A Geared Continuous Hinge is a full-length door hinge that uses two extruded aluminum leaves with meshed gear teeth running the entire height of the door, locked together by a top and bottom cap rather than a knuckle pin. You'll find them on the main entrance doors of hospitals, schools, and airports — places where doors cycle thousands of times a day. The geared design spreads load across the full leaf rather than concentrating it on three or four knuckles, which eliminates door sag and stops the hinge from racking out of square under abuse. Properly specified, one will outlast the door it's mounted to.
Geared Continuous Hinge Interactive Calculator
Vary door weight, width, hinge length, and product rating to see the bending moment per inch carried by a geared continuous hinge.
Equation Used
The calculator uses the article equation for bending moment per inch of geared continuous hinge. Door weight is multiplied by the center-of-gravity distance from the hinge axis, then divided by active hinge length. The result is compared with the selected hinge rating.
- Door center of gravity is at half the door width.
- Door weight is uniformly transferred along the active hinge mesh length.
- Static bending moment only; impact, abuse, and frame misalignment are not included.
- Rating is compared directly to calculated lb-in/in moment demand.
How the Geared Continuous Hinge Works
The mechanism is simple but unusual. Instead of a single pin running through interleaved knuckles like a piano hinge or butt hinge, a geared continuous hinge uses two extruded aluminum leaves whose inner edges are cut as gear racks. The teeth on one leaf mesh with the teeth on the other, and the two leaves are held in contact by a thin bearing channel — usually a polyacetal (Delrin) thrust bearing strip — that runs the full length and is captured by aluminum end caps top and bottom. There is no pin. The pivot axis is the pitch line of the gear mesh.
When you open the door, each tooth rolls against its mating tooth instead of sliding around a pin. That rolling action is what gives the hinge its life — typical ANSI A156.26 grade-1 hinges are tested to 25 million cycles, an order of magnitude beyond a butt hinge. Load on the door (a 250 lb steel slab, say) is distributed continuously along the 83-inch leaf, so the bending moment per inch of hinge is tiny. That is why a geared continuous hinge does not let a heavy door sag — there is no leverage arm trying to pull three knuckles out of a frame.
Tolerances matter. The aluminum extrusion has to hold tooth profile within roughly ±0.05 mm across the full length, or you'll feel the binding when you swing the door. If the leaves are not mounted dead straight on the frame and door — say the frame is bowed by 2 mm over the height — the gear teeth will bind at the bow and you'll get a stiff spot mid-swing. The most common failure modes are exactly that: a bowed frame causing tooth-end loading, the bearing strip squeezing out because the end cap screws backed off, or grit packing into the tooth roots on doors near a building exterior. Lubrication with a dry PTFE film, not grease, is the spec — grease holds dust and accelerates wear in the tooth roots.
Key Components
- Frame Leaf (extruded aluminum): Mounts to the door frame and carries one half of the gear rack along its full length. Typically 6063-T6 aluminum extruded to ±0.1 mm dimensional tolerance, anodized for surface hardness around 400 HV. Length matches the door height — common stock is 79, 83, 85, and 95 inches.
- Door Leaf (extruded aluminum): Mirror of the frame leaf, mounted to the door edge. The gear teeth on this leaf mesh with the frame leaf at a fixed pitch line — usually 0.5 inch tooth pitch. Both leaves must be coplanar within 1.5 mm over the full length or the mesh will bind.
- Bearing Channel (Delrin or UHMW strip): Runs the full length between the two leaves and carries axial thrust load when the door hangs. A typical strip is 6 mm wide × 3 mm thick polyacetal, rated for around 14 MPa continuous bearing pressure. It self-lubricates and is the only wear part you'll ever replace.
- End Caps (top and bottom): Cast or machined aluminum caps that capture the bearing channel and hold the two leaves in alignment. Fastened with two #10-24 stainless screws each. If these back out under vibration, the bearing strip walks and the hinge develops vertical play.
- Mounting Fasteners: Typically #12-24 stainless flat-heads at 4-inch spacing, giving roughly 20 fasteners per leaf on an 83-inch hinge. The redundancy is the point — pull one out and the hinge does not care. Compare a butt hinge where losing one of four screws on the top hinge will cause immediate sag.
Where the Geared Continuous Hinge Is Used
Geared continuous hinges show up wherever a door gets abused. High-frequency openings, oversized leaves, and security applications all favour the continuous-mesh design over conventional knuckle hinges because the load path is fundamentally different. Where a butt hinge concentrates the door's weight onto a few square inches of pin and knuckle, the geared hinge spreads it over 80-plus inches of meshed teeth, and that is what kills door sag, frame distortion, and the slow racking failure you see on neglected commercial entrances.
- Healthcare: Patient room and corridor doors at facilities like Cleveland Clinic — Select Hinges SL11 and SL57 series are specified for 25-million-cycle service on doors that cycle 4,000+ times per day.
- Education: Classroom and corridor doors in K-12 schools, where Pemko FM200 and FM300 full-mortise geared hinges replace traditional butts to stop students racking the door off-square.
