Fault and maintenance knowledge of engine valve train

If it is found that the camshaft follower positioning screw is loose and oil leakage, the camshaft follower roller and the corresponding camshaft convex peach surface are abnormally worn, it can be determined that the cause of the vehicle failure is caused by the valve train failure.

After the camshaft follower positioning screw is loosened, its movement cannot be fixed in the up and down direction, and the rotation operation occurs. As a result, the camshaft follower roller and the camshaft ridge are abnormally worn, and the worn camshaft follower is stuck, triggering the valve pusher to move downward, the force is bent, and finally the valve plate is detached. Failure phenomenon.

The valve shaft camshaft follower transmits the camshaft displacement to the valve push rod, which is then transmitted to the rocker arm by the valve push rod, and the rocker arm compresses the valve to open the valve.

The fault is caused by the camshaft follower being stuck and unable to move up and down smoothly. When the exhaust valve returns to the valve spring, the camshaft follower can not move downward, and the exhaust valve cannot be returned. During the end of the upward exhaust, the exhaust valve that is not completely closed is struck, causing the valve bridge to fall off and the valve stem to bend.

What should we pay attention to when repairing?

Through the above description, we can understand that abnormal valve trains can cause abnormal noise, black smoke, and insufficient power.

In order to quickly troubleshoot, we can start by checking the valve train and quickly find the faulty parts and fault sources. From another perspective, we need to regularly check and adjust the valve clearance according to the requirements of the engine maintenance manual to prevent engine failure caused by excessive clearance or too small clearance.

How to extend the service life of the valve train

01 valve light grinding

There are four issues to be aware of:

First, ensure that the valve head is concentric with the rod, otherwise it should be straightened first;

Second, the amount of light grinding is as small as possible under the premise that the complete cone can be ground;

The third is to improve the surface finish as much as possible;

Fourth, the end of the valve stem should be flattened.

02 valve seat reaming

Valve seat reaming is usually manual, and three issues should be given special attention:

First, on the basis of eliminating depressions, spots, and being able to ream the complete cone, the smaller the amount of reaming, the better;

Secondly, the force should be uniform when reaming, the knife should be lighter and less hinged to observe, so as to ensure less reaming and higher finish;

The third is to test with the valve to determine the position and width of the working face.

03 valve grinding

The grinding of the valve is divided into two cases:

First, the valve and the seat have only slight pitting, and do not need grinding by grinding and reaming: first grind with coarse diamond, grind the pitting, then grind with fine diamond, and finally apply oil to grind until the seal meets the requirements. , the width meets the requirements;

Secondly, the valve and the seat have been polished by the light grinding and reaming: only when the sealing is not up to the requirements, the grinding should be carried out, but the operation must be careful not to use excessive force. It is strictly forbidden to beat the valve up and down, otherwise the concave will appear. Shaped sand marks, affecting the quality of maintenance.

04 valve guide reaming

The clearance between the valve stem and the valve guide is the key to determining the life of the valve guide. Therefore, when replacing the new valve guide, the reaming should strictly control the fit clearance, and the lower limit of the clearance of each model can be used to effectively extend the service life.

05 camshaft bearing scraping

The scraping of camshaft bearings is a manual operation, which guarantees a certain degree of difficulty. For the convenience of scraping, it is usually processed outside the cylinder, so pay special attention to four problems:

First, it is necessary to determine the diameter of the inner hole after the bearing is scraped;

Second, it is necessary to pay attention to maintaining the coaxiality between the inner bore of the bearing and the outer circle during scraping;

The third is to test the scraping edge and the journal, and ensure that the contact marks are evenly distributed;

Fourth, when pressing the bearing into the seat hole, pay attention to the positive oil hole.

06 valve foot clearance adjustment

In the normal use of the parts of the valve train, the valve foot clearance will change with the wear of the parts. If the cam, valve stem end face and tappet contact surface wear, the clearance will become larger, and the clearance between the valve head and the valve seat will become smaller. Therefore, the intermediate value of the clearance should be taken during the adjustment.

Technical Articles

Cummins, Engine, fault

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