Is there an old iron that may encounter such a problem?
The dynamic torque is 30Nm for different connection pairs, but the static torque of some connection pairs is 33Nm or higher (higher than the dynamic torque), and some of the connection pairs have only 26Nm or less (lower than the dynamic torque), why is this?
What factors affect dynamic torque and static torque? What is the connection and difference between the two? How should it be controlled in the actual application process?
Today, Screw Jun answers your questions through this article.
Dynamic torque is the torque measured with the gun sensor during the tightening process of the bolt, which is the torque achieved by the bolt to overcome the dynamic friction. Both torque wrenches and power tools can apply dynamic torque.
The following figure shows the tightening curve of a high-precision gun, and the end point of the curve is the dynamic torque.
Static torque is the maximum torque achieved by overcoming static friction in the tightening direction after the bolt of the fastener is tightened.
The torque value measured by the dial or digital torque wrench that is generally used is static torque.
The purpose of dynamic torque measurement is to ensure that the torque during tightening is acceptable. The purpose of static torque measurement is to verify that there is no variation in the dynamic process and at the same time to ensure the reliability of the clamping force of the coupling after tightening.
The static torque should be consistent with the dynamic torque. But in reality, the relationship between the two is mainly affected by two aspects: static friction and elastic attenuation.
1. The influence of static friction force.
Due to the presence of static friction, additional torque is required to overcome the static friction during the inspection process and move the bolt. As a result, there is a tendency for static torque to be higher than dynamic torque.
In some cases, we measure static torque that is higher than the dynamic torque. This does not mean that there is a greater clamping force, but only the effect of static friction.
2.Effect of elastic attenuation.
Since the static torque is measured after tightening. After a certain period of time, creep and embedding will occur in the material under force. Torque attenuation is generated. Elastic attenuation causes static torque to produce a tendency to be smaller than dynamic torque.
Normally, the static torque on the hard connection is generally greater than the dynamic torque, as shown in the figure below, the torque at point B (static torque) is higher than the torque at point A (dynamic torque).
The dynamic torque on the soft connection is generally greater than the static torque, as shown in the figure below, the torque at point B (static torque) is lower than the torque at point A (dynamic torque).
Depending on the connection method of the fastener, the dynamic torque and static torque acting on the coupling pair are also different. Comparison is made with data on the static and dynamic torques of typical hard-connected and soft-connected fasteners.
Let's take a practical example to see the difference between static torque and dynamic torque in soft connection and hard connection.
Table 1 shows 10 sets of dynamic and static torque measurements in the hard-link method. The dynamic mean is calculated to be 9184n.m, the static mean is 10139n.m。
Table 1 Torque measurement data of a hard connection method
Table 2 shows the 10 sets of dynamic and static torque measurements for the soft connection method. The dynamic mean is calculated to be 9042n.m, the static mean is 7778n.m。
From the comparison of the above two sets of data, it can be concluded that for the threaded pair in the form of hard connection, the static torque is greater than the dynamic torque, while for the threaded pair in the form of soft connection, the static torque is less than the dynamic torque.
Table 2 Torque measurement data of a soft connection method
The torque standard given in the general document is dynamic torque, while the torque detected by the actual inspector with a dial torque wrench is a static torque. In general, there is a difference between dynamic torque and static torque.
In order to better control the torque of the bolt, the bolt torque of the automobile assembly can be controlled in two parts, one is to use dynamic torque control during the tightening process, and the other is to supervise the assembled parts with static torque. This can more accurately and truly reflect the actual torque of the bolt.
Table 3 Dynamic torque test data
Based on the above data, the average value of the 30 sets of dynamic torques is 9833n.m, and then calculate the standard deviation (sigma) as 051. Finally, the upper and lower limits and target values of the dynamic control specifications are calculated according to the 3 Sigma principle.
The upper limit of the Table 3 Dynamic Torque Data Collection Table is 9986n.m, the lower limit value is 9682n.m。The upper and lower limits are rounded averaging, and the dynamic torque control range is 97Nm～100n.m。
Table 4 Static torque test data
Based on the above data, the average static torque of 30 groups is calculated to be 8542n.m, and then calculate the standard deviation (sigma) as 171. Finally, the upper limit, lower limit and target value of the static control specification are calculated according to the 3 sigma principle.
Table 4 Static torque data collection table The upper limit of torque is 9055n.m, the lower limit value is 029n.m。The upper and lower limits are rounded averaging, and the static torque control range is 80Nm～91n.m。
The softness and hardness of the connector.
Softer parts to be connected: e.g. aluminum.
Fastener size: small diameter, long bolt is soft.
Coating: Special coating materials are soft.
Whether there are gaskets: Some gaskets are on the soft side.
Temperature influence: steel and aluminum connection, different materials have different coefficients of thermal expansion.
Tightening equipment and tightening method.
Tightening tool: The error of the tightening equipment.
Tightening method: one-step tightening and multi-step tightening, angle method tightening and torque method tightening.
Torque detection. Personnel: the same wrench, different personnel test out of the value is different, static torque measurement personnel should be trained and certified to work.
Detection tools and methods: Differences in tools and methods can affect detection results.
FirstDynamic torque and static torque are two important indicators to ensure the quality of the connection pair, and the two are different and have a certain relationship. The purpose of dynamic torque measurement is to ensure that the torque during tightening is acceptable.
The purpose of static torque measurement is to verify that there is no variation in the dynamic process and at the same time to ensure the reliability of the clamping force of the coupling after tightening.
The factors that affect the relationship between dynamic torque and static torque include the soft and hard of the connecting pair, the tightening equipment and tightening method, and torque detection.
SecondIn practical application, it is necessary to distinguish the connection status of the connecting pair, establish the key dynamic and static torque control charts, usually according to the test data of the actual connecting pair, use statistical methods to determine the control table, and regularly optimize the control table.
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