Spring help please!~

I need an answer for dummies about how to choose an extension spring.

I am trying to spring load my parking brake so that the handle requires force to pull out, and pulls back into it's detent when the force is released. Distances are appx 45mm length when relaxed, and appx 120mm when loaded. Force required is just a matter of feel, but I am thinking a kg or so when fully loaded. I am very flexible on this.

Problem is that every coil spring I have tried from the hardware store when stretched that far loses it's set, and will no longer recoil to it's relaxed position.

What should I be looking at to solve this problem? Am I stretching the spring too far by asking for a 3x extension, or should I be looking for a different coil diameter spring? A different material?

Any input would be greatly appreciated!

Reid

Hi Reid,
I too have the same issues at times with spring selection and in the end it usually comes down to the good old trial and error method! When I was building my Boeing rudder sets recently I tried three different springs on the brake action until I achieved the desired feel. I wouldn't stretch an extension spring more than twice it's length as a rule. You might try asking your question directly to a spring manufacturer, they are usually quite helpful.
Cheers, Gwyn (Westozy)

This is going to sound silly but if you think of what the parking brake does.
- It rotates the cam to push the brake shoes open. This cam is spring loaded.

With that in mind you may want to look at a cars (automobile) brake assembly for your answer. You may want to try an auto parts store for a brake shoe spring then add a cantilever to the spring. This will simulate a brake assembly and let you adjust the force by changing the fulcrum on the cantilever. The cantilever (bell crank) will be your cam. The greater the distance from the fulcrum on the PULL side will give you less resistance.

How are you doin' haven't heard from you for a long time. I've been sick as a dog as of late (pretty bad cold). It broke today though so things should be back to normal for me in a few more days. Yesterday though sore throat, sinus congestion, the whole nine yards. I was burning the candle at both ends for a time and it caught up to me.

Hi Reid,

perhaps you could use a couple of elastic staps. The ones used for model airplanes to fix the wings on the fuselage are quite strong.

Cheers, Bastian

Thanks for the ideas gents.

Things are going fine, Ron. I have had a bit of a lull in my building, but am working on putting some of the dusty pieces together again. Had a great trip a few weeks ago to Washington DC and the National Air and Space Museum and Udvar-Hazy Center. Got to see a lot of neat aircraft there! Inspired me a little to get my butt in gear and build.

They had a 152 at Udvar Hazy that was on static display, that you could sit in and try the controls. Gave the impression that I was right on track with my controls. The docent thought I was a little strange when I asked to get in, and was mainly interested in the the feel of the stabilizer trim. He was telling me about pitch and roll, and I was interested in only turning the trim wheel. LOL.
He looked even more puzzled when I told him why I was doing it, and looked relieved when I finished and moved on!

I think I will just adjust my spring length to one that is about 100mm long, then I should have the ability for tight recoil, with less than 100% extension. Will have to change my spring mount, but that won't be the first design change in this project!

That did it! Lengthening the resting length relative to the extended length solved the problem. Thanks for letting me use this forum as a sounding board.

Reid