The camshaft and bearings are important driving parts and components in the valvetrain. During use, if the camshaft and bearings are worn, resulting in insufficient engine power, unstable idle speed, difficulty in starting, etc., the camshaft and bearings will be damaged. What are the causes of wear and tear? When we perform maintenance on camshafts and bearings, we need to pay attention to what aspects can reduce their wear. Below, China Bearing Network (China Bearing Network) will share relevant knowledge based on our understanding.

1. Common causes of camshaft and bearing wear

The heat treatment hardness of the camshaft is not enough, or the chrome plating hardness of the R30 arc surface of the valve rocker arm exceeds the hardness of the camshaft (it is easy to wear when there is a lack of engine oil lubrication), causing abnormal wear of the camshaft. A normal camshaft is shown in Figures 1 and 2. The camshaft Abnormal wear is shown in Figures 3 and 4.

2. The R30 arc surface of the valve rocker arm that is in contact with the cam lift is out of parallel with the rocker arm shaft hole (standard parallelism <0.02mm), causing the R30 arc surface to contact the inclined surface of the cam lift part. In severe cases, There is only point contact, which increases the contact stress and causes abnormal wear.

3. The chrome plating layer on the arc surface of the valve rocker arm is of poor quality and the chromium layer falls off and adheres to the cam lift part, aggravating the cam wear. The valve rocker arm is abnormally worn, as shown in Figures 5 and 6.

4. The burrs in the oil groove of the camshaft journal are not removed, which scratches the inner hole of the bushing and in turn strains the camshaft journal.

5. The inner and outer circles of the CB125T camshaft bushing are not concentric, which not only causes unilateral wear of the bushing, but also causes abnormal wear of the cam journal.

6. The clearance between the CB125T camshaft journal and the inner hole of the bushing is too small, and the engine oil cannot play the role of a floating bearing. If the gap is too large, there will be excessive oil leakage and lubrication cannot be guaranteed, resulting in abnormal wear.

7. The center of the seat hole of the cylinder head and valve rocker arm seat assembly bearing is deviated, or the arc depth of the seat hole is too small. After the cylinder head nut is tightened, the rocker arm seat hole is severely deformed, causing the bearing seat ring to be severely squeezed and out of round. , the ball deforms and is damaged and makes abnormal noise.

8. The oil passage for camshaft lubrication was blocked by sealant liquid during maintenance, resulting in abnormal wear due to lack of organic oil or insufficient lubrication. There are a few engines where the oil pump filter is clogged and the engine overheats due to lack of oil supply. If the engine overheats due to other reasons, the camshaft, journal, and bearing bushings will also experience abnormal wear. Friction parts such as pistons and cylinders are also damaged due to lack of lubrication.

9. On the journal surfaces at both ends of the CB125T camshaft, there are left and right spiral grooves processed as shown in Figure 7, which are mainly used to guide the oil to flow to the cylinder head oil pool (used to lubricate the cam lift). There is a positioning keyway about 4mm wide and 6mm long at the left end. If the R1.5×1.5 guide oil groove at the left end of the camshaft is set as a keyway, the lubricating oil will enter the left end of the cam from the cylinder head casting oil passage through the bushing oil hole. After the guide oil groove, most of the lubricating oil did not enter the cylinder head oil pool along the R1.5×1.5 guide oil groove, but flowed from the 4mm x 6mm positioning keyway to the outside of the bushing, causing the journal at the left end of the cam to lack normal lubrication. Abnormal wear and tear occurs.

10. The installation of the horizontal engine oil pump and the uneven plane of the oil pump groove of the white box body cause the sealing surface to leak and the oil pump pressure to reduce, resulting in insufficient lubrication of the cam bearing farthest from the lubricating oil passage, eventually leading to wear of the cam and rocker arm.

2. Pay attention to the following points when inspecting camshafts and bearings:

There are many reasons for abnormal wear of the camshaft and bearings (or bushings). It will be helpful to pay attention to the following points during daily use and maintenance.

1. When the engine is running at idle speed (such as the CG125 engine), loosen the screws on the cylinder head cover. If there is rapid oil leakage, as shown in Figure 8, it means that the lubricating oil circuit is normal (Honda CH125 scooter, you can also loosen the screws on the cylinder head cover. Open the oil screw on the cylinder head cover and observe whether the oil flows out). If no oil flows out or the oil flows out very slowly, you should first check whether the oil pump filter is blocked by foreign matter, and maintain and clean it as appropriate. If the filter is damaged, replace it with a new one in time. If the oil pump filter screen is not clogged, carefully check whether the gap between the inner and outer rotors of the oil pump exceeds the service limit and replace relevant parts or the oil pump assembly according to wear and tear.

