Effective area

[bluesman0007]

This is an elementary question but we have a little debate going on. We have a device, a simple cylinder of sorts. Imagine a bolt without threads, the shank of the bolt goes through a head cap of sorts, it has an o-ring seal on the shank. The head of the bolt is merely what keeps the thing from going through the head cap. There is clearance around the head and no sealing. Isn't the effective area of the bolt the area of the shank where it is sealed? Thanks in advance.

 

[israelkk]

yes, the pressure from both sides of the bolt head (less the shank area) is the same.

 

[bluesman0007]

Thanks israelkk,

To clairify my question a little, and you may have answered it already. If I were to calculate the force this would output I would use the area of the shank and not the head of the bolt. I was trying to get the point across to a fellow worker but his contension is that the bolt deadheaded against the cap would then make the effective area the head of the bolt and not the sealed shank. Am I going out of my mind... Thanks.

 

[israelkk]

If the bolt head side that touches the cap has a fine surface finish and the cap has the same fine surface finish there may be an "effective area made of some spots" that is sealed from the pressure arround the head. To overcome this the piston head is designed to have a smal raised ring near the shank that limits this area the raised ring surface finish should be quire rough surface finish. Sometimes the ring is also cut in some places to prevent close circular machined lines. I hope I was clear enough.

 

[bluesman0007]

Thanks israelkk,

We were both right to a point I guess, I have always thought that the most reliable calculation would come from the sealed member but there was a possibilty of fit of the head making some increase in area. Thank you for bearing with me on such an elementary question.

 

[budt]

It does not matter how big the bolt head is or how well it mates to the cylinder end, the only effective area pushing the Shank (Rod) is the Shank (Rod)diameter.

To prove it to yourself and your coworker take a 1 1/2" diameter cylinder with a 5/8" rod diameter and remove the piston seals so fluid (Oil or Air)can get past the piston. Set the cylinder on its Cap End (Vertical Rod up)and pressurize the Cap End Port with a regulated supply of 80 PSI. Now start stacking weight on the extended rod and see how much it takes to push the rod back into the cylinder with the 80 PSI pushing against the whole inside area. Note: You will have to plug the rod end port for this experiment to trap air in the cylinder.

Since the only effective area pushing out is .625 X .625 X .7854= .306 Sq. In. the rod will start retracting when the weight reaches .306 Sq. In X 80 PSI= or about 25#. Now try the same experiment with the piston seals in place and the rod end port open. The effective area is now 1.767 Sq. In. and it will take over 141# to push the rod back.

No matter how well or how poorly the piston fits agaist the cylinder head it is the cylinder head that is stopping its pressure times area not the external load. As long as the head is held on with more force than the piston area is pushing with it will stop as soon as it contacts the head and will be pushed back when force on the rod is greater than pressure times area on the rod surface.

Another way to prove the point consider a bolt with shank that is light press fitted into the head. When the bolt head contacts the cylinder head it would stop and if pressure continued to build it would finally be high enough to slide the shank out out of the head without any movement of the head which is held stationary.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[israelkk]

budt

I meant that if you want to push the piston from the shank side you may have to overcome larger force than just the force based on the shank area. For example if you have a return spring at the shank side and the pressure is on the piston head side the force that the spring will have to exert to return the piston may be higher if the head of the piston is in tight mate with the cap.

 

[budt]

israelkk;

I agree a piston that has a smooth mating with the end of the cylinder may not move away from the end quickly. Especially if you add oil thinking it needs lubrication. The oil just makes it harder for air to get into the sealed mating surface.

In my experience it does not require any extra force to get it going but some method of getting air in that sealed area. I have removed pistons and used a hand grinder to cut some grooves across the face of the piston or head so air can get behind the whole piston area quickly. It's surprising how quickly they react after a little hand grinding.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us