6.6. Bent shaft

Check necks of a bent shaft for a roughness. Any noticeable roughness indicates regrindings of a bent shaft or its replacement the need.
If the bent shaft was перешлифован, check lack of agnails around openings for lubricant. Remove the found agnails with a file or a shaber and carefully clear openings and channels of shaving.
Using a micrometer, measure diameter of radical and conrod necks of a bent shaft and compare results to specifications. Measure necks in several points both on diameter, and on length that will allow to reveal ovality and conicity if they are.
Check the surfaces of contact on each end of a bent shaft for wear and other damages. If the neck is strongly worn-out from an epiploon, then, perhaps, replacement of a bent shaft will be required.
Wear of a conrod neck is characterized by knocks of metal tone during the operation of the engine, especially under loading, with a small frequency of rotation, and reduction of pressure of oil.
Wear of radical necks of a bent shaft is characterized by strong vibration of the engine and knocks of metal tone, amplifying at increase in frequency of rotation of the engine, and also reduction of pressure of oil.
Even if radical and conrod bearings are subject to replacement at capital repairs of the engine, they need to be examined carefully as on their defects it is possible to judge technical condition of the engine.
The bearing can fail because of a lack of lubricant, hit of pollution, foreign debris, an overload of the engine or corrosion. The cause of failure of the bearing should be removed before the engine is repeatedly assembled.
For survey of bearings remove and spread out them in the same order in what they were established on the engine. It will allow to define the corresponding neck of a bent shaft and will facilitate search of malfunctions.
Foreign debris can get to the engine in various ways. Metal particles appear in engine oil as a result of normal wear of the engine. Small particles together with oil can get to bearings and it is easy to take root into their soft material. Larger particles, getting to the bearing, can scratch the bearing or a neck of a bent shaft. To prevent failure of the bearing for this reason, carefully clear all internal surfaces of the engine and you keep them clean at assembly of the engine. It is recommended to observe also required frequency of replacement of oil with the filter.
Insufficient lubricant of necks of a bent shaft can be caused by many reasons: high temperature of oil, engine overload, oil leak and so forth.
The manner of driving of the car also exerts impact on durability of bearings. At completely open butterfly valve at a low frequency of rotation of a bent shaft of the engine load of bearings raises and there is an expression from a zone of contact of an oil film. High loadings lead to emergence of cracks in a working part of the bearing that in turn can lead to a separation of an antifrictional layer from a steel basis.
The movement on short distances leads to corrosion of bearings as a result the engine does not come to the stabilized working temperature at which water vapor and corrosion gases are removed. Being condensed in engine oil, they form acid and a deposit. Acid together with engine oil gets to bearings and causes corrosion.
The wrong selection of bearings at assembly of the engine also becomes the reason of failure of bearings. In the bearings installed with a preliminary tightness there is a gap, insufficient for providing normal lubricant.
At installation of a bent shaft it is necessary to consider the following.

Fig. 3.4-38. Bent shaft: 1 – the aligning plug; 2 – asterisk; 3 – persistent ring; 4 – bent shaft; 5 – cover of bearings; 6 – bolt; 7 – persistent half ring; 8 – insert of the radical bearing; 9 – plug; 10 – adjusting pin; 11 – persistent half ring;
12 – insert of the radical bearing


Persistent half rings of 7 and 11 (fig. 3.4-38) are established only on the fourth radical bearing and are used for adjustment of an axial gap of a bent shaft.
Lubricant flutes of persistent half rings have to be directed outside.
Inserts 8 and 12 without lubricant flute are established from covers of bearings.
At installation it is necessary to use new bolts of fastening of covers of bearings.

Fig. 3.4-39. Sequence of an inhaling of bolts of fastening of covers of radical bearings



Tighten bolts of fastening of covers of radical bearings in the sequence shown in the figure 3.4-39 in three stages:
The 1st – the moment of 30 N · m;
The 2nd – the moment of 50 N · m;
– tighten the 3rd on a corner 90 °.

Measurement of axial and radial gaps of a bent shaft

Fig. 3.3-50. Installation of an arm with the indicator of hour type for measurement of an axial gap of a bent shaft



Establish on the block of cylinders of the engine an arm with the indicator of hour type so that the measuring tip of the indicator rested against a counterbalance of a bent shaft (see fig. 3.3-50).
Move a bent shaft along an axis to one party against the stop and establish an arrow of the indicator of hour type on 0. Take a bent shaft along an axis to other party against the stop and record the value shown by the indicator. Nominal rate of an axial gap of a bent shaft of 0,090-0,251 mm, maximum permissible – 0,30 mm.
For measurement of a radial gap in bearings of a bent shaft it is necessary to use the calibrated plastic core of Plastigage.
Unscrew bolts and uncover the radical bearing and a radical insert. Clear a radical insert, a cover of the bearing and a neck of a bent shaft.
Cut off a piece of the plastic calibrated core which length is equal to bearing width, and lay it along an axis of a bent shaft on a neck of the radical bearing.
Establish a cover of the radical bearing with a radical insert and fix it by bolts, having tightened them the moment
30 N · m. At the same time do not turn a bent shaft.
Unscrew bolts and again uncover the radical bearing. Compare width of the deformed plastic core to the measuring scale put on packing of a plastic core. Determine a radial gap by a scale. Nominal rate of a radial gap of a bent shaft of 0,018-0,045 mm, maximum permissible – 0,10 mm.