Small Air Dashpot Design Guidelines?

[ThomasUtley]

All,

First post after searching your excellent archives but coming up short. I would like to fabricate a small diameter (1" OD max) air dashpot/damper/(dampener?) cylinder. It is to be incorporated into a piece of furniture to control the speed at which a hidden compartment opens and avoid damage when the moving portion hits the end of its travel. I'm looking for design guidelines that someone like me who was once a mechanical engineer but is now more of a paper pusher and avid woodworker/metal fabricator can follow.

Design Constraints:

Packaging space for the finished device is 1-1/16" max thickness X 15" wide X 12" tall. There will likely be two of them minimum, one on each side of a tall cabinet. I can add more pairs if required to accommodate the mass of the moving assembly.

Min travel of the moving system is 6". I anticipate the dashpot cylinder will need to be at least 8" long to accommodate piston thickness, valving, and endcaps/mounting features.

The weight of the moving portion of the cabinet will be 75 lbf max. The mechanism is lifted with long extension springs which will balance 75 lbf when the mechanism is fully open (least amount of spring tension). When the mechanism is latched (most spring tension), the springs will generate another ~36 lbf of tension (total of 111 lbf offset by the same 75 lbf weight pushing down).

The dashpot must control motion only in the upward direction. When the user pushes the mechanism down to latch, he should not be fighting the dashpot. I anticipate needing some kind of one-way valve for this purpose.

I would like the entire mechanism to rise smoothly over the entire 6" of travel and take, say, 4-6 seconds to rise with no hard accelerations/decelerations that would upset electronics or home decor sitting on top of the cabinet.

Now that you have the basic background, where can I find some guidelines to design the dashpot(s)? Hoping it can be done with only one such small cylinder per side of the cabinet. I have access to machine tools including my home-built CNC router capable of milling non-ferrous metals, so I'm not scared of fabricating it once designed. I'm anticipating the cylinder bore-to-piston interface being the biggest obstacle, but believe I can handle the challenge.

Uploaded screenshot of the mechanism concept attached. This is roughly to scale after characterizing the spring rates of the springs I intend to use.

http://files.engineering.com/getfil...61c-59ca4fccf2bb&file=Spring_Lift_Concept.png

Thanks!

Tom Utley
Tucson, Arizona

 

[hydtools]

Try Airpot to buy them. Or do you want to make your own?

http://www.airpot.com/

Ted

 

[ThomasUtley]

Thanks, Ted. I've searched Airpot's online catalog but haven't found one with the travel I need in a small diameter package. If I could find one off the shelf I'd definitely consider using so long as the price weren't more than the cost of the materials to make my own. This was a late add to the cabinet design, unfortunately, or I could have left room for a larger diameter cylinder.

One fundamental question I'm trying to get my head around is whether a pull (vacuum) damper or a push (pressure) damper is better in an application like this. I've seen online video demo videos showing vacuum damps immediately, but seems like I'd be limited to the atmospheric pressure at my elevation, whereas with positive pressure I could drive it to the limits of the chosen materials...

Thanks,

Tom