Improved Pearl throw off lever

Can anyone explained to me how a throw off lever works on a No. 11 Improved Pearl? I understand what it does but I am not sure how the lever (and what its connected to) actually prevents the platen from closing all the way.

Here are 2 images. One a close up image from the side, with the flywheel removed from the shaft and one from behind, where it connects. Thanks in advance. Matt

image: throwoff.jpg


image: throwoff2.jpg


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I had one of these years ago but never took the opportunity to study the throwoff mechanism. However, it looks like the impression toggle back shaft is an eccentric rotated by the link from the end of the throwoff handle. If it’s not working it may be dry — oiling and gently rotating the end where the link is connected back and forth a little at a time should free it up.

The eccentric is supposed to move the toggle mechanism slightly toward the rear of the press, thus moving the platen away from the type forme.

The concept is based on eccentrics. Well, some people call us letterpressers “eccentric”. So for those not into mechanics, the dictionary definition of an eccentric is “off-center”. To help those without a dictionary but who are into politics, today that might be a liberal leader. For everyone else, just read on.

I suggest you look at the Golding Patent #532,548 which can be accessed in Google patents. In fig 2 of his patent, the shaded item labeled “f” is the eccentric. Now read the description on page 2 of the patent text, page paras #5-50.

If that didn’t help or if you can’t find the patent drawing and description or you are just lazy, try this: Open a hard back book to about the middle. Turn it over so the cover is facing up and the text is facing down on a table. Take a ruler and measure the wing spread of the book from cover to cover. Say it’s 12”. Now put the ruler under the book in the center. Twist the ruler axially. The center binding should be lifted slightly by the ruler as it turns. Watch how the wing spread of the book cover changes from 12” to say 11”.

Imagine that as the wing spread of the book cover gets shorter, the pressure on the platen decreases. The pressure is “off”.

When you rotate the eccentric, the eccentric accomplishes the same function as lifting the center binding of the book. It causes the distance of the iron frame inside the press to be reduced in length. The platen is pushed a lesser distance.

Now, go back to your press. Unplug the motor or stop treadling. Turnover the flywheel by hand until the platen is just ready to make the impression. Don’t move the flywheel. Stand on the left side of the press. Focus your sight on the internal frame (just inside your red line in throwoff2.jpg) which has an “X” cast as internal bracing. Move the throwoff lever and this frame will move. Now put your hand on the outside of the platen and move the throwoff lever. You will feel the platen move. It’s the eccentric that’s keeping the platen from making an impression.

Thanks for the replies. Sorry my press is partly dissembled from moving it so I can’t examine it. But I was looking at it the other night and wondering how exactly that worked.