Yoke lifting power check

A yoke is an electromagnet or permanent magnet whose legs are placed on the surface, driving flux from one pole through the part to the other. The field between the poles is longitudinal to the line joining them, so a yoke reveals discontinuities running roughly perpendicular to that line. Two placements at right angles are needed for full coverage.

There is no current setting to calculate, so the codes verify the yoke by what it can hold: the force needed to pull a steel block off the poles. Lift force is a direct proxy for the flux the yoke can push through a joint, since holding force rises with the square of the flux density in the pole faces. If a yoke can hold a specified weight it is producing at least a defined flux, and the field in the part will be adequate for the geometry it was qualified on.

The two limits differ because of how the fields behave. An AC yoke produces a field concentrated in a thin skin, which is excellent for surface-breaking discontinuities and needs less brute flux, so 4.5 kg (10 lb) is enough. A DC or permanent magnet drives flux deeper into the section and is used where slightly subsurface indications matter, so it must demonstrate 18 kg (40 lb).

The number only means something at a stated pole spacing. Flux leaks out of the magnetic circuit as the legs are opened, so lift force falls sharply as spacing increases. The test must be done at the maximum spacing that will be used in the examination - a yoke that lifts 5 kg at 75 mm may not lift 4.5 kg at 150 mm. Lifting power is verified at the intervals set by the code and again whenever the yoke has been dropped, repaired or has been out of service.

AC yoke:              lift >= 4.5 kg (10 lb)
DC or permanent magnet: lift >= 18 kg (40 lb)
The lift test must be performed at or above the maximum pole spacing to be used
Margin = measured lift - required lift