Help with constant force spring(s) 3

[78finn]

Hi there,

I am a lowly Industrial Designer so please go easy. I'm sure a lot of my terminology will be completely wrong and I am happy to be corrected or advised otherwise, I'm not precious.

I am designing a sit/stand console (medical) that requires a really (REALLY) cheap, but affective counterbalance system that can accomodate circa 60kg of weight and 600mm travel. I have considered the following:

1. Drive motor/threaded bar
2. Gas strut
3. Cable/pulley and counterweight. Counterweight could be several sheets of mild steel plate or concrete I guess.

Motor driven I like but will be too expensive and for my application would require level sensing tech/software. Gas strut not really an option for me, due to travel extension (could consider a lateral expansion cylinder, but gets messy) and de-gassing being an issue over time, which is a BIG negative for my application.

The other option I have been considering is a constant force spring or number of constant force springs. However, I understand how a constant force spring works (at a high level), but what would be a suitable spec given my requirements? Is it possible, what spring would be required and would I need more than one spring given the travel, force requirements and limitations of constant force spring manufacturing techniques?

I have estimated 588N, but I'm not sure of that's right? Would love your help! Please try and explain as you might to small child or a golden retriever

 

[drawoh]

78finn,

A perfect constant force spring would be a violation of Hooke's law. You can design a linkage that magnifies the movement such that the actual spring stretches from around 14% of its length to around 15%. Something similar could be done with a gas spring and a reservoir.

Go to Associated Spring Raymond, and look for constant force springs. These likely are not heavy enough for you.

--
JHG

 

[EdStainless]

These are often done with either large clock spring (coil pancake) or gas strut, then combined with a simple cable arrangement.
You need to pull evenly in order to keep it level and minimize wear, a cable system can help with this.
There are spring devices used for tool counterbalance that are nearly constant force and adjustable.
You might start with one of those.

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P.E. Metallurgy, consulting work welcomed

 

[hendersdc]

78finn,

I have also seen sit/stand desks that use a hand crank with linear guides and acme screws or a scissor mechanism, would one of those approaches meet your cost needs? Down side is they aren't very quick to move from min to max height positions.

Googling "hand crank sit stand desk" will give you some examples.

 

[TheTick]

You know what's cheap? Clients who send an industrial designer to do an engineer's work.

 

[zeusfaber]

Before you go too far down the path of constant force, how much do you need the force to vary? We don't all weigh 60 kg.

A.