Excessive off-stroking

Figure 9-50. Crankshaft Offset To Grind Connecting Rod Bearing Journals

TM-9-6115-604-34 Generator Set Diesel Engine Driven Tactical Skid Mtd 5 Kw. 1 Phase-2 Wire 1 Phase -3 Wire 3 Phase -4 Wire 120. 120/240 and 120/208 Volts Manual
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TM 9-6115-604-34 NAVFAC P-8-633-34 CAUTION Excessive off-stroking changes the compression ratio of a diesel engine a significant amount. Shortening the stroke reduces the compression ration and can lead to poor ignition, hard starting and severe loss of fuel economy in cold weather. Lengthening the stroke may also cause a loss of fuel economy due to the increased friction and lower amount of air for combustion. Also a lengthened stroke may result in valve to piston interference, and overload the connecting rod bearings. Off-stroking shall be limited to shortening the stroke a maximum of 0.020 inch (0.51 mm). Lengthening the stroke is not permitted. NOTE The rod bearing journals may be damaged more deeply on one side than the other. The amount of grinding on these may be reduced by off-stroking or changing the crankshaft throw dimension. This will save one or more of the undersizes, and lengthen the life of the crankshaft. When off-stroking the crankshaft, both heads of the grinding machine shall be offset the same amount to ensure that parallelism and taper specifications are maintained. For smooth and efficient operation of the engine, the stroke of all throws shall be the same within 0.005 inch (0.013 mm). (10) If the crankshaft is being off-stroked to reduce the amount of grinding needed to remove defects, move each throw head an equal amount to shorten the stroke a maximum of 0.020 inch (0.51 mm). Each additional throw shall be off-stroked the same amount to tolerance of ±0.005 inch (±0.13 mm). (11) Install steady rests on the journals to stabilize the crankshaft during grinding. Keep a light pressure on the steady rests during grinding to support the crankshaft, but do not apply enough pressure to bow the crankshaft or cause misalignment of the journals. (12) Using a micrometer, measure the journal to determine the undersize to which the journal should be ground (Figure 9-41). All rod bearing journals shall be ground to the same standard undersize, though not necessarily to the same undersize as the main bearing journals. Rod and main bearing journal out-of- roundness, taper, and parallelism specifications are listed in Table 9-6. Table 9-6. Rod Bearing Journal Specifications Out-of-Round - 0.0005 inch (0.013 mm) TIR maximum. Journal Taper - 0.0005 inch (0.013 mm) TIR maximum measurable in length of journal. Parallelism - 0.0005 inch (0.013 mm) maximum measurable in length of journal. (13) Determine how much material needs to be removed for standard undersize desired. For example: if the measurement shows the journal is worn 0.001 inch (0.03 mm) and the desired undersize is 0.010 inch (0.25 mm) under, set the dial indicator to show that 0.009 inch (0.23 mm) of material is to be removed. (14) Start and adjust the grinding machine for a crankshaft speed of 50 rpm, and a grinding wheel speed of 6500 feet (1981 m) per minute. (15) Turn on the coolant, center the wheel on the journal, and carefully bring the wheel until it just touches the journal. (16) Traverse the wheel across the journal until it starts to grind into the fillet area of either journal, and just touches the journal side wall. NOTE As the wheel grinds the journal, the tension on the steady rest must be increased because of the decreasing size of the journal. This also stabilizes the journal after any high spots or out-of- round condition has been ground out. (17) While keeping a careful check on the journal size, slowly feed the wheel in until the correct undersize has been reached, and set the index on the feed indicator to 0. Let the grinding wheel dwell at this point until spark-out occurs; this will improve the surface finish and help prevent grinder bum damage. 9-88

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