Linear Motion Parts: Which type of Linear Bearings?

Last updated on 12th July, 2020

Over the years, I’ve built and cannibalised a lot of 3D printers, laser etchers and CNC machines. With the COVID19 lockdown in full swing, I am having a good sort out of some of these old projects. Now I want to take a dive into what I’ve learnt about some of the parts themselves. This post is going to focus on one very key type of part: linear bearings.

Here I am going to cover:

1. Design considerations
2. Roller bearings on a guide
3. Telescopic Slides
4. Plain Linear Bearings
5. Linear “Bush” Bearings
6. Linear bearing rails and guides
7. Guide bearings
8. Summary

Design considerations

Simply put, linear bearings are a part or mechanism which allows a body (such as a gantry) to travel smoothly in one direction (axis). When trying to determine which type of linear bearing to use, you need to determine two very critical factors for your application: Load and rigidity. For the load you need consider the mass of your moving elements (on that axis), kinematic loads and any additional loads from your application (e.g. a CNC spindle cutting). Now rigidity is slightly more difficult. The design of your overall system, and not just the type of bearing, is the main driver here. Imagine a well specified LM16UU Bush Bearing, but mount this on a 2m horizontal unsupported rail. The rail is going to deflect somewhat in in the centre!

Other key considerations when selecting a linear bearing are lubrication, ease of cleaning, free degrees of freedom and the all important cost! A huge word of warning on cost: ‘cheaping’ out on some of this stuff might seem like a good idea at the time. However, when your system just doesn’t work in the way you intended, you end up redesigning, rebuilding and buying twice. You know the phrase…Buy cheap, buy twice.

So with cost in mind, I’ve tried to categorise the following into types of linear bearing or system and put them roughly in terms of cost. By no means is the order reflective of “how good” the type of linear bearing is: selecting the right linear bearing for the job is key here.

Roller bearing on a guide

A roller bearing is very simple system in principle, but can quickly increase in complexity. In this system, a radial bearing runs along a flat surface (or guide). The radial bearing can be a plain bearing (imagine a brass roller), a ball bearing or something like a roller bearing.

The guide can be any straight flat surface. However, a guide which helps to restrain the bearing is commonly used. For example, a very popular choice for 3D Printers is using a “V-Slot Pulley”. This is a rubber wheel with integrated ball bearings designed to sit on an aluminium extrusion.

One really important point to note about roller bearings on a guide is the degrees of freedom of the system. For instance, the 3D-Printer “V-Slot Pulley” arrangement always needs 2 sets of bearings – one at the top of the rail and one at the bottom. It then relies on the V shape to stop forward and backwards motion and the multiple bearings stops twisting about the axis. This works on 3D printers because the loads are light, but might not be the best solution for a heavy duty application.

“Roller bearings on a guide” are usually a cost effective solution for lightweight applications. You can get a set of 10 “V-Slot Pulleys” for as little as £8 or cheaper. Another great point is that the axis travel is only limited by your guide. However, don’t forget the comment about rigidity! So if your looking for a huge 2x2meter 3D Printer, “Roller bearings on a guide” are a good option.

Telescopic slides

Telescopic slides, commonly thought of as drawer runners, are basically a more refined version of the roller bearing on a guide element. They typically incorporate roller bearings or balls on a steel shaped housing or frame. Telescopic slides are usually for cantilever applications and mounted on vertical surfaces. Imagine a drawer runner: it’s mounted on the (vertical) side of the drawer and extends out of the cupboard. However, they can also be mounted on a horizontal plane similar to a linear rail.

Telescope slides typically extend slightly less than their compressed length but can sometimes extend further. Slides can be mounted in pairs to avoid the cantilever type set up and can give smooth motion. Check out CopperDropDesigns Instructables post for an example telescopic slides in a small CNC machine.

Plain linear bearings

Plain bearing used on a linear system, also known as a slipper bearing, is a type of bearing which which relies on minimal friction between the bearing and it’s guide. This is done by careful material selection between the bearing and guide. Another option is to use a hydrostatic lubrication film between the two surfaces. The lubrication layer can be a pressurised liquid or a gas. Check out Physik Instrumente USA on Precision Motion Control air bearings for a great example of this.

Now obviously, a precision air bearing set up is going to cost a pretty penny. But generally speaking; plain bearings are cost effective solutions. You can pick up 8 IGUS RJ4JP-01-08 bearings to replace your old LM8UU bush bearings for around £10.

Linear “bush” bearings

Linear bush bearings come in many forms. The most common is know as the “LM” series and include bearings such as the LM10UU. These types of bearing have a series of ball bearings inside which run along a track in contact with the guide rail. The ball bearings run down this track and are recycled through a secondary channel.

Linear bush bearings are designed to use small ball bearing to minimise the contact area, and hence, the friction. The small contact area causes a large force. Therefore, bush bearing have to be run on a hardened steel shaft which has been suitably sized to fit the bearing. Although using a piece of non-hardened tube or bar will work for a while, the hardened steel balls in the bearing will damage the surface of the shaft over time. See the above photo as proof!

The LM series come in all sorts of shapes and sizes. For example, in the image above you can see (left to right): a flange mounted “LMH10UU”, a standard “LM10UU”, a LM bearing mounted in a aluminium housing “SC10UU” and finally a open bearing mounted in a “SBR10UU”. In case you hadn’t reliesed, there is a bit of a pattern to the part numbering:

Linear bearing Rails and Guides

Linear bearing mounted on guide rails are another type of bearing system. This group can have a fair bit of cross over with the Bush Bearings group but also includes the the popular “H-Series” bearings.

The H series refers to the cross sectional profile of the guide rail (kind of looks like a H?!) the the bearing “slides” or “slider block” runs up and down the length of the guide. The guide is supported along it’s full length. This is usually by means of some recessed cap head screws. Inside the slicer, a series of small ball bearings run along the guide before being recycled through a secondary channel. These types of bearings can also be referred to as “recirculating linear bearings”. The H-Shape of the guide ensures the balls are retained in the profile. This means the bearing is fully supported in multiple degrees of freedom.

Typically, the slider itself comes with four threaded mounting holes. The mating surface is ground flat to the running surface of the bearing. Because of this, you can be fairly sure that whatever you attach to the slider block, will be parallel to the mounting surface of the guide rail.

This bearing type goes by many names, but are sold in standard sizes and lengths. The size refers to the width of the guide rail and ranges from 8mm up to large sizes of 30mm plus. Lengths simply refer to the length of the guide. Although it is possible, it is not recommended to join multiple guides together to form a longer length. The reason is that aligning the two guides is very difficult and can cause damage to the bearing.

Guide Bearings

For completeness, I have included this final group: Guide Bearings. It’s not a typical bearing arrangement you can use on DIY projects unless you can hack an old machine tool. Guide bearings, or machine guides, are what you would find on industrial machines but can also be found on things like vices. Two or more surfaces are machined flat and usually have a thin layer of lubricant between them.

The flat surfaces often have a distinctive pattern on the surface which is the result of scraping. Hand scraping is a technique to flatten the guides whilst leaving imperfections in the surface to hold lubrication. In some cases, guide bearings might have a hydrostatic lubrication system where oil, air or another fluid is pressurised between the two surfaces.

Final thoughts

In conclusion, that is a bit of a brain dump on linear bearings. But don’t forget, there are other topics of interest! The most obvious one is now you have a body that can move smoothly, you need to move it with something? This is where linear drive components come in! Also we have motors, couplings, shaft alignment and frame design. Do you what to see something else? Drop a comment below, get in touch by email or check out the contact me page. But for now, I’ll leave you with a summary table.