameter. I CANNOT use a ratchet because I need much more resolution for the angular position where I "lock" the shaft in place. I essentially need infinitesimal angular adjustment. The housing that surrounds the shaft is about 1.000 inch in diameter, so I CANNOT take advantage of a large diameter brake rotor or similar.

I want to complete this task in a fully mechanical design. So I do not want to use any solenoids, stepper motors, gear motors, electronics, MR fluids, or anything of the sort.

I have already tried several designs (3D printed) including:

--Dual torsion spring wrap clutch/brake where the opposing torsion springs prevent the shaft from rotating until the spring ends/tangs are squeezed together -Too much rotational "play" once spring tangs are released.
--Tangential brake shoes that wedge themselves between the shaft and ID of the drum in which the shaft rotates - Way too sensitive to slight variations in part geometry and just not enough holding torque for the small hub/drum diameter.
--Coaster brake design - just not enough holding torque for the small hub/drum diameter.

Any suggestions based upon past experience in this area or just creative brainstorming would be greatly appreciated.

TIA,
--Neal

 

[JohnRBaker]

I suspect that the dual torsion spring arrangement is the closest that you're going to get to what you're looking for.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

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[dvd]

I struggle to read most posts, but maybe these type of shaft collars would do the job. A picture usually makes the problem statement more clear. Drill through the collar to mount to your contraption (only one side) -

https://www.mcmaster.com/shaft-locks/tight-hold-quick-release-clamping-shaft-collars/

 

[Pud]

Dividing head?


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[BrianE22]

"Infinitesimal" sounds like a friction type device. Friction varies quite a bit though. How close to the 22 in/lbs. does it need to be? Repeatability?

 

[CWB1]

If a brake poses an issue, could your application use a simple gear reduction and eliminate the need for the brake? An operator could always spin their end of the reduction, your low torque backfeed OTOH couldnt spin the other.

 

[geometrixeng]

Thank you Everybody for your suggestions.

I believe that JohnRBaker's reply may be the best path. I just have to work out the kinks. And yes BrianE22, this probably has to be a friction device in order to achieve the "infintesimal" requirement. I also need to maintain the specified holding torque for about 100,000 cycles. Thanks Pud. I Googled "dividing head" and I do not think that this will meet the requirements as this would take too many manual steps to manually intervene when all I really want to do is have a "push button" release. And DVD, I like the idea. I have thought about this but just haven't yet figured out how to integrate this "mechanism" into the existing housing.

So I am going to keep brainstorming and trying to get this to work. I will post an update once I get this to work.

Thank You,
--Neal

 

[geometrixeng]

Thank you CWB1. I was actually able to do this with a gear motor. The torque required to backdrive the gear motor exceeded the torque requirement. However one of the other requirements for this project is that I cannot use any motors or electronics. Thank you.

 

[MintJulep]

Use a tapered connection. Like a fishing pole uses between sections.

Spring to pull the taper together.

"Button" to push them apart.

Essentially, a cone clutch, with a large L/D aspect ratio.

https://www.mech4study.com/2018/03/...onstruction-advantages-and-disadvantages.html

 

[kjoiner]

Hello,

I was thinking of a one way needle bearing roller clutch. The bi-directional use, however, points to John Baker's torsion spring clutch ideas as probably the best solution.

 

[EdStainless]

The traditional method is a worm gear drive, by changing the angle of the worm you change the 'back drive' force required.


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[Compositepro]

Search for timing or phasing couplings. Geared differentials can be used to change shaft timing even on the fly.

https://candycontrols.com/product-category/phase-shifting-couplings/

 

[Jboggs]

I don't know if these products will work for your specific application but my expereince with them over the years has been good.

https://www.fennerdrives.com/keyless-locking-devices/?=