The armature tube (also called the guide tube, stop tube, or solenoid tube) is a thin-walled, non-magnetic tube that surrounds the plunger and separates the solenoid coil from the valve fluid. It must be thin enough to allow maximum magnetic flux to pass from the coil to the plunger, yet strong enough to withstand internal pressure. Manufacturing this component requires specialized thin-wall tube processing capabilities.
Armature Tube Functional Requirements
- Minimal Wall Thickness: Typically 0.15–0.50 mm to maximize magnetic coupling between coil and plunger.
- Non-Magnetic Material: Must be non-magnetic (paramagnetic) to avoid shunting the magnetic field. Austenitic stainless steels are required.
- Precision ID Surface: The inner diameter directly guides the plunger. Surface finish Ra 0.4 μm and consistent ID over the full length.
- Concentricity: OD-to-ID concentricity within 0.02 mm to ensure uniform magnetic gap around the plunger.
- Hermetic Seal: The closed end must be leak-tight. Helium leak rate < 1×10⁻⁹ mbar·L/s.
- Pressure Rating: Must withstand the valve's rated working pressure (typically 100–350 bar).
Material Selection
| Material |
Grade |
Magnetic Permeability |
Corrosion Resistance |
Weldability |
Availability (Thin-Wall Tube) |
| Stainless Steel |
304, 304L |
< 1.05 (non-magnetic) |
Good |
Excellent |
Readily available as seamless |
| Stainless Steel |
305 |
< 1.02 (very low) |
Good |
Good |
Moderate availability |
| Stainless Steel |
316L |
< 1.05 |
Excellent |
Excellent |
Readily available |
| Stainless Steel |
304H |
< 1.05 |
Good |
Good |
Limited in thin-wall |
Grade 305 stainless steel is often preferred for armature tubes because it remains non-magnetic even after cold working (unlike 304, which can develop slight magnetism when drawn).
Tube Stock Manufacturing
The armature tube starts as precision-drawn tubing:
Seamless Tube Drawing Process:
Extruded or pierced hollow → Cold pilgering → Intermediate annealing →
Cold drawing (multiple passes) → Final annealing → Straightening →
Ultrasonic inspection → Cut to length
Key Parameters:
- Drawing reduction per pass: 15–30%
- Wall thickness tolerance: ±0.03 mm (precision drawn)
- OD tolerance: ±0.02 mm
- Straightness: 0.5 mm/m
- Surface finish (as-drawn ID): Ra 0.8–1.6 μm
Machining Operations
Step 1 — Cut to Length:
Seamless tubes are cut to blank length with ±0.1 mm tolerance. End flatness within 0.05 mm.
Step 2 — End Face Preparation:
Both ends are faced on a CNC lathe. The open end is prepared for welding and the closed end (if integral) is formed.
Step 3 — ID Honing or Burnishing:
The inner diameter must achieve Ra 0.4 μm surface finish for smooth plunger movement:
| Method |
Stock Removal |
Achievable Ra |
Cycle Time |
Best For |
| Diamond honing |
0.01–0.03 mm |
0.2–0.4 μm |
30–60 sec |
High-volume, tight specs |
| Ball burnishing |
0.005–0.01 mm |
0.1–0.2 μm |
10–20 sec |
Very thin walls (< 0.3 mm) |
| Roller burnishing |
0.005–0.02 mm |
0.2–0.4 μm |
15–30 sec |
Medium production |
Ball burnishing is preferred for ultra-thin wall tubes (< 0.3 mm) because it imparts compressive residual stress without risk of buckling.
Step 4 — OD Machining:
The outer diameter is turned or centerless ground to achieve:
- OD tolerance: ±0.02 mm
- Surface finish: Ra 0.8 μm
- Concentricity to ID: Within 0.02 mm
Step 5 — End Closure (Welding):
The closed end of the tube is sealed by one of these methods:
| Method |
Joint Strength |
Leak Rate |
Cycle Time |
Cost |
| Laser welding |
90% of base metal |
< 1×10⁻¹⁰ mbar·L/s |
2–5 sec |
High (equipment) |
| Orbital TIG welding |
85% of base metal |
< 1×10⁻⁹ mbar·L/s |
10–20 sec |
Medium |
| Resistance welding |
70% of base metal |
< 1×10⁻⁸ mbar·L/s |
1–2 sec |
Low |
| Swaging (mechanical closure) |
— |
< 1×10⁻⁶ mbar·L/s |
5–10 sec |
Low |
Laser welding is the preferred method for high-performance solenoid valves due to its combination of speed, precision, and reliability. Typical parameters for fiber laser welding:
- Power: 200–500 W (pulsed or continuous)
- Welding speed: 0.5–2.0 m/min
- Shielding gas: Argon, 15–20 L/min
- Penetration: 60–80% of wall thickness
- Heat input: Controlled to prevent ID surface damage
Step 6 — Annealing (Stress Relief):
After welding, the assembly is annealed at 400–500°C for 30–60 minutes in a hydrogen atmosphere to:
- Relieve residual stress from drawing, machining, and welding
- Restore non-magnetic properties in the heat-affected zone
- Improve corrosion resistance
Critical Quality Parameters
| Parameter |
Specification |
Measurement Method |
| Wall thickness |
±0.03 mm |
Ultrasonic gauge, micrometer |
| ID diameter |
H7 tolerance |
Air gauge |
| ID surface finish |
Ra 0.4 μm |
Profilometer |
| OD-to-ID concentricity |
0.02 mm |
Dial indicator |
| Straightness |
0.05 mm / 100 mm |
V-block test |
| Closed-end flatness |
0.02 mm |
Optical flat |
| Leak rate |
< 1×10⁻⁹ mbar·L/s |
He mass spectrometer |
| Magnetic permeability |
< 1.05 |
Permeability meter |
Common Manufacturing Defects
| Defect |
Cause |
Prevention |
| ID scratch from weld spatter |
Weld penetration control |
Use back-purge gas, reduce power |
| Ovality after welding |
Thermal distortion |
Use copper heat sink fixture |
| Wall thickness variation |
Inconsistent tube stock |
Specify tighter incoming tube tolerance |
| Magnetic zone near weld |
Ferrite formation in HAZ |
Solution anneal after welding |
| ID surface roughness insufficient |
Worn burnishing tool |
Replace tool every 20,000 parts |
Summary
Armature tube manufacturing is a specialized process combining precision tube drawing, thin-wall ID finishing, and hermetic laser welding. The key challenge is maintaining wall thickness uniformity within ±0.03 mm and concentricity within 0.02 mm while keeping tube walls as thin as 0.15 mm for magnetic performance. Laser welding provides the leak-tight closure required for high-pressure solenoid valves, with helium leak rates below 1×10⁻⁹ mbar·L/s.
Need precision armature tubes for your solenoid valve production? Contact us with your specifications (tube OD, wall thickness, length, and pressure rating) for a manufacturing assessment and quotation.
