1、 Double nut locking

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 1

Double screw thread is used to prevent loosing. The structure shown in Fig. (a) is not allowed. The structure shown in Fig. (b) should be adopted. Flat nut is used at the bottom and thick nut is used at the top. However, considering that flat nut wrench can not be used at the bottom and can not be tightened, only two thick nuts can be used, as shown in Fig. (c).


2、 The two dowels should be of different lengths

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 2

The assembly of large box often needs to set several locating pins. It is forbidden to make all locating pins the same height, as shown in figure a, because it is difficult to align several locating pins at the same time. The locating pins should be made into different lengths, as shown in Figure B. It is easier to align one locating pin first when closing the box.


3、 In gear transmission, the correct configuration of big and small gears

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 3

In order to facilitate the installation and avoid step wear during the running of the gear, the width of the pinion should be 5 ~ 10 mm wider than that of the large gear, as shown in Fig. C. However, if the pinion is made of plastic, the tooth width of the pinion should be smaller than that of the large gear, as shown in Fig. D, so as to avoid grinding dents on the gear.


4、 Screw connection often disassembled and assembled

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 4

The screw connection is shown in Figure a. the characteristic of this connection is that the screw is directly screwed into the threaded hole to be connected without nut. The structure is simple and compact. It is suitable for those structures that cannot be connected by bolts. The screw connection should not be used in the occasions with large mining force or frequent disassembly. If it is disassembled frequently, the thread will be easily worn and the connected parts may be scrapped, If the screw hole is made of steel or surface steel, H ≈ D; if the screw hole is made of cast iron, H = (1.25 ~ 1.5) d; if the screw hole is made of aluminum alloy, H = (1.5 ~ 2.5) d, threaded hole H1 = H + (2 ~ 2.5) P (P is thread pitch), and drilling depth h2 = H1 + (0.5 ~ 1) d.


5、 Intermediate coupling bearing of high speed shaft

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 5

When the coupling is installed on the shaft end suspension with high rotation, the structure shown in Fig. A is forbidden. The length of the cantilever should be reduced. The larger the cantilever is, the greater the deformation and unbalanced weight will be. Therefore, when the coupling is installed on the cantilever end, in addition to the stress of the ruler to reduce the weight of the coupling, the ruler should also be close to the bearing.


6、 Deep hole machining of small diameter

Do You Know the Commonplace 6 Mistakes in Machanical  engineering 6

It is difficult to process the small diameter deep hole with high cost and low efficiency. It is forbidden to design deep and small diameter lubrication holes on the rotating shaft, as shown in Figure A. if possible, open larger holes as far as possible. If necessary, make different diameters as shown in Figure B.

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