Mil-I-24768 & NEMA Specifications for Cotton Phenolic Rolled (convolute wound) Tubes
Only 2 of the 4 cotton phenolic grades identified by the Mil-I-24768 and NEMA catalogue of materials have specifications for convolute wound (rolled ) tubes and they only have a limited number of performance properties (axial compression, density & moisture limitations and for LE dielectric strength).
Mil-I-24768/16 type FBG and NEMA C is a 10 oz. /sq. (50x50 thread count) cotton fabric impregnated with a phenolic resin formulation. Grade C is considered a mechanical grade whose strong suits are high impact strength and the lowest cost of the cotton laminate family. It is a popular choice for gears, bearings, marine rudder bearings, mechanical sleeves and transformer insulation.
Mil-I-24768 /13 type FBE and NEMA LE is a 4oz/sq. yd. (76x76 thread count) also impregnated with a phenolic resin formulation. Unlike grade C, both the Mil-I/13 & LE specification have a dielectric strength requirement.
The higher the thread count: the smaller the yarn diameters: the lesser the volume for water molecules: = better dimensional stability which makes it ideal for close tolerance fabricated parts. Smaller diameter yards also promotes a higher resin content that contributes to its very good dielectric strengths and low dielectric loss properties. Worth noting is Mil-I-24768/13 and NEMA LE mechanical properties are similar to grade C but with the enhancement noted above.
A post baking process for all cotton grades of sheet and tubes, will enhance dimensional stability, lower maximum moisture pickup properties and improve dielectric strengths.
Grade LE tubes are also manufactured in a 102x102 finer weave fabric with even better dielectric strength and dimensional stability and is used primarily for ball bearing retainer applications.
The temperature index for all cotton phenolic grades with up to .062” wall thickness is 85°C mechanical & electrical. The index for all grades with a wall thickness greater than .062” is 125°C mechanical and 115°C electrical.