2. When the engine is overheating, you should not take chances and be too lazy to investigate. You must check whether the ignition system is normal, whether the combustible mixture is too thin, whether the clutch is slipping, whether the cylinder head and cylinder block radiator are covered with a large amount of soil, and Analyze whether the lubricating oil pump is working abnormally, etc., until the overheating fault is eliminated, and the hidden danger of abnormal wear of the camshaft due to excessive temperature is eliminated.

3. The working surface of the cam is always in contact with the valve rocker arm. Therefore, it is very important to check the working surface of the cam. During inspection, use silk cloth (or lint-free clean cloth) to wipe off the lubricating oil on the cam working surface, and carefully check the cam working surface. If the friction marks are uniform and slight, it is normal wear; if the wear marks are larger, If it is deep, and there are obvious pull marks or ablation marks on the working surface, the working arc surface of the valve rocker arm should be inspected. If the camshaft surface is in point contact with the arc of the valve rocker arm R30, it may be because the parallelism between the rocker arm hole and the arc contact surface is too poor. After confirmation, the valve rocker arm should be replaced.

4. Measure the cam height. If the service limit has been exceeded, it should be replaced. The standard values and usage limit values of cam heights for common vehicle models are listed in Table 1. At the same time, it should also be noted that if the motorcycle has traveled a long distance and the camshaft has been worn to the limit, it means that there is a problem with the quality of the camshaft, that is, the lubricating oil passage of the engine to the camshaft oil pool is blocked or partially blocked. , as well as the oil pump pressure drop, etc., should be carefully checked until the normal operation of the lubrication system is restored. Otherwise, even if the camshaft is replaced with a new one, abnormal wear will still occur if it is not used for a long time. Please pay close attention to it.

5. To inspect the single-sided wear of the CB125T camshaft bushing, if possible, a digital micrometer should be used for inspection. If the inspection conditions are not met, the cylinder head and rocker arm seat assembly can be removed first, and the timing chain can be removed. wheel and camshaft, leaving only the bushing on the cylinder head, and inserting the hollow positioning pin on the bushing into the cylinder head positioning pin groove. Place a square block into the cylinder head oil pool, and place it close to the cam lining by hand. Cover side. Then attach the magnetic dial indicator holder to the top of the carburetor or the side of the frame, and adjust the dial indicator head so that it extends into the inner hole of the bushing, leaving about 0.20mm of compression margin. At this time, gently rotate the bushing so that the hollow positioning pin on the bushing slowly leaves the cylinder head positioning pin groove until the positioning pin turns to the side opposite the cylinder head positioning pin groove. During this process, carefully observe the dial indicator pointer reading, subtract the minimum value from the maximum value, and then divide it by two, which is the actual difference in concentricity of the bushing (its concentricity is 0.02mm), and print it on the surface of the bushing. Make visible markings with a marker. Then, turn the bushing over (that is, rotate the bushing hole direction 180°), and then detect it according to the above method. If the wear is in the up and down direction, it may be caused by the tensioning mechanism being adjusted too tightly. If it is worn in the left, right or other directions, the situation is more complicated. There may be a problem with the cylinder head seat hole processing, or there may be a problem with the rocker arm seat hole processing (including the position of the locating pin hole). Wear on one side of the bushing will cause the camshaft to run to one side (that is, away from the center of the rocker arm seat hole). After the hole is worn into an oval shape, the radial runout of the timing sprocket will be too large, and the chain will be abnormally worn. In severe cases, , affecting the opening and closing position of the valve, not only causing abnormal operating noise, but also causing changes in the valve timing phase. Bushings whose concentricity is seriously out of tolerance should be replaced and should not be replaced reluctantly to avoid endless troubles.

6. In the CB125T valve mechanism, the support journal of the camshaft is installed in the bushing, and the bushing is positioned in the approximate full circle formed between the two semicircles of the cylinder head and the rocker arm seat. After press-fitting, the inner diameter of the bushing and the clearance between the left and right journals of the camshaft should be 0.063~0.096mm. The purpose of designing this gap of a few tenths of a millimeter is mainly to consider that lubricating oil under a certain pressure can suspend the camshaft journal in the bushing hole and prevent the camshaft journal from direct contact and friction with the bushing inner hole. Some maintenance personnel mistakenly believe that the smaller the gap between the camshaft journal and the bushing, the better, so that there will be no oil leakage between the bushing inner hole and the journal. They don’t know that the journal surfaces at both ends of the CB125T camshaft are machined with left and right spiral grooves, which are mainly used to guide the oil to flow to the cylinder head oil pool. As long as the gap is appropriate, the lubricating oil will not leak out from this gap (possibly There is a small amount of leakage, but it does not affect the lubrication here at all). Therefore, when replacing the camshaft and bushing, the clearance is deliberately made small, which ultimately leads to direct friction between the journal and the bushing and abnormal wear. Therefore, before assembly, inspection must be carried out to ensure that there is a suitable oil gap between the cam journal and the bushing.

