As a professional in the field of elevator safety and inspection, I have encountered numerous cases involving top overrun incidents, particularly in screw gear driven elevators. These systems, which rely on a screw gear mechanism for power transmission, are prevalent in older installations and pose unique risks due to their mechanical design. Top overrun, where the elevator car overshoots the top landing and crashes into the shaft overhead, is a severe safety hazard that can lead to catastrophic failures. In this article, I will delve into the motion process, regulatory requirements, root causes, and preventive measures for top overrun in screw gear driven elevators. My goal is to provide a comprehensive technical foundation for inspectors and maintenance personnel, emphasizing the critical role of the screw gear in these events.
The screw gear drive, a type of worm gear system, is integral to many elevator configurations. It consists of a worm (screw) that meshes with a worm wheel (gear), providing high reduction ratios and self-locking capabilities. However, this very design can contribute to failure modes leading to top overrun. Throughout this discussion, I will repeatedly reference the screw gear to underscore its importance in both causing and preventing these incidents.
Understanding the top overrun motion process is essential for diagnosing and mitigating risks. In a screw gear driven elevator, during normal operation, the car moves vertically guided by rails, counterbalanced by a counterweight. When a top overrun occurs, the car continues ascending past the top floor at normal or increased speed. This typically happens when the counterweight becomes heavier than the car, creating a net upward force. The dynamics can be described using Newton’s second law. Let the mass of the car be \( m_c \), the mass of the counterweight be \( m_w \), and the net force driving the car upward be \( F_{net} \). Then:
$$ F_{net} = (m_w – m_c)g – F_{friction} $$
where \( g \) is acceleration due to gravity, and \( F_{friction} \) represents frictional losses in the screw gear and other components. If the braking system fails, this net force accelerates the car upward. The velocity \( v \) as a function of time \( t \) can be approximated by:
$$ v(t) = v_0 + a t $$
where \( v_0 \) is the initial velocity (often the rated speed), and \( a \) is the acceleration given by \( a = F_{net} / m_c \). As the car accelerates, it may exceed the rated speed, triggering overspeed protection devices. For screw gear drives, the upward overspeed protection often involves a rope gripper or similar device. If this fails, the counterweight impacts its buffer at the bottom of the shaft. The impact force can be modeled using impulse-momentum theory. When the counterweight buffer is fully compressed, the counterweight stops abruptly, causing the car to be propelled upward due to inertia. This can lead to a free-fall scenario if the car detaches from the ropes, followed by engagement of the safety gear if downward speed exceeds limits. The entire process highlights the interplay between the screw gear’s mechanical integrity and the safety systems.
Regulatory standards, such as GB7588-2003 (equivalent to EN 81-1), outline specific requirements to prevent top overrun in screw gear driven elevators. These focus primarily on the brake and upward overspeed protection device. The brake must be capable of stopping the elevator under 125% rated load descending at rated speed, with deceleration not exceeding that from safety gear or buffer engagement. Crucially, the brake’s mechanical components must be arranged in two independent sets, so that if one set fails, sufficient braking force remains. Electrically, cutting off the brake current must involve at least two independent devices. For the screw gear system, this means the brake must effectively hold the worm shaft or drum even under wear conditions.
The upward overspeed protection device must include speed monitoring and deceleration elements. It should activate when the car’s upward speed exceeds 115% of rated speed, up to a limit defined by standards, and bring the car to a stop or reduce speed to within buffer design range. This device can act on the car, counterweight, rope system, or screw gear drive itself. For screw gear drives, common implementations include rope grippers acting on suspension ropes or brakes acting on the drive shaft. The effectiveness of these devices is paramount in compensating for potential screw gear failures.
Now, let’s analyze the root causes of top overrun in screw gear driven elevators in detail. I will categorize them and use tables to summarize, while emphasizing the role of the screw gear throughout.
