I have dedicated my work to understanding the severe safety issue of car top impact accidents in worm gear driven elevators. Over the past decade, the number of elevators in service has grown dramatically worldwide. According to official statistics, by the end of 2021, there were approximately 8.8 million elevators in operation in China alone, accounting for nearly half of all special equipment. While elevators greatly facilitate daily travel, accidents such as car top impact, car bottom impact, collisions, entrapment, and shearing still occur periodically. Among these, the car top impact accident, where the elevator car overshoots the top floor and strikes the pit ceiling, is particularly dangerous because it often involves the failure of multiple safety systems. In worm gear driven elevators, the unique transmission mechanism — a worm engaging with a worm gear — is susceptible to fatigue wear, pitting, tooth breakage, and other failure modes under heavy duty and high frequency operation. Coupled with potential failures of the brake and overspeed governor systems, these factors can lead to catastrophic car top impacts. In this article, I will analyze the dynamic process of a car top impact in worm gear driven elevators, review the relevant safety requirements, identify the root causes, and propose effective preventive measures. I will also introduce mathematical formulas and summary tables to provide a clear, quantifiable framework for understanding and mitigating this hazard.
To begin, I will describe the physical process of a car top impact in a worm gear driven elevator. Under normal operation, the car travels upward at rated speed. When the car passes the top landing without stopping, it continues to rise into the overhead space. This scenario typically occurs when the counterweight is heavier than the empty car, because in most worm gear driven elevators the counterweight is designed to balance the car plus a rated load (usually around 40% to 50% of the rated load). With an empty or lightly loaded car, the counterweight exerts a downward force greater than the upward force required to hold the car, resulting in a net upward acceleration of the car. If the brake fails or the traction force is insufficient, the car accelerates upward. The governing equation of motion for the car-counterweight system, neglecting rope mass and friction, can be written as:
$$m_{cw} g – m_{car} g = (m_{car} + m_{cw}) a$$
where \(m_{cw}\) is the mass of the counterweight, \(m_{car}\) is the mass of the car (including any load), \(g\) is gravitational acceleration, and \(a\) is the net acceleration of the entire system. For an empty car, \(m_{car} = m_{empty}\), and if the balance coefficient \(\beta\) is defined as the ratio of counterweight mass to (car mass + rated load mass), then \(m_{cw} = \beta (m_{empty} + m_{rated})\). Under no‑load condition, the net upward acceleration is:
$$a = \frac{g ( \beta (m_{empty} + m_{rated}) – m_{empty})}{m_{empty} + \beta (m_{empty} + m_{rated})}$$
This acceleration is usually modest (fraction of g) but sustained. If the brake is not applied, the car continues to accelerate until it reaches the end of the travel. The velocity as a function of time becomes:
$$v(t) = v_0 + a t$$
where \(v_0\) is the speed at which the car passed the top landing (typically rated speed). The distance traveled above the top floor is:
$$s(t) = v_0 t + \frac{1}{2} a t^2$$
When the counterweight hits its buffer at the bottom of the pit, the counterweight suddenly stops. The car, still moving upward, may then be thrown upward relative to the counterweight due to the inertia of the car and the elasticity of the hoist ropes. In extreme cases, the car speed can become high enough to cause the car top to strike the overhead structure. For worm gear driven elevators that lack an overspeed governor on the counterweight side, the car may even exceed the tripping speed of the car‑side safety gear. I will now discuss the safety requirements that are designed to prevent this sequence of events.
The prevention of car top impacts in worm gear driven elevators relies primarily on two devices: the brake and the upward overspeed protection device. According to the widely adopted safety code (e.g., GB 7588‑2003), the brake must be capable of stopping the elevator car with 125% rated load at rated speed when traveling downward. The braking system must consist of two independent sets of mechanical parts, so that if one set fails, the other can still bring the car to a safe stop. The brake release must be controlled by at least two independent electrical devices, and if one contactor fails to open, the elevator must be prevented from restarting at the next direction change. The overspeed protection device for upward movement (called the upward overspeed protection device) must detect car speed exceeding 115% of rated speed and then activate a braking mechanism. This mechanism can act on the car, the counterweight, the suspension ropes, or the traction sheave. Common types include car‑mounted safety gears, rope grippers, and additional braking on the worm gear shaft. For worm gear driven elevators manufactured before 2005, many were not equipped with upward overspeed protection. I summarize the key requirements in Table 1.
