Tensioner Idea

[R.G.]

While I'm at it:

The felt pad tensioner seems to be what everybody uses. But my mad-scientist side is nagging at me. How about we put together a pair of offset rollers, one of which is fixed, and the other of which is spring loaded to force the wire into a non-straight path. As you pull on the wire, the wire tension forces the sprung roller to move to straighten the wire path. Using this, we can sense wire tension. I'm thinking that the sprung roller is a bit of music wire which bends when the wire moves it. There is a magnet on the music wire and a Hall effect device to sense magnet movement. This is the position sense.

For the actual tension holder, we use the felt pad, but on one pad we put a chunk of iron, and on the other we put an electromagnet. When we want more tension, the current is increased in one direction in the coil of wire. When we want less, the current is decreased. Zero current is zero attractive force, and therefore almost zero friction.

This mess is controlled by a servo system with a few opamps which read the Hall effect device's position and increase the current in the tensioner pad coil to keep the Hall device in the same relative position. Coil driver is a small power-amp IC like the TDA2030 or LM386.

I think this can be set up to give very fast, high compliance tension control, even with an odd-sized bobbing tugging at the wire. The tension itself is electronically adjustable by changing the servo reference voltage. I bet this can be done with a quad opamp and a power amp IC, some resistors and caps.

[David Schwab]

Nice idea!

[R.G.]

I found the same thing in the archives of the previous forum, except not servo'ed. The objections were that the felt would wear and so the tension per current-applied would decrease.

The servo idea would sidestep that by measuring the actual tension and keeping that constant even if current had to change to get the tension up.

I obviously don't know the rate at which the felt pads would wear, so one might have to change the pads a lot.

Having the feedback signal from the tension sensor also lets the circuit ring the alarm bell when it can't hold tension any more or when the wire breaks.

On the other hand, some of the other suggestions like taking one turn around a tension spool and then using an electronic brake on the tension spool were good too. I think the servo could be used in those cases as well.

It'll take me a bit to read all the pertinent data in the old forum and other research areas.

[dpm]

might be of some use -

http://cgi.ebay.com.au/Magnetic-wire...QQcmdZViewItem

[R.G.]

@dpm:
Yes, it was useful. It put me to thinking about pinch rollers, and I think I now have a Mark 2.

McMaster-Carr has rubber-tired pinch rollers available as stock. With two rollers, one forces the wire against the friction roller, and the friction roller is then braked by the tension servo. Right now this is looking like an electromagnet pulling a friction ring against the roller itself, but I'm still working on the details of the brake and sensor. I think it will end up being something that can be made from fairly cheap bought-parts instead of being a major machining process.

[kevinT]

might be of some use -

http://cgi.ebay.com.au/Magnetic-wire...QQcmdZViewItem

WARNING: DO NOT BUY ANYTHING FROM JWTOOLS ON EBAY. This guy is an asshole and he sells junk. He gave me the runaround for 2 months trying to get my cash back on a coil winder that was total junk. I finally had to file an official complaint with paypal. Then he had enough nerve to ask me to retract my negative feedback. ...told him to go jump in a lake.

Another guy emailed me on ebay asking for advice because he was having the same problem with the winder and the seller.

Buy from this guy at your own risk.


If you want a really nice tensioner (IMO) take a look at the Tanac MT-100 for about $610. I have one, Randy from Walker pickups uses one, nordstrand uses one, and i found out that DiMarzio uses these as well.

http://tanac.com/mt/mt.html

[R.G.]

OK, that took a little thinking and searching.

The Mark 3 tensioner is a pair of "quad skates" roller skate wheels set up to be a pinch roller and a friction roller. There is a felt friction pad which bears against the non-wire section of the friction roller, and this is pulled against the wheel by the pull of an electromagnet on the other side of a pivot. The electromagnet is a small DC solenoid.

A set of eight quad skate wheels with bearings can be had for under $20, possibly cheaper. Each wheel has two ball bearings in it and rotates on an 8mm/ 7/16" diameter axle. The wheel is urethane, and will never wear out under wire loading.

The pivot arm for the tensioner pad has minimal movement. It rests against the friction roller all the time, and so the braking solenoid has almost no movement. A solenoid's force is proportional to both current and position. With position constant, it's a substantially linear function of current, ideal for applying force to a brake. It's not clear if I'll need to pull the brake pad off the friction wheel entirely.

A concensus of wire makers recommend wire tensions from 20 grams to 200 grams for the range of wire gauges I see mentioned here. So the friction needs are quite low.

The tension reading arm for the servo also serves as a tension scale for the person watching the winding. It's a single nylon wheel/pulley mounted on a length of music wire. The music wire is chosen for full deflection toward the wire path at 400grams (there are two strands of wire at up to 200g pulling on it) and down to 0G. It is calibrated with a pull-tension meter; this can be as simple as putting known-weight coins on the end of the wire and making sure it trails down with little friction or it can be a real pull-scale.

The sensing arm has an epoxied-on aluminum "flag" that uncovers an LDR as it's pulled to higher tension. An LED suspended over the LDR makes for a light source. The resistance of the LDR makes the friction servo hold at the selected point.

