Calling all mechanical wizards

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Check out the last spring in the yellow 'Coiled Diameters and Load' chart (the one that's 0.031 material thickness).

It looks like exactly what I need. It's only 2" circumference and 1" wide. It'll give me my 20 lbs resistance over the displacement range I need. It'll only do that for 2500 cycles, but that will be acceptable.

Bob

Close, but no cigar.

Compressor, with storage tank large compared to cylinder volume... pressure rather than a stored vacuum.

...Jim Thompson

That's right! The torsional spring garage doors provide a nearly constant force. Hmmmm....

Thanks.

Bob

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t. A

x).

=A0 =A0 ...Jim Thompson

=A0 | =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

Stored vacuum? Like a bottle full of nothing?

Anyway, it sounds like a good idea to me - you'd need only a bit more than a square inch of piston area, some lubrication and a purge valve. K.I.S.S.

-- Joe

Shock absorbers keep nitrogen pressurized, at high pressure, for years. O-rings work. So why not vacuum?

Pressurizing with a liquid that had the right vapor pressure might work, too. Propane would be a candidate for testing maybe. I used to have a lot of fun with propane.

John

I just checked mine. The cable pays out of a narrow drum, one on each side. Looks like the payout radius falls as the cable feeds out, which may compensate for the curve of the torsion springs. Looks like sort of a 1/x function working against a 1-x function, which could be pretty good over a reasonable range of values.

John

I don't have a vacuum pump in my garage. I do have a compressor.

...Jim Thompson

And vacuum is free. All you need is a check valve in the end of the cylinder, pressure operated or maybe mechanically operated by the piston travel. Something as simple as a tire valve stem, or a little ball thingie.

Stringing a racket, slow leaks wouldn't matter.

John

Hey! I do matter, and stop calling me a "slow leak". 8--)

It's an interesting problem I'm trying to solve. The final tie-off after the last string is tensioned involves a pull that tends to loosen the last section of tensioned string. So, I need to apply a counter force to the soon-to-be-closed knot while pulling on the string to tighten that knot. Also, the thing that attaches to the counter force device needs to become part of the racquet's knot since the knot will be closed around it. It's a tough problem to solve.

I want to be able to release the final string clamp (after tie off) and end up with zero tension loss. Without the standard technique (merely yanking back-and-forth to close the knot), the final string section is about a minor third lower in pitch (roughly 18%) than it should be, but it varies from string job to string job. It's all about minimizing the loss of tension with time (a big problem, imho).

Thanks for your and everyone's help.

Bob

Me neither, but I concur. :-)

I found one for 19.8 lb:

Have Fun! Rich

Betcha they don't! You're talking about the roll-up type door, right? When it's all up, flat against the ceiling, you can hold it in place with a fingertip.

But when it's down, it has to support the full weight of the door, give or take a few lb. In transit, from 1x to .01x door-weight, depending on how much of it is rolled up.

Hope This Helps! Rich

With the door up there is very little vertical weight, but I'm guessing it simalar to the spring system.

greg

The spring-based counterbalances for my windows use an interesting mechanism. It's a very stout spring that connects via a multi-stage block & tackle. The spring doesn't move very far because of the block so the force is fairly constant.

Actually, What's _is_ wrong with a 20# weight, a cable and a pulley... pulley mounted on the edge of the stringing table??

...Jim Thompson

That's what I'm currently using (a 20 lb sledge hammer). The placement of the pulley has to move as different strings are tied (mains vs crosses vs left/right side). The system works well. I'm just looking for something that makes the whole process easier to set up and move around as needed.

Bob

All the electronic wizards here have forgotten that the constant-force spring problem was solved hundreds of years ago - it's called a clock spring. The ability to produce a constant force over many rotations is fundamental to their purpose of keeping accurate time. Get a clock-spring from a decent-sized grandfather clock and attach the end of your racquet string on the circumference of the barrel.

A lever arm to rotate the barrel away from a stop would make it easy to load.

Clifford Heath

A short coilspring with one end fixed and the other at some fixed radius on an elliptical pulley, with the link to your string in the elliptical groove so the variation in pulley radius compensates for the variation in spring force. Use a large enough pulley so it only goes 1/4 turn or so to make it mechanically very simple - that would be a diameter of about 10" which is larger than your 2" cube but it is simple and should have great spring life :-). Multiple pulleys and/or gears would shrink it but now it's more expensive.

----- Regards, Carl Ijames

a 1.25 square inch piston with a vacuum behind it?

a DC motor fed the correct current working a rack and pinion?

a much longer spring? (you said "about")

an ordinary spring working a ramped pulley. ramped pulley as used on a compound bow:

maybe a gas strut with a reservoir? (hmm that's my first idea again)

how about a motor-driven pulley with a clutch that slips at the force you want?