Visual inspection should precede all other inspection procedures. Parts should not be cleaned before preliminary visual inspection, since indications of a failure often may be detected from the residual deposits of metallic particles in some recesses in the engine. Several terms are used to describe defects detected in engine parts during inspection.
Some of the more common terms and definitions are:
- Abrasion - An area of roughened scratches or marks usually caused by foreign matter between moving parts or surfaces.
- Brinelling - One or more indentations on bearing races usually caused by high static loads or application of force during installation or removal. Indentations are rounded or spherical due to the impression left by the contacting balls or rollers of the bearing.
- Burning - Surface damage due to excessive heat. It is usually caused by improper fit, defective lubrication, or over temperature operation.
- Burnishing - Polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.
- Burr - A sharp or roughened projection of metal usually resulting from machine processing.
- Chafing - Describes a condition caused by a rubbing action between two parts under light pressure which results in wear.
- Chipping - The breaking away of pieces of material, which is usually caused by excessive stress concentration or careless handling.
- Corrosion - Loss of metal by a chemical or electrochemical action. The corrosion products generally are easily removed by mechanical means. Iron rust is an example of corrosion.
- Crack -A partial separation of material usually caused by vibration, overloading, internal stresses, defective assembly, or fatigue. Depth may be a few thousandths to the full thickness of the piece.
- Cut - Loss of metal, usually to an appreciable depth over a relatively longer and narrow area, by mechanical means, as would occur with the use of a saw blade, chisel or sharp-edged stone striking a glancing blow.
- Dent - A small, rounded depression in a surface usually caused by the part being struck with a rounded object.
- Erosion - Loss of metal from the surface by mechanical action of foreign objects, such as grit or fine sand. The corroded area will be rough and may be lined in the, direction in which the foreign material moved relative to the surface.
- Flaking - The breaking loose of small pieces of metal or coated surfaces, which is usually caused by defective plating or excessive, loading.
- Fretting - A condition of surface erosion caused by minute movement between two parts usually clamped together with considerable unit pressure.
- Galling - A severe condition of chafing or fretting in which a transfer of metal from one part to another occurs. It is usually caused by a slight movement of mated parts having limited relative motion and under high loads.
- Gouging - A furrowing condition in which a ,displacement of metal has occurred (atom effect). It is usually caused by a piece of metal or foreign material between close moving parts.
- Grooving - A recess or channel with rounded and smooth edges usually caused by faulty alignment of parts.
- Inclusion -Presence of foreign or extraneous material wholly within a portion of metal. Such material is introduced during the manufacture of rod, bar, or tubing by rolling or forging.
- Nick - A sharp sided gouge or depression with a “V” shaped bottom which is generally the result of careless handling of tools and parts.
- Peening - A series of blunt depressions in a surface.
- Pick Up or Scuffing - A build up or rolling of metal from one area to another, which is usually caused by insufficient lubrication, clearances, or foreign matter.
- Pitting - Small hollows of irregular shape in the surface, usually caused by corrosion or minute mechanical chipping of surfaces.
- Scoring - A series of deep scratches caused by foreign particles between moving parts, or careless assembly or disassembly techniques.
- Scratches - Shallow, thin lines or marks, varying in degree of depth and width, caused by presence of fine foreign particles during operation or contact with other parts during handling.
- Stain - A change in colour, locally, causing a noticeably different appearance from the surrounding area.
- Upsetting - A displacement of material beyond the normal contour or surface (a local bulge or bump). Usually indicates no metal loss.
Defects in nonmagnetic parts can be found by careful visual inspection along with a suitable etching process. If it is thought that a crack exists in an aluminium part, clean it by brushing or grid blasting very carefully to avoid scratching the surface. Cover the part with a solution made from 1¼ lbs. of sodium hydroxide and 1 pint of water at room temperature. Rinse the part thoroughly with water after about 1 minute’s contact with the solution. Immediately neutralize the part with a solution of one part nitric acid and three parts of water heated to 100° F. Keep the part in this solution until the black deposit is dissolved. Dry the part with compressed air. If a crack exists, the edges will turn black after this treatment, thus aiding in its detection. For magnesium parts, a 1070 solution of acetic acid at room temperature can be applied for a maximum of 1 minute. The part should then be rinsed with a solution of 1 ounce of household ammonia in 1 gallon of water.
Examine all gears for evidence of pitting or excessive wear. These conditions are of particular importance when they occur on the teeth; deep pit marks in this area are sufficient cause to reject the gear. Bearing surfaces of all gears should be free from deep scratches. However, minor abrasions usually can be dressed out with a fine abrasive cloth.
All bearing surfaces should be examined for scores, galling, and wear. Considerable scratching and light scoring of aluminium bearing surfaces in the engine will do no harm and should not be considered a reason for rejecting the part, provided it falls within the clearances set forth in the Table of Limits in the engine manufacturer’s overhaul manual. Even though the part comes within the specific clearance limits, it will not be satisfactory for re-assembly in the engine unless inspection shows the part to be free from other serious defects.
Ball bearings should be inspected visually and by feel for roughness, flat spots on balls, flaking or pitting of races, or scoring on the outside of races. All journals should be checked for galling, scores, misalignment, or out-of-round condition. Shafts, pins, etc., should be checked for straightness. This may be done in most cases by using V-blocks and a dial indicator.
Pitted surfaces in highly stressed areas resulting from corrosion can cause ultimate failure of the part. The following areas should be examined carefully for evidence of such corrosion:
- Interior surfaces of piston pins.
- The fillets at the edges of crankshaft main and crankpin journal surfaces.
- Thrust bearing races.
If pitting exists on any of the surfaces mentioned to the extent that it cannot be removed by polishing with crocus cloth or other mild abrasive, the part usually must be rejected.
Parts, such as threaded fasteners or plugs, should be inspected to determine the condition of the threads. Badly worn or mutilated threads cannot be tolerated; the parts should be rejected. However, small defects such as slight nicks or burrs may be dressed out with a small file, fine abrasive cloth, or stone. If the part appears to be distorted, badly galled, or mutilated by over tightening, or from the use of improper tools, replace it with a new one.
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