Cylinder Body

Common failure modes can be roughly divided into four categories: wear, deformation, corrosion, and hole cracking. Methods for repairing the main bearing bore size include traditional methods and two new processes: zinc-aluminum alloy electroplating and nickel wire argon arc welding. Key points to note during the repair process. Keywords: Cylinder body bearing hole repair

1 Preface

The engine is the heart of various power equipment such as automobiles, ships, and internal combustion engines. The cylinder body is a basic and valuable part of the engine, and its value can account for more than one-fourth of the entire engine, especially the Price of imported cylinder bodies is astonishing. The unit Price of the Cummins K50 engine cylinder body equipped with the MK-36 mining truck is as high as 500,000 yuan, which is equivalent to the Price of a high-end car. It can be seen how significant the economic significance of cylinder body repair is.

The common failure modes of the cylinder body can be roughly divided into the following four categories: 1. Wear. Wear and "burn" of the main bearing bore (hereinafter sometimes referred to as the bearing bore), camshaft bore, and thrust bearing bore end face; 2. Deformation. Deformation of the main bearing cap (hereinafter sometimes referred to as the bearing cap) due to force inward causes loosening of the stop, and the entire body deforms due to fracture of the crankshaft and connecting rod assembly; 3. Corrosion. Serious failure of the cylinder head gasket sealing surface and wet cylinder liner flange sealing surface due to cavitation and water erosion; 4. Hole cracks. Loosening and falling off of connecting rod bolts, problems with the piston assembly, etc., can damage the cylinder liner hole or punch holes or cracks in different parts of the cylinder body. Some other reasons may also cause cylinder body cracks. Most of the above failure modes can be repaired using traditional conventional processes. Among them, the first two failure modes are the most common and have the most repair value. Therefore, in recent years, some large users, professional repair plants, and relevant scientific research units have carried out a lot of research work on these failure modes. Based on many years of exploration and practice, the author discusses some basic understandings of the repair of the main bearing bore of the cylinder body.

2 Methods for Restoring Main Bearing Bore Size

2.1 Several Traditional Processes

The common failure modes of the main bearing bore are: inward deformation of the main bearing cap causing loosening of its stop, resulting in horizontal misalignment of the upper and lower halves and exceeding the cylindricity of the bearing bore; "burning" of a bearing bore due to "bearing failure"; and due to severe deformation of the bearing bore, its Size, shape, and position accuracy are out of tolerance to varying degrees; deformation of the entire cylinder body will also cause deformation of the bearing bore and exceeding the coaxiality between the various bearing bores.

The key to repairing the main bearing bore is the process method for restoring the Size. Traditional methods include: manual arc cast iron welding, inlay, increased bearing backing, brazing, and spraying. In fact, brazing and spraying are the least desirable. Because brazing requires local heating of the cylinder body welding area to thousands of degrees, the stress deformation and other damage caused by such high temperatures to the cylinder body are obvious. The bonding strength of spraying is less than 30N/mm2, and it cannot withstand the "hammering" effect of fatigue cycles produced by the reciprocating motion of the piston for a long time. Coating shedding is a matter of time. The method of increasing the bearing backing is to bore the bearing bore diameter by 1mm and then use a main bearing with a bearing backing diameter increased by 1mm specially made by the engine manufacturer. This process is still feasible, but only a few engine manufacturers develop this special bearing. Even if there is such a bearing, most users and their repair departments are unlikely to have it in stock. Only professional repair plants may have this spare part, which will inevitably delay the repair time. The second disadvantage of this method is that the manufacturer only produces bearings with a bearing backing diameter increased by 1mm. If the bearing bore damage is severe or there is secondary damage, this method will be ineffective. Its third disadvantage is that if the bearing bore wear is very small (for example, about 0.10mm), 1mm also needs to be bored off the bore diameter. The inlay method is as follows: the manufacturer supplies a main bearing cap with a machining allowance, and the cylinder body half bore is bored away by a few millimeters in radius, and then the half bore steel sleeve with machining allowance is riveted to the bearing bore. After machining to the standard Size. This process is also feasible, but in addition to the several disadvantages of the bearing backing increase method, there is an additional layer of steel sleeve between the main bearing and the bearing bore, and the adhesion between layers cannot be too good, which will inevitably reduce the heat dissipation capacity of the bearing bore. In addition, the bearing clearance is difficult to control, which increases the possibility of "bearing failure" and shaft seizure. In fact, the manual arc welding method is still commonly used to restore the bearing bore. This process is more difficult and requires the welder to have rich experience and a strong sense of responsibility. Because cast iron welding is very easy to produce a large number of pores, undercuts, and hard spots, this will greatly reduce the number of contact points between the bearing backing and the bearing bore, thus affecting heat dissipation and laying the hidden danger of "bearing failure" and shaft seizure. The repair of the main bearing positioning groove on the bearing bore is also a detail that is very prone to problems. The cylinder body manufacturer uses special equipment to process this groove, while the repairer manually repairs it, which cannot be very accurate. It is very easy for the main bearing to be unstable in the bearing bore, that is, the so-called "breathing" phenomenon. When the "breathing" reaches a certain degree, the main bearing will rotate in the bearing bore, the oil path will be interrupted, and "bearing failure" is inevitable. The biggest hidden danger of welding is the problem of stress and deformation. Sometimes, no matter how the main bore is inspected before installation, it is qualified. After the failure, when the bore is inspected again, it is seriously deformed and out of round. The root cause lies in the welding stress.

