Lead screws are an excellent linear motion product that converts rotary motion to linear motion very economically and until recently offered only moderate accuracy and repeatability.
Lead screws are an excellent linear motion product that converts rotary motion to linear motion very economically and until recently offered only moderate accuracy and repeatability. Today's top manufacturers are producing standard lead screws with +/- 0.003"/inch lead accuracy and even better on request. These companies are also producing wide ranges of sizes and leads to choose from and readily produce custom sizes and shapes of nuts to fit these screws. Here are four things that every engineer should keep in mind when selecting any lead screw.
- Use PTFE coatings when possible: Its a cheap and very effective to extend teh life of your lead screw assembly. The nut is always the first thing to fail and adding a PTFE coating to a lead screw will extend the life of the nut by a factor of 8-10X. Some manufacturers use water-based PTFE coatings that can wear over time and can cause contamination so be sure to request solvent based coatings that do not outgass or wear. Most good PTFE coatings should last 25 million to 80 million inches of travel without wear even in aggressive applications.
- Ask for custom nuts: Unlike ball screws and ball nuts, its very easy to machine the different plastics that lead screw nuts are manufactured from so try to incorporate as many other different parts that are connected to the nut into its design. Nuts are typically machined out of bar stock orplastic sheets or blocks.
- End Machining / Straightening: The materials used to manufacture lead screws lend themselves to easy machining and can be done on any conventional lathe, mill or OD grinding machine. This is easy if you are just machining one or two pieces for prototypes and can in most cases be quicer than waiting on the manufaturer. However, if you are ordering several screws and teh machining needs to be very accurate, its best tolet the manufacturer do the work fr you since its something they do every day. The manufacturers will typicall include the straighteninng processes in the cost of machining if you are having them do the machining for you.
- Flats: Mounting a lead screw in to your machine or device and adding bearings, lock nuts, encoders and mating parts can get a little tricky when you need to hold onto the screw without damaging the threads or the machined ends. Adding a couple flats on the outside diameter of the screw in the threaded section will go a long way towards helping you install a lead screw without damaging any of its critical dimensions. The flats will allow you to Flats can be machined onto the screw in a way that allows the nut to travel over them so you don't lose any travel by adding them.