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Toss Out Those Strings!

by Jerry Coopmans

Scotch Brake tension strings, that is. Spinners have been using the Scotch Brake (SB) system for centuries and for good reason. It provides tension from very fine to fairly heavy with a simple adjustment of the brake band (string). The SB is a flyer lead system and works by putting just enough drag (tension) on the bobbin inside a spinning flyer to draw the yarn on when one relaxes the pull on the yarn. The usual configuration starts with a spring on one end, tied to the tension band which wraps around the bobbin in a groove, connects to another spring and finally to some sort of knob/shaft arrangement to tighten the whole assembly. The springs accommodate variations in bobbin eccentricity, starting/stopping and reduces the loss of friction as the string heats from friction. A spring on each end is necessary to accommodate both spinning and plying. Although the SB is the most common brake, Irish (bobbin lead) and double drive are also used.

While the SB works well, it has some drawbacks. One is the fact that each time the bobbin is removed, it is necessary to back off the tension, remove the band, change the bobbin and recalibrate the tension. Another is that tension is affected by temperature and humidity and sometimes requires frequent adjustments. It also prevents the flyer arms from overhanging the end of the bobbin, possibly limiting bobbin fill on the distal end. One more is finding the best material to replace it with when it breaks and then tying all the knots properly. Cotton string, hemp, mono-filament and even dental floss are commonly used; a lark’s-head on a bight knot and other fun fastening techinques are often involved.

In building several eSpinners over the last decade or so, I wanted something better. So last winter I started looking into using magnetic force to do the braking. It turns out that using Lenz forces resulting from eddy currents for adding drag and braking has been done for centuries. Jean Bernard Foucault discovered the principle in the early 1800s and eddy current brakes were in commercial use by the 1890s. Today magnetic eddy current brakes are commonly used in light rail trains, roller coasters, rowing machines, and even many bait casting fishing reels.

The premise is quite simple and is documented quite well on the web (see references below). In Fig. 1, when electrically conductive disc D rotates in the presence of a magnetic field (B), eddy currents (red lines) are generated which produce a magnetic field. This magnetic field opposes the flux (Lenz's law) from the magnet (N pole shown) creating drag. Aluminum is most often used for the disc and neodymium magnets work well. The magnets can be on one side of the disc or both.

Eddy Current Disc

Fig. 1 The basic idea of a single sided eddy current brake. - Wikipedia

While this Lenz force can be used to slow down nearly anything, it will in this case be used to replace that pesky piece of string slowing down bobbins everywhere. The references are loaded with optimal magnet arrangements including Halbach arrays of many magnets to just single magnets. Halbach arrays which were first used in linear accelerators are an arrangement of five or more magnets (Fig. 2) resulting in a unidirectional magnetic force. Studies show these to be the most effective for Lenz braking. But for slowing down a bobbin enough to get good draw-on, we don't need to get quite so fancy.

Halbach Array

Fig. 2 The basic Halbach magnet array. The arrow represents the north pole of the magnet. - LWM

A single magnet will provide acceptable draw-in, but it has as much force on the opposite side of the magnet which may be a problem. By arranging two or more magnets with alternating poles and a soft iron back, the force can be concentrated in one direction, similar to a Halbach array (which doesn't need the back-iron). In the example below, I am using just two magnets. The electrically conductive disc needs to be mechanically attached to the bobbin (Fig. 3). In my early efforts, I used an 1/8 inch aluminum disc attached with carpet tape to the bobbin. My current designs are using all aluminum bobbins.

The adjustment mechanism is another consideration. The Lenz force vary with the inverse square of the magnetic field/distance, so although using a screw adjuster with the magnet on the end would work, the adjustment would not be very linear. Instead, I vary the angle of the magnets with respect to the rotating disc. Maximum braking occurs when the magnets are parallel to the disc, reducing more or less linearly to near zero when they are at a right angle to the disc.

Early Lenz brake

Fig. 3 In this early prototype, I use a 1/4" aluminum tube to rotate the magnets from parallel to 90 degrees. The 4.5" diameter, 1/8" thick aluminum disc is attached to the bobbin with carpet tape. When parallel to the disc, the magnets are about 3/16" away. The result is a no-fuss, set-and-forget tensioning mechanism.

In testing over the past year, the Lenz brake has proven to be very smooth and reliable. It tends to be mostly a non-touchy "set and forget" process, requiring increasing the braking slightly as the bobbin starts getting full. When replacing the bobbin, nothing needs to be hooked back up; just start spinning. Yarn-lock is very smooth and precise. One difference which needs to be considered, is that Lenz forces become very small at low speeds. While this doesn't affect treadle wheels, it could affect eSpinners, especially with full bobbins (more mass). The result is the lack of draw-on just before the flyer stops which might cause a minor backlash. This can usually be compensated for by extending the stop-ramp time, increasing the brake setting or just technique. The reason for this is the non-linear action of the typical Scotch Tensioner string caused by slight stretching of the string due to heating when it's slipping. As the bobbin slows, the string cools and snugs up somewhat, increasing the braking as the bobbin comes to a stop.

I encourage spinners to explore Lenz force brakes and enjoy the benefits of a simpler and more stable approach to bobbin tensioning. While I've shown just permanent magnet examples, the principle works the same with electromagnets which offer the potential for closing the loop. Add-on kits could be made and marketed so the spinning community could improve on the simple designs shown here. I have been using these magnetic tensioners on my designs for an affordable advanced eSpinner I've been working on (and off) over the past year and am very pleased with their smooth and predictable behaviour.

Please use the Contact link above if you have any questions.

(1) Eddy Current Braking Study for Brake Disc of Aluminium, Copper and Zink, Baharom, et al

(2) Experiments with eddy currents: the eddy current brake, Gonzalez

(3) Permanent Magnet Electrodynamic Brakes Design Principles and Scaling Laws, Thompson

(4) Eddy current brake, Wikipedia

Aluminum discs - Metal Remnants, Inc.

Neodymium Magnets - K&J Magnetics, Inc.

Jerry Coopmans is a retired engineer with a warm spot in his heart for weavers and spinners. He created back in '99 with the intention of providing ad-free no-cost resources for fiber artists everywhere.

Creative Commons License
Toss Out Those Strings by Jerry Coopmans is licensed under a Creative Commons Attribution 4.0 International License. 11/2015