In recent years, the number of elevators in service worldwide has grown rapidly. According to statistics from national regulatory bodies, by the end of 2021, the total number of elevators in China alone reached 8.80 million units, accounting for 48.45% of all special equipment. While elevators greatly facilitate daily travel, safety accidents such as car overshoot, free fall, collision, entrapment, and shearing continue to occur. Among these, car overshoot—where the elevator car runs beyond the top terminal landing and impacts the overhead structure—is particularly hazardous and often involves severe consequences. As a professional engaged in elevator inspection, we have observed that worm gear driven elevators are especially susceptible to this failure mode due to the unique characteristics of their drive system. The worm gear pair, operating under high loads and frequent starts, is prone to fatigue wear, pitting, tooth fracture, and other deterioration mechanisms. When combined with failures of the braking system or safety devices, these conditions can lead directly to tragic overshoot accidents. This article, based on our extensive field experience and theoretical analysis, aims to dissect the car overshoot process in worm gear elevators, identify root causes, and propose effective preventive measures. We will present our findings in a structured manner, incorporating technical tables and mathematical formulas to provide a comprehensive reference for inspection personnel and maintenance teams.
The Car Overshoot Motion Process in Worm Gear Elevators
Car overshoot in a worm gear elevator occurs when the car, traveling at normal speed, passes the top landing and continues upward into the headroom, eventually striking the ceiling or overhead machinery. This scenario is typically initiated when the counterweight is heavier than the car (i.e., when the car is lightly loaded or empty). Under the action of gravity, the counterweight accelerates downward, pulling the car upward at an increasing speed. In worm gear elevators that lack a counterweight-side overspeed governor and safety gear linkage, the car’s upward velocity can quickly exceed the rated speed. The standard protective device against upward overspeed in such systems is an overspeed protection device acting on the suspension ropes, most commonly a rope gripper. When the upward speed exceeds the electrical switch activation speed of the governor, the governor sends a signal to trigger the rope gripper, which clamps the hoisting ropes and brings the car to a stop. If this protection fails, the car continues accelerating upward, the counterweight eventually contacts and compresses its buffer, coming to an abrupt halt. The car, now disconnected from the driving force but carrying significant kinetic energy, is thrown upward in the headroom. The hoisting ropes may jump out of the pulley grooves. If the upward acceleration is sufficiently high or the clearance between the car top and the overhead structure is inadequate, the car top collides with the ceiling. Following the collision, the car may fall freely, and if its downward speed exceeds the mechanical trip speed of the governor, the governor will clamp the governor rope, activating the car safety gear to stop the car on the guide rails.
This entire sequence reveals multiple failure points: the brake, the overspeed governor linkage, the rope gripper, the traction capacity, the terminal protection switches, and the upward overspeed protection device itself. Each of these elements must function reliably to prevent an overshoot. In the following sections, we analyze these failure modes in detail, using tables and mathematical relationships to quantify the risks.
Regulatory Requirements for Car Overshoot Prevention
International standards such as EN 81-20 and the Chinese standard GB 7588-2003 (which is technically equivalent to EN 81-1) specify stringent requirements for braking and upward overspeed protection in worm gear elevators. The key provisions are summarized below.
Braking System Requirements
The brake must be capable of stopping the machine when the car is loaded with 125% rated load and traveling downward at rated speed. The resulting deceleration must not exceed that produced by the safety gear or buffer. All mechanical parts that apply braking force to the drum or disc must be arranged in two independent sets. If one set fails, the remaining set must still provide sufficient braking torque to decelerate a rated-load downward-moving car. Additionally, the electrical supply to the brake must be interrupted by at least two independent electrical devices. If one contactor main contact fails to open when the elevator stops, further movement must be prevented before the next direction change.
Upward Overspeed Protection Device Requirements
This device must include speed monitoring and deceleration elements. It shall detect an uncontrolled upward speed, with the lower activation threshold set at 115% of rated speed, and shall either stop the car or reduce its speed to within the design range of the counterweight buffer. The device can act on the car (e.g., upward safety gear, rail clamp), on the counterweight (counterweight safety gear), on the suspension ropes (rope gripper), or on the traction sheave (e.g., brake acting directly on the sheave or its shaft). For worm gear elevators, the most common configuration is the combination of a governor (speed monitor) and a rope gripper (deceleration element).
