A positioning device without a linear motor is likely to use a leadscrew or a ball screw (discussed in an accompanying section). The leadscrew or power screw is a threaded shaft on which rides a simple threaded nut. It has no ball bearings. Leadscrews use the helix angle of the thread to convert rotary motion to linear motion. The nut attaches to a load (a movable table or carriage) so that, as the shaft turns, the nut and load transits one way or the other. This simple design is open to wide variations, such as length, thread pitch, coating, and nut design. The shaft can be turned by any of the motors discussed in another section.
With attention to selection and application, leadscrews can work with the efficiency that comes close to ballscrews on many applications – as well as high load capacity and positioning accuracy. In addition, engineers can more easily tailor leadscrews to an application, thanks to a flexible configuration and form factor, the ability to operate without lubricant, quieter operation, and lower cost.
Leadscrew performance depends heavily on the coefficient of friction between the nut and screw, which in turn depends upon the materials of the nut and screw. Leadscrews typically use nuts made of internally lubricated plastic or bearing-grade bronze. Plastic nuts usually travel on stainless-steel screws, while bronze nuts often run on carbon-steel screws. A few simple steps can help determine whether or not a leadscrew is a good fit for an application (and select the most appropriate leadscrew features).
Leadscrew load capacity: When considering whether leadscrews or ballscrews are better for an application, look at the required load capacity. Plastic nuts are suitable for light loads of less than 100 lb, although plastic nut designs for 300 lb and beyond are possible. Bronze nuts, on the other hand, are useful for much heavier loads.
Leadscrew efficiency: The efficiency of leadscrews typically ranges from 20% to 80%, a value highly dependent upon helix angle. Helix angle is the arctangent of the lead divided by the pitch diameter. It is the angle of the advancement of the thread. As a general rule, higher helix angles mean higher efficiency. A higher helix angle is more efficient because less of the energy driving the leadscrew goes into overcoming friction. This is because the number of times the screw must rotate to get a given linear displacement is lower on a high helix screw. One disadvantage of a high-helix angle is that it necessitates more torque to turn the screw.
Leadscrew speed: Leadscrews come in leads from less than 0.050 to 2.00 in./rev and more. This range can deliver jog speeds to 70 in./sec. This leadscrew feature can provide advantages in many applications. For example, devices that must accurately position payloads can use a leadscrew with a low helix angle to get high positioning resolution. Other applications benefit from fast jog speeds and low screw rpm, providing quiet operation and long life. The maximum rotational speed of a leadscrew is limited by the critical speed of the screw – the speed at which resonance occurs. Leadscrew nuts can be driven at high rpm, but depending on the applied load, heat buildup may limit duty cycles.
