When you design a 3D printer, a CNC machine or a cartesian robot, one of the first design decisions you will encounter is choosing between linear rails and linear rods. And in many cases, the answer is not simple as it requires taking many factors into consideration.
However, if I were to design a machine as a proof of concept, and I didn’t have a lot of time in hand, I could simplify the problem based on a few factors.
If cost is not a limiting factor, linear rails are better at they could achieve higher precision, withstand higher loads, operate at higher speeds, and occupy less space. However, linear rods are cheaper, easier to install, and could be used at a structural beam.
When I am trying to design a machine that will work every time for an extended period, I work in a deeper level of detail. In this article, I will discuss the major factors that influence the selection of linear rails and linear rods.
General Design Considerations
In general, linear rails could be used individually which makes for a more compact design. However, linear rails require support along the rail. Typically supports come the form of aluminum extrusions or a part of frame of the machine.
Linear rods must be used in pairs to achieve one dimensional motion, which increases their footprint. However, they are typically supported at the ends and do not require additional support. In some cases, supports, such as aluminum extrusion, are used for ease of alignment.
These basic considerations are usually good enough to eliminate one of the options. For example, if my main goal was to create a compact 3D printer, linear rails might be a better option, given their ability to function individually.
However, if my goal was to create a wider support, linear rods might be a better option. Since they are used in pars, is possible to spread the pair apart to achieve the desired width.
Cost of Linear Rails and Linear Rods
Based on the prices shown in McMaster-Carr, a major retailer of mechanical parts in the US, I collected the prices for several linear rail widths and linear rod diameters to compare the prices of each. Given the wide variety of materials and specifications, I limited the data to the most basic linear rails and rods.
The linear rails used for this comparison are the standard 440 Stainless Steel guide rails found in this link here in McMaster-Carr. The linear rods are the 420 Stainless Steel linear motion shafts found in this link here. To account for using linear rods in pairs, I doubled their price.
| Rail width/Rod diameter | Linear Rail cost/length | Linear Rod cost/length [double] |
| 5 mm | $0.40/mm | $0.16/mm |
| 10 mm | $0.56/mm | $0.17/mm |
| 12 mm | $0.46/mm | $0.14/mm |
| 15 mm | $0.63/mm | $0.17/mm |
| 20 mm | $0.68/mm | $0.23/mm |
Linear rails cost 2.5 to 4 times as much as a pair of linear rods per length at the same width and diameter. Depending on the material and finish selection, the price may vary. This does not consider the cost of fixturing parts and carriages, given their variety in specifications and costs.
Although the cost of linear rails is far more expensive compared to linear rods, the cost difference alone is not enough to select one over the other. Linear rails have various advantage and disadvantages compared to linear rods that could justify the cost difference.
The differences between the two could be categorized as mechanical or fabrication differences. I will discuss both differences in-depth below.
Mechanical Considerations
The mechanical considerations are determined based on the desired performance of a given design. In my workflow, I tend to create these specifications prior to any physical prototyping or CAD designs. It is important to be as specific as possible when creating design goals to minimize errors.
For example, when I build a 3D printer, some of the main goals are build volume, overall size and cost. Each of these three goals will impact the selection of linear rails or linear rods. If cost reduction is an important factor, linear rods might be a better candidate.
If you’re interested in building a cheap 3D printer, I wrote this article (Building your own 3D printer: is it cheaper?), where I discussed the main cost reduction factors in building a DIY 3D printer.
Loading Capacity of Linear Rails and Linear Rods
The loading capacity of linear bearings is relatively complex and depends on the definition used to determine its value. For this comparison, I used the rated dynamic load capacity of the linear bearing carriage.
The linear bearing carriage is the part that slides along the rail or the rod. And the rated dynamic load capacity is the recommended limit a single carriage could withstand while in motion. For a safer design, I recommend choosing a linear rail or rod with an additional 20% or more dynamic loading capacity from the intended design.
To ensure that the comparison is not biased towards linear rails or rods, I doubled the loading capacity of the linear rod carriages. This doubling will maintain the assumption that two linear rods are required to achieve the performance of a single linear rail.