Root Cause Analysis of Car Overshoot in Worm Gear Elevators
Based on our inspection data and failure investigations, the primary causes of car overshoot in worm gear elevators can be categorized into five major areas: brake failure, governor-rope gripper linkage failure, insufficient traction capacity, terminal protection switch failure, and upward overspeed protection device failure. Each area is examined below, supported by tables that list specific failure modes and their underlying mechanisms.
Brake Failure
| Failure Mode | Mechanism | Consequence |
|---|---|---|
| Compression spring fatigue or breakage | Repeated loading cycles cause spring relaxation or fracture, reducing clamping force. | Insufficient braking torque; car may drift upward when empty. |
| Electrical contactor welding or sticking | Arc welding or contamination prevents contacts from opening, keeping brake coil energized. | Brake remains released; no braking action. |
| Excessive brake shoe clearance | Wear of friction linings or improper adjustment increases air gap beyond specification. | Delayed brake application; reduced effective torque. |
| Overheating of brake drum and shoes | Frequent high-load stops generate heat, reducing friction coefficient and causing thermal expansion. | Brake fade; potential seizure or cracking. |
| Mechanical linkage failure (pins, arms, bolts) | Corrosion, fatigue, or improper lubrication leads to seizure or breakage of moving parts. | Brake cannot close; total loss of braking. |
| Worm gear transmission failure (key shear, bearing collapse, tooth stripping) | Overload or wear causes separation between brake and traction sheave. | Brake acts on worm shaft but not on sheave; car moves freely. |
The brake is the first line of defense. In worm gear elevators, the brake typically acts on the high-speed shaft of the motor or the worm shaft. However, if the worm gear pair itself fails—such as when all worm wheel teeth are stripped off or the coupling bolts shear—the brake may still be intact but mechanically disconnected from the traction sheave. This catastrophic failure mode, though rare, is extremely dangerous and must be addressed through regular inspection of the transmission elements.
Governor-Rope Gripper Linkage Failure
| Failure Mode | Mechanism | Consequence |
|---|---|---|
| Governor overspeed setting incorrect | Mechanical wear or misadjustment changes the trip speed; governor may not activate at required 115% rated speed. | No signal sent to rope gripper; overshoot proceeds unchecked. |
| Governor electrical contact failure | Contacts become welded, corroded, or misaligned; circuit remains closed despite governor overspeed. | Brake not commanded; rope gripper not triggered. |
| Safety circuit fault | Wiring break, relay failure, or controller logic error prevents execution of governor signal. | Same as above: no braking action. |
| Rope gripper misadjustment | Cable tension too slack or too tight; operating mechanism jammed; reset screw over-tightened. | Rope gripper does not engage or provides insufficient clamping force. |
| Governor rope groove wear | Groove worn smooth, reducing friction between rope and sheave. | Governor rope slips; overspeed detection fails to transmit force. |
| Car safety gear linkage switch failure | Limit switch contacts stuck; mechanical linkage corroded or jammed. | Safety gear not activated even if governor triggers. |
The governor-rope gripper linkage is a critical safety chain. During our inspections, we frequently encounter cases where the rope gripper operating cable is incorrectly tensioned, or the reset screw is left in a position that prevents full closure. Regular testing of the entire sequence—simulating overspeed with an electronic or mechanical test device—is essential to verify proper function.
