Coefficient of Friction Tester

Determines how easily the wire can be wound into coils

A coefficient of friction tester is a lab instrument used to measure how much resistance there is when one material slides over another. It gives a numeric coefficient of friction (CoF) that describes how “slippery” or “grippy” a material is.

Why CoF matters on magnet wire

For enamelled (magnet) wire, the coefficient of friction is mainly about how easily the wire can be wound into coils without damaging the insulation. The same basic CoF ideas and testers apply, but the test geometry and what you care about are adapted to long, thin, coated conductors instead of flat films.​

During coil winding, the wire is under tension and constantly sliding over guides, formers, and over other turns of wire, so friction directly affects winding speed, filling factor, and insulation damage risk. Lower CoF helps the wire feed smoothly at higher speeds, reduces scraping of the enamel, and maintains dielectric integrity of the insulation.​

Manufacturers often design enamels and overcoats (for example, self‑lubricating topcoats or wax treatments) specifically to reduce CoF and improve “windability.” Technical brochures for magnet wire explicitly quote “low coefficient of friction” as a key feature for high filling factors and less soiling of winding machinery.​

What is tested on enamelled wire

Instead of a flat strip, the “sample” is the round or rectangular magnet wire itself, with its enamel system (base coat, top coat, lubricants). Tests typically focus on: wire‑to‑wire friction like turns rubbing against each other in a coil, important for high slot fill and avoiding insulation wear.​

Some magnet‑wire developments specifically report static friction coefficients, for example values dropping from about 0.13 (non‑lubricated) down to 0.03–0.05 with self‑lubricated enamels, to show improved winding behavior.​

How CoF is measured on magnet wire

The same physical principle applies: CoF is friction force divided by normal force, but the fixtures are adapted to wires. Common approaches include:​

• Static tests: The static coefficient of friction μs is determined by measuring the inclining angle α of a plane when a block begins to slip on the track made from the wire specimen.  The plane is then slowly inclined
  (approximately 1°/1”) until the block slides down the track. At that moment, the angle of inclination α is read from the scale and the static coefficient of friction is calculated.​

• Dynamic tests: The coefficient of friction is defined as μ=Fr/Fn, where Fr is the friction power and Fn is the load applied to the wire. The test wire is moved at a speed of 15m/1’ between a basic plate and pre-loaded sapphires. The sapphires are mechanically connected to a load cell to detect the force of friction. The values detected shall be displayed and recorded at desired intervals.​

Each method yields a coefficient that can be used comparatively (between enamel systems, lubricants, batches) as an industry‑standard way to quantify “slipperiness”​.

How this affects enamel and lubrication choices

Coil designers and wire makers balance several properties at once: abrasion resistance, adhesion of enamel to copper, thermal rating, and CoF. For example, certain polyamideimide or polyester‑imide systems, sometimes with silicone or wax additives, are engineered both for high thermal endurance and low CoF to survive high‑speed winding without insulation cracking or abrasion.​

In practice, a target CoF range is chosen: too high and the wire scuffs, sticks, and limits winding speed; too low and layers may shift too easily under vibration or mechanical stress. By measuring CoF on enamelled wire with appropriate fixtures, manufacturers can tune enamel formulations and surface lubricants to hit that desired processing window.

If you are looking for specific detailed information of the test instruments, please click here:
SST4 Coefficient of Friction tester (static)
SST4-F Coefficient of Friction tester (static for flat wires)
SST5 Coefficient of Friction tester (static/dynamic)

 Please contact our sales department for more information.

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