| Component | Requirement | Test Condition |
|---|---|---|
| Brake | Must stop car with 125% rated load at rated speed downward. Two independent braking sets. | Downward direction, rated load × 1.25 |
| Brake Electrical Control | At least two independent electrical devices to interrupt brake current. | Any stop condition |
| Upward Overspeed Protection | Must detect speed ≥ 115% of rated speed and bring car to stop (or slow down to buffer design range). | Upward overspeed test |
| Speed Monitoring | Governor or electronic device with mechanical trip speed setting. | Periodic verification |
| Action on Ropes | Rope gripper (e.g., jaw‑type) must clamp suspension ropes effectively. | Simulated overspeed |
With the requirements established, I now turn to the root cause analysis of car top impacts in worm gear driven elevators. Based on my field investigations and failure data, the primary causes can be categorized into five groups: brake failure, failure of the governor‑brake/rope‑gripper interlock, insufficient traction capacity, failure of terminal protection devices, and absence or failure of the upward overspeed protection device. I will discuss each in detail, incorporating quantitative relationships where possible.
(1) Brake failure or insufficient braking torque. The worm gear driven elevator brake is typically a spring‑applied, electromagnetically released drum or disc brake. Common failure modes include: relaxation or fracture of the brake springs, causing reduced clamping force; sticking of brake electrical contacts that prevent the brake from releasing; excessive clearance between brake shoes and brake wheel; overheating of the brake drum due to frequent or prolonged sliding, which reduces the coefficient of friction; and mechanical failures such as shear of the brake arm pivot pin or loosening of the shaft key. The braking torque \(T_b\) is given by:
$$T_b = n \cdot F_s \cdot \mu \cdot r$$
where \(n\) is the number of brake shoes, \(F_s\) is the spring force per shoe, \(\mu\) is the friction coefficient between shoe and drum, and \(r\) is the effective radius. When the spring force degrades or the friction coefficient drops, the braking torque falls below the required value. In a worm gear driven elevator, the worm gear pair itself provides some inherent resistance due to the sliding friction, but this is not reliable for emergency stopping. I have seen cases where the worm gear teeth were completely worn away (stripped), disconnecting the brake from the traction sheave. Such mechanical disconnection leaves the car without any braking power, leading directly to a car top impact if the counterweight is heavier.
(2) Failure of governor‑brake/rope‑gripper interlock. The governor is responsible for detecting overspeed and triggering both the electrical safety circuit and the mechanical action of the brake or rope gripper. Several failure modes can occur: the governor trip speed setting may drift out of tolerance (e.g., too high, allowing the car to exceed safe speed); the governor electrical switch may fail to activate due to contact welding or broken wires; the safety circuit may have a fault that prevents it from opening even when the governor switch operates, so the brake remains released; the rope gripper mechanism may be improperly adjusted — for example, the actuating cable may be too slack, the reset screw may be over‑tightened, or the clamping jaws may not align correctly, resulting in insufficient gripping force. Additionally, in worm gear driven elevators, the governor rope may slip in its groove if the groove is worn or the rope tension is low, preventing the governor from transmitting the required force to the safety gear linkage. The condition for the governor to effectively activate the safety gear is:
$$F_{rope} \geq \frac{F_{safety}}{\eta}$$
where \(F_{rope}\) is the tension in the governor rope (determined by the counterweight or tensioning device), \(F_{safety}\) is the force needed to operate the safety gear linkage, and \(\eta\) is the efficiency of the lever mechanism. If the rope tension is insufficient, the safety gear will not engage even if the governor mechanically trips. I have also observed that the safety gear electrical switch (often a limit switch on the car top) can stick in the closed position, so that even if the safety gear mechanically engages, the control circuit does not cut off the motor. This can lead to further acceleration.
(3) Insufficient traction capacity. The traction capacity of a worm gear driven elevator depends on the friction between the hoist ropes and the traction sheave grooves. Over time, the groove profile wears down, and the rope diameter decreases slightly, causing the rope to sit deeper in the groove. The effective friction coefficient can change. The fundamental traction equation is:
$$\frac{T_1}{T_2} \leq e^{\mu \theta}$$
where \(T_1\) is the tension on the heavier side (e.g., counterweight side when car is empty), \(T_2\) is the tension on the lighter side (car side), \(\mu\) is the equivalent friction coefficient in the groove, and \(\theta\) is the angle of wrap. For a typical worm gear driven elevator, the wrap angle is usually around 150° to 180° (in radians, \(\theta = 2.62\) to 3.14). The code requires that under all loading conditions, the ratio must not exceed the exponential limit. When the balance coefficient is too high (i.e., counterweight too heavy), the ratio \(T_1/T_2\) becomes large, and traction may be lost. For example, if the balance coefficient exceeds 0.55, then with an empty car, the counterweight side may be 1.2 times heavier than the car side, leading to potential rope slip. I calculate the critical balance coefficient from the traction condition as:
$$\beta_{crit} = \frac{e^{\mu \theta} \cdot m_{empty} – m_{empty} – m_{rated}}{e^{\mu \theta} \cdot (m_{empty} + m_{rated}) – (m_{empty} + m_{rated})}$$
Any balance coefficient above this critical value increases the risk of rope sliding during emergency braking or even during normal operation if the brake fails. In worm gear driven elevators, the worm gear itself can also exhibit a self‑locking property under certain conditions, but this is imperfect and degrades with wear. I always emphasize checking the balance coefficient during periodic inspections.
