Portable Magnetisation Equipment

Outcomes

After this chapter, the learner should be able to:

• select among permanent magnets, yokes and prods;
• define the effective area and favoured discontinuity direction for each;
• identify equipment-specific verification checks;
• control contact, arcing and surface damage; and
• explain specialised magnetic-rubber use.

8.1 Permanent U-shaped magnets

A suitable U-shaped permanent magnet produces a local longitudinal field between its poles without an electrical supply. It can be valuable where electrical equipment is restricted, access is remote, or a simple local surface test is needed.

Limitations include fixed field strength, difficult placement and removal, attraction of loose ferromagnetic debris, uncertain contact on irregular surfaces and restricted coverage. The magnet must be handled so it does not snap onto the component or trap fingers. A field or performance demonstration is required by the technique; a permanent label stating a nominal pull is not enough.

The favoured discontinuity lies broadly transverse to the pole-to-pole field. Two placements approximately 90 degrees apart are needed for multi-directional coverage.

8.2 Electromagnetic yokes

A yoke uses a coil around a high-permeability core to produce a longitudinal field between adjustable poles. Laminated cores reduce eddy-current loss in AC units. AC yokes are highly effective for surface discontinuities. DC or rectified yokes may extend response below the surface but can leave stronger residual magnetism and may be restricted by the procedure.

Before use, check:

• body, cable, switch and strain relief;
• pole faces and articulation;
• electrical safety status;
• stated pole-spacing range;
• lift or performance verification;
• field direction and effective area; and
• duty cycle.

The supplied source gives a nominal AC yoke lift check of 4.5 kg at the recommended pole spacing. Confirm the governing requirement, block geometry and equipment instructions during approval; do not transfer the value to DC equipment without a controlling basis.

8.3 Yoke scan discipline

Both poles must make stable contact. Curved or rough surfaces reduce magnetic coupling. Keep the pole-to-pole line consistent with the technique sketch. Apply media without moving the yoke, inspect the area, then lift and reposition.

A lift check demonstrates that the yoke can produce mechanical attraction under the test arrangement. It does not alone prove that every point on a coated, curved weld has adequate direction and sensitivity. Overall system performance and field demonstration remain important.

8.4 Prods

Prods pass current through the component between two contact points, creating a distorted circular field around each current path. A linear discontinuity along the line joining the prod tips is generally favoured because it crosses the local circular field.

Advantages include adjustable current, local examination and portability when used with a suitable power unit. Major hazards and limitations include:

• contact arcing and metallurgical burns;
• poor contact on scale, paint or curvature;
• exclusion zones close to each prod;
• high current and heavy cables;
• complex field distribution;
• need for overlapping two-direction placements; and
• restrictions on critical or finished surfaces.

Use soft, clean contact tips suited to the component. Establish full contact before energising and release current before lifting. Prods should not be used where the controlling document prohibits arc strike risk.

8.5 Prod field assessment

The supplied source uses formulae based on target tangential field H and prod spacing d:

• rectangular zone: I = 2.5 x H x d
• circular zone through the contacts: I = 3 x H x d

In that formula set, I is amperes for the stated current basis, H is entered in kA/m and d in mm. The rectangular relationship is described for spacings up to 200 mm, and both relationships assume a sufficiently large surface radius. These are training formulae requiring formal verification.

Example: For a circular zone, H = 2 kA/m and d = 150 mm, using the source-model RMS basis for time-varying current:

I = 3 x 2 x 150 = 900 A RMS

The calculated value is a starting setting. Confirm actual field/performance and component restrictions.

8.6 Clamps and cable contacts

Current-flow clamps can provide better contact than handheld prods where component geometry permits. They still pass current through the part and retain the same burn, current-path and circular-field considerations. Confirm that the return path does not create unplanned fields or heat vulnerable joints.

8.7 Magnetic rubber

Magnetic rubber is a specialised recording technique in which ferromagnetic particles are suspended in a curable rubber compound. The compound is introduced to a difficult-access surface or cavity while an applied or residual field is present. After curing, the flexible replica is removed and examined for particle accumulations.

It can preserve a three-dimensional record from bores or inaccessible geometry. Limitations include long processing time, material compatibility, cure control, lower immediacy, and the need to maintain field and position as specified. It is a specialist process and should not be improvised from general casting rubber.

8.8 Overall system performance

For portable systems, a representative component or approved test piece with known discontinuities provides the strongest end-to-end check. It challenges equipment, field, media, timing and viewing together. The check piece must be clean, stable, traceable and free from a retained indication left by the previous check.

8.9 Common traps

• Applying a yoke with one pole partly off the component.
• Treating a lift check as proof of complete examination sensitivity.
• Energising prods before both tips are firmly seated.
• Using prod spacing outside the approved formula range.
• Ignoring the current return cable and its unintended field.
• Assuming a permanent magnet cannot lose useful performance.

Chapter summary

Portable equipment provides flexible local magnetisation but demands disciplined placement and verification. Yokes and permanent magnets create longitudinal fields; prods pass current through the component to create local circular fields. The Level 2 technician must manage effective area, direction, contact, arcing and end-to-end performance.