Learning Forum

[gambra77]

Hi everyone

Im stdying the stress in a body that suffers a force in a inner face.

As you can see the body is splitted with a slot but a bolt that prevents its separation. 

The bolt has no nut in the oposite face and I don't know how to calculate the compression force it transmits to the body. When the force is aplied the main body suffers a little deformation that i want to prevent with this bolt compression.

Maybe its no tnecessary any preload and is enough with the contact between de bolt and the body (bonded)? Or do i need any special constrain?

Aitor.

[Ashish Khemka]

Hello,

Split the screw in three parts. The head portion which will be in contact with the body (may be bonded or frictional contact), the central shank portion on which preload can be applied and third portion can be bonded to the other hole. Solve analysis in 2 steps - define preload on central portion in first step and lock it in second step. Apply internal force in second step. This way you can compare results at 2 time points as well - what is the deformation due to preload and preload + force.

 

17.6.2.9. Bolt Pretension (ansys.com)

 

Regards,

Ashish Khemka

[gambra77]

HI,

So a bolt without a nut also has a pretension force? Because there is no nut that elongates the body.

[peteroznewman]

Ashish provided a good description that I can explain a bit further.  The physical bolt is threaded, but the simulated bolt is represented as a cylidrical head and a cylindrical shank. The bolt shank is in the threaded hole. Use the plane of the part where the bolt emerges from the threaded hole to split the face of the bolt shank. Then the face that is inside the hole can be bonded to the hole and that will prevent it from moving.  The underside of the head of the bolt will be bonded to the face that it is touching.  The cylindrical face of the bolt shank between the head and the split line at the threaded hole is where the bolt preload is applied. During the solution, Ansys will split the mesh at the center of the bolt shank and put constraints like a translation joint that will pull the two halves of the shank toward each other using the preload force in step 1, then hold that displacement fixed in step 2. That force will cause the bolt shank to become slightly shorter and the other part will compress the two legs together.

[gambra77]

Thanks Peter, i´ve divided the bolt in two and did what you said. But i have a problem, how much force do i need to apply in the preload? Because it has a diameter of 8mm and the tightening torque is 31Nm and if i apply the T=dxkxF formula (F being the preload) the force is around 25KN[KN] and with this force, both legs broke because of the stress. And lastly, the 1 and 2 steps that you said are an automatic tool or i need to put them manually?

[peteroznewman]

Add a pipe to the assembly that fits inside the cylidrical cavity behind the split. The pipe diameter should be slightly smaller than the ID of the cavity. Add frictional contact between the OD of the pipe and the ID of the cavity. As the bolt shortens, the arms will grip the pipe and the pipe will prevent the arms from collapsing at the split. At the other end of the pipe, add a fixed support, otherwise the pipe will cause the solution to fail.

[gambra77]

Thanks your answers are very useful as always.