4-9. GENERAL MAINTENANCE PROCEDURES - Continued
If discoloration can be wiped off, such marks can usually be traced to oil-burn stains which are not serious.
Rolling. This damage usually occurs only on plastic gears. Rolling is when material is pushed out of
shape but does not break off. It is usually caused by heavy, even loads and overheating.
Visually inspect all castings and weldments for cracks. Parts that carry a great load should be inspected by the
magnetic particle method. Non-ferrous parts may be inspected by the fluorescent penetrant method.
Magnetic Particle Inspection
This type of inspection can only be carried out on metals that can be magnetized. If in doubt check whether a
magnet sticks to the surface. It should only be carried out if parts are not easily replaceable (cost or special
order), or parts have been reworked or reground, or parts are subject to high stress. Magnetizing current
depends on parts being magnetized for solid section parts, current shall be 1000 amps per diameter inch for
variable diameter thickness, current shall be adjusted for diameter inspected. ·
Parts shall be rejected if:
Indications of non-metallic inclusions are present (foreign body - solid, liquid, or gaseous) and these are
longer than one inch, or indications are closer than 1/8 in. apart.
Any evidence of cracks are seen
Scattered short sharp bursts are seen. Bursts are caused by metals working at temperatures that
weaken and break the material. Bursts are not usually seen until the metal is cut through to the burst area.
Separate short wavy lines in the same general direction are seen. Flakes are caused by improper
cooling. They are not usually seen until the metal is cut through to the flake area.
Fine, sharp, tightly packed lines are seen (grinding cracks). These are usually caused by a glazed
grinding wheel. The wheel, instead of cutting the material, rubs and overheats the material. The lines are thermal cracks
similar to heat treat and hardening cracks.
Fluorescent Penetrant Inspection
This type of inspection can be carried on any metal but is usually reserved for non-magnetic material (e.g.
aluminum, stainless steel). The parts shall be warm prior to applying penetrant. The penetrant may be applied
by dipping, painting or spraying. All surfaces to be examined shall be completely covered. Penetrant time for
various metals is as follows:
Aluminum Alloy - no less than 20 minutes
Magnesium Alloy - no less than 20 minutes
Brass or Bronze - no less than 30 minutes
Ferrous Alloys - no less than 30 minutes