shop press design

I'm building a shop press about like this one: http://www.northerntool.com/shop/tools/product_604_604
I found tons of 8" by .25 wall channel iron in the scrap pile. So, the
two verticals are 6' long and the four horizontals are 30" long. I'll plan on two 10" long pins at each end of each horizontal.
OK, how large a pin is needed for a 20 ton press. How about 30 ton?
or put another way: eight spots in pure shear need to hold 20 or 30 ton. how big a pin?
Extra bonus question: Two 8" channel irons 27" long anchored on each end. How much force is the maximum in the center?
Karl
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Karl, one number 12 taper pin at each of the four corners will take (by themselves) 30 tons... not with any safety margin, but they'd take the load in shear. You don't need through-pins unless you plan to move one of the reaction beams. If so, you'll need four pins per corner of each beam to take 30 tons.
One problem with pins... they don't resist twisting. You'll need BOLTS. 5/8" grade 8 UN-plated fasteners tightend well will do it.
8" x 1/4" channel will just do 30 tons, doubled, with a safety margin of about 3. (I design for four). The real deal is to determine the amount of deflection. Over 30", I'd keep it at or below 1/32" for a good fatigue life.
I just finished a 30-ton press for a customer, and the numbers work out to a minimum 7" x 12.25lb channel doubled (about 3/8" flange and web thickness), one on each side of the tension elements, for each reaction beam. Across those needs to be a minimum 12" square x 1" plate against which the cylinders can bear.
Lloyd
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For a pin in shear the max load is given by the shear strength times the cross sectional area. Using 25 ksi (%,000 psi) as the yield shear strength for mild steel and a load of 20 tons (@,000 lbs) spread over 8 pins = 5,000 lbs load on each pin, and 5,000 lbs / 25,000 lbs/in^2 = 0.2 in^2, the diameter is 2*sqrt(0.2 in^2 / pi)=.504". That's how to do the math, with zero safety margin and a perfectly distributed load, and since Lloyd has actually built a press and I haven't, I'd definitely take his advice on construction.
I assume that by 8"x.25 wall channel you mean C channel C8x11.5, 8" height, 2.26 flange width, 0.22 web thickness, 0.39 flange thickness, 11.5 lb/ft. For one channel 27" long simply supported on the ends and with the load concentrated at the center a load of 30,000 lbs results in a deflection of 0.013" and a maximum stress of 25 ksi, so two channels could just support 60,000 lbs or 30 tons with no safety margin, again assuming 25 ksi as the yield strength. My calculation doesn't quite gibe with Lloyds, I get less deflection and a lower max load than he does. Either we are using slightly different channel dimensions or different yield strength.
----- Regards, Carl Ijames "Lloyd E. Sponenburgh" wrote in message

Karl, one number 12 taper pin at each of the four corners will take (by themselves) 30 tons... not with any safety margin, but they'd take the load in shear. You don't need through-pins unless you plan to move one of the reaction beams. If so, you'll need four pins per corner of each beam to take 30 tons.
One problem with pins... they don't resist twisting. You'll need BOLTS. 5/8" grade 8 UN-plated fasteners tightend well will do it.
8" x 1/4" channel will just do 30 tons, doubled, with a safety margin of about 3. (I design for four). The real deal is to determine the amount of deflection. Over 30", I'd keep it at or below 1/32" for a good fatigue life.
I just finished a 30-ton press for a customer, and the numbers work out to a minimum 7" x 12.25lb channel doubled (about 3/8" flange and web thickness), one on each side of the tension elements, for each reaction beam. Across those needs to be a minimum 12" square x 1" plate against which the cylinders can bear.
Lloyd
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On Mon, 23 Apr 2012 18:18:17 -0500, Karl Townsend wrote:

The 2 pins on my HF#32879 20-ton press are 3/4". I added air-over. <http://www.harborfreight.com/20-ton-shop-press-32879.html> The vertical channels are 2" x 3.5" x 0.2" channel and the top and bottom horizontals are 2" x 4.8" x about 0.2. Underbuilt compared to your plan! Note, at the site you linked, the page for the 50-ton model says "Positioning pins are 1in. steel" and picture shows 4 pins for that model.

--
jiw

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That particular press actually starts to fold up and get longer at about 10 tons actual force. In addition, the metal starts to tear around the pins at about the same force.
I have several friends who believed Horrible Fright, and bought them, only to rebuild them extensively.
They're so badly underrated as to be dangerous.
LLoyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message

I measured them to find the minimum material dimensions with no safety factor, then upped it with the next larger size stock and bolts when I increased the height capacity of my press.
It's an interesting exercise to calculate the stresses at the full press rating and compare them to A36 steel's properties.
jsw
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Karl Townsend wrote:

Karl
If you are going to do any broaching you should use a longer stroke cylinder with at least a foot of travel.
John
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