Coil winder pt. 3: Tensioner mechanics and woes

Winding instrument pickups presents a few interesting challenges in terms of wire tension. We will step through all these issues, compare them with available technologies, and propose solutions.

SECTION A) – THE UNIQUE CHALLENGES OF PICKUP WINDING

1 – wire is unusually fine compared with typical coils (~41-45 awg)
2 – turns count is unusually high (~5,000-10,000), encouraging a high winding speed
3 – coil is multi-layered (so the coil diameter increases with each layer)
4 – the winding window is small (~0.25”-0.5”)
5 – aspect ratio of the coil form (bobbin or core of pole pieces) is unusually high (~ >9:1)
6 – major axis of the core has a large flat area

The corresponding problems presented by each point:
1 – issues with breakage & stretching. A need for high-precision
2 – high RPM winding is desirable to reduce winding time. Potential for error is greater
3 – length of wire per-revolution increases with each successive layer
4 – again, a need for precision. Frequent and abrupt traverse direction changes
5 – tension/wire-pull-speed changes significantly twice per revolution
6 – maintaining sufficient and even tension across the flats relative to the rounds on each layer. maintaining equal tension of the wire across the flats on each successive layer relative to the other layers

SECTION B) – WHY WORRY ABOUT TENSION: THE ISSUES

1 – wire breakage due to over-tension or rapid tension changes
2 – coil shorts from insulation breakage due to over-stretching or rapid elongation changes
3 – excessive risk of microphonics and handling noise due to under-tensioning
4 – consistency and repeatability from one coil to another

Issues 1 and 2 can be considered the ‘major’ issues since they lead to winding failure or (even worse) the pickup failing while in use by a customer. Issue 3 can be considered a secondary issue since it can usually be somewhat or completely addressed by wax/epoxy potting. And Issue 4 can be considered a conditionally-serious issue since it is more important in some situations than others (for example: ‘scatter-wound’ pickups can be expected to vary, matched coils for common-mode rejection doesn’t apply to single-coils, test/sort/match of batches of coils can offset build variation, obviously it does not apply to custom one-off builds, etc, etc).

These issues can all be addressed by the use of a proper tensioner. HOWEVER, they are not commonly used. The reason is simple: there are not really any tensioners on the market that fully address the issues in action A while being both accessible and affordable. (I will try to resist going off on a tangential tirade about the price of some of the tensioners on the market, must… resist… urge… to rant…). We will look deeper into tensioner problems and solutions in a later section.

SECTION C) – COMMERCIAL TENSIONERS: METHODS, PROBLEMS & COMPROMISES

Just about every pickup builder that does appreciable volume uses some sort of tensioner, but often they modify the setup or the units themselves to compensate for their inherent shortcomings (more on that later). The biggest major issue is avoiding breakage while maintaining at least semi-consistent tension. In theory, tensioners are designed with this in mind, but fall short in practice usually due to a combination of section A issues 1, 2 & 5 (thin wire + high-speed + high aspect-ratio).

The typical arrangement of a commercial tensioner can be boiled down to ‘drag-wheel + dancer arm + dynamometer’ (plus whatever additional gizmos they hang off it… pre-tension pads, roller guides, pigtails/eyelets, etc). The drag-wheel is typically magnetic, mechanical, or electronic; the dancer-arm is typically either flexible or rigid; the dancer arm is either coupled to the clutch to vary braking force, or independent. Electronic CNC tensioners are typically built into winding machines (although stand-alines are available), and are widely-used by large-volume winders (EMG, PRS, generic pickups, etc). These are the most effective, but by nature they are large, expensive, require power & editing of the software, etc, which usually puts them out of range of the small-business builder, so we will look at the others.

That leaves us with mechanical/magnetic clutch + flexible/rigid arm:
1 – non-dancer arm designs = various cheaper models
2 – (NON-coupled) mechanical/magnetic clutch + flexible/rigid arm = various cheaper models
3 – mechanical clutch + flexible arm = Azonic tel-a-tension 3000/4000 series
4 – mechanical clutch + rigid arm = Meteor ME483 and similar (note: later models can also have magnetic clutch)
5 – magnetic clutch + rigid arm = Altic S.A.R.L./European style (some later Meteors)
6 – magnetic clutch + flexible arm = Tanak/generic-asian MT/MTA or TC series and similar

So let’s break down the components:
Mechanical clutch: This is the oldest style of clutch, and it is essentially either a drum brake/band brake, or just a simple felt pad and cam or flywheel. I have seen patents going back a hundred years. The clutch uses either a textile or leather belt around a drum, which is attached to a spring and thumbscrew. The other style uses a felt pad on a dial screw. Both styles obviously function by causing friction against the clutch wheel, which produces drag on the wire.

Magnetic clutch: This method uses either a magnetic clutch cam or flywheel+magnet arrangement. As the magnets are adjusted closer to the cam/flywheel, eddy currents are produced. This increases drag on the cam/flywheel, which is coupled via rotating shaft to the clutch/brake pulley, which produces drag on the wire.

Rigid arm: A rigid cantilevered arm on a pivot that ‘dances’ in response to variations in tension. These arms are biased against the wire tension by adjustable springs + angle 99% of the time. Often they are coupled to the braking mechanism in a way that increases friction/eddy-currents when the arm is moved by tension, which smooths out the variations in tension.

