"One-Way Linear" Escapement? 1

[ThomasUtley]

All,

Need some ideas for controlling the speed at which an assembly which is being lifted by springs against gravity can travel upward. In general terms, the assembly being lifted weighs 50-75 lbf while being pulled upward by spring tension exerting approximately 111 lbf at the beginning of motion (most spring extension). The weight being lifted is a range because of a fixed component (the moving mechanism) and a variable component (user items which may or may not be present when the assembly is released).

The assembly rises 6", which reduces the spring tension to just balance the 75 lbf max load when spring tension is at its lowest.

Packaging space is limited. I have two pockets, one on either side of the mechanism, measuring approximately 1" thick X 15" wide X 12" tall. The 1" thickness seems to be the real limiting dimension for concepts I've come up with so far.

I would like the assembly to rise smoothly over a few seconds without slamming against stops at the end of its travel. The motion control device must work in only the upward direction and not fight the user trying to push the assembly back down when finished.

I have considered air dashpots and friction braking so far but been stymied by the lack of space for the dashpot diameter and lack of tunability/stability over time for the friction applications.

Yesterday I was reminded of mechanical escapements used in clockworks. Is there such a thing as a one-way linear escapement design that would slowly allow the mechanism to rise but not inhibit quick movement on the way down?

I'm open to any other suggestions as well. Nothing off the table if you have an idea you'd like to share.

Thanks!

Tom

 

[GregLocock]

Consumer appliances use dashpots with either liquid or foam rubber to get this effect. The latter is cheaper in volume manufacture but may not be suitable for your relatively large forces. Design the shape of your foam rubber so that on the downward stroke it folds out of the way. A mountain bike's front fork provides an off the shelf demonstration of this.

You need a better spec, life, contaminants etc.

Cheers

Greg Locock


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[ThomasUtley]

Thanks, Greg.

Good point on the spec details, which may help center responses on better solutions to my problem. Long story short, it's a one-off mechanism that is part of a piece of high end custom furniture. The piece is an upright dresser. The customer desires a hidden compartment above the top drawer and wants the top to rise vertically instead of tilting which would require moving anything on top of the dresser before accessing the compartment below.

The environment is very benign, of course, but it does need to last many decades with no wear or adjustment. It should not have any potential to leak fluids or fail from pressurized gases leaking down to the point of failure.

Finally, I've searched online for dashpots that fit within the 1" space limitation and have not yet found an air dashpot with a 6" stroke that could handle the energy inputs involved. Would love to be shown a viable alternative, however.

Screenshot of the proposed layout attached.

Thanks,

Tom

 

[ThomasUtley]

Sorry, missed the button to attach the screenshot in the last post. Hopefully this one will work as intended.

Thanks,

Tom

 

[ThomasUtley]

And another screen shot showing the latch release mechanism layout. Essentially four posts passing through latch release bars which are activated by a decorative medallion on the front face of the piece. The design of the medallions hides the fact that the center portion is movable to release the top.

What is shown here in red is actually dark ebony wood. The moving parts are brass for the most part with the exception of the long springs and linear bearings which are standard THK units.

 

[hydtools]

Use a TV lift. Power up, power down. No springs, no dampers, no leaks.

http://www.tvlift.com/full-size-tv-lift.html

Ted

 

[MikeHalloran]

Take a look at the variety of bellows inside a player piano, and look closely at an accordion.

I'm thinking a 1"x15" accordion- type bellows is do-able, certainly not easy, in leather and paper, such that it would be extended 6" as the top rises, drawing in air through an orifice. As the top is pressed down, the bellows would release air through a free flowing leather check valve. Since an externally pressurized bellows is stable wrt to buckling, you don't need a fancy mechanism to guide the bellows, just a telescoping box around it to gently trap and guide it while you're collapsing it.




Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Nice. Watch for wear of the pleats against the sides.

Cheers

Greg Locock


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[zekeman]

You have an early 20th century solution to a problem solved in 1940 and is now easily done by a simple bang-bang DC servo driving a lead screw or a winch


And why should the operator need to use 50lb to bring it down?

 

[ThomasUtley]

All,

I appreciate the great ideas! However, since it is a free-standing piece of furniture, I and my client would both prefer to have no electrical power input required to operate the mechanism. Call us traditionalists, but the bellows concept sounds more appealing for this application than stepper motors. Now if only a bellows could be made to reliably last for several decades...alas.

I'm oscillating between the air dashpot concept and the escapement. I'm still investigating escapement designs looking for a linear layout that can be run backwards without resistance...the idea of it tick-tick-ticking open appeals very much to the client who's into unique mechanisms, but I'm concerned he might be waiting around all day for the darned thing to open.

Thinking the compression dashpot may win the day. Rather, a series of them since I'd need to handle maybe 60-70 psi using only a single small diameter cylinder to damp the spring tension down to something approaching a crawl.

 

[zekeman]

A 2 inch diameter 6 inch stroke passive air cylinder might do it.
Probably 3 inches of acceleration,then deceleration when the damping force cv exceeds the spring force minus the weight
Before you do it maybe a dynamic analysis?

 

[hydtools]

Use the window counterweight idea. Ropes, pulleys, and lead weights. Perfectly balanced. Move as fast or as slow as the client wishes. Machine the weights to closely fit tubes for an airpot-type effect, if desired.

Ted

 

[1gibson]

Small hand held soccer/basket ball pump(s), use the needle as an orifice (crude but adjustable if necessary.)

If you need even slower travel, manifold the lines of multiple pumps to a single orifice/needle.

This could be a "dollar store" solution for proof of concept, then you could get higher quality name brand pumps (adidas, nike, etc)for longevity.

Bonus points for fancy mechanisms that you can activate with the compressed air. Personally, I think I would go for a flow indicator (stainless or brass ball inside a glass tube) for effect. You may even be able to use that directly, instead of the needle/orifice.

 

[GregLocock]

How long do the bellows in an accordion last? use PTFE rubbing strips and it'll outlast you or the owner.

Cheers

Greg Locock


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[MintJulep]

pantograph and spring.

 

[berkshire]

ThomasUtley
Have you investigated gas springs for this project. The rate of spring assist, and the valving is variable to whatever you like these days.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[bithkits]

How about a centrifugal governor that is only engaged on the upward stroke of motion?

Adriaan.
I am a Mechatronics Engineer from South Africa.

http://www.martin-electronics.co.za