Additionally, linear rod carriages are commonly offered in two types: low and high loading capacity. I included both in this comparison. The links for the data could be found here for the linear rail carriage, and here for the linear rod carriage.
| Rail width/Rod diameter | Linear Rail Carriage dynamic load capacity | Linear Rod Carriage low dynamic load capacity [doubled] |
| 5 mm | 68kg | 34kg |
| 10 mm | 86kg | 72kg |
| 12 mm | 336kg | 100kg |
| 20 mm | 1045kg | 177kg |
As you can see above, the dynamic loading of the linear rail carriage is always higher than linear rod carriage. Additionally, the capacity of the linear carriage increases faster than a linear rod carriage as the size increases.
| Rail width/Rod diameter | Linear Rail Carriage dynamic load capacity | Linear Rod Carriage high dynamic load capacity [doubled] |
| 5 mm | 68kg | 50kg |
| 10 mm | 86kg | 118kg |
| 12 mm | 336kg | 164kg |
| 20 mm | 1045kg | 282kg |
A similar trend can be seen when comparing linear rail carriages to high dynamic load linear rod carriages. Except for a 10 mm diameter rod carriage, the linear rail is consistently higher in terms of the dynamic load capacity.
Since I didn’t factor in the cost of the carriages in the price comparison earlier. I included it here to see if there are any noticeable differences.
| Rail width/Rod diameter | Linear Rail Carriage cost | Linear Rod Carriage low dynamic load capacity cost [doubled] |
| 5 mm | $77.64 | $70.60 |
| 10 mm | $91.77 | $91.08 |
| 12 mm | $82.76 | $90.52 |
| 20 mm | $220.60 | $136.40 |
In the case of the low dynamic load linear rod carriage, the price difference was negligible except for the 20 mm case. However, the price difference is justified since linear rail carriage is capable of withstanding 8 times the load in the 20 mm case.
| Rail width/Rod diameter | Linear Rail Carriage cost | Linear Rod Carriage low dynamic load capacity cost [doubled] |
| 5 mm | $77.64 | $133.60 |
| 10 mm | $91.77 | $182.48 |
| 12 mm | $82.76 | $185.16 |
| 20 mm | $220.60 | $270.00 |
In the case of the high dynamic load linear rod carriage, the cost of the linear rail carriage is consistently lower, which leads to the following conclusion.
A linear rail carriage can withstand a higher dynamic load compared to a pair of linear rod carriages of the same diameter of the width of the rail. Additionally, the cost of a linear rail carriage is the same or lower than that of a linear rod carriage.
Accuracy and Precision of Linear Rails and Linear Rods
In general, linear rails offer greater precision and accuracy compared to linear rods. Both come in different tolerance classes; however, achieving the optimal accuracy levels is dependent on the assembly and manufacturing process of the machine that relies on linear bearings.
Linear rails are designed such that the carriage can only move in one direction. Linear rails, on the other hand, allow the carriage to travel along the rod and rotate around it. This additional degree of freedom reduces the accuracy of linear rods compared to linear rails.
Additionally, linear rail carriages are commonly designed with circulating ball bearings which allow the carriage to move smoothly across the rail. Linear rod carriages are commonly do not have circulating ball bearings, which causes binding and additional friction.
Parallelism is major contributor to the accuracy of linear bearing assembly. However, it is only a factor when linear rails are used in pairs. If the distance between two rails changes along the direction of motion, the carriages will bind, causing an increase in friction and a loss in precision.
In light weight or compact applications, where a single linear rail is sufficient, parallelism will not be an issue. However, linear rods must be used in pairs. This makes parallelism an issue for linear rods. I will further discuss the issue of parallelism in the alignment section later in this article.
Both, linear rails and linear rods, depend on the assembly process to achieve their optimal precision. However, it is easier to achieve a higher degree parallelism and flatness with linear rails compared linear rods. Again, I discuss that in detail in the alignment section in this article.
If you would like to read more about the accuracy specifications, ranges and classes of linear rails, read this paper here from THK, a major linear rail manufacturer.
For more information about the accuracy of linear rails, raid this article here from Thomson Linear Motion. The article discusses the theory behind the accuracy measurements of linear shafts.
Stiffness and Rigidity of Linear Rails and Linear Rods
Linear rails are stiffer than linear rods due to the difference in the mounting method. Linear rails take advantage of the additional stiffens of the mounting surface using more screws per unit length. Linear rails are commonly fixed at the ends, making them susceptible to bending and vibrations.
As you can see in the schematic above, linear rails distribute their resistance to loads among many nuts or screws. This distribution makes them less likely to bend at a result of excessive load, and less likely to vibrate due to fast motion.
On the other hand, linear rods utilize fixtures the ends of the smooth rod to support the rod. Although the fixtures could be stronger than individual nuts, they are less capable of preventing bending and vibrations due their distance from the applied loads.
