In automobile repair work, we often hear some “fit” concepts, such as clearance fit, interference fit, transition fit and so on. What do they mean? What are the applications in cars? Let’s analyze this problem.
The so-called “fit” refers to the relationship between tolerance zones of holes and shafts with the same basic size and mutual combination, which is generally said to be the relative relationship between holes and shafts. In the national standard, there are two kinds of standard system, namely the basic hole system and the basic shaft system. In general, the base hole system is preferred.
The so-called basic hole system refers to a system in which the basic deviation is used as the tolerance zone of a certain hole and the tolerance zone of the shaft with different basic deviation forms various matching. The hole made of base hole is the reference hole, and the lower tolerance deviation is zero. Generally speaking, the size of the hole is fixed and the matching property is adjusted by changing the size of the shaft.
The so-called basic axis system refers to a system in which the basic deviation is used as the tolerance zone of a certain shaft and the tolerance zone of a hole with different basic deviation forms various matching. The axis made of base axis is the datum axis, and the upper tolerance deviation is zero. Generally speaking, the size of the shaft is fixed, and the matching property is adjusted by changing the size of the hole.
According to the different relative position of tolerance zone of hole and shaft, clearance fit, interference fit and transition fit can be formed by two datum systems.
Clearance fit
When the algebraic difference obtained by subtracting the size of the matched shaft from the size of the hole is a positive number, it is called gap, and the size of the hole is greater than that of the shaft. This kind of fit with clearance (including minimum clearance of zero) is called clearance fit. Due to the existence of clearance, relative movement between the shaft and the hole is allowed. For example, if the size of the hole is 20.02mm and the size of the shaft is 19.98mm, then the fit clearance between the two is 20.02-19.98 = 0.04mm, and the shaft can rotate or slide freely in the hole.
Sliding bearings are generally used to support the gap fit parts, which are commonly known as “copper sleeve” and “bearing bush”. The inner surface of the bearing is usually cast or sprayed with a layer of special antifriction materials, such as Babbitt alloy, tungsten tin alloy, etc. Under the condition of liquid lubrication, the sliding surface is separated by the lubricating oil without direct contact, which can greatly reduce the friction loss and surface wear. The oil film also has certain vibration absorption capacity and can reduce the size of parts. The disadvantage is that the starting friction resistance is large. There are many applications of sliding bearing in automobile, such as curved bearing bush, connecting rod bush, cam bearing bush, transmission gear, etc.
In automobiles, there are many clearance fits. As long as there is relative movement between two parts, there must be clearance between them, such as piston and cylinder, crankshaft and cylinder block, connecting rod, gearbox shaft and housing, valve and valve guide, gear and gear shaft, wheel and axle, etc. According to the different motion relations, the gap size of clearance fit is also different, generally between 0 ~ 0.10 mm. For some very precise clearance fit, they are ground in pairs during manufacturing and are not allowed to be exchanged in the later stage. For example, the plunger of the high-pressure pump and the needle valve of the fuel injector have a clearance of 0.002 ~ 0.005mm, which are extremely precise. If the fuel is not clean enough and contains a small amount of impurities, it may cause the needle valve of the injector to be pulled and stuck.
For example, the normal clearance between crankshaft and cylinder block and connecting rod is between 0.04 mm and 0.07 mm. If the clearance is less than the clearance, the fit between the crankshaft, cylinder block and connecting rod is too tight, the running resistance is large, and it is not conducive to the formation of lubricating oil film, which is easy to cause the failure of Bush burning due to poor lubrication; if the clearance is greater than this, the fit between crankshaft and cylinder block and connecting rod is too loose, and the impact force is too large during operation, which will lead to noise increase and abnormal wear of bearing bush The lubricating oil film is easy to be damaged, too much oil leakage, low oil pressure, and even serious mechanical failures such as tile melting, crankshaft breaking and so on. Therefore, in the overhaul of the engine, to fine adjust the gap between the two, beyond the allowable range of error, we must find out the reason and make corresponding treatment.
There is also the fit between the piston and the cylinder, which is a very special clearance fit. Because the fuel burns in this part, the temperature of this part is very high. The cooling strength and material of piston and cylinder are different, and the expansion degree under high temperature is also different. The piston is generally made of aluminum. Its head is directly contacted with the flame, and the temperature will be as high as 600 ~ 800 ° C, while the piston skirt is less heated, so the piston will present an oval shape after being heated Circular deformation. Therefore, the clearance of this part is larger at room temperature and smaller during normal operation. Generally, the clearance between piston skirt and cylinder is 0.10 ~ 0.15mm, and the normal working time gap is less than 0.05mm. If the clearance is too small, or the engine temperature is too high, the piston will expand excessively, which will cause the piston to lock up in the cylinder, which is commonly known as “cylinder pulling” or “cylinder bursting”; if the gap between them is too large, it will cause cylinder knocking fault.