Lear Bead Detector Head

(Background of design)

A brief overview of the history of bead detection on magnet wire may provide some insight into the path that Lear followed in the design of the detector, now, with 14 years operation and thousands built.

As long as magnet wire has been manufactured, using an enamel coating as the insulation, the smoothness of the enameled surface has served as a measure of consistency (and perceived quality) of the process. Both the manufacturer and user of the wire have used this as a primary indicator of quality together with other tests such as high voltage continuity (HVC). Unfortunately, HVC insulation tests and some other wire parameters, can be measured in a fairly straightforward manner, this is not the case with beads nor is there a recognized definition of what a bead is.

Without a definition, the bead population, size and distribution, has been found to be an important (although indirect) indicator of the eventual performance of the wire when installed in a motor, transformer, and many other applications. Historically, the detection device, in most cases, has been the pinched fingers of the operator or user of the wire and his judgment. Also, for practical reasons, only a small fraction of the manufactured wire can be tested in this manner, leaving it to the users winding machinery to malfunction due to the beady wire.

Getting to the point of this article, we (Lear) then sought to design an online test system that matched the technique used by operators and users of the wire. It was our thinking that the human performing the test with his fingers was very sensitive to an abrupt change in the surface texture of the wire but insensitive to a longer term diameter change which may have a smooth entry and exit to/ from the bead. For example, a 2 mil high bead distributed over a 10 mil wire length could be felt by the human fingers whereas, a gradual 2 mil change over a one inch length would not be felt and, in fact, should be filtered out of the wire profile measurements.

We were not trying to design a diameter gauge. Been there, done that.

Matching the operators (and users) technique and experience was then the focus of the design for a full time on-line bead detector. This, in our opinion, would also lead to quicker acceptance by avoiding the requirement to get worldwide industry acceptance to a dimensional 3D standard.

For a variety of reasons, including cost, maintenance, and physical size, we decided against optics and other approaches to observe the surface of the wire, but instead, chose to touch the wire surface with small “reeds”.

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