Choosing the wrong miniature linear guide usually shows up months later β as fretting on the raceway, a carriage that develops play, or a positioning error that cannot be dialled out. This guide walks through the six decisions that determine whether a 7 mm or a 20 mm guide is right for your axis.
Sizing a miniature linear guide is not a matter of picking the smallest rail that physically fits. Service life scales with the cube of the load ratio, so a guide that is 30 % under-rated can lose more than half its travel life. Work through the steps below in order.
1. Define the load and the moments
Start with the real load seen by the carriage, not the nominal payload. Add the mass of the fixture, the tooling, the cable management and any process force such as drilling thrust or insertion force. Then look at where that load sits relative to the carriage centre.
A load offset from the carriage centre generates moments, and moments are usually what limits a miniature axis:
- MR β rolling moment around the travel axis
- MP β pitching moment (nose-down / nose-up)
- MY β yawing moment (side-to-side)
For combined loading, convert everything into an equivalent load P before comparing against the rating. Never check a single direction in isolation and assume the rest is fine.
2. Choose the rail width β then verify it
Rail width sets the basic dynamic load rating. As a starting point for compact automation equipment:
| Rail width | Typical role | Dynamic load C (standard carriage) |
|---|---|---|
| 7 mm | Micro mechanisms, optics, small sensors | 100 kgf |
| 9 mm | The general-purpose entry point for precision automation | 190 kgf |
| 12 mm | Inspection, medical and new-energy equipment | 290 kgf |
| 15 mm | Higher-load precision equipment | 470 kgf |
| 20 mm | Load-oriented compact machinery | 678 kgf |
Step up one size when the axis carries significant offset load, when vibration is present, or when the machine must run for years between rebuilds. Space is the only reason to stay small.
3. Pick the carriage length
Carriage length governs moment capacity and rigidity far more than it governs load rating. On the same 9 mm rail, a long carriage can carry several times the moment of a short one. If your load overhangs, the long carriage is usually cheaper than moving up a rail size — and it keeps your mounting height unchanged.
- C β standard: balanced default for most axes
- L β long: higher moments, better rigidity, longer travel envelope
- S β short: tight packaging, minimal moment demand
4. Apply a static safety factor
Rate the guide on static load rating C0 using a safety factor matched to the application. Permanent deformation is not recoverable β it is set once by the worst load the axis will ever see.
| Operating condition | Suggested static factor |
|---|---|
| General industrial machinery | 2.0 β 2.3 |
| With vibration or impact | 3.0 β 4.0 |
| Machine tools, general load | 2.0 β 2.5 |
| Machine tools, shock load | 3.0 β 7.0 |
5. Estimate travel life
Nominal travel life follows the standard rolling-bearing relationship:
C: basic dynamic load rating | P: calculated equivalent load | Fc: contact factor | Fw: load factor
The cubic exponent is why selection margin is cheap and under-sizing is expensive: doubling the load rating gives eight times the nominal life. Final life also depends on load spectrum, mounting accuracy, lubrication, contamination, speed and temperature β so treat the calculation as a screening tool, not a guarantee.
6. Confirm preload, accuracy grade and rail length
Preload removes internal clearance and raises rigidity at the cost of slightly higher running friction. Specify preload when the axis must hold position under reversing load, or when deflection directly affects the process. For light, low-force axes, a lighter preload keeps friction and heat down.
Accuracy grade determines achievable straightness and repeatability. Finally, specify the rail length together with the hole arrangement:
- Standard hole pitch: 15 mm (size 7), 20 mm (size 9), 25 mm (size 12), 40 mm (size 15), 60 mm (size 20)
- Hole counts run from 2 up to 16 depending on length
- Custom lengths are cut within the manufacturing range; hole positions and end distances are confirmed on the order drawing
Bring us the axis
If you would rather not run the numbers, send us the load (including moments), stroke, speed, installation orientation, accuracy requirement, environment and annual quantity. DDJ engineers will come back with the recommended series, size, carriage style, preload and rail configuration β and a drawing to check against.