Additionally, the issue of bending gets worse as the linear rod gets longer. However, a linear rail has nuts that are distributed along the rail, making length a relatively negligible issue.
If linear rails become excessively long, they are more likely to depend on the stiffness and rigidity of the frame they are attached to. The most common frames in 3D printers are made from aluminum extrusions, which are far stiffer than a floating linear rod.
However, if the machine in intended for low precision applications where the deflection and vibrations of the load are negligible, linear rods might be a suitable candidate.
If you are interested in learning more about how beams bend and what kinds of fixtures reduce bending and vibrations, I recommend reading this article here on Wikipedia on Euler-Bernoulli beam theory.
Speeds of Linear Rails and Linear Rods
Generally, the maximum speed for linear bearings is 2 m/s. Special linear bearings, such as the THK FHS linear rail can reach speeds up to 15 m/s. However, linear rods, compared to a rail of similar price, offer lower drag and higher smoothness allowing the same motor to achieve higher speeds.
If the budget is fixed, and speed is your main goal, linear rods might be the better option. This mainly due the price difference between linear rails and rods. Since linear rods are cheaper, it is possible to pay for a higher quality linear rod that could withstand higher speeds.
However, lower end linear rods will not withstand more than the average 2 m/s speed without binding. This issue will increase vibrations, noise and reduce the life expectancy of a machine. Note that excessive binding increases friction and could produce enough heat for electronics to burn.
Keep in mind that applications that exceed speeds of 2 m/s are extremely specialized. Using linear bearings that have the capacity to operate at speeds beyond 2 m/s is a waste if these speeds will never be used.
Check out the THK FHS linear rails here in the THK product catalog.
Fabrication Considerations
Another aspect that should be considered when choosing linear rails or linear rods is fabrication. Linear rails and linear rods are assembled and aligned using different tools and parts. The availability of these tools and parts could pose a limitation that will eliminate one or the other.
In high precision applications such as industrial CNC machines or SLA 3D printers, precise alignment is essential to achieve the highest performance possible. However, you will not be able to achieve precise alignment without being able to measure it.
In this section, I will discuss the fabrication limitations of linear rails and rods. These should help you take into consideration your ability to use linear rails or rods effectively.
Required Parts for Linear Rails and Linear Rods
Generally, a linear rail requires hex slot screws and a supporting body, commonly aluminum extrusions. Linear rods require shaft supports attached at both ends of the linear rod. The main difference is that a Linear rod can be used as a structural beam, unlike a linear rail.
Both linear rails and linear rods require hex slot screws, however, a linear rod requires fewer screws to attach the shaft supports to a fixed body. Linear rails, however, require hex slot screws along the length of the rail to be attached to a fixed body.
As I have discussed earlier in the article, the additional screws used for a linear rail increase its rigidity. However, this comes at the cost of drilling the required holes or using T-slot nuts for the aluminum extrusions.
The additional material used to support a linear rail, commonly aluminum extrusions, will increase the cost. Additionally, if you don’t use aluminum rails, you will have to drill holes in the fixed body to attach a linear rail to an assembly.
Assembly and Alignment of Linear Rails and Linear Rods
The assembly process of linear rails is more complex compared to linear rods. However, with the right equipment, linear rails could achieve higher accuracy and greater rigidity. Note that the tools and equipment used to perfectly align a pair of linear rails are expensive; however, it is possible to achieve reasonable results without them.
This video on YouTube is the first of a three-part series on how to assemble and align a pair of linear rails with high precision:
This method of assembling linear rails is time consuming and tedious; however, it is necessary for applications that require high precision. Some applications, such as 3D printers, do not require the same level of precision. For these applications, there is a less complicated method to assemble linear rails.
This video on YouTube how linear rails could be attached to an Ender-3 3D printer, and uses a simple alignment process:
Note that it is not necessary to use a pair of linear rails for certain applications. Depending on your design, you might be able to skip the alignment step if you use a single linear rail.
Linear rods, however, do not require extensive alignment, partly due to their lower precision carriages, which allow for slight imperfections. The alignment of a pair of linear rods depends on the precise placement of the end supports.
Aligning the end supports could be achieved in many ways, but it could be achieved with using a tool as simple as a ruler to mark the locations of the holes before drilling them. If the holes were placed correctly, the linear rods could be installed by hand.
This video on YouTube shows how a pair of linear rods is installed using 3D printed end supports:
If you own a limited set of tools and equipment, and do not need high precision, linear rods might be the better option for you. As you have seen in the videos above, installing and aligning linear rails is a difficult procedure that might not be worth the cost.