METHODS OF DRIVE
Sprocket Drive
The film is driven through the machine by means of sprocketed rollers engaged with the sprocket holes of the film. The drive is often called "positive" and the film runs at constant speed. The disadvantages are the risk of damage to the film sprocket holes and the expense and high maintenance cost of the sprockets. The slack caused by wetting and the tautness caused by drying is compensated for by the bottom loop of each strand hanging free with a "diabolo" type roller suspended in each loop. High-speed modern processors use sprocketed rollers at the bottom of each rack as well.
Tendency Drive
The film is threaded over and under rollers fixed to the top and bottom of each rack in the same helical path.
A constant speed drive roller positioned at one point near the beginning of the film path is run continuously at the required speed and feeds film into the process. Every rack has its top roller driven and as slack is created by the feed roller it is taken up by the top rollers.
Tendency drive machines are still in use although they suffer from the problem that any break or stop in the film does not stop film being fed into the machine, so that a pile of wet spaghetti is the usual result!
Demand Drive
Demand drive is the converse of tendency drive.
The same helical rank is used but the constant speed drive roller, known here as the "pacer" is at the drying end of the machine. The film runs over bobbins, either pivoted with weights or with a spring mechanism. Tension on the film pulls the bobbin in contact with a plastic coated drive roller below, rotating it and transporting the film.
In some versions, the driven shafts are inside the sprung rollers rather than below.
Some have the drive rollers at the bottom of the tanks; most have them at the top of the tanks.
Unlike the tendency drive if the film breaks, the film up to the break stops, but the pacer would then strip the film out of the processor beyond the break, so a break detector is fitted to stop the pacer immediately.
Soft Touch Tyres
The friction drive systems like tendency and demand are kind to the film but at high speeds, the film can aquaplane and lose traction. The bobbins have soft plastic tyres of various patterns with raised block patterns or treads such that only small areas of film are in contact with the film. The drive is positive but there is risk of damage to the film if a slip occurs, causing, usually small backing scratches.
Modern High Speed Drive Processors
Today processors for camera films are almost always top drive shaft demand drive soft touch machines up to a maximum of 150 ft/min [2700 m/hr] and print processors are exactly the same but up to 250 ft/min [4500m/hr]. Faster print processors operate up to 700 ft/min [12,800 m/hr] but generally use modern sprocket drives with a variable height bottom roller.