Bearings vs. Bushings
The previous post discussed the two dominant types of linear motion systems used in 3D printers: lead screws and belt drives. These are the systems that actually move the build platform and/or extruder. Lead screws and timing belts are used to transfer electrical energy to movement.
There is another important part of most 3D printer motion systems though. On most 3D printers, the build platform and the extruder slide over smooth rods as they move.
Motion over the smooth rods is achieved with either bushings or bearings.
Bushings, also called sleeve bearings, slide over smooth rods and provide an extremely low friction motion which minimizes power consumption, noise, and wear on parts. Bushings look like plain metal tubes, but they are actually fairly sophisticated parts.
Bushings are typically made from a bronze powder. The powder is fused together such that tiny pours are present in the metal. The bushings are then impregnated with oil (about 20% oil by volume). Then, as the bushing contacts a shaft, the oil is drawn to the surface of the bushing via capillary action so that the bushing constantly deposits a thin film of lubricating oil onto the shaft. In other words, bronze bushings are self-lubricating.
Self-lubrication is the first major advantage of using bushings over ball bearings. Another major benefit is cost. Bronze bushings cost between six and ten times less than linear ball bearings. The third major benefit to bushings is that they are generally quieter than ball bearings. Fourth, bushings can be used on either hardened or non-hardened shafts, whereas linear ball bearings can only be used on the more expensive hardened shafts. Finally, bushings generally require less maintenance than linear ball bearings.
Bearings have a couple of downsides as well. First, bushings can have what is commonly referred to as the “stick and slip” problem. Bronze bushings, unlike linear ball bearings, need to overcome static friction forces before moving. Especially if the linear motion system is worn, or not properly aligned, this means the bushings can move in a kind of jerky, uneven movement. Second, bushings, especially cheap ones, can have wider tolerances making for a slightly worse fit on the smooth rods.
Linear Ball Bearings
The alternative to bronze bushings for motion over smooth shafts in a 3D printer is linear ball bearings. Bushings are pretty fancy bits of metallurgy, but linear ball bearings really are fine pieces of engineering. For a good explanation of how linear ball bearings work (and radial ball bearings), check out the video below by Tomas Sanladerer.
Linear ball bearings have a number of advantages over bushings. First and foremost, the motion provided by linear ball bearings is generally smoother than bushings. Since linear ball bearings roll instead of sliding, they do not have static friction to overcome before moving like bushings do. Second, linear ball bearings are also generally built to tighter tolerances than bushings. The tighter fit on the smooth rods means less slop and less backlash.
Linear ball bearings have a few disadvantages as well, chief among them is cost. As mentioned above, linear ball bearings cost between six and ten times as much as bushings, assuming you are purchasing decent bearings. Adding to this increased cost is the fact that linear ball bearings must be used with hardened, preferably chrome-plated, shafts. If linear bearings are used on softer metal shafts, the balls will basically cut into the shaft over time, which increases backlash. The second disadvantage is that linear ball bearings require more maintenance than bushings. Inside a linear ball bearing, the ball bearings themselves contact the smooth shaft directly. This means the shaft must be lubricated from time to time in order to keep the system running well. This also means dirt and dust can make its way into the ball bearings, which again hurts performance. Along the same lines, linear ball bearings can have problems if not properly maintained. With lack of maintenance, the linear ball bearings can lose their smooth motion, can run louder, and could even jam.
Linear Ball Bearing Naming
There are a few types of linear ball bearings that are used in a huge proportion of all 3D printers on the market. The size and design of linear ball bearings is denoted by an alphanumeric code used to identify the bearings. For example, the LM8UU linear ball bearings in the photograph above are extraordinarily popular in the 3D printing community because of their low price and relatively good quality. The “LM” denotes that the bearing is intended for use in linear motion. The number denotes the diameter of the shaft that the bearing will fit on, in this case an 8mm diameter shaft. The final one or two letters denote the design of the bearing. The letters usually identify the type of seals used on the ends of the bearing to keep the lubricating grease on the inside and dust out. The “UU” indicates that the bearing has been double shielded and sealed.