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Bolts Can Prop Up A Good Role

This paper discusses the Bolts tightening control method from the actual situation of torque and clamping force in Bolts connection.

When the torque is rotated, the screw is stretched, and the screw elongation produces clamping force to clamp the connecting piece tightly. We know that the torque exerted is not as simple as clamping force, in general formula:

Force (f) * torque (l) = torque m

In other words, the more Boltss rotate, the greater the torque. But 90% of the torque is consumed by friction, and only 10% is converted to clamping force. For example, when you tighten a process for a 10n M torque Bolts, what we really need is that 1n M axial torque, most of the torque is consumed by friction.

What is the relationship between friction and clamping force? As already mentioned, in general, follow the 50-40-10 principle, that is, 50% of the Boltss under the head of friction, 40% of the thread pair of friction, 10% of clamping force. But in some conditions the proportion of clamping force can be changed.

For example, when a worker's master picks up a Bolts and discovers that the thread has a touch or an impurity, what kind of clamping force does this Bolts produce if you put it in the hole? It is generally considered that there is a defect in the thread pair (impurity, bump, etc.) in accordance with the Assembly torque assembly, there is 50% of the Bolts head under the friction, 45% threads of friction in the pair, only 5% of the clamping force we want. At this time the Bolts assembly torque is achieved, but far from the clamping force we need. If the Boltss in the flywheel, crankshaft, such as the movement of the very easy to fall off, which is what we often say "false tight."

And the softening of the elastic material will weaken the clamping force, which is usually said to be the soft connection of the torque attenuation. For example, the cylinder cover cushion material soft we use two times to tighten the method to reduce the clamping force attenuation, as well as oil disk Boltss often occur clamping force attenuation, is because the Bolts under the oil disk gasket (the reason for soft materials).

Imagine we need the screw elongation and produce clamping force, the greater the torque screw can extend the longer, is the greater the better the torque? The more torque we impose on the Assembly, the greater the Bolts elongation, the more the Bolts exceeds the yield strength limit will occur the stress rupture.

Thereby losing the link effect of the Bolts.

In practical work, whether it is the pressure between two connected bodies or the axial preload of Boltss, it is difficult to detect, it is difficult to direct control, so people have adopted the following methods to control indirectly.

1. Torque control Method (t)

The torque control method is the most initial and simplest control method, it is based on the thread connection, the axial clamping force F tightening and tightening torque T proportional relationship, can be a formula Tuk F to indicate that this k is the torque factor. When a screw is designed, his axial clamping force F is known, tightening torque t to set our tightening torque is also regulated by the process department. However, the assembly shop often appears tightening torque to achieve but the assembly of the Bolts is still unqualified, this is why?

The key is in this torque coefficient, the torque coefficient k variation of the main wave factor is the composite friction coefficient u, that is, Boltss, screw hole precision, impurities, whether the bump will affect the composite friction coefficient u. And this k value and temperature also have relations, after the Japanese Sumitomo Company through the experiment to prove that the ambient temperature of 1 Shan, the torque coefficient k decreased by 0.31%. is the torque control method accurate? For everyone to deepen the impact, according to the German Engineers Association tighten the experimental report that when the torque T error is ± 0 o'clock (that is, no error torque) Bolts axial clamping force error can reach ± 27.2%.

Application steps: direct or indirect control of the torque, the actual target torque is usually the yield torque $number to 85%, used in the tight elastic area, 90% of the load torque to overcome friction, preload accuracy Seath

The advantage of the torque control method is: Low cost, can be used to tighten the tool torque wrench to check tightening quality.

Its disadvantage is: tightening accuracy is not enough, can not give full play to the material potential, environmental impact of large (temperature, Bolts thread, impurities, bump, etc.).

Torque-angle control method is to first screw the Boltss to a small torque, generally will be tightening torque $number (by process Validation), and then start from this point, twist a specified angle control method.

This method is based on a certain angle, the Bolts produces a certain axial elongation and the connecting piece is compressed. The purpose of this is to screw the Boltss to the close contact surface, and overcome some uneven factors such as uneven surface, and the demand for the back of the axial clamping force from the corner. After calculating the rotation angle, the influence of frictional resistance on the axial clamping force no longer exists, so the precision is higher than the simple torque control method, the key point of the torque control method is to measure the starting point of the corner, once the corner is determined, we can get a fairly high tightening precision.

Because of the relatively advanced tightening method has produced a tool to adapt to productivity, is the electric tightening tool, it is driven by the motor-drive teeth-elbow gear-sensors and other components, can be relatively easy to set the early warning torque and starting angle.

Application steps: Applying a fixed torque (starting (opening) torque), turning the fastener to the predetermined corner, and at the initial stage of yielding tightening, is also used in the elastic area. , it is necessary to use the test to determine the starting (opening) torque and angle parameters, the preload 15.

The torque-angle control method (TA) has the advantages of high precision and can obtain a larger axial clamping force.

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