Interesting encounter today at the farmers market. When I was looking to sit down to enjoy my danish and espresso, there was no table free. So I shared a table with an older couple. We got to talking about things and surprise, surprise the gentlemen used to feed a C&P letterpress many years ago in a little town west of Red Deer. I asked about the speed, and he insisted that they were doing about a hundred pieces a minute top speed. Hm, that seems a pretty fast! The other production number I have heard a few times is about 2000 impressions per hour (iph), that comes to 33 pieces a minute.
I am going to aim for half of that, 1000 iph. My press has a 24″ diameter drive pulley. The smallest I am likely to get for the motor shaft pulley is 2″ diameter. So that works out to a 12 to 1 speed ratio.
With 7 turns per impression on my press, that is 7000 rotations per hour, or 116.7RPM.
On the motor side that is 116.7 times 12 = 1400RPM. For an 1800RMP motor, operating at 1400/1800 = 78% of maximum speed is not that bad.
But there is something else I have not mentioned before. I would like to increase the time available for the paper removal and placing task (while the platen is open) by manipulating (reducing) the drive speed depending on the press position (when the platen is open). During the three truns of the drive wheel while the platen is open I’d like to reduce the drive speed to about 40% (for a 1800RPM motor that would be 720 RPM). And during the four turns while the platen is closing/opening I’d like to run it at full speed. My plan right now is to use an inverter duty 3/4 HP motor driven by a VFD to be able to handle the cyclic acceleration/deceleration.
An 1800RPM motor actually runs at about 1750RPM at full load. Assuming that it will take half a turn (on the press) to accelerate from 720RPM to 1750RPM, and half a trun to decelerate back to 720, that makes one turn at an average speed of (720 + 1750) / 2 = 1235RPM.
So here is what my prediction is for overall rate:
3.5 turns at 1750RPM (at the press that is like running at 1250iph)
2.5 turns at 720RPM (at the press that is like running at 514iph)
1 turn at 1235RPM (at the press that is like running at 882iph)
Working out the average speed for the time weighting that each speed applies:
(3.5/7) * 1250 + (2.5/7) * 514 + (1/7) * 882 = 934.6 iph average speed, pretty close to the 1000 iph I was aiming at, and I gain a safety advantage with longer platen opening time. Let’s see what that would actually work out to: 720RPM / 12 is 60RPM at the press, that is 1 rotation per second, so for the 2.5 turns the platen is open – that makes it 2.5 seconds. Plus the bit of time the drive is speeding up and slowing down. So maybe close to three seconds time for the paper shuffle. That seems lots. We’ll see! One other feature I have been thinking about is to have two buttons on the feed table that have to be depressed before the press will return into the high-speed mode (or possibly if they are not pressed bring the press to an emergency stop via dynamic breaking). If you can think of holes in this reasoning, I’d like to hear what you are thinking.