(4) Failure of terminal protection devices. Every worm gear driven elevator is equipped with forced slowdown switches, limit switches, and final limit switches at the top and bottom of the hoistway. These are designed to cut power to the motor and apply the brake when the car overshoots. However, if the car is already moving upward under the influence of an unbalanced counterweight and the brake has failed, these switches cannot stop the car because they only remove electrical power — they do not apply mechanical braking. The switches can also fail due to misalignment: the cam on the car may strike the switch roller incorrectly, or the switch contacts may be welded shut. In such cases, the car continues upward until it hits the mechanical stops or the pit ceiling. The displacement required for the forced slowdown switch to activate is typically set at about 1.5 meters above the top floor; if the car speed is high, the stopping distance may exceed that interval. The deceleration needed to stop within the available overtravel \(s_{over}\) is:
$$a_{stop} = \frac{v^2}{2 s_{over}}$$
If the required deceleration exceeds the maximum achievable deceleration by the brake (or by safety gear), a car top impact becomes inevitable.
(5) Absence or failure of upward overspeed protection. The most significant preventive measure against car top impacts in worm gear driven elevators is the upward overspeed protection device. For elevators manufactured before 2005, many worm gear driven units lacked this device altogether. Even in newer installations, the device can fail. The speed‑monitoring element (governor) may fail to detect overspeed if its trip speed is set too high or if the governor rope slips. The braking element (rope gripper or extra brake) may fail to engage due to corrosion, dirt buildup, or mechanical seizure. For example, the rope gripper jaws must be able to clamp the suspension ropes with enough force to prevent further movement. The required clamping force is given by:
$$F_{clamp} \geq \frac{m_{car} g}{n_{ropes} \cdot \mu_{rope}}$$
where \(n_{ropes}\) is the number of suspension ropes and \(\mu_{rope}\) is the friction coefficient between the gripper jaw and the rope. If the gripper is not periodically cleaned and lubricated, the friction coefficient drops, and the clamping force becomes inadequate. In worm gear driven elevators, the upward overspeed protection device is especially critical because the worm gear itself does not provide inherent overspeed protection — in fact, a worn worm gear can break free and rotate independently. Table 2 summarizes the five root causes and their typical failure modes.
| Category | Specific Failure Mode | Consequence |
|---|---|---|
| Brake failure | Spring relaxation, friction material wear, electrical contact welding, mechanical linkage fracture | Insufficient braking torque; car free to accelerate |
| Governor interlock failure | Governor speed setting drift, rope slip in groove, switch failure, gripper misadjustment | Overspeed not detected or not acted upon |
| Insufficient traction | Groove wear, rope diameter reduction, excessive balance coefficient | Rope slip under emergency braking or overrun |
| Terminal protection failure | Switch misalignment, contact welding, electrical circuit fault | Power not removed; car continues upward |
| Upward overspeed protector failure | Governor trip at wrong speed, gripper corroded or jammed, reset screw overtightened | No mechanical stopping means for upward overspeed |
To prevent car top impacts in worm gear driven elevators, I have derived a set of practical measures corresponding to each root cause. These measures should be implemented by elevator owners, maintenance companies, and inspection authorities. I present them in Table 3 for clarity.