2.2 Two Applicable New Processes

2.2.1 Zinc-Aluminum Alloy Electroplating

The bearing bore out-of-tolerance caused by pressure wear and slow deformation is often not very serious, and the out-of-tolerance value in diameter is generally not greater than 0.5mm. The best process for restoring the Size of this type of bearing bore is metal electroplating. The only commercially available plating solutions commonly used to restore Size are nickel and copper. However, both of these plating solutions have low bonding strength, and the former is too hard and the latter is too brittle, neither of which can be manually scraped and machined.

In order to solve these problems, after half a year of repeated experiments, we have developed a zinc-aluminum alloy plating solution, the formula of which is:

Zinc salt: 500g/L

Aluminum salt: 80g/L

Additive Ⅰ: 30g/L

Additive Ⅱ: 24g/L

PH: 4

This plating solution does not require a nickel underlayer and can be directly plated on the main bearing bore. Its brush plating process, parameters, and precautions are the same as conventional brush plating. The plating layer of this plating solution has low hardness, good toughness, high bonding strength, and good machinability. We have used this plating solution to repair hundreds of cylinder bodies, and the effect is very ideal.

2.2.2 Nickel Wire TIG Welding

This method is relatively simple; it is a type of tungsten inert gas welding using pure nickel welding wire and manual operation. The cross-sectional area of the welding wire should not exceed 10mm². Before welding, pay special attention to heating and degreasing. The advantages of this cast iron surfacing process are: no undercutting, no pores, thin weld layer, no hard spots, and high bonding strength. Because the welding current can be controlled within 100A, the welding stress deformation is significantly lower than that of manual arc welding. This method is most suitable for surfacing the bore that has been severely 'burned' due to 'roller bearings'.

3 Key Points of Bore Repair Process

The main procedures for repairing cylinder block bores are: checking the plan—repairing the bearing cap—pre-boring—heating and degreasing—restoring the bore size—boring the bore—inspection and acceptance. Repair personnel should pay sufficient attention to some key points in this process.

3.1 Key Points of Bearing Cap Repair

According to the positioning situation, bearing caps can be divided into single-lip positioning bearing caps (in-line engines) and double-lip positioning bearing caps (V-type engines). The repair of single-lip positioning bearing caps, especially those with bolt holes inclined to the mounting surface, involves restoring the half-hole size and repairing the two end faces of the positioning lip. The restoration of the bearing cap half-hole size can be divided into the following situations: For the bearing cap half-hole shown in Figure 1, as long as the 'burn' is not too serious, the half-hole can be sunk by milling the mounting surface to create machining allowance. However, if the cylinder block half-hole needs to be plated, the bearing cap should not be bored in this way, to avoid uneven hardness between the upper and lower half-holes causing the machining to be out of round. For the bearing caps shown in Figures 2 and 3, it is best not to sink the half-hole too much by milling the mounting surface, because this will cause the side pull bolt hole of the bearing cap in Figure 2 to be vertically misaligned with the bolt hole on the cylinder block and cannot be installed; similarly, it will also cause the lower end of the bearing cap bolt hole in Figure 3 to be horizontally misaligned with the bolt hole on the cylinder block, causing 'strain' and affecting the cylindricity of the bore. A more reasonable choice for restoring the size of these two types of bearing cap half-holes is: If the hole diameter exceeds the limit by a small amount, the bearing cap half-hole can be sunk or sunk in combination with plating; If the hole diameter exceeds the limit by a large amount, nickel wire TIG welding should be chosen. The welded bearing cap should be subjected to high-temperature tempering to eliminate welding stress. And the mounting surface of the bearing cap should be ground with a surface plate to ensure sufficient contact points.

