# Design a coil with a variable pitch but a constant length?

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1. May 1, 2015

### brent garson

I believe what I am attempting to do is not possible but here goes. I want to design a fixed overall length coil which I can manually adjust the pitch on. Optimally one would be able to grab the coil and by twisting it in place would be able to change it's pitch by changing its diameter but not changing its overall length. One end of this coil is going to be connected to a motor and the length needs to remain constant, really only the diameter can change though maybe there is another way? Now you know why I think this is not physically possible. Is it? Thanks!!

2. May 1, 2015

### Staff: Mentor

Can you say what the overall application is? That would be a big help in figuring out a way to make it work. Is the coil being used as a spring? What characteristic are you trying to adjust and why?

3. May 1, 2015

### brent garson

The coil would be used to move items of different dimensions from point a to point b on a linear plane. Presently, when you want to change from one size object to another a different coil pitch is required resulting in the need to change the coil. The only constant we need to maintain is the overall length of the coil, its diameter can and I assume will change? Thanks for taking an interest in this.

4. May 1, 2015

### Staff: Mentor

Can you just use a fixed pitch coil and a stepper motor instead (vary the number of steps to the stepper motor to get different displacements)?

Alternately, just use different limit switch placements for the various size items...

5. May 1, 2015

### brent garson

The coil moves a specific size item along a linear plane, the reason the pitch is critical is it helps to insure that the item properly transits the linear plane. Pitches that are to small for instance won't allow a larger item to fit with in the pitch and if the pitch is to large than smaller items might not successfully transit the linear plane. This is why overall length is so important but the coil's diameter is not. The items need to transit a fixed distance regardless of their size so in a perfect world one could literally grab the coil and cause its diameter to shrink or expand, keeping its length constant but changing the pitch of the spiral to better fit the items being placed within it. The items themselves being transited are larger always taller then the coil so that the coil is pushing the along from point a to point b.

6. May 1, 2015

### Staff: Mentor

Oh, the coil itself is in contact with each item, and is what pushes the items along. Can you post a picture?

Is there a reason that you don't just use a conveyor belt or similar linear movement apparatus?

7. May 1, 2015

### brent garson

The items we are moving preferably are moved standing up and they tend to flop over when vertically positioned, this is why the coil is such a good means to transport them. The issue is we change items from time to time and when we do we have to change spirals if the required pitch is materially different. This is an operational problem. If there was a way to take a coil and change its pitch, again not concerned about it diameter, we would eliminate what's today an issue.

8. May 2, 2015

### Baluncore

When you change the pitch of the helix, do you change the rate of rotation to regulate the speed of translation ?

If you change the pitch by changing the radius then do you need to change the height of the helix axis relative to the surface to compensate for the change in radius?

Is the axis of the helix free for a horizontal shaft or would that obstruct the product being moved?

9. May 7, 2015

### brent garson

The issue we are trying to solve for is to be able to use a coil to transit across a horizontal surface a group of identical sized items and then to be able to do the same with a different sized group of items without having to change the coil itself but by somehow being able to manipulate the coil to reduce its pitch so that the new group items which are of a different size then the previous group will still successfully transit the across the horizontal surface. Speed is not a consideration, only the overall length of the coil has to remain constant. we could for example collapse a portion of the spiral, particularly where it connects to the motor driving it, so if we were able to reduce the pitch but by so doing this might cause the overall length to be elongated then at the same time cause the last several sections of the coil to collapse to keep the overall coil's length constant. Is this possible?

10. May 8, 2015

### jack action

The only way it could work is by collapsing end coils together. You install a spring with the smallest pitch you need. When a larger pitch is needed - say you need 3 coils less - you collapse the 3 end coils together. The true length is slightly shorter (maybe you could have a «storage» space for the end coils in your design) and the diameter remains the same.

The method you describe (reducing the diameter to keep the same length) is probably impossible as you need to re-bend the spring in a certain way (as in forming). I tried to imagine a spring with square coils, where corner pieces would slide into side pieces to be able to change its diameter (4 corner & 4 side pieces per coil). The problem is as you change the diameter, you will need to add or remove corner and side pieces to keep the correct length (to compensate for the total wire length that is now changed). So even with a round coil, you can see than the coil need to be completely re-formed with some sort guide/mold if you want to change the pitch with the diameter. Just «twisting it» won't do the trick: You have no control over which coil(s) change its diameter (there might only be one).