Maglock Effective Holding Force at the Door Handle
A 1,200 lb maglock does not resist 1,200 lb at the handle, and where it is mounted decides how much it does.
Example
You enter
- Lock rated holding force (lb) 1200
- Door width (in) 36
- Hinge to lock centre (in) 3
- Hinge to handle or pull (in) 36
- Holding loss from armature gap and contamination (%) 50
- Voltage measured at the lock (V) 20
- Lock rated voltage (V) 24
- Target resistance at the handle (lb) 600
You get
- Lever ratio 0.0833
- Force resisting a pull at the handle 100 lb
- Best case force (lb) 1100
- Improvement factor 11
- Effective force (lb) 34.722
- Rating needed at this lock position 7200 lb
Details, formula, and sources
The door is a lever about its hinge line and the magnet works against that lever. Mount a 1,200 lb magnet 3 in from the hinge on a 36 in door and the ratio is 1 to 12: it resists about 100 lb at the pull -- one determined person. Move the identical magnet to 33 in from the hinge, near the strike edge, and it resists 1,100 lb. ELEVEN TIMES BETTER FROM THE SAME HARDWARE, at no cost. Lock position is the whole design, and a magnet mounted where the header was convenient is a magnet mostly thrown away. Two further reductions apply before anyone pulls. Rated holding force assumes full face contact between armature and magnet with no gap, and a warped door, paint, dirt, or a misaligned armature drops it sharply -- a sixteenth of an inch can cost half of it. And the magnet must have its rated voltage AT THE LOCK: an electromagnet's force follows the square of the flux and so roughly the square of the supply voltage, so 20 V on a 24 V lock costs about 31%, not 17%. A well-mounted magnet that is dirty and undervolted ends up back where a badly mounted one started -- that badly mounted 100 lb becomes 35 lb with both losses applied. THE EGRESS CAUTION IS NOT OPTIONAL. A magnet holds until it is de-energized, so on any door required for egress the release arrangement -- request to exit, motion sensing, fire alarm interface, and power failure behaviour -- is a code matter, and a magnet that stays locked when the building is on fire is a fatality mechanism regardless of its rating. A lever-ratio calculation from a rated holding force. Real holding force depends on armature contact, alignment, cleanliness, door rigidity, temperature, and supply voltage, none of which this measures and all of which reduce it. It does not evaluate the door, frame, or the fasteners securing the magnet and armature, which are frequently the actual failure point, and it does not size power or wiring. The adopted building and fire codes, the lock manufacturer's listing and installation instructions, and the AHJ govern.
force at the handle = rated holding force x (lock distance from the hinge) / (handle distance from the hinge); an electromagnet's force follows the square of the supply voltage; the entered gap derate multiplies what is left.
The lever relation for a door hinged at one edge, by name, with the adopted codes named as governing electrically locked egress. Rated holding force is the manufacturer's figure and assumes full face contact at rated voltage.
A lever ratio on dimensions the user measures, against a rated force from the manufacturer; no product table is reproduced.
Estimate. AHJ and licensed professional govern.
Field names used by the API: rated_holding_lb, door_width_in, lock_distance_from_hinge_in, handle_distance_from_hinge_in, gap_derate_pct, voltage_at_lock, rated_voltage, target_resistance_lb, lever_ratio, force_at_handle_lb, best_case_force_lb, improvement_factor, effective_force_lb, rating_needed_lb
- Lock position is the whole design near the hinge throws most of the rating away; near the strike edge keeps itlever mechanics
- Force follows the square of voltage so a small drop at the lock costs more holding force than it appears toelectromagnetics
- Rated force assumes no gap a warped door, paint, dirt, or a misaligned armature drops it sharplylock manufacturer data