oh boy, need a spring and it's complicated
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  1. #1
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    Default oh boy, need a spring and it's complicated

    I'm working on an old piece of American iron. There is a gear which is driven by a ratchet/pawl. The ratchet pinion is fastened to the shaft but the gear is free to rotate on the shaft. The spring keeps the pawl engaged. Here's a pic:



    The image quality isn't the greatest - sorry - I grabbed it off of Youtube. But it should convey the idea. As you can hopefully see, there is a spring which keeps the pawl engaged.

    My machine is missing this spring. Truth be told, I'm not 100% certain that the spring is original. The bolt and nut holding the pawl onto the gear spoke don't look like the rest of the fasteners on this machine.

    Anyway, I have never made a spring. Now I have to make a sort of complex one. Can anyone walk me through exactly how to form and heat treat a spring like this?

    metalmagpie

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    Practice bending it with a coat hanger or other cheap wire to get the size & shape right before wasting good stock on prototypes.

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    That one will be pretty easy. Buy music wire in the right diameter and make a mandrel which is smaller than the I d you need. Machinery Handbook offers some guidance. But some experimentation will be needed. You can cold form the whole thing with no heat treating. Check out some YouTube. This Old Tony showed some simple demonstrations. Use pliers for the ends. Not glamorous but works well. I have made a few, mostly just retaining clips.

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    Those are a lot easier to make than you might imagine. But you could find a torsion spring to use as starting material using the parametric search at Century Spring, just enter the specs you know and see if something close comes up that can be modified to what you need. You might end up with a few spare...

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    While not super difficult to make that looks like an off the shelf torsion spring.....

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    Search "torsion spring" on Mcmaster-Carr. You can buy that spring. Just figure out what dimensions you need and guess at the wire size.

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    Quote Originally Posted by Garwood View Post
    Search "torsion spring" on Mcmaster-Carr. You can buy that spring. Just figure out what dimensions you need and guess at the wire size.
    X 2 on that - not much to choose from though

    FYI McMaster-Carr

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    Ditto! I have made a variety of springs and never had to do any heat treat. Music wire is soft enough to work but hard enough to serve as a spring. In fact, if you try to alter a commercially made spring, you will find the exact same combination of properties. I suspect even the spring makers just use music wire or, at least, they could. But they may use un-hardened wire due to the spring-back factor.

    I altered a spring in my toaster oven a few days ago. It held the door closed and after some years of constant use, it broke. I had one long, straight arm and broke where that arm joined the coil. I un-would one turn and made round loops in both the broken ends and used them to re-join it. I used drill bits as mandrels and vise grips to hold the wire on them. Turned a partial circle with each grip. Worked like a charm.

    The big thing is the spring back so you must use a mandrel that is somewhat smaller than the desired diameter. This number is around 70% to 85% from my experience, but a quick trial with your actual music wire should tell you exactly how much is needed. I will warn you that it is not possible to correct a bad diameter after it has been formed on a mandrel that is too large. The spring will just act like a spring and will not deform any further with a small change in the mandrel size. All tests of mandrel diameter must be from scratch, from straight wire.

    As to the diameter of the wire, that may be difficult to determine. There are ways to calculate a spring constant but you would also need to know or at least estimate the force needed. In your case and if that photo is close to your situation, then I would just start with about 1/16" diameter music wire and see how it works out. Then you can step up or down in wire size to get the action you want. Seat-of-the-pants engineering? You bet. But I would not be surprised if a lot of professional mechanical engineers do the same.



    Quote Originally Posted by NC Rick View Post
    That one will be pretty easy. Buy music wire in the right diameter and make a mandrel which is smaller than the I d you need. Machinery Handbook offers some guidance. But some experimentation will be needed. You can cold form the whole thing with no heat treating. Check out some YouTube. This Old Tony showed some simple demonstrations. Use pliers for the ends. Not glamorous but works well. I have made a few, mostly just retaining clips.

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    Quote Originally Posted by metalmagpie View Post
    Can anyone walk me through exactly how to form and heat treat a spring like this?
    Two years ago I needed to make a spring for the oil pump of a 90-year old motorcycle I would ride across the U.S. Tolerances were pretty tight since the ID had to fit around a plunger without rubbing on it and the same for the OD and the body of the pump. Things were tight enough that the dia. of the wire itself couldn't be off by more than 0.001". Anyway, McMaster-Carr had the wire (Spring-Back Multipurpose 304 Stainless Steel Wire), and interpolating between values in a table in Machinery's Handbook I determined a 0.260"-dia. mandrel would be needed. I wound the spring and the size was perfect.

    To wind the spring I set my lathe to the pitch of the spring I needed (I copied the heavily worn one that had been in the pump), had a piece of metal with a hole drilled in it for the wire on the toolpost, and manually fed the wire while applying tension with my hands to bend the wire around the mandrel as it slowly turned in the lathe (I'd have to look it up, but the dia. of the wire was somewhere near 0.040"). I started and ended the winding a few turns on either end of the actual spring I needed since the material just beyond the actual spring affected the neighboring turn.

    Searching around I found a 1974 Army manual that had tested springs of roughly similar sizes as mine made with various stress relieving temperatures after winding. The data in that manual showed springs stress relieved after winding at 450 +/- 10 oF for 30 min. had the longest life so that's what I used.

    The oil pump got me across the U.S. without failing, during which the spring underwent an estimated 5.2M cycles. Your ratchet probably won't need to operate 5.2M cycles, but I don't remember how long springs that weren't stress relieved lasted in the Army tests.

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    Common name is mouse trap springs.McMaster has what you need.Need to know weather RH or LH.I have a few that look to be the same size.Get an accurate size drawing or a better description and I will send you one.As mentioned already very easy to make.

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    Quote Originally Posted by NC Rick View Post
    That one will be pretty easy. Buy music wire in the right diameter and make a mandrel which is smaller than the I d you need. Machinery Handbook offers some guidance. But some experimentation will be needed. You can cold form the whole thing with no heat treating. Check out some YouTube. This Old Tony showed some simple demonstrations. Use pliers for the ends. Not glamorous but works well. I have made a few, mostly just retaining clips.
    And I would add use only smooth jaw pliers or vise pads to make the bends at the ends. Any dings or nicks and it will fail early. You don't want too sharp of a bend. I stoned a radius on one side of the jaws for bending wire, etc.

    One other hint. If the music wire comes in a coil bend it around the mandrel opposite to the existing curve to reduce springback.

    Usually for a first timer it takes one or two practice springs before you get the hang of it.


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