The sales engineer pushes a linear motor. It’s faster, more accurate, and has no backlash. The price is four times the ballscrew option. The job is a pick-and-place at 300 mm travel. You don’t need a linear motor. You need a ballscrew that can hit the cycle time.
What a linear motor actually buys you
A linear motor is a direct drive: the moving coil floats on a magnetic field, no mechanical transmission. That means no backlash, no wear, and acceleration limited only by the force-to-mass ratio. You can get 5 g acceleration and 5 m/s speed on a well-designed stage.
You also pay for it: the magnet track along the entire travel, the linear encoder, the cooling, and the control electronics. And the attractive force between the coil and the magnet track is 10 times the rated thrust, which means the bearings have to handle a huge constant load.
Where the ballscrew still wins
A ballscrew at 1000 mm travel is accurate to a few microns, repeatable to a micron, and costs a fraction of a linear motor. For most machine axes, that’s plenty. The ballscrew’s speed limit is the critical speed: above about 1500 rpm, the screw whip resonates. On a 20 mm lead screw at 1000 mm travel, that limits you to about 1 m/s.
If your axis moves 300 mm at 0.5 m/s, a ballscrew is fine. If it needs to move 1 m at 3 m/s in 0.5 seconds, the ballscrew can’t. That’s the point where linear motors make sense: long travel, high speed, high acceleration, and cycle time measured in hundredths of a second.
Acceleration is the real tradeoff
A ballscrew has inertia. The screw itself spins up and down, which adds to the motor load. On short moves, most of the cycle time is spent accelerating and decelerating the screw. A direct linear motor has no rotating mass, so the acceleration is limited only by the payload and the force.
For pick-and-place with 300 mm strokes, the ballscrew spends half its time just spinning up. A linear motor does the same move in a third of the time. That’s where the linear motor earns its cost: not on accuracy, but on cycle time.
The maintenance gap
A ballscrew needs lubrication. The nut recirculates balls, and the lube breaks down over time. A contaminated lube or a missed relube interval doubles the wear. Preloaded nuts also lose preload over years of use, which shows up as backlash.
A linear motor has no contact. No wear, no lube, no preload to lose. On a 24/7 machine, the linear motor’s maintenance advantage is real. On a one-shift machine, the ballscrew will outlast the plant.
Cooling and safety
Linear motors generate heat in the coil. That heat goes into the machine structure, which causes thermal growth. A precision linear motor stage needs liquid cooling to keep the stator temperature stable. Skip the cooling and the accuracy drifts with duty cycle.
The magnetic field is also a safety issue. Loose tools, wristwatches, and ferromagnetic parts get pulled into the magnet track. The motor has to be covered or fenced. A ballscrew is mechanical: it’s dangerous when rotating, but it doesn’t grab tools off the workbench.
Bottom line
Choose a ballscrew for travel under 1 m, speeds under 1 m/s, and one or two-shift duty. Choose a linear motor for high-speed, high-acceleration short strokes where cycle time is money. Don’t buy a linear motor for accuracy; buy it for throughput. The accuracy difference between a quality ballscrew and a linear motor is smaller than the price difference suggests.
