Translating rotary motion to linear motion is a basic part of mechatronic design. Take a look at the nearest 3D-printer or CNC router — at least the Cartesian variety — and you’ll see some mechanism that converts the rotation of the the motor shafts into the smooth linear motion needed for each axis.

Hobby-grade machines are as likely as not to use pulleys and timing belts to achieve this translation, and that generally meets the needs of the machine. But in some machines, the stretchiness of a belt won’t cut it, and the designer may turn to some variety of screw drive to do the job.

Lead Screws

We have all seen CNC projects where the builder has built a linear actuator from a length of hardware store threaded rod. Chopped to size with a hacksaw, held in place with a couple of bearings, and attached to a stepper with a coupling of some type, these screw drives do a decent job of producing linear motion. But it’s far from a perfect solution, mechanically speaking.

The main problem with this arrangement is the thread profile. As we mentioned in our post on screw threads, the V-thread profile on threaded fasteners is optimized for providing a high axial clamping force and a “non-overhauling” property, or the tendency for the fastener to self-lock. This requires a high friction arrangement, which is not optimal for a screw drive.

A lead screw, on the other hand, has a thread optimized for reducing friction. Lead screws commonly use a different thread profile such as the trapezoidal Acme profile. Also, the lead of the thread, or the distance along the screw’s length that one full turn of the thread covers, tends to vary more in lead screws than with threaded fasteners. Precise positioning and non-overhauling can be obtained with a shorter lead; long lead lengths will allow the lead screw to translate fewer rotations into longer linear travel. Lead screws also differ from threaded fasteners in that they often have multiple starts. Each individual thread is called a start, and lead screws often have two or more threads nested together along the surface of the screw.

Every screw needs a nut, and lead screws are no exception. The lead screw nut is a part with an internal thread that mates with the lead screw’s external thread, and provides some way to secure the nut to the machine and transmit the force produced by the rotation of the lead screw. Because the mating surface of the lead screw and nut tends to be much longer than would be the case for a threaded fastener, a lead nut tends to be made from materials that will reduce friction as much as possible. Bronze is commonly selected for its natural lubricity; sintered bronze is often used too since it can be impregnated with lubricants. Plastics are not …read more

Source:: Hackaday