Elevator Safety Gear is a mechanical brake mounted under the elevator car that clamps the steel guide rails to stop a free-falling or overspeeding car. Unlike a friction-only hoist brake at the machine, it acts directly on the rails so a snapped rope or failed motor brake cannot defeat it. An overspeed governor trips the gear when car speed exceeds 115% of rated. The result is a controlled deceleration of 0.2g to 1.0g and a stopped car within a few hundred millimetres of travel.
Elevator Safety Gear Interactive Calculator
Vary the wedge ramp angle and rail gap to see the self-energizing clamp multiplier, wedge stroke to contact, and gap penalty.
Equation Used
The wedge ramp converts downward motion into side clamping force on the guide rail. A smaller ramp angle gives a larger ideal clamp multiplier, while a larger retracted rail gap requires more wedge travel before contact.
- Ideal self-energizing wedge with friction losses neglected.
- Ramp angle theta is measured from the vertical guide direction.
- Rail gap is the retracted clearance on one side of the guide rail.
- Calculator estimates wedge geometry and force ratio, not full dynamic stopping distance.
Operating Principle of the Elevator Safety Gear
The Elevator Safety Gear, also called the Safety Catch for Elevators in older British lift code, works by physically wedging hardened blocks against the machined faces of the guide rails. A separate device — the overspeed governor mounted in the machine room — monitors car velocity through a continuous loop of governor rope tied to the safety linkage on the car frame. When the car exceeds the tripping speed (typically 115% of rated, capped at 0.3 m/s above rated for slow lifts), the governor jaws grip the rope. The car keeps falling, the rope stops, and that relative motion yanks the safety lever which drives the wedges up into the rails.
Why wedges and not pads? Because the gear has to work whether the rails are dry, lightly oiled, or freshly lubricated for the guide shoes. Wedges are self-energising — the harder the car tries to fall, the deeper they bite. The hardened wedge faces are typically 55-62 HRC running against rail flanges of 200-250 HBN. Get this hardness pair wrong and you either glaze the wedge (no bite) or gall the rail (rail must be replaced — at $80-150/m for T89/B or T127/B profiles, that gets expensive fast). The standard wedge-rail gap when retracted is 2-3 mm per side. If you let it drift to 5 mm because the linkage pivots wore, the gear strokes through air before contact and your stopping distance balloons.
Failure modes are predictable. Glazed wedges from one full test trip without redress. Governor rope slip if the sheave groove is worn smooth — the governor jaws clamp the rope but the rope slides through them, no trip. And linkage seizure from corrosion in damp shafts — the jaws grip, but the lever on the car can't move the wedges. EN 81-20 clause 5.6.2 requires the gear to be tested under full load at rated speed at commissioning, then retested with empty car at reduced speed annually.
Key Components
- Wedge or Roller Block (the gripping element): Two hardened blocks per rail, one each side, shaped to climb a ramp inside the safety housing and clamp the rail flange. Surface hardness 55-62 HRC, ramp angle typically 15-20° for instantaneous types, with a calibrated spring stack on progressive types to limit clamp force.
- Safety Housing or Yoke: Bolted to the bottom crosshead of the car sling. Locates both wedge sets, takes the full vertical reaction load during a stop — for a 1,600 kg duty load car at 1.0g deceleration that is roughly 32 kN per rail. Housing is forged or fabricated steel, never cast iron.
- Overspeed Governor: Centrifugal device in the machine room that runs off a 6-8 mm governor rope looped to the car. Trips at 115% of rated speed (EN 81-20 clause 5.6.2.2.1.1). Two stages: first stage cuts power to the drive, second stage clamps the rope to fire the safeties.
- Governor Rope and Tensioner: Continuous loop of 6 mm or 8 mm steel wire rope, tensioned by a weighted sheave in the pit (typically 30-60 kg). Tension must keep the rope from slipping in the governor sheave groove during a trip — slip means no firing.
- Safety Linkage and Lift Lever: Mechanical pivot system that converts governor-rope arrest into upward motion on both wedge sets simultaneously. Synchronisation is critical — both rails must engage within 50 ms of each other or the car twists and jams in the shaft.
