Misc. Overthinking those "soft close" drawer slides - can you kick them in?

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The discussion centers around the mechanics of soft close, undermount drawer slides, particularly focusing on the function of the dashpot, which acts as a shock absorber. The design features a dashpot that engages and disengages with the moving slide, allowing for a smooth closing action when the drawer is pushed in. However, concerns arise regarding the potential for damage if the drawer is closed too forcefully, as the abrupt transition from no damping to damping can create a hard shock that may wear out the plastic components involved. The conversation also touches on the engineering philosophy of creating "good enough" products, suggesting that while stronger designs could be made, they may not be economically viable. Additionally, there are references to the broader applications of dashpots in automotive engineering and other mechanisms, highlighting their role in managing forces and vibrations.
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I bought a pair of soft close, undermount type drawer slides. Being a card carrying PF member, the first thing I did was examine the soft close feature to see how it works. Apart from general curiosity, I have seen video advertisements where they demonstrate how you can close the drawer with a smart little kick, which is then supposed to be absorbed by the soft close feature.

The fixed slide carries a dashpot (with presumably a spring hidden inside). The dashpot is coupled to the moving slide via a hook type catch. When you pull the moving slide out, then at some point the catch unhooks from the moving slide and hooks onto the fixed slide, so that the dashpot no longer follows the moving part. From this point on you have nearly zero damping. The spring is now latched in the fully-extended state and is waiting for the drawer to be closed.

If you push the moving slide back inwards (or let it move back under its own weight), then again at some point it unhooks the dashpot from the fixed slide and latches itself onto the dashpot's shaft. This action now allows the internal spring to kick in and slowly draw the slide into the fully closed position.

A potential flaw is revealed if you try to see how the impact of a fully loaded, kicked-in drawer might be absorbed. Ideally, the damping should kick in gradually, or else a buffer spring should decelerate the drawer at a reasonable rate before the final closing velocity is reached. In the system that I bought, if you allow the moving slide to fall nearly vertically into place, you can feel a pretty hard shock as the latch flips and the damping kicks in abruptly. The brunt of this shock is taken by the latch itself, which is made from a couple of small plastic parts.

So in this particular design, those small plastic parts probably won't last if they have to transmit that large decelerating force from the dashpot to the drawer, slowing it down significantly within hardly a millimeter of travel.

So if you have looked at this kind of mechanism, maybe yours was better designed? If you have also overthunk this issue, what are your thoughts?
 
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Well my thoughts are: what in Heaven's name is a dashpot?

Oh. Its a shock absorber, just like in an auto suspension. Why didn't you say so?
 
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DaveC426913 said:
Well my thoughts are: what in Heaven's name is a dashpot?

Oh. Its a shock absorber, just like in an auto suspension. Why didn't you say so?
Dave, perhaps it's just not a Canadian thing. I've been aware of dashpots since I the 1960's
 
Dash-pot! Parbuckle my frustum with a whippletree.
What will we discover next on the Physics Forums?
 
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So, you're asking "I bought a thing. If I kick it hard enough might it break?"
I'll venture a guess: yes, you could break it. Or Bruce Lee, he could probably break it.

1744957135338.png


You could design a stronger version, I'm sure. But would you make as much money in the furniture business as the people who designed this version? Engineering is the art of good enough. Good enough is the goal.

Granted, there are things at the big box HW store that aren't really good enough. IDK. Don't kick it.
 
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phinds said:
I've been aware of dashpots since I the 1960's
In the 60s I played with motor cars and the word 'dashpot' appeared in the context of SU carburetters. There are many hits from a search of the word - including a fast way to churn butter.
It could be mentioned that a dashpot or a damper very often (always?) has an asymmetrical damping characteristic; a piston moves one way easier than the other and that's the clever bit. Motorcar dampers allow the wheels to deflect upwards easily but keeps them from falling into potholes and damaging the suspension. It's done with a flap valve on holes in the piston. Dead smart..
 
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If your refrigerator/freezer has 'ice thru the door,' there is an excellent chance that you already own a dashpot - it is a common way to slowly/positively close the ice door. Wait for your wife to leave before taking it apart.
 
DaveE said:
.. Engineering is the art of good enough. Good enough is the goal.
I once heard this described more pointedly:
"Engineering is the art of doing for a nickel what any moron can do for a dollar."
 
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DaveC426913 said:
Oh. Its a shock absorber, just like in an auto suspension. Why didn't you say so?
If you use an automotive reference, use an appropriate one: carburetor, as mentioned by @sophiecentaur .

It was used to prevent stalling when the throttle was suddenly let go.

2010-05-26_163852_dashpot.jpg

datsun-dashpot.jpg

c8of-9510-d_01-jpg.jpg
 
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jack action said:
If you use an automotive reference, use an appropriate one...
I did.

Shocks in autos have a slower rebound (re-extension) rate than the compression rate by varying the flow of oil through the piston head - just like the pashdot brashplot smashalot thingy the OP mentioned.

1744987724662.png
1744987707714.png
 
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DaveC426913 said:
pashdot brashplot smashalot
Smashpot? Opa!
 
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DaveC426913 said:
I did.

Shocks in autos have a slower rebound (re-extension) rate than the compression rate by varying the flow of oil through the piston head - just like the pashdot brashplot smashalot thingy the OP mentioned.

View attachment 360032View attachment 360031
Yes, but shock absorbers are meant to reduce vibrations, not softly stop the closing of something.
 
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  • #13
Swamp Thing said:
So in this particular design, those small plastic parts probably won't last if they have to transmit that large decelerating force from the dashpot to the drawer, slowing it down significantly within hardly a millimeter of travel.

Next Level Overthinking:
A mitigating factor might be that, rather than bleeding through the valve, the air might compress in the initial phase and so reduce the transient stress on the plastic parts.

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Overthinking Ultimate Tier:
I had a vision of the lubricant "dieseling" under that rapid compression, with unfortunate consequences to this man...
DaveE said:
1744957135338.png
 
  • #14
jack action said:
Yes, but shock absorbers are meant to reduce vibrations,
This is another example of engineers' malapropism. 'Shock' implies (to me at least) dealing with a one-off shock (buffers at the end of a railway line, for instance) but a motor car damper is at work all the time, suppressing resonances between the road and the suspension springs. I had a defective rear damper and it failed to damp a rear wheel from bouncing along the road at a resonance frequency. Other drivers were worried by the motion of the wheel and told me to put a new one on; job done and no more bouncing so the wheel constantly followed the road surface which is what's needed.
 

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