Magnetic Grippers for Thin Steel Sheets: Air Gaps, Double-Sheet Pickup, and Release

A convincing magnetic lift is not enough to approve thin-sheet handling. The gripper must hold the intended sheet, avoid carrying a second sheet and release at the required station. For thin steel, sheet thickness, ferromagnetic grade, oil adhesion, stack contact and the magnetic circuit all influence whether an upper pick also lifts a second sheet. The release and single-sheet checks need their own tests.

This article is about the full chain of a single-sheet magnetic pick: holding force, the air gap and sheet thickness that set it, double-sheet detection, and release. It is written for the buyer who has decided magnetic gripping is viable for their steel and now has to confirm that single-sheet handling actually works.

Characterize the Actual Steel and Stack

The starting point is the steel itself and the way it is stacked, described as a range rather than a single value. The buyer should specify the sheet family explicitly: thickness from minimum to maximum, coating type, oil presence, and surface condition, plus how the sheets sit in the stack. Thin material can limit usable magnetic holding force, while its flexibility and stack adhesion create additional pickup and release conditions to check. A second sheet can be carried because of magnetic interaction, oil adhesion, burrs or the pressure difference created as sheets separate. Test the mechanism on the actual stack rather than assuming thickness alone predicts double pickup.

Include the thinnest sheet at the largest credible pickup gap when checking minimum holding-force margin, then test double pickup, release and distortion independently across the stack range. A nominal-condition trial leaves the rest of that operating range unqualified.

Read Holding-Force Conditions

Read the stated conditions before using a datasheet force in a production calculation. For example, Schmalz's SGM-HP 20 data sheet describes its holding forces as static values under optimal conditions and distinguishes configurations with and without the friction ring. Your production sheet may have a different grade, thickness, coating, contact area or pickup gap.

The mistake to avoid is taking a catalog peak value and applying it to the thinnest, most coated sheet in the family. Holding force depends on the magnet design, sheet thickness, material, contact coverage and gap. Increasing the gap can reduce the available force; a thickness curve must be read for the particular gripper rather than replaced with a generic correction factor. A number measured on a thick bare plate does not describe a thin painted sheet, so it cannot establish the available holding force for that different condition.

The honest engineering position is that the holding-force-versus-air-gap curve has to be supplied for the actual sheet in question, at the actual thickness and surface condition.

The SMC China MHM catalog, CAT.CS100-131A, gives thickness-versus-force curves for its named sizes and calls them theoretical reference values for low-carbon flat steel. It requires actual adsorption trials because material and shape change the result. That mainland regional catalog is useful selection guidance, not a test report for your steel family.

For the earlier choice among vacuum, mechanical, magnetic and soft tooling, consult the robot gripper selection guide; this article covers what to test once a magnetic approach is shortlisted. If a supplier cannot produce that curve for your sheet, the holding-force claim is unverified for your application, not disproven, but unverified, and it should not be used to approve a pick.

Evaluate Air Gaps and Surface Layers

The pickup gap is the separation between the gripper's magnetic contact surface and the steel. Non-magnetic coatings, contamination, roughness and sheet distortion can increase the effective gap or reduce contact coverage. Record these conditions instead of assuming either perfect contact or a fixed production gap.

This is why the buyer should require the holding-force curve across the sheet family's range, with the air gap stated for each point. In Goudsmit's TPGC100088 measured-force document, the 0.5 mm St37-2 (S235JR) sample with Ra 1.03 µm is listed at 299 N with zero gap and 238 N at a 1 mm gap. Those are measured force values for that setup, not approved working loads or a rating for another gripper. The product page separately advises a 3 mm minimum sheet thickness, so the thinner test sample does not establish an approved operating application.

So the specification should tie every holding-force figure to its air gap and its surface condition. Ask also for the test conditions behind any number: plate thickness, material grade, surface condition, air gap, and magnet configuration. A number without its conditions cannot be compared to another number, and comparisons are exactly what a buyer needs when choosing between suppliers.

