The quality of the product is assessed by comprehensive indicators such as the performance, use and life of the product. In order to ensure the quality of the product, several process assembly requirements are proposed for the product, and these assembly requirements should be guaranteed during the assembly process. Describe the assembly methods and precautions for bolt fasteners and some common hydraulic components.
Whether a company can remain invincible in the fierce competition in the same industry, the quality of its products is a decisive factor. Of course, there are many factors that determine the quality of products. From design, assembly to testing, a series of processes will affect the products. quality.
It should be said that China's requirements for assembly at this stage are not very strict. In many enterprises, there is an idea of ​​designing light craftsmanship. The design of products is important, but the quality of a product is also closely related to the level of assembly.
I believe many people know that the difference in quality and performance between the whole machine products assembled with pure imported parts and the imported original machine products in China is still relatively large. In fact, one of the problems lies in the assembly process. The importance of the process of guiding the assembly is self-evident.
Assembly is the process of fitting or joining parts or components into semi-finished or finished products according to the specified technical requirements. The assembly of the whole machine is the final stage in the production process. It includes assembly, adjustment, inspection and testing, and the final quality of the product is guaranteed by assembly. The fastening of the bolts and the connection of the hydraulic components are the two major points in the technical requirements of the assembly process.
1. Assembly of bolt fasteners
Threaded joints are widely used in mechanical structures. The quality of threaded joints is not only affected by machining accuracy, but also has a great relationship with assembly technology. If the order of tightening the nuts is incorrect, the uneven force will deform the parts and reduce the assembly accuracy.
For threaded connections on moving parts, if the tightening force is insufficient, the life of the connecting parts will be greatly shortened and even cause an accident. Most threads are pre-tensioned when connected to enhance the rigidity, tightness, looseness and slip resistance of the connection. Proper control of the preload is the key to ensuring the quality of the threaded connection.
Because the preload of the threaded connection will have an effect on the total load of the thread, the critical load of the joint, the ability to resist lateral loads, and the sealing performance of the joint. Excessive or too small preload is harmful, so the magnitude and accuracy of the preload are important.
Therefore, for important threaded connections, the preload force must be specified. For medium and small bolts, the constant torque method (with a fixed torque wrench) is used to control the preload. As for the tightening torque, the problem is not explained here. The company's technical documents have specific regulations and a list of tightening torques that can be relied upon.
The following points should be noted in the assembly process:
(1) When tightening bolts or nuts with a pneumatic wrench, be sure to maintain the vertical angle of the air wrench to the pre-tightened joint to prevent misalignment. Many workshop workers are habitually tightened with a sleeve directly, and rarely pay attention to the angle, so it is easy to deflect and fail to meet the mechanical requirements of the joint surface.
(2) When tightening bolts and nuts, it is forbidden to use improper wrenches and sleeves.
(3) In accordance with the fastener assembly specified in the drawings or process documents, it is not permissible to replace low-performance fasteners with low-performance fasteners or high-performance fasteners.
There are generally several types of bolts on one station in the workshop. Because the assembly cycle is faster, sometimes the workers pay little attention to this point and use it. Therefore, two types of bolts often exist on one assembly surface. phenomenon.
(4) Generally speaking, bolts and screws should be screwed into 2~3 pitches before being assembled, and then tightened with a wrench or pneumatic tool. Especially for hydraulic flange connection bolts, you can manually screw in more than 2~3 pitches, then use a wrench or a low-speed small torque pneumatic wrench to tighten the two times according to the diagonal principle, and then use at least the corresponding torque wrench to turn 30° reaches the specified torque.
The author found in the workshop that individual workers put the bolt into the socket of the pneumatic wrench and align it with the screw hole. The labor intensity is reduced and the efficiency is high, but it is easy to be biased because the power of the air wrench is very large. Even if the screw is not properly screwed, it will be strongly screwed in, which will seriously damage the mechanical properties of the bolt.
(5) When applying a thread-locking sealant to the external thread, it shall be applied within a length of 15 mm from the end of the thread.
(6) When the group bolts and nuts are tightened, they should be tightened in a certain order according to the shape of the connected parts and the distribution of the bolts (usually 2~3 times). Groups of bolts or nuts arranged in a rectangular shape shall be tightened from the middle and gradually expand symmetrically to both sides, see Figure 1 (a); the set of bolts or nuts arranged in a square or circular shape shall be symmetrically tightened, see Figure 1 (b) ).
Figure 1 Group bolt tightening sequence diagram
If there is a locating pin, tighten it from the bolt near the locating pin. In order to prevent the bolts that are tightened one by one from being deformed due to excessive changes in the pre-tightening force, the method of tightening three times and tightening in reverse order is adopted. which is:
The first tightening torque is based on 50% Ty (Ty is the tightening torque specified in the relevant technical documents), and the bolts are symmetrically tightened in sequence;
The second time is based on 80% Ty, and the bolts are symmetrically tightened in reverse order;
The third time is based on 100% Ty, and the bolts are tightened symmetrically in sequence.
2. Assembly of common hydraulic parts
Oil leakage is one of the ills of construction machinery, which is not only related to design, but also an unreasonable assembly of hydraulic components.
The following are some issues that should be noted during the assembly of hydraulic components:
(1) Check that the mating surface and joint thread are clean. Wipe the mating surface and joint thread with a lint-free paper or silk material until the requirements are met.
(2) Electrophoretic paint is not allowed at the joint surface. The presence of electrophoretic paint can affect the quality of the assembly and the sealing of the hydraulic components.
(3) Avoid scratching the hydraulic components during the assembly process. The protective cap at the joint surface can be removed during assembly and is not allowed to be removed in advance.
(4) When placing the O-ring, wipe the hand cleanly, put the O-ring into the O-shaped groove by hand, flatten it, do not wear gloves to place the O-ring, and do not allow grease to be applied. After the installation, the O-ring and the flange groove should be carefully checked. If it is not suitable, it should be placed aside and not assembled.
Because, if the position of the O-ring is not suitable, the O-ring will be deformed or even damaged during the fastening process, resulting in uneven pressing force on the mating surface, loss of sealing performance, and oil leakage. It should be said that It was not very good before. Grease is not allowed to avoid contamination of hydraulic oil and to prevent deterioration of hydraulic oil.
(5) When tightening the joint body, the gas trigger is not allowed. The joint must be screwed to the end manually, and then tightened to the specified torque with a torque wrench of the corresponding specification. However, as far as the author's observations are concerned, many workers' drawings are convenient and quick, and they are directly tightened with a gas trigger. This should be avoided.
(6) When assembling the hose, the hose connector is required to be connected to the oil port without distortion or torque on the hose due to assembly and use. Therefore, the correct installation sequence is: first tighten the elbow end of the hose. Tighten the straight end afterwards; the hose with the hinge should first tighten the joint end. If there is distortion or torque during the assembly process, the above principle must be followed. Loosen the straight end first, and then tighten the other end.
(7) When fitting a joint body with an O-ring or a 24° cone hose, a small amount of oil corresponding to the system should be applied to the thread and the tapered surface to enhance lubrication and sealing.
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