A linear guide spec sheet gives you several load numbers for the same part, and they do not all mean the same thing. This article explains what dynamic and static ratings describe, why static ratings are always higher, what per-block versus per-axis capacity means, and how to check a real axis before you order.
If you have two catalogues open in front of you, one listing 18.0 kN and another listing 29.5 kN for what looks like the same size carriage, the number that matters depends on what your axis is actually doing. Dynamic and static ratings answer two different questions, and mixing them up is one of the most common reasons a rail gets oversized, undersized, or quietly overloaded.
This article is about reading those numbers — not about deriving them. It walks through what each rating describes, why static ratings always sit above dynamic ones, what changes when a load is shared across several carriages, and a short checking method you can apply before placing an order.
Key takeaways
- Dynamic rating describes load capacity while the carriage is moving; static rating describes load it can hold while stationary or under momentary shock.
- In published data, static ratings are consistently higher than dynamic ratings — for example HGH20CA at 18.0 kN dynamic and 29.5 kN static.
- Ratings are usually quoted per block. When several blocks share one load, the axis capacity is the sum of the blocks, not the value of one block.
- The static limit is what you check for holding loads and shock events — not the dynamic number.
In this article
- What a linear guide load rating describes
- Dynamic rating: load in motion
- Static rating: load at rest and under shock
- Why static ratings exceed dynamic ratings
- Per block versus per axis
- Load direction and rail cross-section
- A practical check before ordering
- FAQ
What a linear guide load rating describes
A load rating is a published capacity figure for one specific piece: a carriage block, or in some catalogues a rail. It tells you the load that piece is designed to carry under defined conditions. It does not tell you how long it will last, how flat your mounting surface has to be, or how much preload you need — those are separate parameters on the same page.
When buyers search for “what is a linear load” or check a “linear rail load rating,” the confusion usually comes from the same source: a catalogue page shows several numbers close together, in the same unit, with similar-looking labels.
Dynamic rating: load in motion
The dynamic load rating is the capacity used when the carriage travels along the rail. It is the number that pairs with stroke, speed and duty pattern in sizing work. If your axis moves, the dynamic rating is the first figure you compare against your estimated working load.
Examples from published catalogue data:
- HGH20CA carriage block: 18.0 kN dynamic
- HGR20 rail: 16.7 kN dynamic
- MGN12C miniature carriage: 3.7 kN dynamic
Notice that the carriage and the rail of the same nominal size can carry different dynamic figures. That is normal — they are separate components with separate load paths, and both have to be checked.
Static rating: load at rest and under shock
The static load rating describes the load the component can hold while stationary, and the level it can tolerate during a momentary event such as an impact or a hard stop. For the same parts above:
- HGH20CA carriage block: 29.5 kN static
- HGR20 rail: 26.7 kN static
- MGN12C miniature carriage: 5.4 kN static
The static figure is the one you use for holding loads — a fixture parked in a fixed position, a vertical axis sitting under its own weight, or a load applied while the machine is not indexing. It is also the figure that applies to the short spike during a collision or an emergency stop.
Why static ratings exceed dynamic ratings
Every published pair in this article follows the same pattern: static is higher than dynamic. HGH20CA runs 18.0 kN dynamic against 29.5 kN static; HGR20 runs 16.7 kN against 26.7 kN; MGN12C runs 3.7 kN against 5.4 kN. Larger sizes keep the same relationship — HGR45 rail with HGW45CA block is listed at 62.8 kN dynamic and 112 kN static.
The reason is mechanical. Under a stationary load, the rolling elements and raceway rest in contact and the load is carried by that contact geometry. Once the carriage moves, the balls or rollers continuously enter and leave the loaded zone, contact stress cycles, and the allowable load is reduced to keep the contact stresses within the design range over travel. The static rating is therefore the higher of the two, and it is not an “upgraded” version of the dynamic rating — it answers a different question.
Per block versus per axis
This is where a lot of sizing goes wrong. Catalogue ratings are typically quoted per block. A single-block number describes one carriage on one rail. Your axis usually carries load with several blocks on two rails, and the load is divided between them.
