Phenolic Tube
Phenolic tube — in paper phenolic grades — is the cost-efficient electrical insulating sleeve for transformer coil winding forms, relay bobbin tubes, and insulating conduit in power distribution and industrial electrical equipment where mechanical loads on the sleeve wall are light and voltage isolation is the primary performance requirement.
TL;DR — Phenolic Tube Quick Facts
| Property | Value |
|---|---|
| Typical OD range | 1/2" to 12" (custom configurations available) |
| Minimum wall thickness | 1/8" (thin-wall); 1/4" recommended for structural use |
| Standard length | 4 ft (48") |
| Wall tolerance | ±0.010" on wall thickness |
| Color | Natural tan/brown |
| Lead time | 1–5 days (standard OD/wall); 2–4 weeks (custom) |
| Price tier | $ — lowest cost phenolic tube form |
Standard Sizes — Phenolic Tube
Paper phenolic tube is produced by spirally winding kraft paper plies impregnated with phenolic resin around a mandrel, then curing under heat and pressure. This produces a cylindrical form with circumferentially oriented paper plies:
| Parameter | Range |
|---|---|
| OD | 1/2" – 12" (typical range; confirm larger sizes) |
| Wall thickness | 1/8" minimum (electrical-only duty); 1/4" for structural |
| ID (derived) | OD minus 2 × wall |
| Length | 4 ft standard; longer custom lengths available |
| Wall tolerance | ±0.010" |
| OD tolerance | +0.031" / −0.000" for most configurations |
Common stocked configurations: 1", 1½", 2", 3", 4", 6" OD with 1/8" and 1/4" walls. Confirm stock at time of inquiry — paper phenolic tube configurations vary by distributor.
Paper phenolic tube is available in NEMA Grade X (general) and Grade XX (moisture resistant). For applications in humid environments or near water, specify Grade XX.
Properties Relevant to Tube Form
Paper phenolic tube's spiral-wound construction produces a tube with good hoop strength (against internal pressure and OD compression) but lower axial and transverse shear strength than fabric-reinforced phenolic tube. These tubes are engineering insulating sleeves, not load-bearing bushings.
| Property | Grade X | Grade XX |
|---|---|---|
| Hoop/Compressive Strength (wall) | 20,000–25,000 psi | 20,000–25,000 psi |
| Flexural Strength (axial) | 10,000–12,000 psi | 9,500–11,500 psi |
| Dielectric Strength (wall) | 300–400 V/mil | 350–450 V/mil |
| Water Absorption (24 h, wall) | 1.0–2.0% | 0.5–1.0% |
| Continuous Use Temperature | 230°F (110°C) | 230°F (110°C) |
| Density | 1.30–1.35 g/cc | 1.30–1.35 g/cc |
Paper phenolic tube is significantly lighter than fabric phenolic tube at the same ID/OD combination, which matters for hanging or suspended installations where tube weight adds to structural loads.
Critical limitation: Do not use paper phenolic tube as a load-bearing sleeve bearing or bushing. The paper ply structure does not tolerate the sustained radial compressive loads of bearing service — use cotton phenolic or canvas phenolic tube for those applications.
For the full property datasheet, see Phenolic Paper properties.
Typical Applications — Tube Form
Paper phenolic tube applications are concentrated in electrical equipment manufacturing and power distribution:
Transformer coil winding forms — The largest single application for paper phenolic tube. The tube serves as the bobbin or core form around which the primary and secondary windings are wound in distribution transformers, instrumentation transformers, and current transformers. Typical Grade XX tube — the moisture exposure inside a potted transformer during impregnation and service justifies the improved water resistance.
Relay and contactor coil bobbins — Small-diameter (1/2"–2") paper phenolic tube cut and machined into coil formers for relay coils, solenoid coils, and contactor coil assemblies. The tube provides structural support for the magnet wire winding and isolates the winding from the relay frame.
Electrical conduit and wire pass-through sleeves — Paper phenolic tube sections used as insulating pass-through sleeves through panel walls, barrier plates, and isolation panels, providing electrical isolation while protecting the wire insulation from abrasion at the panel penetration.
Bus bar insulating sleeves — Sections of Grade XX tube sleeved over bus bar bolts and fasteners in switchgear assemblies, preventing fault currents from tracking through hardware to grounded enclosure surfaces.
Insulating couplings and joiners — Short sections of paper phenolic tube as insulating unions between metallic piping sections in cathodically protected systems and between metal shafting segments where electrical isolation is required.
PCB standoff tubes — Older industrial panel assemblies use paper phenolic tube sections as printed circuit board standoffs, maintaining accurate board-to-board spacing and electrical isolation in control racks.
Potting and encapsulation forms — Paper phenolic tube used as a disposable form into which epoxy or polyurethane potting compound is cast around electronic assemblies; the tube bonds to the potting compound and becomes part of the finished assembly.
Machining Notes — Tube
Paper phenolic tube machines more easily than fabric-reinforced phenolic tube, but the spiral paper construction requires attention to support during machining:
OD turning: Sharp HSS or carbide tooling, 300–500 SFM, light feed (0.002–0.005 IPR). Paper phenolic tube walls can collapse inward under excessive cutting force; use a mandrel support inside the tube bore for ODs below 3" with walls under 1/4".
Bore finishing: Paper phenolic tube bores can be finished by boring or reaming for close-fit sleeve work. Single-point boring at 0.001" final pass; spring-back is minimal (0.0005"–0.001") versus fabric grades. Reaming with a fluted reamer produces excellent bore finish (Ra 16–32 μin) in paper phenolic.
Cutting to length: A sharp parting blade or a cutoff wheel on a lathe produces clean, square ends. A fine-tooth hand saw (hacksaw) with a miter box is suitable for low-volume cut-to-length work on smaller ODs. Support the tube at both sides of the cut.
Threading: Paper phenolic tube OD can be externally threaded with a die or single-point threading; the wall must be at least 2× the thread pitch thick to avoid stripping. Internal threading of paper phenolic tube is less common — thin walls and the spiral winding direction can cause inter-ply delamination under tap torque.
End facing: Face both tube ends in a lathe using a facing tool to ensure squareness. Paper phenolic tube ends from the saw or cutoff can be slightly non-square due to the spiral winding direction.
Dust and fume: Phenolic resin dust and fume are respiratory hazards. NIOSH P100 respirator; localized exhaust ventilation; no open flame.
See Phenolic Paper Machining Guide.
Standards and Compliance
| Standard | Scope |
|---|---|
| NEMA LI 1 | Grades X and XX for paper phenolic tube |
| ASTM D709 | Standard specification for laminated thermosetting materials |
| MIL-I-24768/2 | Type PBG-XX — moisture-resistant paper phenolic (tube configurations) |
Request a Certificate of Conformance citing NEMA LI 1 Grade X or XX on all orders. Confirm documentation requirements for MIL-spec applications at order entry.
Get a quote on phenolic tube — NEMA Grade X or XX, specify OD, ID, and length
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