YOKE MACHINE – FULL DESCRIPTION

A Yoke Machine (commonly called an MPI Yoke) is a portable, hand-held magnetizing device used in Magnetic Particle Inspection to detect surface and near-surface cracks in ferromagnetic materials. It creates a localized magnetic field between its two legs, allowing magnetic particles to gather at discontinuities.

Yoke machines are widely used in weld inspection, structural steel, pipelines, casting, and fabrication industries.

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✔ What a Yoke Machine Does

A yoke machine is used to:

• Apply a controlled magnetic field to a small area

• Detect cracks, laps, seams, porosity, weld defects

• Perform MPI using dry powder or wet visible/fluorescent particles

• Inspect parts that are large, heavy, or difficult to move

• Conduct field MPI where bench units cannot be used

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✔ Construction of a Yoke Machine

1. Electromagnetic Coil (Inside Housing)

   • Energizes the legs to create a magnetic field

2. Two Pivoting Legs

   • Adjustable to fit various shapes, surfaces, and weld profiles

3. Switch / Trigger

   • Activates the magnetic field

4. Power Source

   • AC yoke → 220V/110V supply

   • DC/Battery yoke → portable inspection

5. Housing

   • Durable, insulated, and shock-resistant

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✔ Types of Yoke Machines

1. AC Yoke (Alternating Current)

• Strong surface sensitivity

• Ideal for detecting fine surface cracks

• Better particle mobility

• Limited penetration depth

• Common type used in weld inspection

2. DC Yoke (Direct Current)

• Deeper magnetic penetration

• Suitable for detecting sub-surface defects

• Less effective for very fine surface cracks

3. Battery-Operated Yoke

• Portable and convenient for field work

• No external power needed

• Usually DC output

4. Dual Mode Yoke (AC/DC)

• Switchable AC and DC

• Highly versatile

• Used in advanced NDT operations

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✔ Working Principle

1. When switched ON, current flows through the internal coil.

2. The yoke legs become magnetized, forming a magnetic field between the poles.

3. When magnetic particles are applied:

   • Defects interrupt the field

   • Leakage field forms at the crack

   • Particles gather, showing the defect clearly

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✔ Key Technical Specifications (Typical)

Lifting power is required per ASTM E1444 / ISO 9934.

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✔ Applications

• Weld inspection (structural, pressure vessels, pipelines)

• Railways & heavy fabrication

• Castings & forgings

• Shipbuilding

• Powerplants

• Automotive & aerospace maintenance

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✔ Advantages

• Portable and lightweight

• Very versatile for field work

• Easy to use and economical

• Effective on irregular shapes

• Works with both dry and wet particles

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✔ Limitations

• Limited to small/localized areas

• Cannot magnetize large components fully

• Needs frequent repositioning

• Generates heat if used continuously

• Requires proper lifting test verification

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If you want, I can also provide:

📌 ALT Image Description of Yoke Machine
📌 Technical Specification Sheet (format ready for documentation)
📌 Yoke vs Coil vs Bench-type MPI comparison
📌 Working diagram / labeled drawing

Just tell me!