Constant speed pushing mechanism (without motor)

[stdold]

Hey there!

I've been browsing for quite a while and couldn't find an answer to the following problem:

A piston should move out over a distance of 25mm with a (nearly) constant speed of 10 to 20 mm/sec pushing against a linear spring (max force: 1N).
I want to realize this using a spring/damper mechanism without any motor (tension spring before).

My questions:
1. Is there any way not to use a constant force spring? (Normal spring + pneumatical damper?!)
2. Do you know any suppliers that sell components (maybe even combined spring dampers) of this size?
3. Could you think of another possibility to achieve this goal?

I should mention that the mechanism is to be used in a handheld device which means it should be as compact as possible.

Thank you very much!
Kind regards,
Stefan

 

[Jboggs]

So, you're saying you want to use a spring (constant or linear), maybe damped (pneumatically), to push a piston and compress another spring (linear) over 25mm at a constant speed? Not sure that is possible. Depends on what kind of dampening you use and how well tuned it is. Is this some mechanism that will be manually cocked for release? Go through the kinematic and dynamic equations of the system. Establish the theoretical numbers for spring rates, dampening rates and ranges, speed variations, etc. and put together a prototype.

What do you have against constant force springs?

 

[TheTick]

If you use a spring with a low rate and high preload, then you won't see as much variance in force from end to end.

 

[ivymike]

attach a big cylinder of compressed air, and regulate pressure vs. time to give the required spring compression?

 

[MintJulep]

Gas spring.

 

[stdold]

Hi guys!
Thanks for your answers.
The mecanism is to be triggered by manually disengaging the tensioned spring.
Actually I've already considered all of these options but none of them seems really good to me.
The gas springs are promising but I couldn't find any of this size, just bigger ones. You know a place to buy those?
Constant force springs usually pull. I need a pushing one (which would force me to use an additional lever). Magnetic constant force ones (that I found) are too strong and expensive. I'm tempted to use a long weak spring and give it a reasonable preload.
However I still need to find an online shop where I could buy dampers of this size. I found some for model cars but there isn't said anything about there constants...
Can you help me there?

Thanks again!

 

[kjoiner]

Take a look at products from Airpel and Ace Controls. Airpel makes dashpots and Ace Controls makes linear and rotary dampers.

Kyle

 

[MintJulep]

Clock mechanism driving a threaded shaft.

 

[ivymike]

mcmaster.com has air springs

http://www.mcmaster.com/#air-springs/=jjrifb

also gas springs

http://www.mcmaster.com/#gas-springs/=jjriki