GB/T 18438-2001 Lifting Magnet Crane Guide

GB/T 18438-2001 is the dedicated standard for lifting magnets used on lifting appliances. It defines the classification, technical requirements, test methods, and inspection rules for these devices. Lifting magnets are load-handling devices used to lift and transport ferromagnetic materials such as steel plates, sections, and scrap metal, and are widely deployed in metallurgical plants and scrap yards.

Lifting Magnet Types and Construction

GB/T 18438-2001 classifies lifting magnets by application and construction as follows: Round lifting magnets (MW series) — with a circular magnet face, these are used for handling irregular ferrous materials such as ingots, billets, and scrap. This is the most widely used type, available in diameters from Φ300 mm to Φ2000 mm across more than 10 specifications. Rectangular lifting magnets (MR series) — with a rectangular magnet face, these are suited for long and flat ferrous materials such as steel plates, structural sections, and rails, with lengths ranging from 500 mm to 3000 mm. Lifting capacity is determined by the magnet face area and the magnetic flux density. Special-purpose lifting magnets — including billet magnets (with multi-pole arrangements to match the elongated shape of billets) and plate magnets (with a flat face and anti-slip raised pads to handle smooth steel plate surfaces).

The basic construction of a lifting magnet includes: a housing (cast steel or welded steel plate with good magnetic permeability), magnetic poles (made of pure iron or low-carbon steel, with the magnetic field path between the two poles forming a closed loop through the material being lifted), coils (copper or aluminum wire wound around the poles, generating the magnetic field when energized), a junction box (waterproof and dustproof), insulation protection, and lifting lugs. The standard requires the Protection Rating (IP) of lifting magnets to be no less than IP55 (dustproof and protected against water jets).

Electrical and Insulation Requirements

The standard specifies the following electrical system requirements for lifting magnets: Power supply — lifting magnets operate on DC power (220 VDC or 110 VDC), converted from AC via a rectifier. DC supply ensures a stable magnetic field and eliminates eddy current losses. Rated voltage deviation — the magnet must be able to lift its rated load at rated voltage within a tolerance of ±10%. Insulation Resistance — the insulation resistance between the coil and the housing, measured with a 500 V Megohmmeter (Insulation Tester), must be ≥5 MΩ in the cold state and ≥1 MΩ in the hot state. Dielectric strength — the coil-to-housing insulation must withstand a power-frequency test voltage of 2 times the rated voltage plus 1000 V for 1 minute without breakdown or flashover. Coil temperature rise — when the magnet is continuously energized at rated voltage (per the design duty cycle, typically FC=50%, i.e., 5 minutes on / 5 minutes off), the coil temperature rise must not exceed the allowable limit for Class F insulation (≤100 K). Demagnetization — the magnet must be equipped with a rapid demagnetization device (reverse-current demagnetization). After demagnetization, the residual magnetism must not exceed 5% of the allowable residual value for the material being handled, ensuring smooth release of the load.

Lifting Capacity and Safety Factor

The standard defines the rated lifting capacity and safety requirements as follows: Rated Lifting Capacity — the maximum allowable load that can be lifted at rated voltage when handling a specified material (standard Q235 steel plate of defined thickness and surface condition). For scrap magnets, the rated lifting capacity refers to the load when lifting loose scrap (as opposed to a solid plate) — typically only 30% to 50% of the capacity for lifting a solid plate. Safety factor — the ratio of static holding force to rated lifting capacity must be ≥2.5, meaning the magnet must generate at least 2.5 times its rated lifting capacity in static force at rated voltage. Redundancy — for magnets used to handle molten metal or other critical loads, dual coils (primary and standby) or dual power supplies (primary and backup) are required, so that if one circuit fails, the other maintains holding force. Power-loss protection — the magnet must be fitted with a power-loss holding device (battery or supercapacitor) that automatically switches to the backup power supply in the event of a crane power failure, maintaining holding force for at least 5 minutes to give the operator sufficient time to lower the load safely. Kelude can supply a full range of lifting magnets to meet the material handling needs of steel mills and scrap yards.

Testing and Inspection

The standard specifies the following tests: Visual inspection — the magnet housing must be free of cracks, porosity, and damage, and the junction box must be properly sealed. Insulation Resistance Test — cold-state insulation resistance must be ≥5 MΩ. Coil resistance measurement — the DC resistance of the coil must match the design value within a deviation of ≤±5%. No-load current measurement — no-load current is measured at rated voltage and compared with the design value to detect turn-to-turn short circuits. Holding force test — using a standard test block (Q235 steel plate of specified thickness and surface roughness), the actual holding force at rated voltage must be no less than 2.5 times the rated lifting capacity. Temperature rise test — the coil is energized continuously at rated conditions until thermal equilibrium is reached, and the temperature rise is measured. The type test additionally includes a dielectric strength test, a waterproof test (IP55 water jet test), and an impact test (simulating impact conditions). Routine inspection in service — before each shift, the cable and junction box must be checked for damage and insulation integrity; insulation resistance must be measured monthly; and actual holding force must be verified quarterly using a test block.

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