Automation projects usually start with a light axis and grow. A transfer shuttle that moved 20 kg in the prototype carries a 90 kg fixture in production, and the axis that was fine in the lab now settles 0.2 mm after every move, or chatters, or wears out rails in six months. The fix is not a bigger motor. It is a different frame.
Four applications where heavy slides pay
Gantry and long-stroke transfer
A gantry cross-beam plus two end effectors plus a fixture plate easily reaches 200 kg. The cross-slide carrying that beam is loaded in pitch as well as vertically, and pitch moment is what destroys a two-block carriage. A 250 mm frame with four preloaded blocks on 25 mm rails carries 120 N·m of pitch moment without measurable deflection at the table edge.
Press tending and heavy pick-and-place
Press tending adds shock. The axis does not just carry the part, it catches it. Peak acceleration forces during a catch can be three times the static load, so the slide is sized on the dynamic case. A cast iron base also damps the impulse, which protects the rail raceways from Brinelling.
Fixture shuttles and pallet changers
A pallet changer moves a loaded fixture in and out of a machining envelope, often with the fixture overhanging. Overhang is a cantilever load: the moment grows with the distance from the carriage centre, not with the weight alone. Keep the centre of mass over the carriage footprint, or specify one frame size up.
Robot seventh axis and linear tracks
A 300 kg robot on a 4 m track needs a rack drive, not a screw. The rail section and the base stiffness dominate; the drive only has to overcome friction and acceleration. On these axes we size the base for a deflection limit of 0.05 mm under the robot at full reach, which is usually the governing case rather than the load rating.
The three load cases you must check
| Case | What it limits | How to check |
|---|---|---|
| Static vertical | Rail and block rating | Fixture + part + tool weight × 1.5 |
| Dynamic acceleration | Belt or screw, coupling, motor | Mass × peak acceleration, plus gravity on vertical axes |
| Moment | Carriage stiffness, rail life | Load × distance from carriage centre, both pitch and yaw |
Moment is the case that gets missed. A 150 kg load centred on a 250 mm carriage is comfortable. The same load 200 mm off centre produces 300 N·m of pitch moment, which exceeds the rating of most 25 mm rail carriages and halves rail life even when it does not visibly deflect.
Rule of thumb
If the centre of mass sits more than one third of the carriage length away from the carriage centre, specify the next frame size up or add a second carriage pair.
- Carriage length
- from the model drawing
- Safe offset
- ≤ 105 mm
- Pitch moment at 105 mm, 150 kg
- 155 N·m
- Rated pitch moment
- 120 N·m → reduce offset or upsize
Screw or rack at heavy load
Below 1200 mm, a Ø25 mm C5 ball screw is the accurate choice: ±0.015 mm positioning with preload and no drive reversals. Above 1200 mm the screw whips, and a rotating 2 m screw at 500 mm/s is asking for vibration and short bearing life. A helical rack keeps the same frame and rails and moves the drive to the base, at roughly ±0.05 mm positioning — fine for transfer, not for machining.
Protection is not optional at this size
A heavy slide usually lives in a dirty environment because it is moving heavy things in a production area. Bellows, double-lip wipers and a sealed cable chain add perhaps 8 % to the price and roughly double rail life in a coolant or swarf environment. Central lubrication through internal drillings means one grease point instead of five, which is the difference between a maintained axis and an unmaintained one.
Integration details that cause rework
Three things go wrong repeatedly on heavy installations. The mounting surface is not flat enough — a 250 mm frame needs 0.05 mm flatness over its length or the base is preloaded into a bow. The cable chain is sized for a light axis and fails within a year. And the motor is sized on steady-state torque, ignoring the acceleration peak, so the drive trips on overload during the first fast cycle.
All three are specification errors rather than manufacturing errors, and all three are avoidable if the load case is written down before the order. Send us the mass, the offset, the acceleration and the mounting surface; we will return the frame size, the drive and the protection in one pass. The large-stroke column range page carries the full rating table.
Common questions
Around 100 kg horizontal, or earlier if the load is offset. Cast iron buys damping and stiffness, not just strength, so the crossover is often lower on a cutting axis than on a pure transfer axis.