A reliability analysis of feller-bunchers in use in the South

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Virginia Polytechnic Institute and State University

Maintenance, failure, and repair data on three types of feller-bunchers were analyzed. The data was tested for trends in time between failures as a function of time; time-between-failure and time-to-repair distributions were compared; the effects of changes in reliability, maintainability, and maintenance were analyzed; and a listing of reasons for failures was compiled.

Four different cooperators supplied information on 32 feller-bunchers covering 41,300 operating hours that included 1814 failures.

The time between failures (TBF) does decrease with machine age with the possible exception of an increase when the machine is first put into service. The TBF seemed to decrease rather rapidly during some initial operating period after which they still decreased, although at a much lower rate. During this subsequent period some machines exhibited a seasonal fluctuation, longer TBF in the summer and shorter in the winter.

There were no significant differences in TBF among the three types of feller-bunchers. The greatest amount of variability occurred among machines of the same type. A comparison of times to repair (TTR) indicated that the type 2 machines had shorter TTR than the types 1 and 3 which were the same.

Most failures of the shear were minor failures involving only replacing hydraulic fittings and hoses, shimming the cutting blades, or packing a cylinder. Because these are common malfunctions, parts were available and repairs did not require extended periods of time and in many cases could be performed by the operator. Failures of the carrier, on the other hand, covered a wider range. Because of this there were times when the part was not available and the repair usually required the attention of a mechanic. Therefore, failures of the carrier are more critical than failures of the shear.

An increase in TBF does increase availability; however, the magnitude of the effect decreases as the TBP increases. There is definitely a point of diminishing returns. However, the longer the average repair or delay time the more pronounced the effect of the TBF. For all three types of feller-bunchers, the point of diminishing returns was less than an average TBF of 40 hours.

Unlike TBF, there is a linear relationship between availability and downtime due to active repair or delay time. And, the shorter the TBF the greater the effect.

The effect of service time is to decrease the effect of the other variables. As the number of failures increase the amount of service time decreases because there will be fewer services. Conversely, as the number of failures decreases the amount of time the machine is down for service increases.

The most common cause of failure for all three types of feller-bunchers was the hydraulic system, accounting for 41.4 percent of all repairs. Other results indicate that a mechanic should have an adequate knowledge of hydraulics and welding.