Flexible arm: A flexible cantilevered arm that is EITHER on a pivot OR fixed, and ‘dances’ in response to variations in tension. These arms are biased against the wire tension either by the mechanical properties of the arm itself + angle, OR by adjustable springs + angle, OR by a combination of material properties, springs, and/or angle.

Now we can look at the pitfalls of the various designs. We can lump a few of the above variations together.
– Category A – NON-dancer-arm tensioners (including felt-pads, finger-tension, etc) cannot adjust the wire drag to match variations in tension. If a bobbin is spinning at 1000RPM, tension will change from edge to flat 32 times per second. They are often good enough for certain types of pickups (as makers like Seymour Duncan and many others have proven— although Duncan uses dancer-arms at various workstations as well) provided the operator has sufficient experience. Often builders will use a tensioner JUST for the dynamometer and guide-pulleys by bypassing the dancer arm (which is often the actuator for both the clutch and the dynamometer) after tension is set, and using the felt-pad pre-tensioners and post-tensioners combined with finger-tension. This method sets average tension with pads and occasionally fingers, and depends on stretch in the wire to absorb variations in tension, so it works best with long runs of wire between spool and bobbin (i.e. several feet up to a pad and pulley, and several feet down to another pad and wire-guide/fingers).

The disadvantages of this setup are

-obviously finger tension is not consistent, but that may be desired effect (scatter-winding for example). Also, friction surfaces against a wire heat up and change their drag coefficient, changing tension depending on how long/fast they have been operated. This leads to inconsistency between turns (if that is a concern), looser turns on the higher layers of the coil (which can lead to microphonics), and low repeatable-accuracy (again, if that is a concern).

– the stretching of the wire to absorb tension variations MAY cause cracking in the insulation, manifesting issues later in the life of the pickup

-the entire winding process has to be supervised (obviously) if using finger tension

-only one coil can be wound at a time (obviously) if using finger tension

-tension can only be set at a particular velocity (specifically the velocity that the measurement was taken at before removing the wire from the dancer arm pulley), so as winding speed changes, the tension adjustment becomes inaccurate

– Category B – (NON-coupled) mechanical/magnetic clutch + flexible/rigid arm = various cheaper models: NON-coupled arm/clutch tensioners (including felt-pads, finger-tension, etc) cannot adjust the wire drag to match variations in tension. The addition of a dancer-arm can at least compensate by absorbing some of the tension into the spring or/and arm-flex plus wire stretch, although the pulses caused by the tension changes cannot always be accurately tracked by the movement of the arm alone since it can over or undershoot depending on the speed at which it can rebound, and the amount of damping inherent in the system. These tensioners also tend to ‘fade’ in the same way as felt pads/fingers and change tension due to drag-coefficient, but they are more consistent and safer than category A.

– Category C – coupled mechanical/magnetic clutch + flexible/rigid arm = the best of the 3 major classifications in terms of consistency, but they still fall prey to the problems inherent in cantilevered dancer-arms, which we will now cover.

SECTION D) – DANCER-ARM ISSUES

While these devices work well in theory, as mentioned before, many fall short in practice usually due to a combination of section A issues 1, 2 & 5 (thin wire + high-speed + high aspect-ratio). So as a result, many have concluded that without trial and error between units, modifications, and experience ‘dialing them in’, it is hard to justify purchasing one if felt pads and clothespins seem to work. For example, many winders use the lower tension Azonic mechanical tensioners without issue, as well as the Meteor units and even generic Asian or Euro units. A note: the more expensive units may not work best. As a rule of thumb, the ‘best’ tensioners use a combination of flexible dancer arm (or well-designed rigid arm), magnetic clutch, felt pre and post-tensioners, and a healthy length of wire between spool and bobbin… so many of the generic import units fit that bill just fine IF you find a decent one.

BUT, the main issues with these tensioners is that the arm simply cannot handle the thin wire and high aspect ratio of the bobbin when combined with moderate-to-high winding speeds. The issue is that nearly ALL of these units use undamped springs or flexible cantilever as bias against the wire tension. Underdamping leads to wild oscillation and overshoot, which leads to snapped wire. Ideally, we would want a critically-damped cantilever, but since these winders developed from textiles-industry technology, have worked well for most situations, and have not progressed mechanically in decades OR have been replaced by CNC electronic units… it is not an issue which has been addressed.

NON-flexible dancer arms have to be damped externally since the springs provide bias. These should be the best option in theory, but in practice they are not since I have not seen one that is properly damped. I have not seen any mods to these other than trial and error adjustments. The better-suited units tend to have multiple biasing and tensioning springs.  Single-spring units tend to be more underdamped and have particularly harsh arm rebound.

However, many builders have managed to ‘fix’ their flexible dancer arms by various methods: changing the natural frequency or adding damping. Natural frequency is a function of modulus of elasticity of the material, moment of inertia (due to shape), length, and load uniformity, so this can be addressed by changing the length, changing the material, or attaching weights at various places along it’s length. I have seen damping improved by taping cards to the arm to create wind resistance, etc.

Ideally, a pickup-specific tensioner would address these issues from the point of initial design. I think we can do better… and cheaper, but that is for a future blog!

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About alexkenis

Guitarist, philosopher, tinkerer

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