| Root Cause | Preventive Action | Inspection Frequency |
|---|---|---|
| Brake failure | Check spring force, shoe wear, clearance, and electrical contacts; replace worn parts; verify braking torque by test with 125% load | Monthly visual, annual torque test |
| Governor interlock failure | Periodic governor speed verification; clean governor rope and groove; adjust cable length; test safety gear operation at low speed | Semiannual (speed test) & annual (trip test) |
| Insufficient traction | Inspect groove profile with gauge; measure rope diameter; adjust balance coefficient if needed; replace sheave if groove wear exceeds limit | Annual or as per wear indication |
| Terminal protection failure | Verify cam‑roller alignment; test each switch with multimeter; ensure electrical circuit integrity | Quarterly |
| Upward overspeed protector failure | For pre‑2005 elevators: retrofit with compliant device (e.g., rope gripper or bidirectional governor); for existing devices: clean, lubricate, adjust gripper gap, test with simulated overspeed | Annual functional test |
In addition to the device‑specific measures, I strongly recommend a comprehensive management system. Maintenance personnel must be trained to recognize early signs of worm gear wear, such as unusual noise, vibration, or oil temperature rise. The worm gear reducer oil should be replaced according to manufacturer specifications, and oil leaks must be fixed promptly because loss of lubrication accelerates tooth wear. I have seen instances where loose bolts on the worm gear housing caused misalignment, leading to uneven tooth contact and eventual tooth stripping. Regular tightening of all bolts on the transmission chain — including the coupling between the motor and worm shaft — is essential. The elevator’s balance coefficient should be re‑verified whenever significant modifications are made (e.g., car remodeling, rope replacement). A simple field test involves measuring the current during full load upward and downward runs; the balance coefficient can be computed from the motor current difference.
Furthermore, the upward overspeed protection device for worm gear driven elevators deserves special attention. Many old elevators still in service were built before the requirement became mandatory. I advocate for a systematic retrofit program: installing a rope gripper or a bidirectional governor with safety gear on the car or counterweight. The cost is modest compared to the potential loss of life. When retrofitting, one must ensure compatibility with the existing worm gear system. For instance, a rope gripper must be mounted on a rigid structure that can withstand the clamping force without deflection. The actuation must be independent of the main brake. I have also seen designs where the brake itself is duplicated — a second brake acting on the worm gear shaft — which can serve as the upward overspeed protector. However, if the worm gear itself fails (e.g., teeth sheared), this brake becomes useless because it is on the worm side, not the wheel side. Therefore, a device acting directly on the suspension ropes or on the car is more reliable.
Mathematically, the probability of a car top impact occurring can be modeled using a fault tree. Let \(P_B\) be the probability of brake failure, \(P_G\) be the probability of governor interlock failure, \(P_T\) be the probability of traction failure, \(P_S\) be the probability of terminal switch failure, and \(P_U\) be the probability of upward overspeed protector failure. Assuming these failures are independent (a simplification), the probability of a car top impact event \(P_{impact}\) is the product of the probabilities that all required barriers fail simultaneously:
$$P_{impact} = P_B \cdot P_G \cdot P_T \cdot P_S \cdot P_U$$
In reality, the dependencies are complex, but this equation highlights the importance of having multiple independent layers of protection. For worm gear driven elevators, if the upward overspeed protector is absent, then \(P_U = 1\), making the product much larger. Reducing each probability through regular maintenance and testing is the key to safety.
I have also analyzed the dynamics of the final phase — when the counterweight hits its buffer. The counterweight buffer is designed to absorb the kinetic energy of the descending counterweight, which at that moment has gained speed due to the free fall under gravity. The car, being lighter, may be thrown upward with a relative velocity. If the ropes are elastic, the car can continue upward after the counterweight stops, potentially reaching a speed that exceeds the safety gear capability. The maximum height \(h_{throw}\) that the car can reach above the point of counterweight impact can be estimated from conservation of energy and momentum, but a simpler upper bound is:
$$h_{throw} \approx \frac{v_{cw}^2}{2g}$$
where \(v_{cw}\) is the counterweight speed at impact. If this height exceeds the available overhead clearance, impact occurs. This underscores the importance of maintaining the buffer stroke and ensuring the counterweight does not bottom out prematurely.
In conclusion, through my comprehensive analysis of the car top impact process in worm gear driven elevators, I have identified the critical roles of the brake, governor interlock, traction system, terminal protection devices, and upward overspeed protection. The worm gear drive, while efficient and smooth, introduces unique failure modes such as tooth wear, shaft key shearing, and oil degradation that can lead to loss of braking. By applying the preventive measures summarized in Table 3, and by using the mathematical formulas provided to quantify risks and performance, I believe that the incidence of car top impacts can be greatly reduced. Elevator owners, maintenance personnel, and inspection authorities should work together to ensure that every worm gear driven elevator is equipped with functioning brakes and upward overspeed protection, and that all components are regularly tested and maintained. Only by treating each potential failure with the rigor of quantitative analysis and systematic inspection can we provide safe vertical transportation for millions of users daily. The worm gear system will continue to be widely used due to its compactness and high torque capacity, but its safety depends entirely on the reliability of its auxiliary safety devices. I urge all stakeholders to take the warnings from this analysis seriously and implement the recommended actions without delay.