Linear guides: the practice handbook from selection to maintenance

A profiled rail guide has two parts: the rail with hardened raceways and the runner block, in which rolling elements circulate between block and rail. Whether those rolling elements are balls or rollers is the first fork, but the structure stays the same. All profiled rail guides share the same hardened raceways, recirculating rolling elements, the same accuracy and preload classes, and the need for regular lubrication.

One runner block takes up loads from several directions and the roll, pitch and yaw moments of an axis, and does so within a tight, defined tolerance. That is why profiled rail guides are the basis of gantries, slides, machine tools and handling axes. The price of this precision is clean sizing and clean installation; both decide the result more than the catalog value alone.

Sizing an axis means making a chain of decisions that build on one another, in this order:

The fundamental choice is between a ball rail guide and a roller rail guide, and it comes down to a single question: does rigidity decide the function, or do load capacity, speed and price? The difference lies in the contact type. Balls touch the raceway in point contact, cylindrical rollers in line contact. That one point drives the rest.

The ball rail guide is the standard in machine building. Its point contact runs easily, tolerates high speeds and forgives small errors in the mounting surface. The roller rail guide takes up more load at the same size and is considerably stiffer, but it demands more precise mounting surfaces and sits in a higher price class. The full comparison by load capacity, rigidity, speed and mounting effort is in ball or roller rail guide.

For the majority of applications, the ball guide is the right and more economical choice. Reach for the roller when rigidity decides process quality, for example in cutting, grinding or under high cutting and process forces. At Bosch Rexroth, the ball rail guides are called BSHP, the roller rail guides RSHP.

Switching from ball to roller is not a minor variant but a deliberate choice for more rigidity against higher price and higher mounting demands. Before switching, also check the overall height and connecting dimensions, because ball and roller systems are not readily interchangeable at the same dimensions. Rigidity can also be raised within limits via preload, without changing the type at all.

Sizing follows a fixed sequence in which each step builds on the one before. In short form, it is four steps.

You choose the size via two figures, both defined per DIN ISO 14728-1. The dynamic load capacity C describes the load under which the guide reaches a defined nominal service life and sizes it against wear. The static load capacity C0 describes the limit load above which permanent deformation occurs at the contact point and protects against standstill, shock and emergency loads. Always check both: C sizes the service life, C0 secures against short-term overload.

A trap waits in catalog comparison. The dynamic load capacity is only valid for a specific reference distance. Bosch Rexroth references it to 100 km and labels it C100, while many competitors reference it to 50 km. By the factor 1.26, the 50 km figure sits higher without the guide carrying more load. Note the reference distance next to every rating and convert third-party catalogs to the same distance. How to compare fairly and determine the size step by step is shown in sizing a linear guide.

The service life follows from the load rating. For ball guides, L10 = (C / P)³ x reference distance: life scales with the third power of the ratio of rating to load, so a small load reserve pays off disproportionately and a small overload punishes disproportionately. The worked example and the factors of real-world life are in calculating service life.

The anchor row per size is the FNS format (flange, normal, standard height), the most-built version in machine building. Other formats differ: long blocks deliver more load rating at identical size, short blocks less. The values below set the order of magnitude per size.

Before you jump the size, adjust block format and count. A long block yields more load rating at the same size, an additional block distributes the load and supports moments. Two blocks per rail is the default, two parallel rails the default for planar guides. Four blocks, two rails with two blocks each, are the standard configuration for gantries, slides and tables. A single rail only carries pure linear load without tilting moment.

As a rough orientation for the size: small handling and test axes run on size 15 to 20, the machine-building core sits at 25 to 35, heavy gantries and machine tools at 45. Always refine this starting value with the calculated loads and the required service life, never the other way around. For sizes 55 and 65 there is deliberately no online configuration: at these loads, moment distribution, rail joints and the mounting surface belong in the sizing, which PTS engineering takes over.

A Bosch Rexroth runner block carries its full configuration in the type code, for example KWD-025-FNS-C1-N-1. The chain has six fields: family, size, carriage form, preload, accuracy and seal. KWD is the ball runner block of the BSHP system, its matching rail is called KSA. The three digits are the size, the three letters the carriage form, the remaining fields set preload, accuracy and seal.

Read the chain left to right and you have described the block in full without opening a catalog. How to decode each position in detail and place the R16 part numbers in context is shown in Rexroth type code.

The three carriage-form letters follow a fixed grammar: position one is the mounting (F for flange, S for narrow), position two the block length (K short, N normal, L long), position three the block height (S standard, H high, N low). A long block carries more than a normal one at the same size, a flange block is bolted through the flange, a narrow block from the top and builds more compactly. Eight carriage forms are carried online: the flange blocks FNS, FLS and FKS and the narrow blocks SNS, SLS, SKS, SNH and SLH.

Alongside the type code, every block carries a ten-character part number, usually starting with R16. It is unique per variant but not a second type code: preload, accuracy and seal cannot be read from the digits. The number stem only groups coarsely by carriage form. Rely on the type code for the specification, and verify the full part number of the specific variant when ordering.

The matching rail is the KSA ball rail. Three rules govern the pairing: the size of block and rail must be identical, the accuracy class must match, and the pitch of the rail must fit. The carriage form of the block is free, every form runs on the rail of the same size.

