Load Rating and Service Life

Two numbers are printed for every block: the dynamic load rating C and the static load rating C0. They answer different questions. C is used to calculate how long the guide will last in motion; C0 is used to check that the balls do not take a permanent impression when the machine is standing still or hitting a stop. Mixing them up is the most common mistake in a load calculation.

C and C0 are not interchangeable

C is the load at which a group of guides reaches a nominal rating life under continuous motion. It scales with the number and size of the balls and with the raceway geometry, and it is the number that goes into the life formula. C0 is the load at which the permanent deformation at the most heavily loaded contact reaches a defined limit. A press that stands still under a heavy load is limited by C0, not by life; a fast pick-and-place axis is limited by C.

The life formula

For ball guides the nominal rating life is L = (C / P)³ × 50 km, where P is the equivalent load on the block. For roller guides the exponent is 10/3 and the base is 100 km: L = (C / P)^10/3 × 100 km. Both are nominal L10 values: the life reached or exceeded by 90% of a group of identical guides under identical conditions.

The exponent is the important part. Because the ratio is cubed, a 20% overload cuts life by roughly half, and halving the load multiplies life by eight. That is why the load figure has to be the real load on the block, including the moment from an overhanging carriage, not the weight on the drawing.

Worked example

Take HGH25CA, C = 26.48 kN, carrying 8 kN per block. L = (26.48 / 8)³ × 50 = 3.31³ × 50 ≈ 1,810 km. At 10 kN on the same block: (2.65)³ × 50 ≈ 930 km. A 25% load increase halves the calculated life — which is why the load estimate matters more than the safety factor at the end. At 5 kN the same block reaches about 7,400 km.

Convert the kilometres to time only when the stroke and cycle rate are known. A 300 mm stroke at 30 cycles per minute covers 0.6 m per cycle, 18 m per minute, about 1 km per hour of running. The same 1,810 km is then roughly 1,800 running hours at that duty, before any allowance for contamination or lubrication.

Load per block on a real carriage

Four blocks share the load, but not equally. With the centre of gravity in the middle, each block takes a quarter; with the load offset, the near blocks take more and the far blocks take less, and the moment adds a vertical component that the preload does not remove. Work out the worst-loaded block and use that figure in the formula. On a two-block carriage the moment term usually dominates, and the calculation is done per block rather than as half the total.

Static safety factor

For the static check, divide C0 by the static load P0. Common practice for machine tools is 1 to 2 for smooth loading, 2 to 3 where there is vibration or occasional impact, and 3 to 5 for heavy shock or a load that can be applied suddenly. These are engineering conventions, not catalogue figures: the right value depends on what the machine does when something goes wrong, and it is agreed with the application.

What this calculation does not cover

Life calculated this way assumes clean lubrication and a correctly aligned rail. Contamination and starvation dominate in practice: a guide running dry in fine dust can fail in weeks while the formula predicts years. That is why the wiper and the re-greasing interval are part of the selection, not an afterthought, and why a larger block is sometimes the cheaper answer than a maintenance schedule the customer will not follow.

Sizes and models

The ratings for the sizes most often compared are on HGH15CA, HGH25CA, HGH35CA, HGH45CA, HGW25CC and MGN12. The full list is on linear guide blocks, rails on linear guide rails, and the roller series on RG series.

What we send with a quotation

When life matters to the decision, the quotation states the assumptions rather than a single number: the load per block, how it was split between blocks, the stroke and cycle rate used, and the resulting L10 in kilometres and running hours. If the assumption is wrong, the customer can change one input and see the effect instead of arguing with a figure. Where the environment is dusty or wet, the calculation is marked as structure-only, because contamination is not in the formula and it is usually what decides the real life.

FAQ