- Reset Mechanism: After a trip, the car must be lifted clear of the wedges to free them — you cannot drive down to release. Most modern progressive gears reset by raising the car 100-200 mm and the wedges drop back under their own weight or a small return spring.
Who Uses the Elevator Safety Gear
Every passenger and goods elevator in a regulated jurisdiction carries a Safety Catch for Elevators. The split between instantaneous and progressive type is decided by rated speed: instantaneous below 0.63 m/s, progressive above. Beyond passenger lifts, the same mechanism appears wherever a vertical car is suspended on ropes or belts and a free-fall would kill someone or wreck product.
- Commercial Passenger Elevators: Otis Gen2 and KONE MonoSpace MRL elevators use progressive safety gear paired with a centrifugal governor for car speeds 1.0-2.5 m/s in mid-rise office buildings.
- High-Rise and Double-Deck Elevators: Mitsubishi NexWay-S and Hitachi UltraRope installations in 200 m+ towers run progressive gears with controlled deceleration of 0.2-1.0g to protect occupants at rated speeds up to 10 m/s.
- Construction Hoists: Alimak Scando 650 rack-and-pinion hoists carry an overspeed centrifugal brake that grips the toothed mast — the same functional concept as a building elevator's safety gear, sized for 2,000 kg payloads.
- Mining Cages: Drum hoists at deep-shaft mines like the Mponeng gold mine use Lilly-controller-tripped safety dogs that engage timber or steel guides if the cage exceeds programmed speed-position limits.
- Freight and Goods Lifts: Schindler 2400 freight elevators rated to 4,500 kg use heavy instantaneous-with-buffer safety gear because rated speed sits below 0.63 m/s and the duty cycle favours simpler hardware.
- Stage and Theatre Lifts: Gala Systems Spiralift orchestra-pit lifts and similar performance-venue platforms include rail-clamping safety gear sized for 1.5-2.0g emergency deceleration to protect performers and equipment.
- Residential Home Elevators: Stiltz Duo and Savaria Vuelift through-floor lifts use compact instantaneous safety gear since rated speed is 0.15-0.20 m/s, well under the progressive threshold.
The Formula Behind the Elevator Safety Gear
The number that drives every safety-gear selection is stopping distance under emergency clamp. EN 81-50 fixes the allowable mean deceleration at 0.2g minimum to 1.0g maximum for a progressive gear with rated load. Under that, you use kinematic energy balance to predict how far the car will travel from the moment the wedges bite to the moment it stops. At the low end of the deceleration range (0.2g) you preserve passenger comfort but the car stops over a longer distance — fine for a slow goods lift, dangerous for a high-speed car that may run out of buffer. At the high end (1.0g) the stop is short but anyone standing in the car gets thrown to the floor. The sweet spot for passenger lifts sits at 0.4-0.6g, which is what most progressive gears like the Wittur LBS or Dynatech ASG-100 are tuned for from the factory.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| s | Stopping distance after wedges engage | m | ft |
| vtrip | Car velocity at the moment safety gear engages (typically 1.15 × rated) | m/s | ft/s |
| amean | Mean deceleration delivered by the safety gear (0.2g to 1.0g per EN 81-50) | m/s² | ft/s² |
| g | Gravitational acceleration (9.81) | m/s² | ft/s² |
Worked Example: Elevator Safety Gear in a 6-stop hospital bed elevator retrofit
You are specifying the progressive safety gear on a 6-stop hospital bed elevator at a regional health centre in Kelowna, BC. Rated speed is 1.6 m/s, rated load 2,000 kg, guide rails are T127/B at 9.16 mm flange thickness. The architect wants the pit buffer stroke kept short, so you need to know how far the car will actually travel after the gear fires at the governor trip speed.