- Aviation: Concourse doors at major airports including DFW and ATL, where doors are 3-foot-wide, 90-inch-tall steel leaves cycling continuously through 18-hour operating days.
- Detention and Correctional: Cell-block doors using McKinney MCK series geared hinges with security stud kits — the pinless design eliminates the pin-removal attack vector entirely.
- Government and Military: Federal buildings specified to GSA standards using ABH A110HD heavy-duty continuous hinges on blast-resistant entry doors weighing up to 600 lbs.
- Retail and Commercial: Anchor-tenant entry doors at shopping malls — Hager 780-112HD continuous hinges on glass-and-aluminum storefront doors that see 8,000+ cycles per day during holiday seasons.
The Formula Behind the Geared Continuous Hinge
The number that drives hinge selection is the bending moment per unit length of hinge, because that is what determines whether the door will sag over time and whether the gear mesh will see local overload. At the low end of typical commercial use — a 100 lb hollow-core wood door on an 83-inch hinge — the moment per inch is trivial and almost any standard-duty hinge will outlast the door. At the high end — a 600 lb security door on the same 83-inch hinge — you are pushing into heavy-duty territory and need to verify the gear-tooth contact stress against the manufacturer's rating. The sweet spot for a standard SL11/FM200 grade product sits around 250-300 lbs of door weight at 36-inch width, which is why those models are the workhorses of the commercial door industry.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Mper_in | Bending moment carried per inch of hinge length | N·m/m | lb·in/in |
| Wdoor | Total door weight including hardware and glazing | N | lb |
| dcg | Horizontal distance from hinge axis to door centre of gravity (typically half the door width) | m | in |
| Lhinge | Active mesh length of the continuous hinge | m | in |
Worked Example: Geared Continuous Hinge in a hospital corridor door retrofit
A facilities team at a 400-bed regional hospital in Cincinnati is replacing failed butt hinges on 180 corridor doors. Each door is a 1¾-inch steel-stiffened leaf, 36 inches wide × 84 inches tall, weighing 220 lbs with the closer and lite kit installed. They are specifying an 83-inch geared continuous hinge and need to verify the bending moment per inch of hinge against the manufacturer's 60 lb·in/in rating for the SL11 HD product line.
Given
- Wdoor = 220 lb
- dcg = 18 in
- Lhinge = 83 in
Solution
Step 1 — at the nominal 220 lb door weight, compute the total bending moment about the hinge axis:
Step 2 — divide across the 83-inch active hinge length to get moment per inch:
That sits comfortably under the 60 lb·in/in rating of the SL11 HD product — about 80% utilization, which is the sweet spot. Plenty of headroom, no oversizing.
Step 3 — at the low end of the typical commercial range, a 100 lb hollow metal door on the same 36-inch width:
That is 36% utilization. The hinge is loafing — you'd never see wear in the gear teeth in 30 years of service. A standard-duty SL11 (rated 35 lb·in/in) would handle this and save 30% on cost.
Step 4 — at the high end, a 450 lb blast-rated door at the same width:
That blows past the HD rating. You either move up to a heavy-duty product like the ABH A110HD (rated 110 lb·in/in) or specify a 95-inch hinge to drop the moment per inch back to 85 lb·in/in. Running the SL11 HD at 97.6 would chew the gear teeth inside two years on a high-frequency door.
Result
The nominal answer is 47. 7 lb·in/in, which means the SL11 HD geared continuous hinge is correctly sized for this 220 lb hospital door with about 20% margin. In practice this translates to a door that swings smoothly for the full 25-million-cycle rated life — roughly 17 years at 4,000 cycles per day — with no perceptible sag and no need for re-shimming. Comparing the three operating points: at 100 lbs the hinge is barely working, at 220 lbs you're in the design sweet spot, and at 450 lbs you are out of spec for this product line and need to step up. If a measured door starts to drag the strike or shows uneven gap at the head within the first year, the most likely causes are: (1) the frame anchors loosening into a hollow CMU wall, letting the frame leaf walk outward 2-3 mm, (2) the closer arm pre-load exceeding spec and pulling the door against the stop at every cycle, or (3) installer used #10 fasteners instead of the specified #12-24, which strip out under repeated impact loading.
Geared Continuous Hinge vs Alternatives
The choice between a geared continuous hinge, a pin-and-barrel continuous hinge (piano hinge), and traditional butt hinges comes down to door weight, cycle frequency, and budget. Each has a clear application zone — pick the wrong one and you'll be back replacing hardware inside 5 years.
