What Are Linear Rails?
Linear rails, also referred to as linear motion guides and linear slides, are common motion components used to achieve smooth sliding motions. They can be found in 3D printers, CNC mills, waterjet cutters, and the like.
A linear rail consists of a stiff, steel rail, along which a carriage slide. Most commonly, the carriage contains recirculating ball bearings that provide contact points between the carriage and the rail, as depicted above. This enables a smooth sliding motion as the balls roll between the surfaces. The shape of the shaft enables the carriage to stay locked on with tight tolerances, restricting the motion to strictly linear directions.
Linear rail 3D printers utilize the numerous advantages of these linear rails to, supposedly, achieve better motion. So, how plausible are these claims? Let’s take a look.
Most 3D printers sport hardened-steel linear rods, used in tandem with linear ball bearings. This combination has worked well for thousands of printers and users, so what more do linear rails bring?
Linear rails are stiffer. Linear rails flex significantly less than linear rods, due to factors such as shape and metal stiffness. Not convinced? Try bending one yourself, and compare it to a typical linear rod. Any decent rail will have far less give. In terms of 3D printing, this leads to greatly reduced backlash and ringing artifacts, yielding cleaner print surfaces and edges.
You can get away with one linear rail. Unlike linear rods, linear rails don’t allow for any rotation of the carriage. This means that, when setting up a motion system, a single rail can achieve strictly linear motion on its own, as shown above. With linear rods, at least two are necessary to prevent rotation, leading to complications with proper alignment and introducing weight considerations. Using a linear rail can create a simpler, lighter, more nimble assembly, which will also reduce 3D print ringing.
Higher precision, smoother motion. Linear rails are also usually manufactured to much tighter tolerances than linear rods. This higher precision translates to reduced play and binding (when the bearings catch and don’t roll smoothly), yielding smoother, more consistent motion.
Easier, more precise mounting. Linear rails are much easier to mount than rods. The rails and carriage both have regularly spaced mounting holes, so a couple of bolts will have everything locked down. This contrasts sharply with linear rods and bearings, whose round shapes require more creative, and often less precise, mounting solutions in 3D printers.
Of course, nothing is perfect. In a few areas, linear rails fall short of rods in 3D printer applications.
Linear rails are expensive. There’s no getting around that. Any decent linear rail will easily cost several times more than a comparable combination of linear rod and bearing. The premium precision, stiffness, and smooth motion come at an equally premium price.
Linear rails must be handled with care. Unlike round linear bearings, the ball bearings in linear rail carriages aren’t usually capped and locked in. If the carriage accidentally falls off the rail, some ball bearings may roll out, leading to inconsistent motion.