Knowledge · Linear Motion
How to size a linear guide: size, load rating, block count
Sizing a linear guide decides the service life, rigidity and cost of the whole axis. Choose the size too small and you risk early failure. Choose it too large and you pay twice. You work from load analysis to size, block format, block count and preload, and along the way you hit the point almost no catalog spells out: why load ratings from different manufacturers are not directly comparable.
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Sizing in five steps
Always size a linear guide in this order. Each step depends on the result of the one before it, and every shortcut you take shows up later in the axis.
- Determine loads and moments: static and dynamic forces per block, roll, pitch and yaw moments, shock and acceleration components.
- Choose the size from the load ratings: dynamic load capacity C for service life, static load capacity C0 against permanent deformation.
- Set the block format and count: flanged or narrow block, standard or long version, number of blocks per rail.
- Match preload and accuracy class: rigidity against friction, accuracy against price.
- Plan rail length and edge distance: usable stroke, block spacing, edge distance T1 and hole pattern.
Dynamic load capacity C and static load capacity C0
Two figures drive the size decision, both defined per DIN ISO 14728-1. The dynamic load capacity C describes the load under which a guide reaches a defined nominal service life. It is the basis of every life calculation. How to derive the nominal life L10 from it is covered in calculating service life.
The static load capacity C0 describes the load at which a permanent deformation of about 0.0001 of the rolling-element diameter occurs at the most heavily loaded contact point. C0 is the limit against standstill loads, shocks and emergencies, not against wear. Rule of thumb: C sizes the service life, C0 protects against short-term overload. Always check both.
L₁₀ = (C₁₀₀ / P)³ · 100 km
- L₁₀
- nominal service life in km of travel
- C₁₀₀
- dynamic load capacity in kN, referenced to 100 km at Bosch Rexroth (competitors often 50 km, then use C50 and 50 km)
- P
- equivalent dynamic load per runner block in kN
- ³
- exponent for ball guides; 10/3 for roller guides
Why load ratings from different manufacturers are not comparable
The dynamic load capacity is only valid for a specific reference distance, which is the most common trap in catalog comparison. Bosch Rexroth references it to 100 km of travel and labels it C100. Many other manufacturers typically reference their C rating to 50 km. The raw numbers from two catalogs are therefore not comparable: the 50 km figure sits higher by construction, without the guide carrying more load.
The conversion factor follows from the life law. For ball guides, life scales with the third power of the load rating. Halve the reference distance from 100 km to 50 km and the stated rating rises by the cube root of 2, about 1.26. It holds that C50 = C100 x 1.26. Convert every third-party catalog to the same reference distance before you compare.
| Reference distance | Load rating (size 25, FNS) | Ratio to C100 |
|---|---|---|
| 100 km (Bosch Rexroth) | C100 = 28.6 kN | 1.00 |
| 50 km (many competitors) | C50 = 36.0 kN | 1.26 |
Load ratings by size
The table below shows the load ratings of the FNS format as an anchor row per size. FNS (flanged, normal, standard height) is the most-built format in machine building. Other formats differ: long blocks (FLS, SLS) deliver considerably more load rating at identical size, short blocks (FKS, SKS) less.
| Size | C100 (kN) | C0 (kN) |
|---|---|---|
| 15 | 6.9-9.9 | 8.9-12.7 |
| 20 | 23.4 | 29.8 |
| 25 | 28.6 | 35.9 |
| 30 | 36.5 | 48.1 |
| 35 | 51.8 | 80.9 |
| 45 | 86.4 | 132.0 |
Setting block format and count
If one block does not carry enough, first increase the block length or count before you jump the size. A long block (suffix L) yields more load rating at identical size, and an additional block distributes the load and supports moments. Jumping a size is the most expensive lever and often not the one you need.
- Two blocks per rail is the default. They support pitch and yaw moments and are production-friendly.
- Two parallel rails is the default for planar guides. A single rail only carries pure linear load without tilting moment.
- Four blocks (two rails with two blocks each) is the standard configuration for gantries, slides and tables.
- Increase block count or block length before size when installation space or price is tight.
- Size C0 against the maximum standstill and shock load, not against the continuous load.
Preload and accuracy class follow from the rigidity requirement and the tolerance chain. As preload climbs, the axis gains rigidity and loses play, while friction and heat rise with it. For a heavy, tilt-loaded axis the preload class you pick differs from the one for an easy-running handling axis.
Rail length and edge distance
The rail length results from usable stroke plus block length plus safety margin. Watch the edge distance T1, the distance from the rail end to the first hole: it decides whether the hole pattern works out and whether the rail end is properly bolted down. PTS cuts rails in-house from 4000 mm bars to your dimension. You plan the cut, hole pitch and edge distance T1 separately in detail.
Size: heuristic and limits
As a starting point: small handling and test axes run on size 15 to 20, the machine-building core sits at size 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, PTS deliberately offers no online configuration. These sizes carry loads high enough that moment distribution, rail joints and the mounting surface belong in the sizing. Here the PTS engineers take over the axis and rail selection, structured through the technical request: you supply load, stroke and cycle, PTS delivers the verified bill of materials.
Size settled, the rest is configuration.
The PTS configurator takes you from size and block format through preload and accuracy to rail length, with live price and availability per system line, no login. For sizes 55 and 65 or complex moment loads, PTS engineering handles the technical sizing.
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