Given
- vrated = 1.6 m/s
- vtrip = 1.84 m/s (1.15 × rated)
- anom = 4.9 (0.5g) m/s²
- alow = 1.96 (0.2g) m/s²
- ahigh = 9.81 (1.0g) m/s²
Solution
Step 1 — calculate trip speed from rated speed. EN 81-20 fixes the governor trip at 115% of rated:
Step 2 — at the nominal 0.5g deceleration that a properly tuned Wittur LBS or Dynatech ASG progressive gear delivers with 2,000 kg in the car:
That 345 mm is the sweet spot for a hospital bed lift — short enough to clear the buffer with margin, gentle enough that a patient on a gurney is not flung off.
Step 3 — at the low end of the EN 81-50 allowable range (0.2g), worn wedges or a glazed clamping face would deliver:
Now you are looking at 864 mm of free fall after the gear fires. That eats most of a typical 1,000 mm pit buffer stroke and leaves no margin for compression. This is why annual drop tests matter — a glazed wedge that produces 0.2g instead of 0.5g looks fine on inspection but more than doubles your stopping distance.
Step 4 — at the high end (1.0g), an over-tightened spring stack or seized progressive damper acts more like an instantaneous gear:
173 mm is a violent stop. Anyone standing unsecured hits the floor. For a hospital lift carrying IV stands and monitors this is unacceptable — the load tears free of its tie-downs even if no one is injured.
Result
At nominal 0. 5g deceleration the car stops in 345 mm of travel after the wedges bite — the gold standard for a hospital bed elevator and well within the available pit buffer envelope. The 0.2g low-end case stops in 864 mm and the 1.0g high-end case in 173 mm, which tells you the design window is narrow: a 2.5× change in deceleration gives a 5× change in stopping distance because the relationship is squared. If your annual drop test measures 600 mm instead of the predicted 345 mm, the most common causes are: (1) glazed wedge faces from a previous trip that was not properly redressed — surface hardness drops and the wedge slides, (2) governor rope slip in a worn sheave groove that delays full engagement by 100-200 ms, or (3) under-tensioned governor rope where the pit tensioner weight has been removed or reduced, letting the rope slip through the jaws.
When to Use a Elevator Safety Gear and When Not To
The choice between safety-gear types is locked by rated speed and code, but within those bands you still pick between several real product families. Progressive gear gives controlled deceleration at higher speeds; instantaneous gear is simpler and cheaper but only legal below 0.63 m/s rated; instantaneous-with-buffer is a hybrid that adds a coil-spring or polyurethane buffer between the wedge and the housing to soften the stop on slow goods lifts.
| Property | Progressive Safety Gear | Instantaneous Safety Gear | Instantaneous with Captive Buffer |
|---|---|---|---|
| Maximum rated car speed | Up to 10+ m/s (high-rise) | 0.63 m/s (EN 81-20 cap) | 1.0 m/s with buffer |
| Mean deceleration during stop | 0.2-1.0g, controlled by spring stack | Up to 25g+ (uncontrolled) | 1-2g, buffered |
| Stopping distance at 1.6 m/s rated | ~345 mm | ~30-50 mm (violent) | ~150-250 mm |
| Hardware cost (per car) | $2,500-6,000 USD | $600-1,200 USD | $1,200-2,500 USD |
| Reset after a trip | Raise car 100-200 mm, automatic wedge return | Raise car, often requires wedge inspection | Raise car, inspect buffer compression |
| Typical application | Passenger lifts >0.63 m/s, hospital, hotel | Slow goods, residential through-floor | Mid-speed freight, dumbwaiters |
| Test interval (full load) | At commissioning, then 5-yearly per EN 81-20 | At commissioning, then annually empty-car | At commissioning, then annually empty-car |
Frequently Asked Questions About Elevator Safety Gear
That is a clamp-force problem, not a trip-speed problem. The governor and the linkage are doing their job — the wedges are firing — but they are not generating the rated friction against the rail. Two causes dominate. First, the spring stack inside the safety housing has been disassembled at some point and reassembled with the wrong preload, so the wedges grip the rail with maybe 40% of design force. Second, the rail running surface has oil contamination from over-lubricated guide shoes above the safety — even a thin oil film cuts the wedge-to-rail friction coefficient from roughly 0.2 down to 0.08.
