Fixed Unit (Nonportable)
A fixed general purpose unit is shown in figure 10-10. This unit provides direct current for wet continuous or residual magnetization procedures. Either circular or longitudinal magnetization may be used, and it may be powered with rectified ac as well as dc. The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position. Its contact plate is mounted on a shaft surrounded by a pressure spring, in order 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 movable head slides horizontally in longitudinal guides and is motor driven. It 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 the wooden grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.
General Purpose Portable 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. This has occurred, particularly on landing gears and engine mounts suspected of having developed cracks in service. Equipment suitable for this purpose, supplying both alternating current and direct current magnetization, is available. A typical example is shown in figure 10-11.
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.
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 length of time for which it is applied is regulated by an automatic cutoff similar to that used in the standard 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 standard general purpose unit as well as the portable unit. The part or assembly being tested may be suspended above the standard unit and the suspension hosed onto the area; excessive 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 will usually be slight. For applications where prod magnetization is acceptable, slight burning is normally not objectionable.
When it is desired to use cable with the standard general purpose unit as a source of power, a contact block is useful. This consists of a wooden block fitted at each end with copper plates spaced to receive the terminals of the cable.
When the contact block is placed between the heads of a standard unit, the regular controls and timing switches of the unit can be used to regulate the magnetizing current. This provides a convenient way of connecting the cable to the source of power and eliminates the necessity for bolted connections.
When magnetizing current is passed through a steel propeller blade for circular magnetization, it is possible to burn the blade tip if necessary precautions are not taken. This possibility can be avoided by the use of a hinged clamp attached to the movable head of the inspection unit.
The clamp is lined with copper braid, which provides good electrical contact by conforming to the curvature of the propeller blade faces. This fixture avoids electrical contact at the thin edge of the blade tip and eliminates high current intensities which may cause burning at the point. The butt end of the blade is supported by a plug mounted on the other head.
The various types of indicating mediums available for magnetic particle inspection may be divided into two general types: wet process materials and dry process materials. The basic requirement for any indicating medium is that it produce acceptable indications of discontinuities in parts.
The contrast provided by a particular indicating medium on the background or part surface is particularly important. The colors most extensively used are black and red for the wet procedure; and black, red, and gray for the dry procedure.
For acceptable operation, the indicating medium must be of high permeability and low retentivity. High permeability ensures that a minimum of magnetic energy will be required to attract the material to flux leakage caused by discontinuities. Low retentivity ensures that the mobility of the magnetic particles will not be hindered; that is, by the particles themselves becoming magnetized and attracting one another.
The magnetic substance for the wet process is usually supplied in paste form. The red paste improves visibility on dark surfaces. Although the exact amount of magnetic substance to be added may vary somewhat, a concentration of 2 ounces of paste per gallon of liquid vehicle has been found generally acceptable. The paste must not be dumped into the suspension liquid in the tank, since the unit agitator and pump cannot be depended on to do the mixing.
The proper procedure for preparing a suspension is to place the correct amount of paste in a container and add small quantities of the suspension liquid, working each addition with a flat paddle until the paste has been diluted to a uniform watery mixture which can then be poured into the tank.
It is important that new magnetic substance always be used in preparing suspensions. When the suspension becomes discolored or otherwise contaminated to the extent that the formation of magnetic particle patterns is interfered with, the unit should be drained, cleaned, and refilled with clean suspension.