I'll make a conceptual sketch and put it up on my web site.

[R.G.]

OK, got the sketch. Go see http://www.geofex.com/FX_images/wiretensioner3.pdf for it.

I actually drew up two versions. One uses cheap(er) skate wheels and bearings, the other uses idler rollers from McMaster-Carr. The third page is the start of the brake servo, and is as yet unfinished. The cheap skate-wheel version is bigger, the fancy one is smaller.

It's designed around tensions of 20-200g on the wire. Control eyelets could be fishing-rod stuff or a glass bead epoxied into a bit of aluminum angle.

How it works:
1. De-reeled wire from the spool under zero tension enters the control eyelet to force it to a constant position for feeding the roller assembly.
2. Urethane-tired pinch wheel forces wire against urethane-tired friction wheel to keep wire from slipping on friction wheel at all tensions, even zero.
3. Friction wheel drag provides all of the tension in the wire except for rotational friction in the two remaining nylon pulleys.
4. A felt pad is pressed against the friction wheel to add braking tension for the wire. This is the souce of all the controllable tension. Pad bears on a large area of the friction wheel and has no direct source of single-spot wear.Pad is arranged so there is almost no movement of the pad from full braking to zero braking. It is possible that a spring to pull it just free of the friction wheel will be needed.
5. A brake lever connects the felt pad to a solenoid.
6. Solenoid is run with little or no movement, only varying pull. Run this way, solenoid pull is directly proportional to current.
7. Wire passes from friction wheel to a low-friction nylon pulley on the tension scale. The purpose of this pulley is to have a varying position with changing tension.
8. Tension measuring pulley is mounted on a length of music wire, fixed at the opposite end. The music wire bends with increasing wire tension. The spring constant of the music wire is such that tensions of up to 500g deflect it to near the other rollers. The position of the music wire is linear with wire tension, so marks on the base corresponding to tension can be made and show wire tension directly.
9. A photo sensor is uncovered as the music-wire tension scale is moved. This provides the electrical feedback of position. It's not clear whether an LDR or photo transistor is needed here. This electrical representation of position of the scale is the input to the electronic tension servo.
10. The last nylon pulley positions the wire for feeding the last control eyelet before the traverse.
11. The last control eyelet keeps traverse and winding position from affecting the wire routing in the winder.

[dpm]

WARNING: DO NOT BUY ANYTHING FROM JWTOOLS ON EBAY. ....Buy from this guy at your own risk.

interesting. I don't have a problem with what I bought from there, IMO it's pretty solid for the money. Certainly something to keep in mind though.

That Tanac gear looks very nice, thanks for the link!

[kevinT]

[QUOTE=dpm;29195]interesting. I don't have a problem with what I bought from there, IMO it's pretty solid for the money. Certainly something to keep in mind though.[QUOTE]

you are fortunate. ...and what you bought looks pretty nice. i probably would have had better luck if i had purchased the item you bought.

However, if there was a problem with it and it didn't work, i believe you would have had a very hard time getting your money back. The guy was a total jerk and a liar. As a business person, i just couldn't treat a customer the way the guy treated me...

I bought the hand cranked version and it was utter junk.

[dpm]

That certainly looks like a old fashioned contraption. I figure what this guy sells is sweatshop surplus.

[R.G.]

I think I'm wasting my time.

[dpm]

I get the general concept but the electronic part is beyond me. Guitar wiring and simple amp mods are about my limit.

My question is - if someone with limited electronic understanding who is good at making stuff were to put this together how long would it take to get up and running bug free? Hard question I know, but time is a huge factor in decision making.

[R.G.]

I get the general concept but the electronic part is beyond me. Guitar wiring and simple amp mods are about my limit.

My question is - if someone with limited electronic understanding who is good at making stuff were to put this together how long would it take to get up and running bug free? Hard question I know, but time is a huge factor in decision making.

Two things come immediately to mind.
(a) the solenoid can be replaced by a screw pulling on a spring. The tension then becomes constant(ish) at whatever tension the pull sets on the friction pad
(b) the electronics doesn't matter much; it will always be a case of sticking a bunch of parts into the right holes and soldering them, then running wires from the board to the solenoid and light sensor. The hard part is the mechanical fabrication. So I'd say mechanical fab time plus two hours.

[Joe Gwinn]

Beware intertia and resonance

Beware sprung mass and intertia (linear and rotational).

The bobbin is quite oblong, so the velocity of the wire must vary periodically and by a factor if the bobbin rpm is more or less constant.

If thin wire is pulled by the bobbin over an overly massive pulley or pinch-roller assembly, the wire will break because it isn't strong enough to accelerate the sprung weight (and rotational inertia).

Also, once started moving, the pulley et al will tend to keep going, and so will overrun the bobbin, causing loose turns.

Also beware mechanical resonances. The wire pulsates twice per turn, so at 1200 rpm, the pulsation frequency is (1200/60)(2)= 40 Hz. If the mass-spring resonant frequency of any part of the tensioner is not well above this frequency, when the resonance is hit, the resonant part will flail wildly, and may physically break off. Some mechanical damping is also helpful in controlling resonances.

In general, keep things very light, so the wire is able to dominate.