Mechanical Engineering
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# Force applied to fulcrum

## Question

Hello. I am designing a machine that uses a pneumatic cylinder to operate a hole punch to punch through sheet metal. I will be using a lever to get a mechanical advantage to provide the force necessary. I am trying to figure out what kind of force would be experienced by the fulcrum or pivot point. I'm not having any luck finding information on how to calculate that. I would appreciate any help that you could provide. I know I haven't provided much detail here but I can give whatever information is needed to calculate that. Thanks.

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Hi there...it is just a simple moment....on the fulcrum itself, it is the total force applied in the down direction.

I'd calculate the moment on one side then the other, then total the vertical forces and the "reaction" is the sum. I would do both as - occasionally - the moment forces can exceed.

typed in a rush sop hopefully makes sense - good luck!

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On 4/17/2021 at 6:10 PM, G B Reid MIMechE, SIMarEST said:

Hi there...it is just a simple moment....on the fulcrum itself, it is the total force applied in the down direction.

I'd calculate the moment on one side then the other, then total the vertical forces and the "reaction" is the sum. I would do both as - occasionally - the moment forces can exceed.

typed in a rush sop hopefully makes sense - good luck!

As indicated by my earlier counter part you'll need to consider bending moments and consequently bending forces on the cross member.  Further to that you need to compute the direct forces on the fulcrum and forces transmitted on fulcrum members depending on its design.  Detail of the system design may assist to arrive at the specifics.

Regards.

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Thank you both for responding!  I did some research on "moments" and learned a little bit, but I still have some questions.  I will put some actual dimensions and other specifics below.  Maybe that will help.  If you do decide to respond I would like to know exactly how you figure that out for future project and just general knowledge.  I am OBVIOUSLY not an engineer and my extent of math knowledge goes to Algebra II in high school, but I enjoy math and love learning.  Thanks again!

As I stated in the original post, I am building a press essentially.  I have a sheet metal punch that requires an external force to operate.  The punch manufacturer says that it will take approximately 2000 lbs of force to punch a 1/2" hole through 22 ga sheet metal.  I have a pneumatic cylinder that will exert 1500 pounds of force.  The plan is to attach the cylinder to one end of a lever 6" away from the pivot.  On the other side of the pivot will be a foot that will travel downward to press on the punch.  That foot will be 3" away from the pivot to provide a potential force of 3000 lbs.  The total amount of travel on the foot end would be 5/8" and on the cylinder end 1 1/4".

I am confident that I can fabricate a lever and a structure to hold the lever that will be able to handle those forces and more.  It seems to me that the weak point in that whole machine is the pivot for the lever.  The pneumatic cylinder is pushing up with a force of 1500 lbs, the foot is pressing down on the punch with a force of 3000 lbs.  I would assume that the upward force on the pivot would be at least 4550 lbs.  I would also assume that there would be some sort of multiplication of force that would occur but I have no idea how to begin figuring that out.

I have attached a picture of what I have so far.  The cylinder and foot are not drawn yet.

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Been a few years since I had to apply force and moment calcs but the key inputs are the force at the air cylinder connection and the distance to the fulcrum, and the force at the punch point with that distance from the fulcrum.

You have the force required to punch the hole and can work back to the force required at the cylinder to achieve this.

I would guess both added together will give you the shear force at the pivot.

Working in SI units makes that calc pretty straightforward I think

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Hi Jake,

This is abit late.  Hope it will be of assistance.

1 hour ago, Mucugia said:

Hi Pablo,

Have to deal with case by case of each shaft diameter,

General rule is use the Yield strength as it defines the onset of plastic deformation.  Two, work with the outer most diameter as for any shaft plasticity start at the outer most layer of the shaft.  Therefore if the shear stress of outer most layer of the shaft is below the yield shear strength, then your shaft is safe. (Assumption; we are dealing with torque loads).

Regards.

Michael

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I've just perused this again and had a thought....you're making a punch...have you calculated whether the force required to punch through the metal can be achieved?

It's a CSA calculation (circumference x thickness) and the shear strength of the material in Pa

Take the CSA (in m^2) and multiply by the shear strength in Pa to get the required force in N.

Good Luck!

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