Tools such as these Craftsman grippers help multiply torque to tighten and loosen bolts and other fasteners.

If you're just getting into motors and drives, here's some advice that will help you get off to a fast start.

Horsepower is useful in determining the total amount of work that must be accomplished in a given span of time. But torque is what brings machinery to life, turning feed screws, cutting tools, and countless other motion axes.

By understanding torque, and its relationship to speed and horsepower, you'll be less likely to make those beginners' mistakes when making decisions on motors and drives.

In a pickle

If you've ever had a tough time opening a pickle jar, then you're probably familiar with torque. Simply put, the reason you're on the outside staring at the pickles inside is that you can't apply enough torque to the lid to break it loose.

So what do you do? You can always get a better grip and try harder. You can use a rubber pad, or washcloth, to help you transmit more torque to the lid without slipping. Or you can use a mechanical device, like a pipe wrench, to multiply your torque-producing capability.

Torque is the product of force and its radial offset from the turning axis. If you care to know exactly how much torque you need to open the jar, for example, you can measure the jar lid and then find a way to gauge the force needed to break the lid free. The equation for torque T is:

T = F x r

where F = force (oz or lb) and r = radius (in. or ft).

Some people, when they can't open jars, tap on the lid with a butter knife. This technique is often successful and it illustrates another important point about torque. In realworld applications, it usually takes more torque to start a machine than to keep it running once it's going. The higher torque is required to overcome "stiction" and break the load loose from its resting position. Tapping the jar lid lowers this breakaway torque.

Stiction associated with a jar may be due to a combination of things, including vacuum pressure, sealing pressure, and friction between the lid and glass. In a machine, stiction depends on the geometry of the moving components as well as the type of bearings that support them. Once stiction has been overcome, a smaller "running" torque must be maintained to keep the system in motion. The proportion between starting and running torque varies with each load type.

A look at loads

Machine loads generally fall into one of three categories; constant torque, constant horsepower, and variable torque. In most applications, torque is independent of the speed at which the machine is driven. Thus, as the machine accelerates, the horsepower required to drive the load increases while torque remains essentially constant.

Torque T is the product of applied force and its offset (radius) from the axis of rotation.

The basic considerations when sizing a drive for a constant-torque load are breakaway torque, running torque, and operating speed. Only after these have been determined is it possible to calculate the required horsepower. Beyond that, there are no other considerations because most variable-speed drives are inherently constant-torque devices.

Another load type – common in metal-working (as in drilling, boring, tapping, turning, milling, and grinding) – is constant-horsepower. Here, torque is inversely proportional to speed.

Consider a drill press. Large holes are typically drilled slowly at high torque, while small holes are drilled quickly at low torque. In either case, horsepower remains constant regardless of speed. Besides metal-working, constant-horsepower may also be required for center-driven winding and some mixing applications.

One way to identify a constanthorsepower load is to examine the machine output. If a machine is designed to produce a fixed number of pounds per hour – whether it's small parts at high speed or large parts at low speed – the drive requirement is apt to be constant-horsepower.

When selecting a drive for these applications, one approach is to match the drive's output with the machine's torque requirement at low speeds. The catch, however, is that the drive is likely to be grossly oversized for the rest of the speed range.

A more practical approach involves the use of variable-torque transmissions such as stepped pulleys, gearshifts, and adjustable-pitch belt drives. A dc (SCR) drive employing armature control at low speeds and field-weakening at high speeds may also work. In addition, some variablefrequency drives can approximate constant-horsepower (above 60 Hz) when operated at constant voltage.

Continue on page 2