×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Mechanism for automatic release at specified travel

Mechanism for automatic release at specified travel

Mechanism for automatic release at specified travel

(OP)
Does anyone know of a mechanism that can release a one-way clutch automatically at a specified travel distance? My need is for a fixed reciprocating motion for both input and output, but the output needs to retract quickly under spring tension at the end of the stroke, independently of the input return stroke. Then the clutch needs to re-engage again when the input completes its return stroke.

A ratchet and pawl probably won't work for me because it's subject to wear and the force on this will be fairly high during the power stroke. I can't risk the tips of the ratchet teeth or pawl breaking under load.

RE: Mechanism for automatic release at specified travel

Hi

Any chance of a sketch of your device, makes it easier for us to see the problem.

desertfox

RE: Mechanism for automatic release at specified travel

A 1 way clutch is probably not the best solution. You probably need a cluch-brake package that will clutch in the forward motion and at the end of stoke sensed by , say a microswitch will release the clutch and momentarily actuate the brake. After a pause, the brake- clutch system is deactivated and the spring return will end the cycle.
If you have air available, then a pneumatic solution is a candidate.  

RE: Mechanism for automatic release at specified travel

(OP)
We are early in the design phase and have no fixed design. That gives us flexibility, so the necessary mechanism can be linear, rotary, arcuate, or any other shape, as long as it accomplishes the goal. Criteria include simplicity, cost-effectivness and most critically failsafe robustness.

Attached is a simple graphic of what the mechanism should accomplish. Since we have nothing designed as yet, we don't have any drawings.

RE: Mechanism for automatic release at specified travel

(OP)
zekeman,

Unfortunately, no outside power source such as pneumatics, hydraulics or electronics will be available for this. It has to be simple, extremely reliable and strictly mechanical. Sorry, I should have specified that.

RE: Mechanism for automatic release at specified travel

hi tinkertech

theres no image with the link

RE: Mechanism for automatic release at specified travel

As a WAG would a disk and pin (crank pin) work?

It could be done with a cam Circular. but would be a little more a little more complicated.

RE: Mechanism for automatic release at specified travel

The "extremely reliable" spec may rule out springs, unless you pay a lot of attention to related details, or the design lifetime is fairly short.

I'm thinking cams.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Mechanism for automatic release at specified travel

I think unclesyd has something-- elliptical gears may give you what you need.

RE: Mechanism for automatic release at specified travel

One of the things I don't see in the description is what is the nature of your prime mover?  Is the input a constant RPM source of torque?  Variable?  Linear stroke input?  What?

One thing that comes to mind is a mechanism sort of like a ratchet-n-pawl on the 2nd element that captures the 1st element at the beginning of stroke, then by virtue of a linear cam will release the 1st element at the correct point in downstream stroke.

The other idea was cams & cam followers.

There's a bunch of books out on the market for clever mechanisms...do a search on Amazon for them.  That may give you the ideas you need.  But ultimately you will have to synthesize the motion requirements and design mechanisms to produce the motion, which is an "old school" machine design task.   

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: Mechanism for automatic release at specified travel

(OP)
Thanks to everyone for their input. The requirements have been refined a bit in the last few days. Basically, what the mechanism has to do is accept downward input from a horizontal lever for a short arc, only about 20º of travel. An output lever has to match that travel, driving a piston downward. At the end of that 20º, input and output levers must disengage and the piston should immediately and quickly retract upward without force from the input lever. Because of the free-return aspect of the retraction, something like the Geneva mechanism won't work. We are looking at something that uses a form of cam, but the limited travel makes cams rather impractical.

RE: Mechanism for automatic release at specified travel

Basically, a 20 degree segment gear  engaging a spring loaded clutched gear could do it.

RE: Mechanism for automatic release at specified travel

IRstuff the link you provided said the book was still copyrighted and only showed previews for all the versions. It looks like it is still in print and for sale from the publisher. (It is a 4 volume set.)

tinktech
A counter weight maybe an option.

RE: Mechanism for automatic release at specified travel

What about a bicycle freehub?  I asked an engineer at Chris King (a high end bike hub manufacturer) how much torque one of their freehubs can handle and he said it could mate up to a Chevy 350 engine.  

RE: Mechanism for automatic release at specified travel

(OP)
Thanks for the tips, but none of those will work. The sticking point has always been that the return MUST be independent of the input. However slowly or quickly the input is moving, the output must retract at a single, high speed. It's like pushing a rock toward a cliff. It doesn't matter if you push it quickly or slowly. The moment it gets over the edge, it will fall at the same rate. All of these mechanisms tie the output retraction to the speed of the input.

Again, thanks to everyone, but we've more or less figured out a mechanism that accomplishes what we need.

RE: Mechanism for automatic release at specified travel

At least conceptually, what mechanism would that be?

RE: Mechanism for automatic release at specified travel

Rube Goldberg type

RE: Mechanism for automatic release at specified travel

How about a simple toggle mechanism with a sliding cross-head constrained in one direction only?  As soom as the line between the pivot on the two levers drops below the pivot of the crosshead, the crosshead slider will pop up away from the constraining surface allowing free return of the piston.  A small spring could be used to allow return of the crosshead slider if the mechanism was to be used in different orientations.

RE: Mechanism for automatic release at specified travel

Sounds like you should study the mechanism of a double action only (DAO) trigger in a gun.  Pull the trigger: cocks the hammer, releases the hammer, hammer spring drives the hammer to strike.  Release the trigger to reset and engage and pick up the hammer.  Repeat.

Ted

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources