Talking about the control method of pre-tightening force of engineering machinery bolt

1 bolt preloading effect

Most bolted joints must be tightened during assembly so that they are pre-stressed before they are subjected to working loads. This pre-energizing force is called preload. The purpose of the pre-tightening is to enhance the reliability and tightness of the connection, to prevent gaps or relative slippage of the connected parts after being loaded, and finally to ensure that the fasteners are reliably connected together. Numerous studies and experiments have shown that the pre-tightening force of the bolt connection will have an important influence on the total bolt load, the connection critical load, the lateral load resistance and the sealing ability of the joint surface. The key to the reliable connection of the fasteners is to control the axial preload of the bolted joints to the appropriate range. If the pre-tightening force is not appropriate, it will not only cause the connected parts to slip, separate or loosen, but also cause damage to the bolts.

2 bolt tightening axial preload selection

The lower limit of the axial pre-tightening force of the bolt is determined by the function of the joint structure, which is to ensure that the fastener is always reliably attached during the working process. The upper limit of the axial preload of the bolt is determined by the bolt (or nut) and the strength and force state of the fastener. This value is to ensure that the bolt and the fastener are not elongated or sheared during preloading and working. Fatigue fracture, tripping and other phenomena. A large number of studies have shown that a larger initial preload can effectively improve the bolting stability of the bolt connection, so the bolt preload should be as high as possible while ensuring that the thread is not damaged. The bolt fastening points for the key parts of the hydraulic excavator are shown in Table 1.

3 bolt tightening method

3.1 Torque method

The torque tightening method is the most commonly used bolt tightening method, that is, the torque value displayed by the torque wrench is used to control the axial force of the connected member. The control torque is mainly realized by a fixed torque tool, including a manual fixed torque wrench, a pneumatic fixed torque wrench, a pulse wrench, a hydraulic wrench, a rechargeable fixed torque wrench, an electric tightening machine, and the like. The advantage of the torque tightening method is that the operation is simple and easy, and the cost is low. The shortcomings are greatly affected by the torque factor. To ensure the accuracy, it is not only necessary to control the precision of the tightening tool, but also to control the dispersion of the torque coefficient.

3.2 Torque - Angle Method

The torque-turn method is to turn the threaded member to a specified angle after tightening to the specified "fit torque". In the initial stage of bolt tightening, the preload force is not linear with the angle of rotation. In this stage, the “fit torque” control mode is selected, and the general torque value is selected from 20% to 30% of the final torque.

Figure 1 shows the relationship between the tightening angle and the preload force during bolt tightening. The OA phase is the “idle” phase; the AB phase is the “close-fitting” process. At this stage, the bolt fasteners and the pre-tightening force of the fasteners are small, but the rotation angle increases rapidly; the BC phase is the “linear” segment. The pre-tightening force is linear with the angle of rotation; after the point C, the bolt enters the yielding phase. According to the preload force, the torque-turn angle method is divided into two types: the elastic zone tightening and the plastic zone tightening. The former occurs in the bolt elastic range, and the latter occurs after the bolt is tightened to the yield point. The change of frictional resistance when the elastic zone is tightened only affects the starting point of the measuring angle. If the starting point is selected properly, it will have little effect on the final pressing force. The plastic zone tightening occurs in the plastic deformation zone, and the pressing force depends only on the yield limit of the bolt material.

The plastic zone tightening method has high requirements on the quality of bolts and fasteners. For this reason, the engineering machinery industry currently adopts the elastic zone tightening method. The main tools include intelligent electric tightening machines and automatic hydraulic wrenches. The advantages of the torque-turn angle method: the influence of the torque coefficient is small, the control precision is higher than the torque method; the screwing to the plastic zone is not affected by the torque coefficient, and the bearing capacity of the bolt can be fully utilized. Shortcomings of the Torque-Rotation method: The control cost is higher than the torque method. It is not suitable for short bolts with small corners; the pre-tightening force is large (especially screwed into the plastic zone), and it is not suitable for occasions where the bolts and bolts with poor plasticity are used repeatedly.

3.3 Pre-elongation method

The bolt pre-elongation method is also called the quality assurance method, that is, during the tightening process or after the tightening is completed, the elongation length of the bolt is measured, and the axial pre-tightening force of the bolt is controlled by the relationship between the pre-tightening force and the bolt length variation. The bolt pre-elongation method is similar to the corner method, and can be divided into two types: the elastic zone and the plastic zone tightening method according to the pre-tightening force.

The bolt pre-elongation method has a very high control accuracy and is often used as an important part of the bolt pre-tightening force control. The common tool for bolt pre-elongation method is hydraulic tensioner. The method is as follows: use hydraulic tensioner to apply tension to the bolt to make it stretch. After reaching the specified elongation or pre-tightening force, gently tighten the nut. Just fine. Advantages of bolt pre-elongation method: suitable for long bolt fastening, the control precision is very high, it can be fastened to the plastic zone of the material, and the bearing capacity of the bolt can be fully utilized. Disadvantages of the bolt pre-elongation method: the tool has higher requirements on the structure of the parts and the control cost is high.

4 bolt tightening detection method

According to the importance of the thread connection of engineering machinery, there are three main methods for detecting bolt tightening: after-the-fact method, torque process detection method and process monitoring method.

4.1 After the law

The after-the-fact method uses the torque method to control the torque test. The specific method is as follows: a mechanical indicating torque wrench or an electronic torque wrench with high precision is used to test the torque in a loose or tight manner, and the test value is multiplied by a coefficient to calculate the assembly torque. The post-test torque and tightening torque have large errors, so this method is generally applied to bolt torque detection in non-critical parts.

4.2 Torque Process Test

The torque process detection method uses the sensor synchronous feedback torque when using the torque method control. Torque wrenches and other tools are commonly used to directly or indirectly detect the tightening process torque. The advantage of the post-event method is that it can synchronously feedback the tightening torque information with higher precision. The disadvantage is that it is impossible to feed back the axial preload force abnormality caused by the thread quality and the like.

4.3 Process Monitoring Method

The process monitoring method achieves the purpose of controlling and monitoring bolt torque by matching and complementary applications of different control methods. Process monitoring methods include torque control / angle monitoring, angle control / torque monitoring, yield point control / torque or angle monitoring. In construction machinery, the most widely used are torque control / angle monitoring, angle control / torque monitoring, and generally tightened by intelligent electric tightening machine.

The process monitoring method can not only monitor whether the tightening target is achieved, but also check whether the thread is qualified, the ability and accuracy of tightening the tool, the connection status of the fastener and the connecting member, and whether the lubrication condition of the connecting member is normal. The process monitoring method can effectively avoid the occurrence of tightening abnormalities, so the key bolts should be tightened by this method.

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