An iron-core linear motor on a gantry was mounted to an 8 mm aluminum plate. The motor was a 30 mm wide, 250 mm long coil assembly rated at 200 N continuous force. The gantry engineer sized the plate for the 200 N thrust and the 50 kg moving mass. The plate flexed and the motor made a knocking sound on every move. The engineer tightened the bolts. The knocking got worse. The engineer was designing for the wrong force entirely. The motor pulls the plate against its own magnet track with about 1,800 N of attractive force — nine times the thrust. The plate was being bent by a force that appears in the motor datasheet but is easy to skip.
Iron-core linear motors have a magnet track and a coil core made of laminated steel. The steel core is attracted to the magnets even with no current. That attraction is the attractive force. It is typically 5 to 15 times the continuous thrust. For a 200 N motor, the attractive force is 1-2 kN. The force is constant, always pulling the coil toward the track, independent of the command. The mounting structure must carry it as a permanent static load. Most first-time linear motor designs forget it.
Where the attractive force goes
The attractive force acts perpendicular to the direction of travel, pulling the coil down against the magnet track. On a gantry, the track is mounted to the gantry beam and the coil is on the carriage. The force bends the carriage plate, the mounting brackets, and the beam. If the plate is thin, the gap between the coil and the track changes. The motor has a nominal air gap of 0.5 mm. If the plate flexes 0.2 mm, the gap opens to 0.7 mm on one side. The thrust drops (thrust is roughly inversely proportional to gap), the force becomes uneven, and the coil starts to knock against the track or scrape the magnets.
The 8 mm aluminum plate was the design error. With 1,800 N pushing perpendicular to a 250 mm span, the plate deflection is around 0.4 mm — enough to double the air gap on one side. The knocking was the coil assembly bottoming out against the magnet surface. Tightening the bolts made it worse because the bolts pulled the plate flat against the frame while the motor kept bending it back. The fix was not more bolts. It was a stiffer structure.
The stiffness calculation
The deflection of the mounting plate under the attractive force:
δ = F × L³ / (48 × E × I)
For the 8 mm aluminum plate: F = 1,800 N, L = 250 mm, E = 70 GPa, I = b × t³/12 = 60 × 8³/12 = 2,560 mm⁴. δ = 1800 × 250³ / (48 × 70000 × 2560) = 1,800 × 15.6 × 10⁶ / (8.6 × 10⁹) ≈ 3.3 mm. Wait — that is off. Units: F in N, L in mm gives N·mm³; E in MPa (N/mm²), I in mm⁴. E = 70,000 MPa. δ = 1800 × 15,625,000 / (48 × 70,000 × 2,560) = 28.1 × 10⁹ / 8.6 × 10⁹ = 3.27 mm. That is a huge deflection. The plate was bending over 3 mm under the attractive force. The air gap was gone. The motor was dragging on the magnets.
Check the number again, because it determines the whole fix. The 60 mm plate width assumption: a 30 mm wide motor coil typically mounts on a 60-80 mm wide carriage plate. The span between the mounting supports: 250 mm. The deflection formula for a simply supported beam with a center load is F·L³/(48·E·I). 3.3 mm of deflection for an 8 mm plate. The air gap is 0.5 mm. The motor physically cannot run like this. The knock was not a noise — it was the coil housing riding on the magnet surface.
The fix that worked
Two changes fixed it. The 8 mm aluminum plate was replaced with a 20 mm steel plate. Steel at 210 GPa with t = 20 mm: I = 60 × 20³/12 = 40,000 mm⁴. δ = 1800 × 15,625,000 / (48 × 210,000 × 40,000) = 28.1 × 10⁹ / 4.03 × 10¹¹ = 0.07 mm. The deflection dropped from 3.3 mm to 0.07 mm — a factor of 47. The air gap stayed at 0.5 mm. The knocking stopped.
The second change: the motor manufacturer’s mounting spec calls for a mounting surface flatness of 0.02 mm over the motor length and a structure stiffness of at least 5 N/μm at the mounting face. The 20 mm steel plate meets it. The 8 mm aluminum plate did not even come close — its stiffness was roughly 0.5 N/μm. The spec was in the manual. It is worth reading the mechanical mounting section of a linear motor datasheet, not just the force and speed columns.
The design checklist for iron-core motors
- Pull the attractive force from the datasheet. If it is not listed, estimate 8x the peak thrust.
- Size the mounting structure for the attractive force as a static load, not the thrust.
- Check the deflection at the coil mounting face. Target: air gap change under 0.05 mm at full attractive force.
- Use steel, not aluminum, for the mounting plate unless the plate is thick.
- Keep the air gap adjustable. Shim the coil mount so the gap can be set to the nominal value after assembly.
- Do not forget the counterforce: the magnet track pulls the beam toward the coil. The gantry beam must carry the same attractive force in the opposite direction.
The last point is the second most common miss. The magnet track is mounted to the beam. The attractive force pulls the beam toward the carriage. If the beam is an aluminum extrusion with a long unsupported span, it bows toward the motor. The gap closes on one end and opens on the other. The motor loses thrust at one end of travel. The fix is a steel beam or a track support that carries the attractive force.
What it cost to learn this
The first gantry prototype ran with the 8 mm plate for two weeks before the knocking became unignorable. The motor coil housing was scored by the magnet surface. The magnet track had a 0.1 mm wear line where the coil scraped it. The motor was replaced under warranty, but the track was not — the track cost $1,400. The 20 mm steel plate cost $90 in material and two hours of machine time. The lesson is cheap in hindsight and expensive in prototype hardware.
Iron-core linear motors pull their own mounting structure with a force 5-15x the thrust. Size the plate for the attractive force, use steel, and keep the deflection under 0.05 mm at the coil face. The knocking gantry was not a mechanical fault — the 8 mm plate was bending 3.3 mm under 1.8 kN and dragging the coil on the magnets.