Preload means there is no longer any clearance between block and rail; instead, a defined internal force is at work. It is achieved with slightly oversized balls that press permanently against both raceways. The guide therefore runs clearance-free, becomes stiffer and responds immediately to a load reversal, without the dead travel of a guide with clearance. Rexroth grades preload into four classes from C0 (no preload) to C3 (high).

More preload is not a free win, though: as rigidity rises, so do displacement force and friction, and too much preload lowers service life. Rexroth states as a limit that preload force should not exceed one third of the bearing load. So choose exactly as much preload as the application needs for stiffness, and no more. If vibrations occur, Rexroth recommends at least class C2. Which class fits which application is covered in choosing preload.

C0 runs without preload and with slight clearance (1 to 10 µm) and is the choice when smooth running matters more than stiffness or when the mounting surface is not perfectly flat. The guide becomes clearance-free from C1 upward, the pragmatic standard: noticeably stiffer than C0, yet still tolerant of small mounting inaccuracies. C2 and C3 belong in stiff, precise machines whose mounting surfaces can actually absorb the higher preload.

The accuracy class limits the tolerance of the height dimension and the parallelism between block and rail. It combines several individual tolerances: the height tolerance and the height difference between several blocks, the width tolerance and the width difference, and the running parallelism over the travel. Rexroth grades it from N through H, P, XP and SP to UP; from N to UP every tolerance tightens, the price rises, the choice narrows. For orientation: a class-N block may deviate in height by ±100 µm, a class-UP block by only ±5 µm.

N is not imprecise but the standard class for general machine building; the high classes are reserved for metrology and machining tasks where a few micrometers count. Preload and accuracy are also not freely combinable: on class-N rails it stays at C0 or C1, because their larger tolerances would create constraint stresses under higher preload, and the high preload C3 requires one of the tight accuracy classes. In practice the simple classes dominate: C1 is the most-ordered preload, N the most-ordered accuracy class. The full tolerance table and the permissible combinations are in accuracy classes.

The rail comes cut to size. Its raceways are hardened; a parting cut with an angle grinder or band saw introduces heat, warps the profile and damages the raceway, while the cooled cut delivers a dimensionally accurate, mounting-ready rail. PTS cuts rails in-house from stocked 4000 mm bars, one-piece up to 4000 mm. If the axis runs longer, it is butted from several rails, and PTS engineering lays that out so that hole pattern and joint line up across all segments.

At the cut, the edge distance T1 matters, the distance from the rail end to the first bore. It has to lie within the limits of the size so that the hole pattern works out and the rail end is properly bolted down. The pitch, that is the hole spacing, is tied to the series: because the same size can carry a different pitch depending on the R-number series, the part number decides on a reorder, the size alone is not enough.

The length follows from the number of bores, the pitch and the edge distances: L = (nB - 1) x T + T1 + T2. For a symmetric cut, L = nB x T - 4 mm, which distributes the edge distances evenly. Size 25 with pitch 60 mm and seven bores gives 416 mm, the most-ordered length of that size at PTS. The length formula and the edge distances per size are covered in cutting and edge distance T1.

A linear guide is only as accurate as its installation. The mounting surface has to be clean, burr-free and flat before the rail goes down, because chips or a burr under the rail produce exactly the deviation you will later read on the dial gauge. One rail serves as the reference and is pressed against a fixed stop edge and fixed there; the second, parallel rail is aligned to the first, not to an edge of its own. That transfers the straightness of the reference rail onto the whole axis.

Tightening runs from the center outward, at the correct torque. What matters is screw size and strength class, not the feel in your wrist: too little torque and the rail walks under load; too much and you pull it bent into the mounting surface. Under shock loads or vibration, secure the screws additionally. The verified tightening torques per size and the step-by-step sequence are in mounting.

One detail decides whether blocks survive: the runner block must not run off the rail, or the rolling elements fall out. Rexroth therefore supplies the blocks on a plastic assembly arbor that holds the balls inside, and the block is transferred onto the rail only as it is pushed on. To remove it, put it back onto an arbor of equal height.

In the end, lubrication decides real service life, not the rated load on the data sheet. The lubricant film separates the rolling elements from the raceway and protects against corrosion; dry running and contamination are the two most common causes of failure, which is why seals and wipers belong to the strategy as much as the grease. Rolling bearing greases on a lithium soap base are common, but what matters is the manufacturer approval: Rexroth, for example, names greases per DIN 51825 K2K and explicitly excludes greases with solid lubricant content such as graphite or MoS2.

Most applications are lubricated with grease, because it stays in the block, adds sealing against dirt and needs no feed line; oil is used at high speeds, high temperatures or where a central lubrication already exists. Increased running noise, a rising travel resistance or dry, bare raceways are signs of under-lubrication and a signal to check the relubrication interval.

A fixed relubrication interval cannot be stated in general; it depends on load, speed, stroke, mounting orientation and environment, and the base figure comes from the manufacturer catalog. Modern Rexroth ball rails reach up to 20,000 km without relubrication. How strongly load and stroke drive the service life, and with it the interval, is shown in the life calculation; which grease goes in, when and how, is in lubrication.