| Category | Specific Causes | Relation to Screw Gear |
|---|---|---|
| Brake Failure | Spring fatigue or fracture, electrical contact welding, excessive clearance, overheating, mechanical detachment | Brake often mounted on screw gear shaft; failure disrupts torque transmission |
| Protection System Failure | Overspeed governor inaccuracy, switch failures, rope gripper misadjustment, safety gear jamming | Governor may monitor screw gear output; rope gripper interacts with ropes driven by screw gear |
| Insufficient Traction | Worn sheave grooves, reduced rope diameter, incorrect balance coefficient | Screw gear drives sheave; wear affects grip and balance |
| Terminal Protection Failure | Misaligned switches, electrical faults, contact welding | Independent but critical when screw gear or brake fails |
| Upward Overspeed Protection Failure | Absence in older units, governor-brake linkage faults, inadequate deceleration | Directly tied to screw gear drive overspeed scenarios |
Brake failure is a predominant cause. In screw gear drives, the brake is typically mounted on the high-speed shaft (worm shaft) to provide holding torque. If the brake’s tension springs relax or break, braking force diminishes. Electrical contacts that weld shut prevent the brake from releasing, causing drag or failure to engage. Excessive clearance between brake linings and the drum reduces effectiveness. Overheating from friction can lower the coefficient of friction, modeled by:
$$ \mu(T) = \mu_0 – k T $$
where \( \mu \) is the temperature-dependent friction coefficient, \( \mu_0 \) is the initial coefficient, \( k \) is a material constant, and \( T \) is temperature rise. This can lead to brake fade. Mechanical failures, such as sheared keys or bolts connecting the worm wheel to the output shaft, disconnect the brake from the drive train. For instance, if the screw gear’s worm wheel teeth strip entirely due to fatigue, the brake may hold the worm shaft but not the sheave, allowing freewheeling. The torque transmission through the screw gear can be expressed as:
$$ T_{out} = T_{in} \eta \cdot i $$
where \( T_{out} \) is output torque, \( T_{in} \) is input torque, \( \eta \) is efficiency, and \( i \) is gear ratio. A failure in the screw gear reduces \( \eta \) to near zero, negating brake control.
Protection system failures often involve the overspeed governor and safety gear. The governor must be calibrated to trigger at correct speeds. If its electrical switch fails to open, the control circuit remains energized, preventing brake activation. For rope grippers, misadjustment of the linkage or insufficient clamping force can render them ineffective. The clamping force \( F_{clamp} \) needed to stop the car can be calculated based on rope tension and friction:
$$ F_{clamp} \geq \frac{m_c a}{\mu_{rope}} $$
where \( \mu_{rope} \) is the friction coefficient between gripper and rope. If the screw gear is worn, causing vibration, it may affect governor accuracy. Safety gear switches, often limit switches, can weld contacts, failing to signal engagement.
Insufficient traction arises from wear in the sheave grooves and ropes. The traction equation for elevators is:
$$ \frac{T_1}{T_2} \leq e^{\mu \alpha} $$
where \( T_1 \) and \( T_2 \) are rope tensions on either side of the sheave, \( \mu \) is friction coefficient, and \( \alpha \) is wrap angle. Wear reduces \( \mu \) and changes groove geometry, lowering the effective \( e^{\mu \alpha} \). This can cause rope slippage, especially if the balance coefficient \( K \) (ratio of counterweight to car mass) is incorrect. For screw gear drives, an imbalanced system exacerbates slippage. The balance coefficient should be between 0.40 and 0.50. If \( K \) is too high, the counterweight is too heavy, and under light load, the braking torque may not overcome the downward moment, leading to upward creep or overrun. The net force equation becomes:
$$ F_{net} = (K m_c – m_c)g = m_c g (K – 1) $$
If \( K > 1 \), \( F_{net} \) is positive upward, promoting overrun if the brake fails.
Terminal protection devices—forced slowdown, limit, and final limit switches—are last-resort safeguards. If these switches are misaligned or electrically faulty, they won’t interrupt power when the car overruns. This is critical when screw gear or brake failures occur, as these switches should stop the motor independently.
Upward overspeed protection is mandatory in newer elevators but absent in many older screw gear units. Even when present, it may fail due to governor inaction or deceleration element faults. For screw gear drives, the protection must account for the drive’s self-locking nature; if the screw gear fails, overspeed can occur rapidly. The activation speed threshold (115% of rated) means that lower-speed overruns due to brake failure might not trigger it, relying instead on terminal switches.
To prevent top overrun, a multi-faceted approach is necessary. I propose the following measures, summarized in a table for clarity.
| Target Area | Preventive Actions | Implementation Details |
|---|---|---|
| Brake System | Regular inspection and maintenance | Check spring tension, electrical contacts, clearance, lining thickness; test brake performance under load |
| Screw Gear Integrity | Monitor wear and lubrication | Inspect worm and wheel teeth for pitting, stripping; replace oil periodically; check keys, bolts, bearings |
| Overspeed Protection | Install or upgrade upward overspeed devices | Add rope grippers or brakes; calibrate governors; test linkage with safety gear |
| Traction System | Assess sheave and rope condition | Measure groove wear and rope diameter; adjust balance coefficient; replace worn components |
| Terminal Protection | Verify switch alignment and function | Align dogs and switches; test electrical circuits; ensure independent operation |
| Management Systems | Enhance training and protocols | Train staff on screw gear specifics; follow maintenance schedules; document inspections |
For brake systems, regular testing should include verifying the dual-circuit independence and torque capacity. The brake torque \( T_b \) must satisfy:
$$ T_b \geq (m_w – m_c) g r / \eta $$
where \( r \) is sheave radius, and \( \eta \) accounts for screw gear efficiency. If springs are weak, replace them. Electrical contacts should be cleaned and tested for welding resistance.