| Property | Geared Continuous Hinge | Pin & Barrel Continuous Hinge | Heavy-Duty Butt Hinges (3 per door) |
|---|---|---|---|
| Rated cycle life | 25 million cycles (ANSI A156.26 Grade 1) | 1.5 million cycles | 2.5 million cycles (ANSI A156.1 Grade 1) |
| Maximum door weight (36" wide) | 600 lbs (HD product) | 150 lbs | 400 lbs (4.5" × 4.5" ball-bearing) |
| Load distribution along door | Continuous — full 83 in | Continuous — full length | Concentrated at 3 points |
| Cost per opening (typical) | $280-$450 | $80-$140 | $60-$120 |
| Installation labor | 20-30 min (pre-machined frame) | 30-45 min | 45-60 min (mortise per hinge) |
| Sag resistance over 10 years | Negligible | Moderate — pin wear causes drop | Poor on heavy doors — top hinge fails first |
| Security (pinless) | Yes — no removable pin | No — pin can be drifted out | No, unless NRP variant specified |
| Best application | High-frequency commercial / institutional | Light cabinets, residential | Standard commercial under 200 lbs |
Frequently Asked Questions About Geared Continuous Hinge
Almost always a frame straightness issue, not the hinge itself. The two leaves have to be coplanar within about 1.5 mm over the full 83 inches. If the frame is bowed inward at mid-height — common on hollow metal frames that were grouted unevenly or shipped racked — the gear mesh tightens at exactly that point and you feel a stiff spot.
Pull a string line down the frame leaf before you blame the hardware. If the frame is bowed more than 1/16 inch over the height, you need to shim behind the leaf with stainless shim stock at the high points, not force the hinge to conform. Forcing it will cold-work the aluminum extrusion and you'll never get a clean swing back.
Yes, but you need to verify two things. First, lead-lined doors typically run 280-400 lbs for a standard 36 × 84 leaf, so you must size the hinge using the moment-per-inch formula and confirm you're under the manufacturer's rating — usually that means an HD-grade product, not standard duty.
Second, the hinge must be specified with a heavy-duty bearing strip and stainless end-cap fasteners, because lead doors carry their centre of gravity slightly outboard of a standard door (the lead sheet sits closer to the strike face), which adds maybe 10-15% to the calculated moment. Select Hinges and ABH both publish lead-door-specific part numbers — use those, don't try to specify a standard HD product and hope.
Driven entirely by the door and frame construction. Full-mortise is the cleanest install — it requires both the door edge and the frame rabbet to be machined to accept the leaves flush, and that's standard on new hollow-metal frames. If you're retrofitting onto an existing frame that wasn't prepped, you usually can't field-mortise it without ruining the fire rating, so half-surface (mortised into door, surface-mounted to frame) is the common retrofit choice.
Full-surface goes on doors and frames where you can't mortise either side — typically wood-clad frames in historic buildings, or where the door is too thin (under 1¾ inch) to take a mortise without compromising the structure. Full-surface is visually busiest but installs in 15 minutes with no machining.
If the hinge itself is straight and the gear mesh feels normal, the drop is almost never the hinge. The two most common causes on newer installs are frame anchor failure and door-leaf deflection.
Pull the trim and check the frame anchors first — on hollow CMU walls, the wire anchors sometimes pull through the grout pocket if the grout was undersized, and the entire frame migrates outward 2-3 mm at the hinge side, which reads as the door dropping at the strike. Second possibility: the door itself is flexing because the internal stiffeners are inadequate for the closer pre-load. A 220 lb door with a heavy-duty closer running at maximum spring tension will twist the leaf over time, especially if the closer arm is mounted parallel rather than perpendicular. Back the closer off to spec spring size for the door width and the drop usually stops progressing.
Yes, but you need a power-transfer (ETW) variant — Select SL57 SDPT, Pemko FM200HDPT, or equivalent. Standard geared hinges have no provision for wire pass-through because the gear mesh runs continuously down the pivot line and there's nowhere for a wire to live.
The ETW variants build a small section of pin-and-knuckle hinge into the middle of the continuous hinge — typically 6 inches at the door's vertical centre — with a hollow pin that takes a 4, 8, or 12-conductor wire bundle. That short knuckle section is the weak point, so don't spec ETW unless you actually need it; the load rating drops about 25% compared to the same model without the transfer.
Technically yes, practically no. The aluminum extrusion will cut clean on a chop saw with a non-ferrous blade, but you have to re-machine the end-cap mounting holes and re-drill for the bearing-strip retention, and most installers don't have the fixtures to hold tolerance on that.
More importantly, cutting voids the manufacturer's warranty and the ANSI listing. For a one-off custom-height door it might be defensible, but for any production work just buy the correct length — the manufacturers stock 79, 83, 85, 95, and 120 inch standard, and they'll cut-to-order in 5-day lead time for under 10% premium. The labor saved on field-cutting a single hinge is rarely worth losing the listing.
On a properly installed hinge running within its load rating, the polyacetal bearing strip will outlast the building's lease term — 20+ years at 4,000 cycles per day is typical. The strip only becomes a wear item when the hinge is overloaded or contaminated.
The two scenarios that kill bearing strips early: doors near exterior entrances where windborne grit packs into the gear roots and abrades the strip from above, and doors where the closer is over-pressured and slamming the door into the stop, which spikes the axial load on the strip at every close cycle. If you start hearing a chirp or feel vertical play in the door, pull the bottom end cap and inspect the strip — replacement is a 10-minute job and the part costs under $20. Don't grease it as a preventive — grease holds dust and is the most common cause of early strip failure on retrofitted hinges.
References & Further Reading
- Wikipedia contributors. Hinge. Wikipedia
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