Some people think that the cylinder head nut is tightened as tightly as possible. This is not the case. For example, the tightening torque of Dongfanghong 802 cylinder head nut is 190~210Nm. Because the tightening torque of the nut is too large and exceeds the specified torque, the surrounding part of the cylinder block is pulled up, causing the cylinder block and the cylinder head to exceed the limit. The value is 0.10 mm), which makes the cylinder head Gasket fail and an air leakage accident occurs.
The damage caused by the tightening of the screw is too large, the torque of the needle of the injector needle is too large, and the deformation of the needle valve hole is affected, which affects the atomization quality; the tightening torque of the oil outlet valve is too large, causing the plunger to deform and the rod The movement is not working and the fuel pump is not working.
Cause Analysis The specified value of the tightening torque is provided by the manufacturer. When the bolt is tightened too tightly (beyond the specified value), the screw will be elongated and broken, the thread will be damaged, and the screw hole of the fastening will be bumped. If the tensile force exceeds the elastic limit of the screw, the screw will be permanently deformed and elongated; if the tensile force exceeds the strength limit, the screw will be broken.
The consequences of over-tightening of the bolt often occur at the weak points of the bolt and the part being fastened, and not all relevant parts are damaged. From the analysis of the force mechanism of the threaded connection, the threaded connection is achieved by the frictional torque between the threads. Under static load conditions, the screw tightening torque is equal to the sum of the frictional torque between the screw threads and the frictional moment of the screw bearing surface (such as the screw connection of the valve chamber cover). The screw tightening torque causes the screw and the connected parts (the valve chamber cover and the cylinder head) to be subjected to a preload force, that is, the screw rod and the cylinder head screw hole are subjected to tension and torque, and the valve chamber cover is subjected to pressure. For a stud connection to a cylinder block and cylinder head that is subjected to an alternating load, the tightening torque is greater than at the static load because a thrust of the combustion gas against the cylinder head is increased.
Countermeasure a. The specified value of the bolt tightening torque provided by the manufacturer has increased from the past. For example, Dongfanghong 75/802 has only six specified values, while Dongfanghong 1002/1202 has 23 specified values ​​(excluding the tightening torque of the fuel injection pump and the delivery valve).
b. The tightening torque for the general bolt is given by the diameter and material. For example, the safe tightening torque of low carbon steel bolts gives the following data by diameter: 10mm is 60-80Nm, 12mm is 80-100Nm, 14mm is 100-120Nm, and other diameters are similar.
c. When the important bolt is tightened with an ordinary wrench without a torque wrench, the force arm should be 30 to 40 times the diameter of the bolt.
d. The agricultural machinery management department should also pay attention to it, vigorously promote the importance of tightening the bolt according to the specified torque value and strengthen the bolt torque management of important parts, including the installation and periodic inspection of the torque wrench.
A fitting is any part used in a piping system to change direction or function and joined to that system by some mechanical means or by welding. The simplest way to change the direction of a pipe would be to bend it.However, besides restricting the flow, this method often distorts the shape of the pipe and reduces the wall strength. Another method of altering direction is with mitered joints, I.e. straight piecesof pipe cut for the required turn and welded together. This type lowers flow efficiency considerably by creating greater turbulence. A single mitered bend has about six times the resistance of a smooth elbow. Fittings are preferable for most piping systems where rate of flow, internal pressure and protection against a corrosive environment are primary design considerations
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