Calculating point loads

[Oldgit]

Hi All
I've been asked to build an olive press and I thought of using a twenty ton hydraulic jack, (I know it looks like overkill, but getting oil out of olives take a lot of pressure).
I can do the calcs for a point load on sheet steel, but it works out to 3/4" thick plate if I calc for a pair of 12" x 12" plates to squeeze the olives between, (a bit on the heavy side) !
Calcs=
Modulus: S=BDcubed/LD Where B=Breath, D=Depth, L=Length.(Length in inches) (Can't do powers on here!)
Bending Moment: M=PL/4 Where P=Point load
To solve for P
P=4M/L Where M=sxS ie. s=bending stress=0.55xyeild strength (say 30,000PSI) S=Modus (See above)
(I hope this helps others with similar problems!)

I thought I could use thinner plate with box sections welded to the back of the plates. I can also do the calcs for box section, but my question is:-
Do the point loads just sum together or does the compersite welded sheet increase the figure because of the welding?
Thanks in advance for any help!

Oldgit

 

[astro_al]

I think you're over analysing it. What exactly is the failure mode thats worrying you?

If the ram is centred on the plate, you're looking at a maximum moment arm of less than 6". Its going to be one hell of an olive to bend 3/4 plate over that distance? If your concern is possible bending loads on the ram, why not mount the press plate to the ram with a pivotting yoke to remove any stresses of that type?

Sounds like any easy build, please clarify if I have misinterpreted your Q.

Cheers, Al

 

[Pat4d]

For shaped sections you need to find an I value and calculate the mid point of the shape for application of the load. I will have a look if I can find PDF example from my college days.

 

[Oldgit]

Thanks for the feedback astro_al and Pat4d

Al,
it's not failure that worries me, it's distortion of the plates! Both of the plates have to be fixed to carry a stack of filters and separaters with olive paste in each filter! The stack would be about 9 or 10 inches high (before compression) between the plates and if the plates became distorted, the stack could fire out of one side, (olive oil is slippery stuff)! I don't want to injure my friend!
Maybe I'm opting for overkill (not a good choice of word), but you can understand my concern for a friends well-being!

I didn't want to use 3/4" plate because of the weight, I was trying to come up with an equivalent using thinner plates and box sections welded to the the backs, but I was not sure if the welding of the two affected the total strength!
Sorry if I seem picky, but I think safety should be the top priority where 20 Ton pressure is concerned.

Thanks in advance for any feedback!

Oldgit

 

[astro_al]

hi, ok thats a bit clearer, ta. Welding creates internal stresses which do affect the maximum loads a given material can cope with, these can be normalised by post weld heat treatment if you have the inclination/budget (not necessary in this case).

20 tonnes isn't that much, look at the structure of a machine mart 20 tonne press for example, its nothing beefy and the spans are much bigger in that example.

Of course you can use thinner plate with a frame behind no problem. You needn't worry about the welding affecting the materials, you're nowhere near the limits of its strength. Perhaps running the plates in a frame would alleviate fears of things getting out of parallel and squirting everywhere? Perhaps some vertical shafts with linear bearings at each corner?

Al

 

[Oldgit]

Many Thanks Al

That clears up a few worries on my part! The corner guides I've seen on an American site and I like the idea of some kind of cage to keep everything where it should be.

I've yet to start construction, so all thoughts and ideas are most welcome!

Thanks again Al

Oldgit

 

[hareng]

Not a dedicated toolmaker but have served time as an estimator. 20 tonne press is very little relatively, but wouldnt like to get my hand in.
If as you say safety is your concern, why are you asking about welding and using thinner plate?

 

[Oldgit]

Hi Hareng

The thinner plate and the box sections welded to the back are to save on the weight, my friend is no strapping, hairy superman, she's a seventy year old and I'm trying to make this press easy to assemble/dis-assemble if she needs to move it at any time!

Hope this makes my ideas clearer, I'm not that brilliant at expressing myself!

Oldgit

 

[dutchman]

Hi Oldgit,

You might want to read Statics and Dynamics by Hibbler on this subject. Was recommended to me by Storm Monkey and it is a good help.

 

[Oldgit]

Hi Dutchman

Thanks for the tip, I can get a second hand copy quite cheap!

Thanks again

Oldgit

 

[dutchman]

If you search Google books, it can become even cheaper.

http://books.google.nl/books?id=iCBye0_lEXsC&printsec=frontcover&dq=hibbeler

You would have to check which title from the series suits your purpose best though.

Good luck!