Figure 2-2. Wear-metal Concentration vs. Operating Hours

NAVAIR 17-15-50.1

TM 38-301-1

T.O. 33-1-37-1

CGTO 33-1-37-1

i. Patterns of Wear. Note that in figure 2-1, the wear-metal concentration level continued to increase

gradually as the equipment continued in operation. In actual practice, this may not happen because of the effect

of fluid consumption and the replenishment of lost fluid by new fluid. Fluid replenishment usually causes the

wear-metal concentration level in a normal engine to "level off" and remain steady. If fluid were replaced as it

was lost (rather than at discrete intervals, as is the actual practice), the effect of this oil replenishment on the

wear-metal concentration level would be as shown in figure 2-2. In this hypothetical example, the wear-metal

concentration level reaches its steady state following the break-in period and then remains fairly constant. This

steady state point is a function of two variables: (1) the rate of fluid consumption and replenishment, and (2) the

rate of wearmetal production by internal friction within the equipment. Theoretically, a steady-state condition is

never reached but is only approached as a limiting condition. In practice, the steady-state point varies due to

changing rates of fluid consumption and wearmetal production.

j. Effects of Fluid Loss/Addition/Change. The smooth curve of figure 2-2 shows the hypothetical result if

fluid was replaced as it was lost. This, of course, is impossible for most items of equipment. Figure 2-3 shows the

effect of periodic fluid addition and a fluid change. In components such as some reciprocating engines, where oil

depletion is rapid and replenishment is frequent, concentrations of wearmetals will change erratically. Under

these conditions it is best for the oil sample to be taken for analysis just prior to the addition of new oil. An

accurate record of time since last oil change or oil addition is a requisite for the evaluator, as he may be misled if

this information is incorrectly reported. Eight parts per million iron at 50 hours since oil change may be normal for

a turbine engine; however, eight parts per million at 2 hours since oil change may indicate impending failure.

Figure 2-2. Wear-metal Concentration vs. Operating Hours

2-5