Insufficient Traction Capacity
Traction capacity depends on the friction between hoisting ropes and sheave grooves, as well as the balance coefficient. The basic traction equation (Euler’s formula) is:
$$ e^{\mu \alpha} \geq \frac{T_1}{T_2} $$
where \(\mu\) is the coefficient of friction, \(\alpha\) is the angle of wrap (in radians), \(T_1\) is the tension on the heavier side, and \(T_2\) on the lighter side. For an ascending car with empty load, the counterweight side is heavier, so:
$$ \frac{T_{cw}}{T_{car}} = \frac{W_{cw}}{W_{car}} $$
If the groove is worn or the rope diameter has decreased, μ drops significantly, reducing the safety margin. Additionally, the balance coefficient \(K\) is defined as:
$$ K = \frac{W_{cw} – W_{car\_empty}}{Q} $$
where \(W_{cw}\) is counterweight weight, \(W_{car\_empty}\) is empty car weight, and \(Q\) is rated load. Standard requires \(0.40 \le K \le 0.50\). If \(K\) is too high (e.g., 0.60 or above), the counterweight is excessively heavy, and when the car is empty, the gravity imbalance overcomes the brake torque, causing the car to creep upward even at rest. During motion, the traction ratio \(T_1/T_2\) increases, potentially exceeding the available friction, leading to rope slip and uncontrolled upward movement.
| Cause | Effect on Traction | Overshoot Risk |
|---|---|---|
| Groove wear (V-groove or U-groove) | Decreased effective μ; rope sinks deeper, reducing wrap angle. | Rope slip under light car load; car accelerates upward. |
| Rope diameter reduction (wear or stretching) | Poor contact with groove; reduced friction area. | Same as above. |
| Excessive balance coefficient (K > 0.50) | Counterweight disproportionately heavy; tension ratio exceeds design. | Brake unable to hold car; upward creep at stops; slip during run. |
| Insufficient wrap angle (e.g., sheave arrangement) | Lower e^{μɑ} factor; reduced safety margin. | Slip under normal load variations. |
| Lubricant or contaminants on rope/sheave | Reduced friction coefficient. | Increased slip probability. |
We have observed cases where the balance coefficient was inadvertently increased after modernization (e.g., adding heavier cab finishes without recalculating counterweight). This subtle shift can push the elevator into an unsafe state. Regular measurement of the balance coefficient using clamp-on ammeters during load tests is a necessary preventive action.
Terminal Protection Switch Failure
The terminal protection system consists of three levels: forced slow-down switch, limit switch, and final limit switch. If the brake fails or traction slips, the car may overshoot even if these switches are functional, but they provide a last resort. However, when the switches themselves are defective, the protection is lost entirely.
| Failure Mode | Mechanism | Consequence |
|---|---|---|
| Switch position misalignment | Cam or roller out of adjustment relative to car-mounted actuating ramps. | Switch not actuated at correct level; car passes without deceleration. |
| Contact welding or corrosion | Electrical contacts stick in closed position; circuit remains intact. | No signal to controller; car continues beyond limit. |
| Mechanical damage (broken roller, bent arm) | Physical damage from previous impact or aging. | Switch inoperative. |
| Wiring disconnection or short circuit | Broken wire or insulation failure. | Protection circuit bypassed or permanently tripped. |
In worm gear elevators, the forced slow-down switch is especially important because the worm gear drive has inherent self-locking only in one direction under static conditions; during dynamic overshoot, the inertia can overcome self-locking. Hence, reliable terminal switches are critical.
Upward Overspeed Protection Device Failure
GB 7588-2003 mandated upward overspeed protection for all new elevators after January 1, 2005. Many older worm gear elevators installed before that date never received retrofits. Even for those equipped, the device may become non-functional over time. The two most common implementations are:
- Governor + brake linkage (where the brake serves as both stopping and overspeed protection)
- Bidirectional governor + rope gripper
| Failure Mode | Mechanism | Consequence |
|---|---|---|
| Speed monitoring component (governor) failure | Governor internal mechanism worn; centrifugal forces cannot overcome friction. | No detection; overshoot proceeds. |
| Deceleration element (rope gripper) failure | Clamping jaws worn; actuator piston stuck; insufficient clamping force. | Device triggers but cannot stop the car; car overshoots with reduced speed. |
| Missing device (pre-2005 elevators) | No upward overspeed protection installed at all. | No protection against overshoot. |
| Incorrect trip speed setting | Governor adjustment drifts; device activates too late or not at all. | Overshoot may exceed buffer design range. |
It is important to note that the upward overspeed device is only required to activate at ≥115% rated speed. If the car overshoots at a lower speed (e.g., due to brake failure while running at normal speed), the device does not engage. In such cases, the terminal protection switches and brake are the only safeguards. This limitation underscores the need for redundant brakes and proper maintenance of all systems.
