What is the formula for yoke lifting power check?

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. Checks a measured yoke lift against the code minimum - 4.5 kg (10 lb) for AC yokes and 18 kg (40 lb) for DC or permanent magnets - at the pole spacing in use.

What should I check before relying on the result?

The test block must be of known weight and traceable; do not estimate it. Lift force falls steeply as the legs are opened. A test at close spacing does not qualify the yoke for wide spacing. Repeat the lift check after the yoke has been dropped, repaired or taken out of service, and at the interval required by the governing code. A yoke field is longitudinal between the poles. Make a second placement at 90 degrees to cover discontinuities in the other direction. Pole faces must sit flat on the surface. Rocking on a curved or scaled surface breaks the magnetic circuit and the lift result no longer represents the field in the part. AC yokes concentrate flux in a thin surface skin; DC and permanent yokes drive it deeper but are harder to demagnetise afterwards. Verify against the governing edition: ASME BPVC Section V (2023 Ed.), Article 7 - yoke lifting power: at least 10 lb (4.5 kg) for …

Yoke lifting power check — MT Calculator

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.

Worked example

Yoke type	ac
Measured lift	6 kg
Pole spacing at test	150 mm
Pole spacing in use	100 mm
Required lift	4.5 kg
Required lift	10 lb
Margin over requirement	1.5 kg
Measured / required	1.33

An AC yoke held a 6 kg block with the legs opened to 150 mm. The code minimum for an AC yoke is 4.5 kg (10 lb), so the margin is 6 - 4.5 = 1.5 kg and the ratio is 6/4.5 = 1.33. The examination will use a maximum spacing of 100 mm, which is less than the 150 mm test spacing, so the test remains valid for the work. Had the yoke been a DC or permanent type, 6 kg would have failed the 18 kg requirement.

Use at your own risk — verify before you act

These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.