7. For the CG125 model (tappet model), the cam lift height, cam gear inner diameter, and cam gear shaft outer diameter need to be detected according to the method shown in Figure 9, and parts that exceed the service limit must be replaced.

8. The camshaft bearing is an important part that supports the operation of the camshaft. If it is severely worn or damaged, it will affect the normal operation of the camshaft to varying degrees. Check whether the bearing or bushing is smooth, whether there is any vibration, whether the inner hole of the bushing is severely worn, etc. If possible, the camshaft can be installed in the bearing or bushing (the valve rocker arm is not installed in the rocker arm support for the time being). Before and after tightening the cylinder head nut, quickly rotate the camshaft and carefully observe and judge the camshaft. Check whether there are any abnormal phenomena such as jitter when rotating. If the bearing rotation sound changes significantly after tightening, it means that the arc depth of the rocker arm support (some are called bearing supports) is too small. You can cut a paper pad with the same shape and size as the bottom surface of the rocker arm support. Use a piece (thickness of about 0.05mm), place it on the corresponding position of the rocker arm support and the cylinder head, install and tighten the rocker arm support, and quickly rotate the camshaft again according to the above method, if the bearing sound returns to normal. If the sound does not improve, the bearing itself may have quality problems and the cam bearing should be replaced.

9. For horizontal engines, if it is confirmed that the oil flow rate of the oil pump is too small, the cause should be carefully investigated to eliminate potential faults. For the oil pump of the GY6 model, if the oil pump installation surface is uneven with the oil pump groove of the right box, you can find a piece of insulating paper and cut a gasket with the same shape and size as the oil pump installation surface (its thickness is 0.05mm) (left and right), place it on the plane of the oil pump groove of the right box body. After tightening the installation screw, use your fingers to rotate the oil pump shaft to ensure that it rotates flexibly and without jamming. After carefully reinstalling the engine, do not install the cylinder head cover for the time being. After starting the engine, carefully observe whether the oil at the seat of the valve rocker arm on the cylinder head has improved from before adding the paper pad (that is, the amount of oil splashing is large). After confirming that it is correct, Reinstall the cylinder head cover and hit the road.

10. All sit-down motorcycle engines adopt a horizontal structure. Due to structural limitations, the bearing seat of the cylinder head camshaft is separate, which is very convenient to disassemble and assemble. However, because it is difficult to guarantee the machining accuracy of the bearing seat hole, and some maintenance personnel perform irregular operations during the disassembly and assembly process, the tightening torque of the bolts fastening the cylinder head is too large, or there are impurities in the bearing seat hole. After thousands of kilometers of operation, the rolling bearings on the camshaft are unevenly stressed, and their raceways are crushed and deformed by steel balls, which increases the clearance, causing early damage to the bearings and causing operating noise in the valvetrain. During maintenance, you can use your fingers to hold the inner ring and outer ring of the bearing respectively to measure the axial and radial clearance, as shown in Figure 10. At this time, the inner and outer rings of the bearing should not have obvious swing (generally the clearance of the bearing is within 5 ~20um). In addition, the bearings should not make abnormal noise when rotating and should be replaced as appropriate. In addition, it is necessary to carefully check whether there are bright spots in the bearing seat hole, whether the imprint marks of the semi-circular arc groove in contact between the outer circle of the bearing and the bearing seat hole are even, and whether there are foreign objects in the bearing seat hole. For imprints with bright spots, appropriate tools can be used to carefully trim them. After trial installation, disassemble and observe the fit imprints until all the bright spots disappear. During the trimming process, attention should be paid to applying force evenly, moving gently, and performing multiple disassembly and assembly inspections to avoid enlarging the bearing seat hole. When tightening the cylinder head bolts, tighten them 2 to 3 times from the inside to the outside to the specified torque.

11. Whether it is a straddle-type vehicle or a scooter, after installing the camshaft, bearing (or bushing) and rocker arm support assembly, the cylinder head cam oil pool must be filled with engine oil, as shown in Figure 11. Make the camshaft lift part completely submerged in engine oil to ensure initial lubrication between the camshaft and rocker arm in the early stages of engine operation. Because, starting from the oil pump, the lubricating oil has to pass through oil passages such as the crankcase, cylinder block, and cylinder head. It takes about 20 seconds to reach the cam oil pool. At this time, the cam and rocker arm are basically not lubricated by oil. If When the rider increases the throttle, the cam and rocker arm are prone to abnormal wear during high-speed operation. This should attract the attention of maintenance personnel and users.