Detect and Separate Double Sheets

Holding force tells you whether the magnet can carry a sheet. It does not tell you whether it is carrying one sheet or two. A double feed can damage a press tool, forming die or fixture, so single-sheet confirmation belongs in the acceptance criteria.

Double-sheet detection is a separate technology with its own options, and it should be specified as its own requirement. ROLAND ELECTRONIC's I21 system is one concrete non-contact, dual-head eddy-current example: its sensor table separates ferrous, aluminium and austenitic material ranges. This supports asking for a sensor-specific window, not assuming that any metal detector fits every steel sheet. Double-sheet detection may use ultrasonic, inductive, magnetic or thickness-sensing approaches, depending on sheet material, coatings and sensor installation. Require a sensor-specific single-versus-double trial on the actual sheets and feed geometry. Mechanical double-sheet detectors sense the difference in stack height or the passage of material. Goudsmit's HSS-P manual, May 2025 v1.1, describes magnetizing adjacent steel sheets so like poles repel and create separation. They do not, by that action, prove residual magnetism has been removed or that a downstream gripper has picked exactly one sheet; use a separate double-sheet check. These are complementary, not interchangeable, and the right combination depends on the sheet and the process.

The point for the buyer is that magnetic holding and single-sheet confirmation are two different requirements. The gripping system should provide a positive confirmation that one sheet, not two, is held, and the cell should define what happens when the confirmation fails: does the pick stop, does the sheet get released and re-tried, or is it routed out of the line.

Verify Release and Residual Effects

Specify release with the same care as pickup. A sheet that is held perfectly and released imperfectly will stick to the gripper, drop late, or drag against the next sheet.

Release depends on the gripper design. In the SMC MHM catalog, a pneumatic piston changes the position of a permanent magnet relative to the workpiece; the catalog also states that a loss of air supply can leave the workpiece held. For other designs, obtain the specified switching or demagnetization sequence. Neither removal of power nor a release command proves that the sheet has departed. Residual attraction or adhesion can leave sheets sticking after the command. After separation and release, check for residual attraction or sticking on the actual sheet stack; do not assume the sheet demagnetizes merely because it has left a magnetic separator. The release path should be validated on the actual sheet and stack, not just described.

Release also needs a defined verification. A drop check, a position check, or a sensor confirming the sheet left the gripper all serve the same purpose: proving the cycle completed. Define the response to a missed release before allowing the next motion or feed.

Run a Single-Sheet Acceptance Matrix

A useful starting request to a magnetic-gripper supplier is a holding-force-versus-air-gap curve measured on a sheet matching their actual thinnest, most coated, most oily condition. Not the catalog curve. Not the thick-plate curve. The curve for the sheet in question. If the supplier cannot provide the curve, that is not a reason to walk away from magnetic gripping. It is a reason to plan a validation pick on your own sheet before committing, and to write the double-sheet and release requirements into the specification so they are designed for rather than hoped for.

The output of this work is a table that follows the pick end to end. One row per sheet condition (thickness, coating, oil, position in the stack), with columns for: holding force required and holding force available at the actual air gap, the double-sheet detection method and its pass criterion, the release method and its verification, and the residual-magnetism check.

Build that table and the magnetic-gripping question becomes concrete. The holding-force claim is tied to the real sheet, not the lab plate. Single-sheet confirmation is a designed feature, not an assumption. Release is verified, not left to chance. Require the complete pick, single-sheet confirmation and release sequence to pass on the actual steel family before accepting the application.

Qualify the sheet family before comparing magnet ratings. Send the minimum and maximum sheet thickness, steel grade, coatings, oil condition, stack photos, sheet dimensions and the downstream press or fixture requirements. Through robot selection support, we can organize the gripper and handling requirements and identify which holding-force curves, double-sheet sensing options and release checks need confirmation from the equipment supplier. The deciding evidence is a single-sheet trial on your actual material, not a catalog peak-force figure or an assumed sensor compatibility.