Quick check: pull the safety covers, look at the wedge faces. If they show a uniform matte grey scuff pattern after the test, friction is fine and the spring stack is the issue. If they look polished or oil-streaked, clean the rail with a non-residue solvent and re-test before touching the springs.
Code forces the progressive gear above 0.63 m/s rated, full stop. At 0.8 m/s you have no choice — instantaneous, even with a buffer, is not compliant under EN 81-20 clause 5.6.2.2.1.3. The captive-buffer variant only buys you headroom up to 1.0 m/s in some legacy interpretations, and most authorities having jurisdiction will not accept it on new installations.
The real decision at 0.8 m/s is which progressive gear: a wedge-type like the Wittur LBS for clean machine-room-less installations, or a roller-type like the Dynatech ASG-100 if your guide rails are older and slightly out of plumb. Roller gears are more forgiving of rail twist up to about 2 mm/m, wedge gears are not.
Probably yes, and that is by design. Progressive safety gear delivers roughly constant clamp force regardless of car mass, so deceleration scales inversely with load. Empty car (say 1,200 kg sling) at 0.7g and full load (3,200 kg gross) at 0.5g is exactly the relationship the spring stack is tuned for. A 1,200 / 3,200 = 0.375 mass ratio against a 0.5 / 0.7 = 0.71 deceleration ratio means the gear is delivering about 1.7× the clamp force needed for proportional scaling — the spring is slightly progressive, which is normal.
The full-load drop test at commissioning is the only real proof. Empty-car annual tests verify that the linkage and governor still fire, not that the deceleration is correct.
Yes, and this is the most common real-world trip scenario. The safety gear does not care why the car is overspeeding — it only sees velocity through the governor. A failed motor brake on a geared traction lift lets the counterweight overhaul the car (if the car is light) or lets the car descend (if loaded). Once velocity exceeds 115% of rated, the governor fires regardless of rope condition.
This is why the gear is on a separate kinematic chain from the hoist machine. A snapped rope is the textbook scenario but in 30 years of field data it represents under 5% of actual safety-gear trips. Worn brake linings and contactor weld-ups account for most of the rest.
Building sway. In towers above roughly 150 m, wind-induced lateral movement can swing the governor rope enough that the centrifugal weights see a momentary radial spike equivalent to overspeed. The governor cannot tell the difference between a fast-spinning sheave and a sheave that is being yanked sideways. KONE and Otis address this on UltraRope and Gen3 high-rise installations with damped governor sheaves and rope-stabilising guides at intermediate floors.
If you are seeing nuisance trips on a 30-storey building during storms, fit a rope tie-back at mid-shaft. The trips are not a fault in the gear — they are a real signal from a governor that does not have lateral-motion compensation.
Depends on score depth. EN 81-20 commissioning guidance allows surface marking up to about 0.5 mm depth on a T-profile guide rail without replacement, provided the running surface for the guide shoes (usually the front face of the flange, not the side gripped by the wedges) is unaffected. Measure with a depth gauge across multiple points.
Scoring deeper than 0.5 mm or any score that crosses onto the guide-shoe running face means rail replacement of that section. The cause is almost always over-hard wedge faces (above 62 HRC) running against an under-spec rail. Check the wedge hardness certificate before you blame the rail.
Because rated speed on residential through-floor lifts like Stiltz Duo or Savaria Vuelift is 0.15-0.20 m/s. Plug those numbers into the stopping-distance formula at even 25g deceleration: s = 0.20² / (2 × 245) = 0.0008 m, less than 1 mm. The peak deceleration is high but the velocity change is so small that the impulse on a passenger is trivial — comparable to stepping off a kerb.
The 0.63 m/s code threshold for instantaneous gear exists precisely because below that speed, the violent-stop deceleration produces a survivable impulse. Above it, the impulse becomes injurious and progressive gear is mandatory.
References & Further Reading
- Wikipedia contributors. Elevator. Wikipedia
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