July 30, 2009 ISSUE #212
Torque is a very important design consideration in screw presses. The more work a press has to do, the more torque that is required.
Gearboxes are the most expensive component in a screw press. Gearboxes are sold according to their torque rating, not their horsepower input or speed output. The bigger the torque rating, the bigger the gearbox, and the higher the price.
Not many years ago the only way we had to change the speed of a screw press was to change the sheaves used in the belt drive between the motor and the gearbox. In these cases we had to keep two situations in mind: if we changed sheaves to reduce the screw speed, and used the same motor, we effectively increased the torque, running the risk of overloading the gearbox. Once we sold a press with two motors, 15 hp and 5 hp. The 15 hp motor and its sheaves were used for high speed operation on light dewatering. The customer switched to the 5 hp motor and its sheaves for an off-season low speed application.
On the other hand, if we changed sheaves to speed up a screw for more throughput capacity, we were apt to have insufficient torque at higher speed. The press would stall out and we would switch to a bigger motor. The torque stayed the same, so the gearbox was safe.
Today, with ubiquitous VFD's, the world of mechanical engineers has changed. At 60 Hertz, a motor puts out the rated torque for the motor speed and horsepower. If the Hertz are reduced to slow down the motor, the torque remains constant. Thus we always know the gearbox is safe.
However if the Hertz are increased, to speed up the press, the horsepower remains constant. Since the rpm are going up and the horsepower is constant, we lose torque as we speed up. Once again, the gearbox is safe; however, we run a chance of stalling out.
A good example of this occurred with a twin screw 16" press which was used on corn silage during this last harvest. We started with a 75 hp 1200 rpm motor, which gave us 157,000 inch-pounds of torque at 60 Hz. To achieve the required throughput capacity, we had to run at 120 Hz (2400 rpm). That reduced our torque to 78,000 in-lb, which was not enough to both shred and press the husks and cobs. The press tripped out on overload.
So we switched to a 150 hp 1800 rpm motor. At 60 Hz, that gave us 213,000 in-lb. However, changing motors like that only raised the screw speed to 1800 rpm. We needed 2400 rpm. So we had to run the new motor at 80 Hz. That gave us the same screw speed in the press as we had before at 120 Hz. However increasing the speed to 80 Hz reduced torque to 142,000 in-lb. Fortunately, this was enough for the job at hand.
It is notable that motor manufacturers have adapted to the new world of VFD's. Most motors, even those rated for 1800 rpm, now have bearings and balance suitable for 4,000 rpm operation. During the last season's pea harvest, one of our customers ran his standard 1800 rpm motor at 180 Hertz (5,400 rpm!). It lasted almost to the end of the harvest period. The load was low, so, while the motor ran quite hot, it did not trip out.
Similarly, on a trouble job in China (50 Hertz), we found that our four pole, premium efficiency explosion proof Baldor motor, rated for 1500/1800 rpm, had "MAX 4,000 RPM" stamped right on the nameplate. This gave us all the latitude we needed to get out of trouble.
NOTE: The equation for torque is a constant, 63,000, multiplied by motor horsepower divided by shaft rpm:
T [in-lb] = (63,025 x motor hp) / screw rpm
Typically Vincent presses range from as low as 3,000 in-lb in a 4" or 6" press on up to as high as 400,000 in-lb in a 24" or 30" press.