A direct-drive linear axis promises high speed and no backlash. The motor is selected from the peak force needed for acceleration, and in service it overheats, loses force, or faults during repeated cycles. Peak force describes the hardest single move. It does not describe how hot the motor gets doing the real duty, and on linear motors the thermal limit usually decides the selection.
How a linear motor differs
A linear motor unrolls a rotary motor: a long track of magnets or coils, the primary, and a moving forcer, the secondary, separated by an air gap. There is no screw, belt, gearbox, or contact, so there is no backlash, no wear from transmission, and very low friction. The trade-off is that the motor produces force directly and must reject its own heat, and the load is carried by separate linear bearings.
Ironless motors have no attractive force between forcer and track and move smoothly, suited to light, precise motion. Iron-core motors produce more force per size but have cogging and a strong magnetic attraction that loads the bearings.
Peak vs continuous force
Peak force is available for short acceleration and deceleration, limited in duration and duty by the drive. Continuous force is what the motor can produce indefinitely without exceeding its temperature rating. A move profile uses high force briefly during acceleration, lower force during constant motion, and high force again decelerating. The motor must meet the peak, but heating follows the RMS or continuous force over the full cycle.
This is the same peak versus RMS logic as a rotary servo, but more severe: a linear motor often runs at high peak force for fast moves while having limited surface to shed heat. A motor that easily meets peak acceleration can still overheat on a repetitive cycle.
Calculating the force profile
The motor must accelerate the moving mass, force equals mass times acceleration, plus friction, plus any cutting or process force and gravity on inclined axes. The acceleration term often dominates. Convert the full motion into force over time for accelerate, constant, decelerate, and dwell, then compute the RMS force. Compare RMS to the continuous rating and the worst instant to the peak rating, with margin.
Don’t forget the moving cable, the drag from the way covers, and any preload in the bearings; these add steady force that raises heating even though they do not appear in the ideal acceleration number.
Thermal management
Linear motors heat the moving coil, and heat must conduct to the machine or be removed by cooling. Air-cooled motors rely on the mount and natural convection; liquid-cooled motors run coolant through the coil body and gain substantially higher continuous force. The cooling circuit, its temperature, and its flow are part of the motor rating; a liquid-cooled motor run without coolant behaves like a much smaller unit and overheats fast.
Heat also affects the machine structure and accuracy. A motor dumping heat into a precision base can grow the axis thermally; isolate or cool it where position stability matters.
The magnetic attraction load
In an iron-core linear motor, the magnets pull the forcer strongly toward the track even with no power. This attraction can be several times the motor’s rated force and loads the linear bearings continuously, shortening their life and increasing deformation. Size the guides for the attraction plus the process loads, not just the moving weight. Ironless motors avoid this, which is one reason they suit delicate and high-precision axes despite lower force.
Air gap and mounting
The force depends on a precise, uniform air gap between forcer and track across the full travel. A bent base, poor alignment, or swarf pulled into the magnets changes the gap and can let the forcer strike the track. Mount the magnet sections to tight straightness and protect the gap from chips; magnetic tracks attract ferritic debris aggressively in a machining environment.
Cabling and dynamic parts
The moving coil needs a flexible power and feedback cable rated for the travel and cycle count; cable failure is a common limit on high-duty linear axes. Encoder feedback, often a linear scale, must resolve the positioning accuracy and be protected from contamination and heat. Account for cable drag in the force and friction budget.
A worked force profile
Take a 10 kg moving axis accelerated at 20 m/s2, requiring 200 N acceleration force, plus 30 N of friction and cable drag. The cycle accelerates for 0.1 s, runs constant at 30 N for 0.3 s, decelerates at 200 N for 0.1 s, then dwells 0.5 s. The RMS force over the full period is dominated by the two acceleration segments and works out to roughly 105 N. A motor with 250 N peak but only 80 N continuous meets the single move yet overheats on repetition, while one with 120 N continuous runs within margin. This shows why checking peak alone systematically undersizes a cycling axis.
Choosing ironless vs iron-core
Ironless, U-channel motors have zero cogging and no attraction, giving smooth motion for light loads, scanning, and precision positioning; their force is lower and they run longer for a given travel. Iron-core flat motors pack high force for heavy acceleration and machine tool axes, but the cogging must be compensated and the bearings sized for attraction. Select from the required smoothness and force density rather than defaulting to the highest force; a smooth ironless axis often outperforms a stiff iron-core one on surface quality and tracking.
Cooling system design
Liquid cooling raises continuous force, often by 50 percent or more, but introduces a chiller, hoses, and maintenance. Coolant temperature should be stable and above the dew point to avoid condensation on the coil, and flow must be monitored; a blocked or lost flow path silently removes the uprating. In multi-axis machines, share a properly sized chiller and verify each motor receives flow. Air cooling suits lower duty and avoids this complexity, but the mounting surface then acts as the heat sink and must be substantial and clean.
Integration with bearings and feedback
The linear motor provides force but no guidance or support, so the bearing arrangement defines straightness and load capacity. Preloaded linear rails or cross-roller guides maintain the air gap under the attraction and process forces. The linear encoder should mount close to the working point to avoid Abbe error, and its scale and readhead stay within the thermal and contamination environment. Motor, bearings, and scale are one system; selecting the motor independently often leaves guidance or feedback that cannot hold the accuracy the direct drive makes possible.
Energy and regeneration
Fast deceleration of a heavy linear axis returns energy, and vertical or inclined linear motors regenerate continuously on the down stroke. Size braking resistors or regenerative units from that duty, as with rotary servos. On long-stroke high-mass shuttles, the returned energy can be large and trips an unready drive during the very fast cycles the motor was chosen to enable.
Environmental protection
Exposed magnets and coils attract chips and are damaged by coolant, so linear motors in machining cells need covers, bellows, or sealed extraction of the gap. Positive air pressure under the covers keeps fine debris out. A motor rated for a clean room is not automatically suitable for a wet, swarf-producing machine; verify the protection and the maintenance access for cleaning.
Commissioning and verification
Tune the current and position loops with the real moving mass and cable drag, and check force and temperature across the worst repeated cycle, not just one stroke. Monitor coil temperature during a soak test at full duty. Verify the air gap along the full travel and confirm cooling flow and feedback signals. Record the force profile, cooling settings, and measured temperature so a replacement reproduces the validated thermal condition.
When a transmission is better
Linear motors are not always the right answer. For very high force at moderate speed, a ballscrew or belt with a rotary motor is cheaper, uses less energy, and provides inherent mechanical advantage and braking. A linear motor earns its cost in high speed, high accuracy, long life, and clean environments. If the duty is dominated by large continuous force in a dirty, budget-limited machine, compare a rotary motor and transmission on total cost and reliability rather than selecting direct drive for its performance alone.
Keeping the thermal and cooling records with the machine also prevents a later maintenance swap from removing the chiller or changing coolant temperature, which would quietly cut the continuous force the original design depended on.
That documentation protects the machine over its full service life.
That documentation protects the machine over its full working service life.
Bottom line
Meet the peak force for acceleration, but select the linear motor from the RMS force over the real cycle against its continuous thermal rating. Provide and maintain the specified cooling, size bearings for magnetic attraction in iron-core motors, and hold a clean, uniform air gap. Direct drive removes transmission limits but exposes the thermal limit; ignoring it is why axes that accelerate perfectly in a single test overheat as soon as they run production.
