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.

32 thoughts on “Mastering Ball Screws

  1. That linked article is NOT a ballscrew. That is a Rolling-ring drive. Ballscrews and rolling ring drives are separate. Just because it contains ball bearings does not make it a ballscrew. I would have objected at the previous article, but I missed it apparently.

    Ball screws are relatively precise and repeatable. A rolling ring drive can be precise, but is much more difficult to get repeatable, as the entire drive can skip steps. If you want to learn about ballscrews and actually get a lot of information, Machine design is a decent source of information on rolling ring, ball screw, and lead screw designs, as well as linear actuators and a pile of other cool topics, sometimes literally (Cryogenic machining).

    Full disclosure: A family member works for Dyantect LSI, a ballscrew manufacturer, and I have toured their plant. Some of the equipment was amazing, and I was in the building as they were preparing to ship out the James-Webb space telescope’s spare ballscrew, which was the 3rd produced, as the 1st was destructively tested and the 2nd installed. I got to see it with my own eyes, although it was not permitted out of it’s protective bagging.

  2. If you think the magic pixies of electronics are something you don’t want to get loose, wait’ll you run one of these off the end and the ball bearings scatter. Hint: Don’t let this happen. It’s not the end of the world, but putting it all back together is a total PITA on industrial equipment – I can’t imagine how it would be at small scale.

  3. You mention at the beginning that lead screw is slower which isn’t necessarily true but missed out the important link of why, the reduced friction because of the bearings allows the larger pitch which increases travel/turn, there is no reason you couldn’t have similar pitch threaded rod if you could overcome the enormous friction when you lose the mechanical advantage of smaller pitch.

  4. I know it isn’t the main point of the article, but I believe you have the terminology reversed in the intro here.

    Threaded rod is the cheap stuff many printers ship with, and trapezoidal lead screws is the next step up.

    1. I don’t think so. You are right, but the article says:

      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.

      ACME threaded rod isn’t the same as common threaded rod.

    2. It doesn’t really matter that much if it’s ball screw or thread rod. In both cases you still need linear rod and that is expensive.

      I am building a Radial (or arc) Delta printer (actually to drill PCBs) because the cheapest linear setup is no linear setup at all.

  5. One thing that’s being overlooked here is the ballscrew mounting process. If you’re going to do it the “recommended” way, you need to buy a pair of angular contact bearings, to axially constrain the ballscrew on one end, and maybe a deep groove bearing, to support the other end of the screw.

    Anglular contact bearings can get quite expensive, depending on the brand, tolerance, diameter and contact angle you choose. For example, I bought three 16 mm diameter screws of various lenghts for ~$50 bucks, and six 30 degree FAG bearings for $60.

    Finally, I find that linear motion product catalogs are a great learning resource. See http://www.hiwin.com/pdf/ballscrews.pdf

  6. Is there a reason that they always seem to be mounted with angular contact bearings rather than a pair of tapered roller bearings? The only reason I can think of is that an angular contact bearing is good for higher rpm, but a lot of machines have no need to spin them past 1500 rpm anyway, so why the extra cost?

    1. Good question. Tapered bearings are probably bulkier, and I don’t know how available are the small inner diameter models and how much drag they have for the same preload, compared to angluar contact bearings.

      Also, 1500 rpm is maybe for stepper systems, DC and AC servos can easily reach 4 times that speed in rapid movements.

      1. That’s true, I know some more modern milling machines can fly on their rapids. For most hobby/home built machines I expect a tapered roller pair would be 90% the benefit for a quarter the cost. Thinking about it now 1500 is probably conservative as well. The wheel bearings in my car would have to be capable of over 2000.
        Also, aren’t lathe spindle bearings often tapered roller bearings? Granted they are a little slower as well.

        1. Yup, lathe and mill spindles use tapered roller bearings, at least the lower speed ones. You’re right about the cost advantage of tapered roller bearings, and I was wrong about them being bulkier, both types are comaprable in size. The main problem I see is the minumum inner diameter of the bearings, as angular contacts can be found starting from 6mm ID. Tapered rollers are limited to bigger diameter screws (think >25mm) wich are not commonly used in home built machines.

    2. The reason is that a rolling ring assembly is used where you need the drive to be reversible. Reversing a tapered roller bearing would dramatically increase backlash when reversed if it even stayed together. Tapered rollers are used where you have longitudinal forces and act like a mix of a normal bearing and a thrust bearing. The thrust part only works for load in one direction only.

      1. In fairness, JoelC said a pair of tapered roller bearings. Installed back to back (or front to front) like a pair or angular contact bearings and preloaded should work with them too.

        1. Granted … but … How can you preload two back to back and angled tapered roller bearings on an under sized shaft and then reverse the angled alignment of the complete two bearing set when you change direction?

          1. Not sure if I understood your comment, so I don’t know if this reply helps. With the pair of bearings mounted in opposite directions, with a spacer between them (the spacer keeps the distance between the both inner or outer races) one of them provides the reaction force in one direction, and the other one does the same thing for the other direction, so there’s always one bearing stopping the other bearing from loosing its preload when the axial thrust is reversed. The preload is adjusted via shaft or housing locknut (depends on the mounting configuration). See figure 1 here http://www.skf.com/us/products/bearings-units-housings/ball-bearings/principles/application-of-bearings/bearing-preload/types-of-preload/index.html?switch=y

          2. One of the features of a rolling ring assembly is that it can be reversed extremely quickly without needing to slow the motor to a stop and then accelerate it back to speed in the opposite direction. This is done by flipping the angled bearing assembly which you describe as two tapered roller bearings.
            Forward [| // |]
            Backward [| \\ |]
            That means for one direction the running surfaces are the top of bearing ‘A’ and the bottom of bearing ‘B’ and for the other direction it would be the top of bearing ‘B’ and the bottom of bearing ‘A’.

            In any case you might as well weld the cases together because any combination of two running surfaces will be at exactly the same speed.

            So if you solve these problems (and they can be solved) then what you are left with is a bearing assembly that has increased complexity, increased cost, reduced reliability and more backlash than the traditional method.

            This is *if* I understood you correct, if not then please forgive me.

            Do a youtube for rolling ring drive to see the reversing motion.

          3. Oh, I think I know what was the problem here RÖB :) According to me, JoelC was talking about angular contact bearings vs tapered roller bearings for constraining the ballscrew on its ends. You’re talking about the bearings on rolling ring drives, the ones that are sort of analogous to a ballnuts.

  7. “a race containing ball bearings”. Nope. A ball bearing is a combination of inner race, outer race, and steel balls. Those are just steel balls or “bearing balls”, but definitely not “ball bearings”.

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