How to calculate required friction too overcome force? (with pretty diagram)

[Shaun_W]

I'm involved in a project and I have to design a device that will "cap" the flow of water coming out of a pipe, which is located underwater. This device has to be transported down by an ROV that we have already made, so that's not a problem.

The device I am creating involves a solenoid being inserted inside the pipe. It is then actuated, pushing the arm out against the wall of the pipe. There will be a buffer material - probably rubber, unless anyone has any better suggestions - which will cause friction between the arm of the solenoid and the wall of the pipe. The solenoid will be keeping the cap in place, which will be stopping the water flow.

Below is the pretty diagram that I drew on Paint:

http://img31.imageshack.us/img31/3723/pipewithsolenoid.jpg

Uploaded with ImageShack.us

As you can see the water will be pushing upwards with a force of WF, the solenoid arm will be pushing against the inside of the pipe with a force of F, there will be a buffer material with area A on both the end of the solenoid and at the end of the arm.

What I need to know is how to calculate the required friction force between the buffer material and the wall of the pipe, if the water force, WF, pushing upwards is 1000N and the diameter of the pipe is 4.1cm?

Thanks!

 

[edgepflow]

Could you sketch a top view of this - I am not sure what you are trying to design. If you are just trying to shutoff flow, could you just use an off-the-shelf shutoff valve?

 

[Shaun_W]

If an off the shelf shutoff valve that can be attached to a 4.1cm diameter plastic PVC plumbing pipe by an underwater ROV exists I could use one.

http://img291.imageshack.us/img291/9392/pipearialview.jpg

Uploaded with ImageShack.us

This sketch isn't quite so pretty, sorry. The black bits at the end is the rubber, to create a base for grip/friction. There will also be a simple plug on top (not shown). The device I am designing will be holding down this plug.

 

[edgepflow]

To just hold the solenoid arm against the force of the water, I believe you would just divide the 1000N by the coefficient of static friction.

Then, you will need to add extra force to seal off the water that is being capped. This is similar to sealing a raised face flange and gasket: you torque the bolts until they make enough force to make enough pressure on the gasket to seal. There are methods to do this - let me know if you need any information.

 

[Shaun_W]

Thanks very much - that was exactly the kind of answer I was looking for. I'll have a look for data regarding static friction coefficients.

Of course, information regarding methods to do this is always appreciated. This, being an extra curricular project, is quite different from my actual studies, which is why I'm struggling so much. But I'm definitely learning things from this project.

 

[edgepflow]

OK, here is an outline of how to figure out the force to seal the cap. It is based on the pressure on the raised face of a flange to form a seal. Try a 2" 150# ANSI flange for all calculations.

1. Determine bolt torque for 150# ANSI class 150 flange (rated to about 260 psig).

This link lists bolt torques for 150 # flanges
http://www.durlon.com/InstallTorque.htm


2. With the bolt torque from Step 1, figure the bolt force from:

http://www.engineersedge.com/calculators/torque_calc.htm

The total force will be the bolt force X number of bolts. Look up number of bolts in a flange catalog.

3. Determine the required sealing pressure by dividing the total force in Step 3 by the area of the raised face of the 2" 150# flange (use a flange catalog for dimensions.

4. Determine the force needed by multiplying the cross section area of the pipe you are trying to seal by the sealing pressure in Step 3.

Don't forget to check that your PVC pipe is strong enough to take this force. You may need to reduce the pressure rating of the seal.

 

[Shaun_W]

Thanks very much. I'll look into those.