Repair and Maintenance of Diesel Engine Connecting Rod Bearings Part 2

Continuing from the previous text, this article will continue to introduce the preparation work before repairing the scraping bearing of diesel engines and the repair of connecting rod bearings.

2. The inspection method for the roundness and taper of the bearing shell hole is to tighten the bearing cover screws to the specified torque, and then measure their roundness and taper using a cylinder gauge. The difference between two perpendicular diameters in the same cross-section is called out of roundness; The difference between the maximum and minimum diameters in the same longitudinal section is called conicity. The roundness and cone shape should both be within 0.02~0.04mm.

3. Inspection of tightness (radial clearance between bearing shell and journal)

(1) Measurement method. Secure the connecting rod with the installed and scraped bearing on the vise, and tighten the connecting rod screws to the specified torque. Measure the diameter of the bearing hole using a cylinder gauge and an outside micrometer. The diameter of the tile hole and the diameter of the shaft neck are equal to the radial clearance. Among them, roundness should be taken into account, and there are specific regulations for the radial clearance between the bearing shell and the shaft neck of each model.

(2) Lead wire and copper foil method. The lead wire method is to place a lead wire with a diameter of about twice the standard clearance between the bearing and the shaft neck, tighten the bearing cover with the specified torque, and then remove the lead wire. The thickness is measured with a micrometer to obtain the radial clearance between the bearing shell and the shaft neck. Copper skin method: Use copper skin with a length of about 30mm, a width of about 10mm, and a thickness equal to the standard clearance (taking the minimum value) (the corners should be rounded, and a thin layer of engine oil should be applied when using) and place it between the bearing and the shaft neck. Tighten the bearing cover bolts according to the specified torque. If the crankshaft or flywheel cannot be pulled by hand, it indicates that the radial clearance between the bearing shell and the journal is too small; If there is resistance that cannot be easily pulled, but the copper piece can be rotated with a slight force after being removed, it indicates suitability; If there is no resistance or the rotation is too loose, it indicates that the radial clearance between the bearing and the journal is too large. If the gap is too large or too small, it can be adjusted by adding or removing shims.

(3) Experience check method. The method is to apply a thin layer of engine oil on the bearing shell, then install it on the shaft neck, tighten the connecting rod bolts according to the specified torque, and shake the connecting rod vigorously by hand. If the bearing shell alloy is Babbitt alloy or nickel based alloy, it can rotate 1/2 to 1 turn by relying on the inertia of the connecting rod itself; If the bearing alloy is copper lead alloy (commonly known as copper tile), it can rotate 1-2 turns; If the bearing alloy is aluminum based alloy (commonly known as aluminum tile), it can rotate 2-3 turns, and then hold the small end of the connecting rod and move it along the crankshaft axis direction without any looseness, it is suitable.

4. After all the connecting rod bearings are scraped and matched, the end clearance of the connecting rod end should also be checked. The clearance between the side of the connecting rod end and the crankshaft arm should not be too large, usually 0.1-0.35mm. If it exceeds 0.5mm, copper should be welded or a layer of bearing alloy should be hung on the side of the connecting rod big end for repair.

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