Phenolic Glass Tube

Phenolic glass tube — manufactured from woven glass cloth with a phenolic resin matrix — is the high-performance thermoset sleeve form for structural transformer coil formers, high-voltage isolation tubes, and heavy-duty insulating sleeves that must combine mechanical strength above 40,000 psi flexural, arc resistance above 120 seconds, and a 300°F (150°C) service temperature.

TL;DR — Phenolic Glass Tube Quick Facts

Standard Sizes — Phenolic Glass Tube

Per NEMA LI 1:

Glass-phenolic tube is a more specialized product than paper or fabric phenolic tube; inventory is typically maintained at fewer stock points. For critical delivery requirements, confirm availability at quotation.

Properties Relevant to Tube Form

Glass-phenolic tube achieves mechanical properties far above paper or fabric phenolic tube, while the 300°F service temperature and high arc resistance open applications that other phenolic tubes cannot serve:

The combination of 300°F service temperature and high hoop strength makes glass-phenolic tube the correct choice for power transformer coil forms where winding self-heating is sustained and the coil form must not creep under winding tension at elevated temperature.

For the full datasheet, see Phenolic Glass properties.

Typical Applications — Tube Form

Glass-phenolic tube serves demanding electrical and structural applications where paper or fabric phenolic tubes are thermally or mechanically insufficient:

Power transformer structural coil forms — Glass-phenolic tube as the mechanical core form for medium and large power transformer windings (above 25 kVA to several MVA range). The tube must support wire winding tension during manufacture, then resist sustained compression from winding clamping hardware in service — all at 300°F. Glass-phenolic meets this requirement where paper phenolic tube would creep.

High-voltage isolation tubes in switchgear — Glass-phenolic tube sections used as insulating distance tubes in medium-voltage (4.16–34.5 kV) switchgear, holding bus bar insulators at specified clearance distances while providing structural rigidity and arc resistance at the tube surface.

Aerospace electrical isolation tubes — Glass-phenolic tube to MIL-I-24768/4 (Type GSG) in aircraft electrical system insulating spacers, antenna support tubes, and airframe electrical isolation sleeves that must withstand combined mechanical load and high-voltage isolation in the aircraft environment.

Industrial rotating machine insulating rings — Glass-phenolic tube segments machined into insulating rings for large motor and generator rotor assemblies where the rotor retaining ring requires both structural strength and electrical isolation between rotating winding and shaft.

Chemical plant high-voltage equipment sleeves — Glass-phenolic tube as insulating sleeves in chemical plant electrical equipment where the tube must resist vapors of acids, caustics, and solvents that would attack paper phenolic tube more rapidly.

High-voltage capacitor and reactor coil forms — Glass-phenolic tube as the winding form for high-voltage capacitors, series reactors, and coupling capacitors in power substations and utility switchyards where service voltage can reach 230 kV and mechanical stability is critical.

Machining Notes — Tube

Glass-phenolic tube requires the same carbide/PCD tooling used for glass-phenolic sheet and rod:

OD turning: Solid-carbide or PCD tools, 500–700 SFM (carbide), 700–1,000 SFM (PCD). Use steady rest support for ODs above 4" and for wall thicknesses below 1/2" to prevent chatter from tube wall resonance.

Bore finishing (ID): Carbide boring bar with positive-rake insert; spring-back 0.001"–0.002". For final bore fits, hone after boring — 180 to 240 grit produces Ra 32–63 μin in glass-phenolic. Use a mandrel support on the OD while honing to prevent ovalization under honing stone pressure.

Cutting to length: Carbide parting blade, 0.080"–0.125" wide. Support tube close to parting zone. Glass-phenolic tube can crack at the parting cut if the tube is not well-supported; use a steady rest or V-block at both sides of the parting zone.

End facing: Face both ends to clean up the parting tool marks and produce a flat, square reference face. Facing at 600–800 SFM with carbide insert; light feed 0.001"–0.002" IPR.

Dust and fiber control: HEPA-rated exhaust ventilation at the cutting zone; NIOSH P100 respirator required at all times when machining glass-phenolic tube. Glass fiber particulate from machining is a serious inhalation hazard. Wet machining (flood coolant) suppresses airborne fiber but creates a slurry — dispose of coolant per local regulations.

See Phenolic Glass Machining Guide.

Standards and Compliance

Request a Certificate of Conformance citing NEMA LI 1 Grade G5. For MIL-spec applications, confirm QPL qualification of the manufacturer at quotation.

More related guides

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

Industries

• Aerospace Plastics
• Electrical Power & Distribution

Compare materials

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

Other phenolic glass forms

• Phenolic Glass Rod
• Phenolic Glass Sheet