What i am doing here is simply coyping a post I made to the LETPRESS list concerning motor driven C&P Presses. There was a query for advice about pulley sizes and drive belt widths. Some printers drive their presses by belting the flywheel. On the right side of the press (if you're standing facing the delivery board) is a place to put a regular drive pulley, directly attached to the same shaft the Fly is mounted and keyed to, only the pulleys used are considerably smaller than the Fly. In the case of my 8x12 NS, the Flywheel is 32" in diametre, which, when belted to a standard 3-phase motor rotating in a counter-clockwise direction, works out very nearly perfect as far as how fast this press makes impressions.
Different C&Ps have different rates of impressions, expressed in terms of Impressions Per Hour, or IPH. I have added yet another measure: Impressions Per Second, or IPS, sort of a no-brainer, and finally Press RPM, which is the speed per minute which the press' drive shaft turns.
With Chandler & Price Gordon Style Letterpresses, and, I suppose, all similar presses, the larger the machine- the more turns of the drive shaft it takes to close the "clamshell", closing the Platten against the type bed to make the impression. It's a lot of iron to move. The larger, the heavier. Correspondingly, the power to maintain rotation of the press' drive shaft needs to be higher. Thus, there is also a minimum Horsepower rating for each press as well, expressed in HP. Here is a list of C&P Letterpress IPH / HP ratings per style and size, as found in the rear chapters of the 1916 edition "American Specimen Book of Type Styles", produced the the American Type Founders (ATF) :
New Series C&P 8x12...................................2600 IPH / 0.25 HP
New Series C&P 10x15..................................2300 IPH / 0.25 HP
New Series C&P 12x18..................................1800 IPH / 0.5 HP
New Series C&P 14-1/2 x 22.........................1500 IPH / 0.5 HP
C&P Regular Models (O.S.) 7x11..................2800 IPH / 0.25 HP
C&P Regular Models (O.S.) 8x12.................2600 IPH / 0.25 HP
C&P Regular Models (O.S.) 10x15...............2300 IPH / 0.25 HP
C&P Regular Models (O.S.) 12x18................1800 IPH / 0.5 HP
C&P Regular Models (O.S.) 14x20................1500 IPH / 0.5 HP
C&P Regular Models (O.S.) 14-1/2x22.........1500 IPH / 0.5 HP
I might add, at this point, some discussion on Horsepower. It's easier to maintain an object in motion once it's in motion. But to get something started from a dead start requires a much higher degree of energy to overcome the sheer weight of the object. As regards presses, it is my own opinion that the HP ratings are actually a maintaining horsepower rather than a start-up horsepower. When you start up one of these presses, it is necessary to give the flywheel a push, lest the motor stress with inrush current as it is momentarily frozen until gravity and weight, inertia, is overcome. This inrush of current is caused by the motor's internal load increasing, and demanding more current from the power source. This creates a lot of heat, which can break down the insulation of the internal stator wiring, causing the motor to burn. There are also shaft bearings which are stressed, and can be damaged. Better to help that nice, expensive motor in the start up process. The proper rotation of the Flywheel is considered to be "top-away", or Counter Clockwise facing the wheel. Of course this means that facing the drive pulley on the other side of the press, you will have clockwise rotation. In my experience, most motors today tend to be CCW ( counter clockwise ) unless reversable.
The following describes how I have computed pulley sizes necessary for any desired rate of impression, based on 4 variables: Motor RPM, Press RPM, Press IPH and desired IPS. All of these dynamics play a part in your calculations for pulley sizes, etc. Remember, I am not an engineer or particularly good at math, and these calculations are purely for "ballpark" use. I find that in most cases, "Ballpark" is good enough, considering materials available.
The original question regarded variable speed motors, which I feel are not necessary. Here was my response:
You may not have to worry about variable speed motors, which can get pretty costy. And variac auto-transformer driven speed controls can be a real can of worms. Standard motor rpms will probably give you the speed you need without controls. If it's any help, here's what I might try:
Start with your press drive shaft turns per impression. I think it takes six revolutions for one impression on a 10x15. Let's say you want to do one impression every three seconds. Since that fly[wheel] must spin six times per impression, figure 6 revolutions x (60 / 3 seconds), or 6 x 20, which gives you 120 rpm on the flywheel, or I guess it's called the drive shaft. I'm afraid I've never mastered the terminology. One impression every two seconds would be faster, 6 x (60 / 2), or 6 x 30, 180 rpm. Now you have a speed to figure your drive pulley ratio, based on your motor's rpm.
If it takes ten turns of your motor's shaft to spin your press's drive shaft once, you have a 10:1 drive relationship. At this ratio, to drive your press at 3 impressions per second, you would need a motor turning 120 press rpm x 10 motor shaft rpm, or 1200 rpm. Either changing the ratio between press and motor shaft, or changing the rpm of the motor will, obviously, affect your speed.
A 10:1 set up at 1700 rpm would give 170 rpm at the press, which you can see will bring you to between 2 - 3 sec. per impression at the rate of 6 turns per impression. Comfortably below max IPH, which I think for your press is around 2300.
To come up with an approximate press to motor drive ratio based upon motor rpm, divide Press(rpm) into Motor(rpm). In my case, the 8x12 spins at 4 revs per impression. At one impression every 2 secs, the press needs to spin at 4x30, or 120 rpm. 120 divided into my motor's 1750 gives me about a 14 or 15 :1 motor pulley to press pulley ratio. This was best achieved with a 2" motor pully driving the 32" flywheel. Everything is an approximation, the math I use is strictly general reference to help me make do with what is available. I use a 5/8" rubber belt, too. It's been with the press for many years, still has years to go on it.
One more suggestion: the motor minimum hp rating claims to be 1/4 hp. But I wonder if these specs take into consideration starting torque and the current inrush to the motor which could shorten the life of the motor? In fact, in 1912, was C&P even thinking in terms of a single electric drive motor? At the last shop I worked at professionally (1974) we were told by the company we purchased our 14-1/2x22" C&P from to make sure that wheel was spinning before we hit the motor switch. The 1/2 hp motor that came with it could not overcome the starting torque. Personally, I think the rated 1/4 hp is necessary to maintain the speed once you've arrived there. All this to say, it wouldn't hurt to go with 1/2 hp or 1 hp if available. I'm sure there are lots of folks who think it's overkill, but I have replaced enough nose bearings to make me a bit skiddish. :>)
I hope this makes anything like sense, and might be helpful.
Good Providence in your endeavours!