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Ship diesel engine with the cylinder head bolts
The cylinder head bolt belongs to the powerful bolt in the diesel engine, and bears the maximum explosive force to separate the cylinder head body and the cylinder head. At the same time, under the action of the bolt pre-tightening force, the airtightness of the cylinder head gasket is ensured.
The bolts are generally designed as a flexible structure with a shaft diameter slightly smaller than the inner diameter of the thread. This structure reduces the magnitude of the load change acting on the bolt and improves the fatigue strength of the threaded connection. Although the pre-tightening stress of the shaft portion is improved, the strength of the shaft and the thread segment tends to be equal, and the structure is reasonable. When the thread outer diameter d≥16mm, the shaft diameter d0=0.8d1 (d1 indicates the inner diameter of the thread) is suitable, and should not be too small.
The recommended radius of the bolt body and the thread transition is R=(0.75～1.0)d, the surface roughness is generally 1.25um or 0.63um, and there is no need to have the knife mark to improve the fatigue strength. For other measures to improve the fatigue strength of bolts, refer to the previous section. The bolts are fastened to the cylinder body and often use tight fitting threads, that is, the threaded medium diameter has an interference fit. When the bolt is screwed into the cylinder, a large radial stress is generated near the thread, the screw hole is the tensile stress, and the thread is the compressive stress. To reduce this stress, a tapered hole is sometimes designed at the end of the bolt, and the depth of the tapered hole cannot exceed the effective length of the thread. In order to reduce the bending stress of the bolt, sometimes the screw is screwed in so that the screwing end surface is in close contact with the bottom plane of the screw hole, thereby being firmly fixed.
Most of the strong bolts are made of alloy structural steel with high tensile strength and yield strength, such as 40Cr, 38CrSi, 35CrMo, 40MnB, etc. For light heavy-duty diesel engines, advanced alloy structural steels such as 18CrNiW and 35CrNi3W are sometimes used.
In order to reduce the stress concentration factor in the thread, it is preferable to increase the radius R of the root of the thread, and the optimum ratio of the pitch S to the pitch S is R/S=0.18 to 0.21.
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