The screw gear itself requires diligent monitoring. Worm gear wear can be modeled using the Hertzian contact stress equation:
$$ \sigma_H = \sqrt{\frac{F E^*}{\pi b \rho}} $$
where \( F \) is tooth load, \( E^* \) is equivalent modulus, \( b \) is face width, and \( \rho \) is relative curvature. High stress leads to pitting and tooth failure. Regular oil analysis can detect metal particles indicating wear. For screw gear drives, using high-viscosity oil with anti-wear additives is crucial. Additionally, check for backlash increase, which signals wear. Backlash \( B \) affects positioning accuracy and can be measured as:
$$ B = \theta_{input} – \theta_{output} / i $$
where \( \theta \) are angular positions. Excessive backlash may require gear replacement.
Upward overspeed protection should be retrofitted in older screw gear elevators. This involves installing a governor that monitors car speed and triggers a rope gripper. The gripper’s clamping force must be calculated based on the maximum expected force during overrun. For screw gear drives, since the drive may fail catastrophically, the protection must act quickly. Testing should simulate overspeed conditions by overriding controls in a safe manner.
Traction system maintenance includes measuring sheave groove depth and rope diameter periodically. The groove wear limit can be defined as a percentage of original depth. For example, if wear exceeds 10%, replacement is needed. Balance coefficient verification involves running the elevator with known loads and measuring current draw or torque. The ideal balance coefficient minimizes motor load. For screw gear drives, an incorrect balance can cause overheating and accelerated wear.
Terminal protection switches should be tested monthly. The forced slowdown switch must activate early enough to allow normal braking. Its position can be calculated based on deceleration distance. For an elevator with rated speed \( v \) and deceleration \( a \), the distance \( d \) needed to stop is:
$$ d = \frac{v^2}{2a} $$
The switch should be placed at a distance greater than \( d \) from the terminal to ensure stop before impact. For screw gear drives, deceleration may vary if the gear is worn, so conservative margins are advised.
Management practices are equally important. Personnel must understand the peculiarities of screw gear drives, such as their self-locking property which can mask problems until failure. Regular training on inspection techniques for screw gear components is essential. Maintenance logs should record screw gear condition, oil changes, and brake tests.
In conclusion, top overrun in screw gear driven elevators is a complex issue rooted in mechanical and electrical failures. The screw gear is central to both the risk and the solution. Through rigorous analysis of motion dynamics, adherence to standards, and proactive maintenance focused on the brake, screw gear integrity, and protection systems, these incidents can be prevented. I hope this detailed exploration provides valuable insights for engineers and inspectors working with screw gear driven elevators. Remember, the screw gear’s health is a bellwether for overall safety; neglecting it invites catastrophic failure. By integrating these preventive measures, we can enhance the reliability and safety of these essential transportation systems.
To further illustrate, let’s consider some quantitative examples. Suppose a screw gear driven elevator has a rated speed of 1 m/s, car mass of 1000 kg, counterweight mass of 1200 kg (balance coefficient 0.48), and sheave radius of 0.3 m. The net upward force under no load is \( (1200 – 1000) \times 9.81 = 1962 \, N \). If the brake fails, acceleration upward is \( 1962 / 1000 = 1.962 \, m/s^2 \). The car would reach 115% rated speed (1.15 m/s) in approximately \( (1.15 – 1) / 1.962 = 0.076 \, s \), covering a distance of about 0.08 m. This shows how quickly overspeed can occur, emphasizing the need for rapid protection response.
For the screw gear, if the worm has a lead angle \( \lambda \) and friction angle \( \phi \), the self-locking condition is \( \lambda < \phi \). If wear increases \( \phi \) or changes \( \lambda \), self-locking may be lost, allowing backdriving. This can contribute to overrun if the brake fails. Regular inspection of the screw gear’s geometry is thus critical.
In summary, a holistic approach encompassing design, maintenance, and regulation is key to mitigating top overrun risks in screw gear driven elevators. The screw gear, as the heart of the drive system, demands unwavering attention to ensure safe and reliable operation.