When blocks share a load evenly, the axis capacity is the sum of the contributing blocks. Compare:
| Configuration | Published figure | What the number describes |
|---|---|---|
| HGH20CA block | 18.0 kN dynamic / 29.5 kN static | One block |
| Two HGH20CA blocks on one rail | Sum of the two blocks | Load shared across two blocks |
| SBR20 dual-block | 22.4 kN per axis | A supported rail configuration quoted per axis rather than per block |
Note the two different conventions in the same table. The HGH figures are per block, so a four-block arrangement on two rails can carry roughly four times the load of one block if the load is evenly distributed and the mounting is rigid. The SBR20 figure is already quoted per axis — the shared capacity is built into the number. If you compare an SBR20 dual-block axis against a single HGH20CA block without noticing which convention each uses, you will draw the wrong conclusion.
Two conditions matter when you sum block ratings:
- The load must actually be distributed across the blocks. A moment load or a poorly supported carriage can put most of the load on one block.
- The mounting surface and rail parallelism affect how evenly the load arrives. A soft or twisted mounting base defeats the arithmetic.
Load direction and rail cross-section
Load rating also depends on direction. A two-row Gothic-arch miniature railway such as the MGN series carries equal load in all four directions — radial, reverse-radial and both lateral directions — from the same two-row contact geometry. Four-row heavy-load types such as HGH and HGR carry radial load, reverse-radial load and load in both lateral directions, with the four rows dividing the work.
This is why a miniature carriage and a heavy-duty carriage of similar external size are not interchangeable. The MGN12C carries 3.7 kN dynamic in a compact profile with equal four-direction capacity. An HGH20CA carries 18.0 kN dynamic with a four-row arrangement suited to higher loads. Which one fits your axis depends on the load magnitude, the direction of that load, and the available mounting envelope.
A practical check before ordering
You can do a useful first pass with the catalogue alone, before any detailed analysis:
- Separate your loads into moving and holding. Anything the carriage carries while indexing goes against the dynamic rating. Anything parked, hanging or subject to impact goes against the static rating.
- Confirm the convention on the page. Check whether the number is per block or per axis. If it is per block, count the blocks that will actually share the load.
- Add the blocks that share the load, then compare the total against your estimate. Do not compare a four-block axis against a single-block figure.
- Check the static limit separately for shock and holding. The static figure is higher than the dynamic one, but it is still a limit. Never exceed it — not for a holding load, and not for an impact event.
- Check direction. Confirm your load direction is one the block geometry is designed to carry.
This is a capacity check, not a life calculation. A part that passes the rating comparison still needs the mounting, lubrication and alignment conditions the catalogue specifies. Rating tables tell you whether the component can carry the load; they do not promise a service life in your machine.
Where these ratings come from in the catalogue
The dynamic and static figures quoted here are published per-block or per-axis values for the relevant series. If you are comparing suppliers, do the comparison on the same basis: same convention, same direction, same unit. A catalogue that quotes per-axis capacity and one that quotes per-block capacity can show very different numbers for mechanically similar hardware, and neither is wrong — they are answering different questions.
Choosing the right guide for your axis
If your axis is a light, compact mechanism with loads in the low kilonewton range, a miniature railway such as the MGN12 linear rail is the matching envelope. For general automation and machine axes, the HGH flange-type carriage block paired with an HGR heavy-duty rail covers the middle of the range. For heavy loads, the HGR45 rail with HGW45CA carriage is listed at 62.8 kN dynamic and 112 kN static.
If you send the working load, load direction, number of blocks and mounting arrangement, a supplier can confirm which size and block count matches — and which figure on the catalogue page applies to your case.
FAQ
Is the dynamic or static rating the “real” capacity?
Both are real; they describe different conditions. Use the dynamic rating for a moving carriage and the static rating for a stationary or shock-loaded one. Neither one replaces the other.
Can I add the ratings of two blocks together?
If the load is genuinely shared between them, yes — the axis capacity is the sum of the contributing blocks. If the load concentrates on one block because of a moment or a soft mounting, the sum overstates the real capacity.
Why is the static rating higher?
Because a stationary contact does not cycle through the loaded zone. Once the carriage moves, the rolling elements repeatedly enter and leave the load path, so the allowable load is reduced to keep contact stresses inside the design range. Published data reflects this consistently.
Does a higher rating mean longer life?
Not by itself. Ratings describe load capacity under defined conditions. Actual service life depends on the load, speed, lubrication, alignment and mounting in your machine, so treat the rating as a capacity check rather than a life prediction.