Mastering Ball Screws

Most inexpensive 3D printers use a type of lead screw to move some part of the printer in the vertical direction. A motor turns a threaded rod and that causes a nut to go up or down. The printer part rides on the nut. This works well, but it is slower than other drive mechanisms (which is why you don’t often see them on the horizontal parts of a printer). Some cheap printers use common threaded rod, which is convenient, but prone to bad behavior since the rods are not always straight, the threads are subject to backlash, and the tolerances are not always the best.

More sophisticated printers use ACME threaded rod or trapezoidal threaded rods. These are made for this type of service and have thread designs that minimize things like backlash. They typically are made to more exacting standards, too. Making the nut softer than the rod (for example, brass or Delrin) is another common optimization.

However, when lead screws aren’t good enough, mechanical designers turn to ball screws. In principle, these are very similar to lead screws but instead of a nut, there is a race containing ball bearings that moves up and down the screw. The ball bearings lead to less friction.

Misumi recently posted a few blog articles about ball screws. Some of the information is basic, but it also covers preloading and friction. Plus they are promising future articles to expand on the topic. If you prefer to watch a video, you might enjoy the one below.

In general,

  • Lead screws cost less than ball screws
  • Unlike ball screws, lead screws are self-locking and do not require a braking system
  • Ball screws can have difficulty with vertical operation
  • Ball screws are more efficient allowing less torque and smaller motors than a lead screw, which also will generate more heat
  • Lead screws require more frequent replacement than ball screws

The Misumi catalog, by the way, is a marvel of mechanical engineering toys. If you’ve followed Hackaday for a while, you might know that all ball screws don’t have threads. While ball screws are uncommon in 3D printers, they show up pretty often in CNC machine designs.

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