Phenolic Glass Silicone Tube

NEMA G7 silicone-glass tube is the highest-temperature-rated sleeve form in the commercial laminate family — combining the circumferential hoop strength of glass-cloth winding with a silicone resin binder that retains 70–80% of room-temperature mechanical properties at 150°C, and provides arc resistance exceeding 180 seconds for coil forms and isolation tubes in Class H insulation systems and high-voltage switching equipment.

TL;DR — NEMA G7 Tube Quick Facts

Standard Sizes — NEMA G7 Tube

G7 tube is a specialty product — confirm availability and lead time at quotation for any non-standard configuration. For critical applications, specify the tube OD, ID, length, and NEMA LI 1 Grade G7 on the purchase order with a documentation requirement.

Properties Relevant to Tube Form

G7 tube's defining characteristics — 180°C service temperature and high arc resistance — are sustained in tube form as in sheet and rod:

Hoop strength of G7 tube is modestly lower than G5 glass-phenolic due to the silicone resin's lower fiber-to-matrix adhesion. For applications where maximum hoop strength at ambient temperature is the driver and thermal requirements are 150°C or below, G5 glass-phenolic tube may be more appropriate and lower cost.

For the full datasheet, see Phenolic Glass Silicone properties.

Typical Applications — Tube Form

G7 tube targets the highest-temperature coil form and isolation sleeve applications in the laminate family:

Class H motor coil winding forms — The primary application. Large AC motors and generators in IEC Class H insulation systems require the coil winding bobbin tube to carry the Class H thermal rating. G7 tube is the standard choice for motor coil forms in motors above 100 hp where thermal loading pushes winding temperatures to the Class H limit. Paper phenolic tube (230°F/110°C) and even G5 glass-phenolic tube (300°F/150°C) cannot serve Class H systems without thermal derating.

Power transformer coil forms at sustained thermal loading — Oil-filled transformers operating in emergency overload cycles or in high-ambient environments (desert, tropical) require the coil form tube to withstand repeated excursions to 150–180°C without creep or dielectric degradation. G7 tube is the thermally safe choice.

High-voltage arc-gap isolation tubes — G7 tube sections used as insulating bodies in vacuum interrupter mounting assemblies, SF6 interrupter supporting tubes, and high-voltage link insulator tubes where arcing at the tube surface during switching must not carbonize the tube to the point of tracking failure. The 180–240 sec arc resistance provides far greater safety margin than G5 or G10 tubes.

Aerospace nacelle and engine bay coil forms — Aircraft jet engine nacelle electrical assemblies include coil forms and insulating sleeves that see sustained elevated temperatures. G7 tube to MIL-I-24768/7 (Type GSG-S) is specified for Class H-rated aircraft electrical components.

Industrial process heating equipment coil forms — Industrial resistance heating elements wound on G7 tube as the coil former in immersion heaters, infrared heaters, and process heating systems where the coil form temperature exceeds the G5 glass-phenolic rating.

High-frequency high-power RF amplifier coil forms — G7 tube as the winding form for RF power amplifier coil inductors where the combination of RF heating, high RF voltage, and elevated ambient temperature exceeds what paper phenolic or G5 glass-phenolic can manage.

Machining Notes — Tube

G7 silicone-glass tube requires the same tooling upgrades as G5 glass-phenolic tube — carbide or PCD mandatory:

OD turning: Solid-carbide or PCD tools, 450–650 SFM (carbide). Use steady rest support for ODs above 3" and wall thicknesses below 3/8". G7's silicone resin may produce a slightly different chip morphology than G5 — more powdery and less stringy — which affects chip evacuation from the cutting zone.

Bore finishing: Carbide boring bar; spring-back 0.001"–0.002". Hone to finish for precision ID dimensions: 180–240 grit achieves Ra 32–63 μin. Use expanding mandrel support on OD during honing to prevent OD deflection.

Cutting to length: Carbide parting blade; support tube close to parting zone. G7's slightly lower interlaminar shear strength (silicone resin) versus G5 or G10 means the tube is slightly more prone to inter-ply cracking if the parting blade is not sharp or if the tube is not well-supported during the cut.

End facing: Face both ends for squareness. G7 end faces may show surface porosity from the silicone cure — this is cosmetic and does not affect performance.

Thread cutting: External threading with carbide threading insert; use adequate cutting lubricant at the die to prevent surface-layer tearing. Internal threading feasible for coarse threads in walls above 3/8".

Dust and fume: Silicone pyrolysis products and glass fiber particulate — HEPA ventilation, NIOSH P100 respirator, no open flame or heat source near machining area.

See Phenolic Glass Silicone Machining Guide.

Standards and Compliance

Request a Certificate of Conformance citing NEMA LI 1 Grade G7 on all orders. For Class H motor and transformer applications, request the thermal classification documentation from the manufacturer.

More related guides

Applications

• Electrical Insulation — Phenolic & Laminate Options
• Aerospace Interior Plastics

Industries

• Aerospace Plastics
• Electrical Power & Distribution
• Defense & Military Plastics

Compare materials

• G7 vs. G9 Laminate Comparison
• Phenolic Paper vs. Glass Phenolic

Other NEMA G7 forms

• Phenolic Glass Silicone Rod
• Phenolic Glass Silicone Sheet