Usually, the bearing cap and cylinder block lip are interference fits, which is determined by the positioning function of the lip. The direction of force on the bearing cap during operation determines that the bearing cap will eventually undergo inward shrinkage deformation, causing the lip to change from an interference fit to a clearance fit. In severe cases, the clearance can reach tens of micrometers. This will inevitably make the size and shape of the bore uncertain, so it is necessary to restore the original interference fit of the bearing cap lip end face first; otherwise, repairing the bore first would be meaningless. The repair process of this end face is to determine the size to be restored on each side by measurement, perform spray welding or surfacing welding, and then process it with a bearing cap lip end face grinding machine or other general-purpose machine tools, but it should be noted that the mounting surface and half-hole end face of the bearing cap must be used as the positioning datum. The bottom surface, as the installation positioning datum, must be ground in advance to ensure sufficient contact points. For the double-lip positioning bearing cap shown in Figure 2, it should be ensured that both lips are interference fits, the interference amount should be between 0.03 and 0.08 mm, and the interference amount of the lip with side pull bolts should be slightly smaller.

3.2 Pre-boring

If a certain half-bore on the cylinder body has sunk, its size must be restored. If plating is used, the upper and lower half-holes should be bored at once, and the boring amount should be minimized. Note that only half of the hole should not be bored to avoid out-of-roundness and make polishing difficult. If surfacing welding is used, at least 1 mm should be evenly bored on the diameter to ensure uniform hardness of the weld layer.

3.3 Boring Main Bearing Holes

In addition to repairing all bores, it is generally not recommended to machine bores on an ordinary boring machine, because although the holes can be aligned well on the boring machine, tightening will cause new offsets, causing the machined holes and unmachined holes to be coaxial. It is best to machine the bores on a dedicated boring machine. The cutter bar of this boring machine is flexibly connected to the machine spindle, and the machine accuracy has basically no effect on the machining accuracy. Insert two positioning sleeves with certain mating accuracy into two standard bores that are as far apart as possible, insert the cutter bar into these two positioning sleeves, install the cutter head on the part to be bored, and then start boring. The dimensional accuracy is solved by the tooling block. The advantages of this machining method are that as long as the cooperation between the cutter bar and the positioning sleeve, and the positioning sleeve and the bore is reasonable, and the mounting surface is clean, then the coaxiality between the bored bore and the positioning hole is easy to ensure. Below is a brief introduction to four practical methods for the coaxiality inspection problem that users are concerned about: First, directly install the crankshaft and rotate it. This method is simple and easy to implement, and can meet general use requirements; it has been adopted by most repair workers; Second, the method of using a test bar and a plug gauge. That is, a test bar with a total length bending of no more than 0.005 mm (the length should be sufficient, the diameter can be about Ø60) is placed in all the bores, after the free swing stops, a 0.02 mm thick plug gauge is inserted between each bore and the test bar, if none can be inserted, it can be considered that the coaxiality error of adjacent bores and all bores of the cylinder is no more than 0.02 mm. Of course, the cylinder also needs to be rotated 90° and the above inspection steps repeated. This method is both accurate and simple; Third, the inspection ring and plug gauge method. That is, an inspection ring is added to the cutter bar of the boring machine, and the clearance between the inspection ring and the cutter bar and the bore is appropriate. Push the inspection ring into the bore to be inspected, insert a plug gauge around it, and determine whether the coaxiality is qualified based on the insertion situation. This method requires precise design and manufacturing of the testing tool's accuracy and matching clearance, and should not be adopted by non-professional factories; Fourth, a long test bar with a down-tolerance of about 0.04mm less than the bore is made, inserted into the cylinder block half-bore, all the bearing caps are installed according to the specified torque and fastening method, and then the test bar is rotated with a handle, and the coaxiality is determined based on the force. If the test bar is coated with red dan powder in advance, when the coaxiality is considered to be a problem, the bearing cap can be opened to scrape the high points, which is also a major advantage of this method. Making a test bar for each repaired cylinder is of course not very economical. This method is suitable for specialized repair with a single type of cylinder block.