Preventive Measures
Based on the root cause analysis, we propose the following comprehensive preventive measures, summarized in the table below:
| No. | Measure | Targeted Failure Mode |
|---|---|---|
| 1 | Retrofit or replace all pre-2005 worm gear elevators with compliant upward overspeed protection devices (e.g., rope gripper + governor assembly). | Missing or outdated device |
| 2 | Inspect brake compression springs annually (measure free height, check for cracks). Test electrical contactors for welding (manual check after power-off). Measure air gap with feeler gauge; replace linings when thickness < 50% of original. Clean and lubricate brake linkages; replace worn pins. Check worm wheel teeth for pitting or fractures via borescope; replace lubricant with manufacturer-specified grade; fix oil leaks. | Brake failure; transmission failure |
| 3 | Test governor-rope gripper linkage semi-annually using overspeed simulation. Verify governor trip speed (calibration test every 2 years). Adjust governor electrical switch and pawl mechanism. Clean safety gear wedges and guide surfaces; ensure clearance within 2 mm. Check rope gripper cable tension; adjust actuating linkage; ensure reset screw is not over-tightened. | Governor/linkage failure |
| 4 | Measure groove wear depth and rope diameter every 6 months. Replace sheave if groove wear exceeds 1 mm for U-groove or 0.5 mm for V-groove. Replace rope if diameter reduction > 10% or if broken wires appear. Verify balance coefficient annually by measuring current during full load up/down runs; readjust counterweight if K < 0.40 or > 0.50. | Insufficient traction |
| 5 | Check position of terminal switch cams relative to car-mounted actuating ramps during 1:1 inspection. Test electrical continuity of each switch. Replace any switch showing signs of corrosion or mechanical damage. Verify that forced slow-down switch activates before limit switch. | Terminal switch failure |
| 6 | Establish a robust maintenance management system. Provide training for technicians on worm gear-specific failure modes. Adhere to the manufacturer’s maintenance schedule. Document all inspection results and trend analysis. | Overall system degradation |
Additionally, we recommend that operators of worm gear elevators pay special attention to the periodic verification of the upward overspeed device. Unlike safety gear on the car side, the rope gripper mechanism is often hidden inside the machine room and may be neglected. A simple functional test—using a portable overspeed test instrument to simulate governor activation while the car is empty and moving upward at slow speed—can reveal hidden defects. The deceleration of the car should be recorded and compared to design values. If the stopping distance exceeds the calculated value, immediate corrective action is required.
The mathematical relationship for the minimum stopping distance under rope gripper action can be derived from energy balance:
$$ \frac{1}{2} m v^2 = F_{\text{grip}} \cdot s $$
where \(m\) is the total mass of the car plus any imbalance, \(v\) is the activation speed, \(F_{\text{grip}}\) is the clamping force, and \(s\) is the stopping distance. In practice, the available friction force must also account for the rope’s stiffness and the dynamic coefficient of friction. For worm gear elevators, the rope gripper must be capable of absorbing the kinetic energy without allowing the car to contact the overhead structure.
Conclusion
Car overshoot accidents in worm gear elevators are preventable through a systematic understanding of the failure mechanisms and rigorous application of maintenance and inspection protocols. The unique vulnerability of worm gear drives—namely the potential for gear stripping and the reliance on a single brake (often acting on the motor shaft rather than the sheave)—demands heightened vigilance. By addressing brake deterioration, ensuring the governor-rope gripper linkage functions correctly, maintaining adequate traction capacity, verifying terminal switches, and retrofitting missing upward overspeed devices, the risk of catastrophic overshoot can be dramatically reduced. Our analysis, summarized in the tables and formulas presented here, provides a practical guide for inspectors and maintenance teams. We strongly encourage all stakeholders to adopt these measures without delay, as the cost of prevention is far lower than the human and economic toll of a single overshoot event.