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Linear guide rails and linear blocks

Linear rail guides and carriages are mechanical components that enable precise movement of loads in a straight line with minimal friction. Their primary function is to carry loads and provide precise guidance. They consist of a guide rail and a linear carriage that moves along the rail.

Advantages of linear guides

These guides are characterized by the best ratio between price and features, such as high precision, high load capacity and stiffness, and long service life. They are also mass-produced by different manufacturers, which contributes to the favourable price.

These characteristics make them very popular and widely used, especially in CNC machines and generally in machine construction.


CNC machine with linear guide rails

Figure 1: CNC machine with linear guide rails

Disadvantages of linear guide rails

As a disadvantage, we can mention a certain degree of friction due to the seals and the preload of the carriage, which can cause slip-stick effect. Therefore, they are not the most suitable for manual movement. It may also be mentioned that the rail guides run better when loaded than when unloaded.

Due to their precision and rigidity, they also require properly prepared mounting surfaces and precision assembly to achieve smooth operation.

We must also be mindful of dirt and dust to which these guides will be exposed. Basic seals allow for relatively good sealing and resistance to dirt. The efficiency of sealing can be further improved with sets of additional seals. However, they are not suitable for the most aggressive environments where abrasive dust is present.

To achieve the expected performance and service life, the guides must be properly lubricated with suitable grease or oil, via the lubrication nipples on the carriages.

Composition

Let us now look at the composition of linear rail guides. The guides consist of a guide rail, which has ground and hardened surfaces, and a linear carriage, which moves along the rail.

Within the linear carriage, balls or rollers can be used as rolling elements. In the case of balls, point contact occurs between the ball and the guide, whereas in the case of rollers, linear contact occurs. Therefore, roller linear guides have significantly higher load capacities and stiffness.

Ball and roller surface contact in a linear carriage


Figure 2: Ball and roller surface contact in a linear carriage

Preload of linear carriages

To achieve the highest possible stiffness in linear guides, preloaded carriages are used. Preload simply means a tight fit between the guide and the carriage. This is most commonly achieved by using a few microns larger balls or rollers inside the carriage. This eliminates play between the guide and the carriage, but on the other hand, results in a slightly shorter service life. 

Precision classes of guide rails

Manufacturers offer the same guideways and carriages in different tolerance classes. Tolerance classes refer to the tolerances of the individual dimensions of the guide and carriage, which affects the guiding precision. For general engineering, the entry two classes Normal and High are usually sufficient. Even in these two cases, we are talking about deviations of a few tens of microns.

Dimensions of the rail carriage (source: Bosch-Rexroth)

Figure 3: Dimensions of the rail carriage (source: Bosch-Rexroth)


Tolerances according to the quality class (source: Bosch-Rexroth)

Figure 4: Tolerances according to the quality class (source: Bosch-Rexroth)
 

 

Guidance tolerances of the linear guide (source: Bosch-Rexroth)

Figure 5: Guidance tolerances of the linear guide (source: Bosch-Rexroth)


 

Dimensions and sizes of linear guides

Linear guides are available in a variety of standard sizes, which are mostly uniform even between different manufacturers. In particular, the main assembly dimensions such as overall height, carriage width, spacing between the holes on the guide and the carriage, and length are standardised. However, it is always necessary to use the guide and carriage from the same manufacturer.

The dimensions can be easily checked and compared in the manufacturers' PDF catalogs, which are also available in our online store Tuli.

Typically, rails are manufactured in a factory length of 4 m. However, they can be supplied cut to the customer's desired length. Precise cutting and preparation of guides for joining into longer lengths are also possible.

In addition to the standard versions, miniature versions are also available, most of which are made of stainless steel. Miniature sliding guides are also available.

Load capacity and layout of guides

As mentioned, linear guides have high load capacities and can withstand loads from all directions. The load capacity is expressed in the manufacturers' catalogues through static (C0) and dynamic (C) load capacity factors, which are used to calculate the service life.

Factors of actual load and other factors describing working conditions (movement speeds, travel length, temperature, vibrations…) are also taken into account in the calculation.

For a rough estimate of load capacity you can take 10-20% of the dynamic load factor C as the load capacity of the carriage. For a carriage size 15, this means over 100 kg.
 

Load capacity factors of linear guides (source: Bosch-Rexroth)

Figure 6: Load capacity factors of linear guides (source: Bosch-Rexroth)
 

However, we must be aware of the moment or rotational loads on the carriage, which carriages bear significantly worse compared to compressive and tensile loads.

Therefore, for a single axis of movement, the placement of two parallel guides with two carriages on each guide is recommended. This converts all moment loads into compressive and tensile loads on the carriages, resulting in significantly higher load capacity and longer service life of the guides.

 

Arrangement of guides and carriages

Figure 7: Arrangement of guides and carriages

 

Producers of linear guide rails

There are many different manufacturers of linear rail guides on the market. And choosing between them is not easy. We roughly divide them into three classes: low, medium and premium.

Low class
In the lower class we typically classify all low-cost Chinese manufacturers. They are most often available in the basic quality class and with little or no preload. Their main advantage is that they are reasonably priced and of sufficiently good quality.

Middle class
In the middle class are mostly Taiwanese manufacturers, such as TBI Motion, Hiwin, PMI, STAF… who usually offer the best price-to-quality ratio. They are also available in different quality and preload classes.

Premium class
In the premium class we include most European and Japanese brands such as Bosch-Rexroth, INA, THK, Schneeberger, NSK, Rosa... which are the right choice if you are looking for the most precise and reliable guides. They are characterized by high-quality standards and a wide range of different options, quality classes, and preloads.

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Comparison
In our experience, the differences between individual manufacturers within the same group are not significant. And it is more a matter of personal preference. Availability and price can also play an important role in the selection.

Generally, in our experience, linear guide rails represent a mature and refined technology, reflected in high reliability. As a result, we have not experienced any major problems or complaints, even in the case of cheaper guideways. However, it is extremely important that the guides are properly selected, installed and maintained.

If you need help with the selection and supply of linear guides, you can always contact us! 

 

About the author


Jure Pleško is co-owner of Tuli, a family business that helps innovative mechanical engineers by supplying them with machine elements to assemble machines and devices. As part of a family tradition, he grew up with machine elements from an early age and also completed his studies in mechanical engineering, earning a Master's degree in Mechanical Engineering. As a 2-year-old child he was also the inspiration for the company name, because at that time he was shouting a lot and hence the name Tuli, which in translation means screaming. Jure is an innovative mechanical engineer himself, which he proved during his student years with the construction of an electric vehicle, which was also awarded by the Slovenian Fund for Human Resources Development and Scholarships.

 

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