Linear guide life calculation (L10)
Calculate the nominal life of a profile rail guide using the manufacturer catalogue method (HIWIN/THK/Rexroth): from the dynamic load rating C, equivalent load P, load factor f_w and reference travel distance L_B follow the effective load, the life in kilometres, and - from the motion profile (stroke and stroke frequency, or a direct speed) - the life in operating hours. Optionally the static load safety factor S0 is also checked.
Calculation
Manufacturer-specific - always check the catalogue (e.g. Rexroth ball guides are often rated at 100 km rather than 50 km). Load ratings are not comparable without the same reference distance.
- Life L_h = 13981 h is below 80 % of the target of 20000 h: design is critical - a larger size, lower load, or lower speed is required.
Guideline: static safety factor S0 >= 4 OK, from 2.5 marginal - required value depends on the application, typically 3 … 5, higher under shock or vibration. Combined loads, preload and mounting tolerances remain manufacturer-specific and are not included here.
Results
- Load factor f_w
- 1.1
- Effective load P_eff
- 4.4 kN
- Nominal life L10
- 25,166 km
- Average speed v_m
- 30 m/min
- Life L_h
- 13,981 h
- Life (2000 h/year)
- 7 a
Sketch: carriage on profile rail, cross-section
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Formulas and fundamentals
Effective Load and Nominal Life
As with a rolling bearing, the effective load is formed from the equivalent dynamic load P and a load factor f_w that accounts for shock and vibration in actual operation, which is not included in the statically determined equivalent load P:
The nominal life then follows from the dynamic load rating C and the reference travel distance L_B:
The exponent p reflects the contact type: ball rail guides have point contact (p = 3, as with ball bearings), roller rail guides have line contact (p = 10/3, as with roller bearings). The roller guide therefore has a flatter life curve - for the same overload its life drops less sharply than that of a ball guide, but it is more sensitive to misalignment.
Reference Travel Distance L_B: a Manufacturer-Specific Convention
The dynamic load rating C is only meaningful together with its reference travel distance L_B: some manufacturers normalise to 50 km of travel (many ball rail guides), others to 100 km (e.g. Rexroth ball guides, most roller rail guides). Load ratings from two manufacturers are therefore not directly comparable - always check the reference distance stated in the catalogue. Approximate conversion between the conventions: C_100 = C_50/2^(1/p).
Speed from the Motion Profile
For cyclic operation with stroke s and stroke frequency n_h, the average speed follows as:
From this, together with the life in kilometres, follows the life in operating hours:
Static Safety Factor
In addition to the nominal (dynamic) life, the static load safety against permanent deformation of the raceways is checked:
Typical required guideline values are S0 = 3 … 5, higher under shock loads or vibration. Combined loads (moment loading), preload and mounting tolerances (parallelism and squareness of the rails to each other) additionally affect the real load capacity and should be verified from the manufacturer's documentation - as a simplification, this calculator treats the maximum carriage load as the equivalent load P.
Worked example
Reference example: a ball rail guide with C = 35 kN, equivalent load P = 4 kN, f_w = 1.0, and reference travel distance L_B = 50 km gives L10 = (35/4)³·50 = 8.75³·50 = 33,496 km. For cyclic operation with stroke s = 500 mm and n_h = 30 strokes per minute, the average speed is v_m = 2·500·30/1000 = 30 m/min, giving a life in hours of L_h = 33,496·1000/(30·60) = 18,609 h, roughly 9.3 years at 2000 operating hours per year. Against a target of L_h,target = 20,000 h, the result is just below target but above 80 % of it - amber rating (check the margin, e.g. by choosing a larger size or reducing speed).
Calculating more realistically with f_w = 1.1 (guideline value for essentially shock-free operation) instead of f_w = 1.0, the life drops to L_h ≈ 13,981 h - below 80 % of the target, red rating. For the additional static check with a static load rating C0 = 52 kN and a maximum static load P0 = 6 kN, S0 = 52/6 = 8.67, well above the typical minimum guideline of roughly 3 to 5 - the static check is uncritical even though the dynamic life falls short of the target. The example shows how dynamic life and static safety must be assessed independently.
Frequently asked questions
How long does a linear guide last?
This depends on the load rating, the actual applied load, speed, and operating conditions - there is no single blanket figure. The calculator gives the nominal life L10 using the manufacturer catalogue method: calculated so that 90 % of a large batch of identical guides under the same conditions reach or exceed this travel distance without fatigue damage to the raceways. Individual guides may fail earlier or last considerably longer.
Ball rail guide or roller rail guide - which is the better choice?
Ball rail guides (point contact) run more quietly, are more cost-effective, and are available in most sizes and accuracy classes - the standard choice for most applications. Roller rail guides (line contact) carry significantly higher loads at the same size and are stiffer, but are more sensitive to misalignment and skewing, and cost more. Roller guides are therefore mainly used for high loads, high stiffness requirements, or tight installation space combined with high load capacity.
Why aren't load ratings of ball or roller rail guides from different manufacturers directly comparable?
The dynamic load rating C is always stated relative to a reference travel distance L_B - typically 50 km or 100 km. Two guides with the same load rating C but a different reference distance have a different actual life: a rating referenced to 100 km gives a higher life than one referenced to 50 km, all else being equal. Always check the reference distance in the relevant manufacturer catalogue before comparing or reusing rated values.
What is the load factor f_w and how do I choose it?
f_w increases the calculated load to account for shock, vibration, and dynamic effects in real operation that are not captured in the statically determined equivalent load P. Guideline values: essentially shock-free 1.0 … 1.2, light shocks 1.2 … 1.5, moderate shocks 1.5 … 2.0, heavy shocks (e.g. press or impact duty) 2.0 … 3.5. When in doubt, choose the upper end of the relevant range or check with the guide manufacturer's applications engineer.
How are combined loads and moments handled?
When a carriage is simultaneously subjected to forces in multiple directions and moments (roll, pitch and yaw moment), the manufacturer determines an equivalent dynamic load P from these using bearing-specific coefficients - the procedure differs by manufacturer and product line and should be taken from the relevant catalogue documentation. This calculator simplifies and assumes an already-determined equivalent load, or, as a simplification, the maximum carriage load - for accurate results under combined loading always use the manufacturer's method.
How does preload affect life?
Preload (interference between carriage and rail) increases stiffness and reduces play, but at the same time raises the internal load on the rolling elements and thereby reduces the calculated life for the same external forces - this effect is already included in the load rating stated for the relevant preload class, provided the matching catalogue rating is used. Higher preload classes (e.g. for precision applications) therefore often have a lower stated life than clearance-free or lightly preloaded variants at the same size.
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