A Torque Devil Is in the Details

Published: October 01, 2009

In center winders, like the classic Rolling Stones song, you can't always get what you want.

Imagine you need to buy a new center-driven winder or unwinder. To determine the size of the brake, clutch, or motor you will need, you need to know the torque and horsepower requirements.

You probably don't know the torque in ft-lb or N-m required for your winding or unwinding process, but it's not hard to figure out. Here's an example of what you might come up with in calculating high and low torque requirements.

Let me know if you see anything wrong with this specification:

• Wanted | Center winder and unwinder
• Tension range | 0.5-3.0 lb/in. (6:1 range)
• Width range | 30-60 in. (2:1 range)
• Cores | 3- and 6-in. inner diameter (0.5-in. wall thickness)
• Roll Size | 40-in. maximum diameter (10:1 range)

Let's calculate the low- and high-end torque requirements. The low-end torque requirement is the lowest tension × the narrowest width × the smallest radius (0.5 lb/in. tension)(30-in. width)(2-in. radius) = 30 in.-lb of torque.

The high-end torque need is the highest tension × the widest width × the largest radius, (3 lb/in. tension)(60-in. width)(20-in. diameter) = 3,600 in.-lb of torque.

We're done. We need a low-end torque of 30 in.-lb and a high-end torque of 3,600 in.-lb. Great. Okay? Hmmm. Gulp.

What's wrong? There is nothing wrong with either of these torque requirements, except that you were hoping to get them on the same winder.

The torque range — the ratio of the high-end torque to the low-end torque — is 120:1 (3,600 in.-lb/30 in.-lb). This range is outside the capability of any one normal brake, clutch, or motor.

Most brakes are pneumatically regulated with a maximum 80 psi and controllable minimum of 2 psi for a range of 40:1. Some disc brakes allow you to use multiple discs to double and triple their torque range, but that involves turning discs on and off.

Motors are sized by horsepower or kilowatts, which is a function of torque and speed. For a given mechanical drive train, a motor will have a 30:1 torque capability, but if you modify the leverage of the drive train, such as with a two-speed gearbox, you could sacrifice speed to get more torque.

Clutches are sized by torque, but as they run at faster speeds, they are limited by heat dissipation and sized by slip-watts, a strange unit that is a function of torque and speed, effectively similar to horsepower.

If you send the above 120:1 torque requirement to an equipment supplier for a quote, there are three possible responses, two of which are bad.

First, they may calculate the high-end torque, combine that with high-end speed, add a safety factor, and provide you with a big motor (and find later they can't get within 4x of your low-end needs).

The second option, which almost never happens, is they calculate the low-end torque and design a system to meet that need, but fall shy of the high end by 3x or 4x.

The third option — and the one I'm hoping for — is the supplier calls and tells you there is a problem with your specification.

What can you do if you have an excessive torque need? You'll have to make some concessions by reducing any of the tension, width, or diameter ranges.

Eliminate the 3-in. cores and narrowest width for the low-tension products. Reduce the maximum roll size for the high-tension products.

The other big torque range trimmer is taper tension — the process of decreasing tension as roll size grows. You can cut the high-end torque need in half by using 50% taper whenever you run the high-tension, wide, large-diameter products.

You may not be able to get what you want, but with the proper specification of torque range, you may get what you need: a winder that works.

Coming next month: How cores can reduce defects.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.