Mechanical press design

[blisus]

I´m tryin to design a manual press based on the eccentric mechanism. The problem is that I’ve not been able to find a formula that takes into consideration friction. The problem could be stated as follows:

Given a Torque T, a power shaft with diameter D supported into two journals with diameter d and eccentricity e, which is the maximum force than can be applied if the friction coefficient, ?, is the same for all friction pairs.

Can somebody provide a link or book where I can find info about this problem?

As an option I could design a mechanism based on a double toggle mechanism. Does any body knows which have, eccentric or toggle, the highest mechanical advantage?

Thank you very much.


Julio Gomez

 

[BillPSU]

Is this going to be driven by human power or some other power source? There are all sorts of presses commercially available new and used. How much tonnage do you need and how fast of actuation is required?

 

[MikeHalloran]

Similarly to the way that engine valve duration is stated at an arbitrary small lift, it might be helpful to evaluate your mechanism designs for their transfer function in terms of force per unit torque at some arbitrary small stroke away from dead center. The most sensible small stroke to use would be the one at which your tooling is beginning to do useful work. As a first approximation, try doing the trig to work out the mechanical advantage in a spreadsheet. You will see that as either eccentric or toggle near 'dead center', the number gets impossibly large.

The real limit to how much force can be applied is the stiffness of the mechanism. Think of the press frame as a spring. Seriously. When you are designing a forcing press, every part of it behaves like a spring, and should be analyzed as such.

Don't worry about the friction just yet; that's what moly grease and rolling element bearings are for.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[TVP]

Julio,

You can find equations that describe the forces, energy, etc. for various presses, including eccentric mechanical presses, in one of the following books:

Metal Forming: Fundamentals and Applications by Taylan Altan, Soo-Ik Oh, and Harold L. Gegel, American Society for Metals, 1983, ISBN: 0-87170-167-7

or

ASM Handbook Volume 14 Forming and Forging, ASM International, 1988, ISBN: 0-87170-007-7

or

Cold and Hot Forging: Fundamentals and Applications by Taylan Altan, Gracious Ngaile, and Gangshu Shen, ASM International, 2005, ISBN: 0-87170-805-1

The first book is now out-of-print, but you should be able to find it at a good engineering library or at a used book store. Either of the other 2 books are available from the ASM International website:

http://www.asminternational.org/Template.cfm?Section=Bookstore&Template=/ecommerce/ecomdefault.cfm

 

[blisus]

Thanks to Mike and TVP for your hints. I´ll try to find that literature.

To BillPSU: The intend is to design a manual operated shear press that cuts mild steel sheets up to 18 or 20 gauge.

I´m out of the US and I'm analyzing the feasibility to commercially produce this product.

I like DIACRO presses system (eccentricity) but I think that the double toggle mechanism has more mechanical advantage.

Julio

 

[MikeHalloran]

A shear is not a press.

Part of a shear typically clamps the sheet to be cut, e.g. by pressing on it, but the shear blade is usually angled, so it doesn't shear the entire width of the sheet at once. The mechanism needs to be able to exert substantial force over a significant stroke.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[blisus]

Thanks for your Mike.

I am also considering a independent system for clamping the sheet.

My main interest at this moment is to decide on the best suitable mechanism that tranforms human force (hand or foot) into one that cuts mild steel.

Which would you consider the minimum angle for the blade? The maximum length to be cut is 24".



Julio Gomez

 

[PersianEng]

For this width and as a manual machine you can simply design a scissor like machine with blade pivoted to the scissor bed and cutting against the bed edge.Here is how itlookslike:

http://www.staples.com/images/products/catalog/products/S0000595.JPG

If you insist to have a vertically moving blade, you can use a crank-rocker mechanism with blade as the rocker bar. Then you need to design a gearbox to provide enough torque to rotate the crank while cutting the sheet metal.

 

[rogzog]

For the cost of your time to design and build a foot operated shear I would think it might be more practical to buy one already built and ready to go. Something like this maybe.

http://www.sterlingmachinery.com/photos/7077.jpg

rogzog