I got the impression that the failure was fatigue related and that the first clue was alignment. Half of the press had a different level than the other half. All 4 did not go.
This was an >>My local machine repair place had some 5" diameter x 14' main posts from a
The total preload on the rods must be at least the total tonnage of the press. The preload for each rod is calculated by using diameter of the rods and how much the must be streched to get the preload in the rod. The nut on the rod is tightened. By using the pitch of the threads the amount of strech is calulated to give an answer in degrees of rotation of the nut. The rod is then heated and the nut is advanced to that position. The rod cools and the preload is in the rod. It works backwards for removing the rods. Heat them up and back off the nuts.
If you didnt have more preload on the rods than the press tonnage, the press would come apart. At any amount of tonnage the pressure would strech the rods and a gap would form every time the press cycled, beating the press to death. With the rods preloaded, the force is just transfered from the uprights to the crank with no movement of the rods.
John
I am referring to a mechanical press. I can see where you're coming from, however.
As I said, we run our does below bottom. We have to in order to get a good hit.
Does anyone have an OBI press which has a tonnage meter? I'd be interested in seeing whether a "below bottom" hit causes the force of the ram to exceed the rating of the press. Everything is flexible, including the frame of the press, the crankshaft and the die being run. When I say "below bottom," that could mean
I'm not sure I understand this figure. It seems that if the preload was equal to the press's capability, then the rods would be loaded up to double the press's capability during normal production (higher during crashes).
Is this a "law of physics" figure or a rule of thumb? I'm not questioning you, I'm just interested (we don't get to build or fix the presses - just break them)
Regards,
Robin
I use my press to bend pins for yarn racks(creels)... I made a V-block for the ram and then put a block of 3" thick by 12" sq. rubber for my table... It was a new chunk of forklift tire rubber (very strong)... I never need to worry about a crash... It work very well... I hold the pin with my hand while it bends it... I've made thousands of pins with no problem... The metal I work with is 3/8" to 1/2" round...35 ton Bliss press...
No light-beam guards eh? ;-)
Regards,
Robin
Think of it this way. Your are pulling on a strong spring at 100 lbs. Along comes someone to help you hold the spring in tension. He applies
75 lbs of force to the spring and leaves you holding 25 lbs. The spring displacement doesnt change but the force is transferred to the other person. You being the press frame. and the other guy is the crank and everything above it.hth
John
I left out, the 100 lbs is the preload. As long as the preload is more than the force transferred, the press frame is held in place. Once the force exceedes the preload the frame below the bearings of the crank are no longer in compression and they move around.
John
Robin added:
Reminds me of one of the set-up guys at my first industrial type job back around '58.
Yep, good old Lefty.
dennis in nca
John,
That makes perfect sense. Thank you.
Regards,
Robin
Oh, the stories...
Regards,
Robin
Robin S. noted:
And now the REST of the story.... :)
This company had about 6 presses and none had even air clutches, only mechanical foot pedals. I'd asked someone why the small press was running with only a chunk of steel on the foot pedal and that's how I learned about Lefty. Man, he was handy with that hook (actually a little more sophisticated than just a plain hook). He could roll a cigarette as fast as anyone.
dennis in nca
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