Fixed (Nonportable) General Purpose Unit
A fixed general purpose unit is shown in Figure 8-19.
This unit provides direct current for wet continuous or residual magnetization procedures. Circular or longitudinal magnetization may be used and it may be powered with rectified alternating current (ac), as well as direct current (dc). The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position with its contact plate mounted on a shaft surrounded by a pressure spring, so that the plate may be moved longitudinally. The plate is maintained in the extended position by the spring until pressure transmitted through the work from the movable head forces it back.
The motor driven movable head slides horizontally in longitudinal guides and is controlled by a switch. The spring allows sufficient overrun of the motor driven head to avoid jamming it and also provides pressure on the ends of the work to ensure good electrical contact.
A plunger operated switch in the fixed head cuts out the forward motion circuit of the movable head motor when the spring has been properly compressed.
In some units the movable head is hand operated, and the contact plate is sometimes arranged for operation by an air ram. Both contact plates are fitted with various fixtures for supporting the work.
The magnetizing circuit is closed by depressing a pushbutton on the front of the unit. It is set to open automatically, usually after about one-half second.
The strength of the magnetizing current may be set manually to the desired value by means of the rheostat or increased to the capacity of the unit by the rheostat short circuiting switch. The current utilized is indicated on the ammeter.
Longitudinal magnetization is produced by the solenoid, which moves in the same guide rail as the movable head and is connected in the electrical circuit by means of a switch.
The suspension liquid is contained in a sump tank and is agitated and circulated by a pump. The suspension is applied to the work through a nozzle. The suspension drains from the work through a nonmetallic grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.
Portable General Purpose Unit
It is often necessary to perform the magnetic particle inspection at locations where fixed general purpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. It is particularly useful for inspecting landing gear and engine mounts suspected of having developed cracks in service. Portable units supply both alternating current and direct current magnetization.
This unit is only a source of magnetizing and demagnetizing current and does not provide a means for supporting the work or applying the suspension. It operates on 200 volt, 60 cycle, alternating current and contains a rectifier for producing direct current when required. [Figure 8-20]
The magnetizing current is supplied through the flexible cables. The cable terminals may be fitted with prods, as shown in the illustration, or with contact clamps. Circular magnetization may be developed by using either the prods or clamps. Longitudinal magnetization is developed by wrapping the cable around the part.
The strength of the magnetizing current is controlled by an eight point tap switch, and the time duration for which it is applied is regulated by an automatic cutoff similar to that used in the fixed general purpose unit.
This portable unit also serves as a demagnetizer and supplies high amperage low voltage alternating current for this purpose. For demagnetization, the alternating current is passed through the part and gradually reduced by means of a current reducer.
In testing large structures with flat surfaces where current must be passed through the part, it is sometimes impossible to use contact clamps. In such cases, contact prods are used.
Prods can be used with the fixed general purpose unit as well as the portable unit. The part or assembly being tested may be held or secured above the standard unit and the suspension hosed onto the area; excess suspension drains into the tank. The dry procedure may also be used.
Prods should be held firmly against the surface being tested. There is a tendency for a high amperage current to cause burning at contact areas, but with proper care, such burning is usually slight. For applications where prod magnetization is acceptable, slight burning is normally acceptable.
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