In the production process of screw pump wells, the rod string experiences torque, causing elastic deformation of the sucker rod and storing a certain amount of elastic potential energy. When the pump stops, the release of elastic potential energy leads to rod reversal. Simultaneously, under the influence of oil-casing pressure difference, the liquid column drives the screw pump rotor to reverse, causing continuous rod reversal. If uncontrolled, high-speed reversal not only results in rod string disconnection and bending of the polished rod but also damages components of the surface drive unit, posing serious safety risks to operators. Traditional horizontal screw pump drive units commonly use a ratchet and pawl brake system. In this design, the pawl is mounted on a cone sleeve plate and can rotate freely. The pawls are distributed around the ratchet and engage externally with it, using springs to ensure normal meshing. However, this ratchet-pawl brake system has a low safety factor, cannot automatically release the reverse torque, requires manual release with high labor intensity, and carries significant potential hazards.
To address these issues, we have designed a novel brake system that combines the self-locking property of worm gear with an overrunning clutch. This system is installed on the main shaft of the screw pump drive unit. During normal operation, the brake system remains inactive. When the system stops and the rod begins to reverse, the brake system engages, providing reverse braking. Then, a release motor is activated to slowly rotate the worm, which drives the worm wheel to rotate gradually. The main shaft and the rod string are constrained by the worm wheel to release the elastic potential energy and liquid level potential energy slowly until they are fully dissipated.
The internal structure of the worm gear brake system is composed of a worm gear pair, an overrunning clutch, and a gearbox housing. The outer ring of the clutch is integrated with the worm wheel, while the gearbox housing is fixed to the reduction gearbox of the screw pump drive unit using bolts. The inner ring of the clutch is connected to the drive unit main shaft via a parallel key. The worm gear is a right-hand single-start worm with a lead angle of γ, and the worm wheel has a large number of teeth to achieve a high reduction ratio and self-locking capability. The materials selected for the worm gear pair are hardened steel for the worm and bronze for the worm wheel to reduce friction and wear.
During normal operation of the screw pump system, the main shaft rotates forward (clockwise when viewed from the top). The inner ring of the overrunning clutch rotates with the shaft, while the outer ring (integrated with the worm wheel) remains stationary because the clutch is in an overrunning state. In this condition, the worm gear pair does not work, and no braking torque is applied to the system. When the system stops, the main shaft begins to reverse (counterclockwise) due to the release of stored energy. The inner ring of the clutch rotates in the opposite direction relative to the outer ring, causing the clutch to engage. The inner ring then drives the outer ring, which is the worm wheel. Since the worm wheel is meshed with the worm, and the worm gear pair is self-locking, the system locks instantly, preventing further reversal. This locking action provides a safe stop, similar to a brake, but without the need for manual intervention.
To release the stored reverse potential energy, an auxiliary electric motor is used to drive the worm. The auxiliary motor is controlled by a circuit that automatically activates after the main motor stops. The power supply for the auxiliary motor comes from an external source, and the input is connected to the normally closed contact of the main motor electromagnetic relay. When the main motor starts, the relay opens, cutting off power to the auxiliary motor. When the main motor stops, the normally closed contact closes, providing power. A time relay delays activation by 30 seconds to ensure the brake has fully engaged. Then, a current detection controller starts the auxiliary motor. As the worm rotates, it drives the worm wheel (which is now the outer ring of the overrunning clutch) to rotate slowly in the reverse direction. The inner ring (connected to the main shaft) tends to rotate due to the remaining torque, but it cannot exceed the speed of the outer ring because the clutch remains engaged only when the inner ring attempts to overrun in the opposite direction. In fact, when the worm drives the outer ring in the reverse direction, the clutch disengages because the relative motion between inner and outer rings is such that the inner ring is moving slower or in the same direction. Thus, the main shaft is not forced to rotate; instead, it is allowed to follow the outer ring as the stored energy is gradually released. The auxiliary motor continues to run until the reverse torque is fully dissipated. At that point, the motor operates under no-load conditions, and the working current drops below a preset threshold. The current detection controller then opens the normally closed switch, stopping the auxiliary motor and extinguishing an indicator light. Maintenance personnel must verify that the indicator light is off and manually check the handwheel for any residual torque before performing repair work. This automatic release process significantly improves safety and reduces labor intensity compared to manual release.
The self-locking property of the worm gear pair is the key to the braking function. When the worm wheel acts as the driving member and rotates in the direction that would cause the worm to rotate, self-locking occurs if the lead angle is sufficiently small relative to the equivalent friction angle. Considering a right-hand worm, the normal force Fn acting at the pitch point P can be decomposed into components. As shown in the force analysis, the axial force Fa1 on the worm is directed leftward, and the radial force Fr1 is directed downward. Since the worm is constrained in axial and radial directions, it can only rotate about its axis. The friction force f on the worm can be decomposed into a circumferential component ft and an axial component fa. For the worm to remain stationary under the action of the reverse torque T2 on the worm wheel, the maximum friction circumferential component must satisfy:
$$ f_t \geq F_{r1} $$
where:
$$ f_t = f \cos \gamma $$
γ is the lead angle of the worm. The friction force is given by:
$$ f = \mu F_n = \mu \frac{F_n’}{\cos \alpha_n} $$
Defining the equivalent friction coefficient μ’ = μ / cos α_n, we have:
$$ f = \mu’ F_n’ $$
where μ is the friction coefficient and α_n is the normal pressure angle. The equivalent friction coefficient is related to the equivalent friction angle Ψ by:
$$ \mu’ = \tan \Psi $$
The radial component Fr1 is:
$$ F_{r1} = F_n’ \sin \gamma $$
Substituting into the equilibrium condition gives:
$$ \mu’ F_n’ \cos \gamma \geq F_n’ \sin \gamma \quad \Rightarrow \quad \tan \Psi \geq \tan \gamma $$
Therefore, the condition for self-locking is:
$$ \Psi \geq \gamma $$
This means the worm lead angle must be less than or equal to the equivalent friction angle. In practical designs, we choose a lead angle of 3° to 5° and ensure the friction coefficient is high enough (e.g., μ ≈ 0.1 to 0.15 for lubricated steel-bronze pairs) to achieve reliable self-locking. The equivalent friction angle Ψ is typically 5° to 7°, so self-locking is easily achieved.
| Lead Angle γ (°) | Friction Coefficient μ | Equivalent Friction Angle Ψ (°) | Self-Locking? (Ψ ≥ γ) |
|---|---|---|---|
| 3 | 0.10 | 5.71 | Yes |
| 4 | 0.10 | 5.71 | Yes |
| 5 | 0.10 | 5.71 | Yes |
| 6 | 0.10 | 5.71 | No |
| 3 | 0.08 | 4.57 | Yes |
| 4 | 0.08 | 4.57 | Yes |
| 5 | 0.08 | 4.57 | No |
For the release operation, we must overcome the self-locking and drive the worm to rotate, which then drives the worm wheel. When an external torque T1 is applied to the worm, the circumferential force Ft1 on the worm is:
$$ F_{t1} = \frac{2 T_1}{d_1} $$
where d1 is the pitch diameter of the worm. The condition for the worm to rotate (i.e., to overcome the friction) is:
$$ F_{\text{ext}} + F_{t1} > f_t $$
Here F_ext represents any additional external force, but in our design, we only rely on the applied torque T1. The axial force Fa1 on the worm generated by the worm wheel torque T2 is:
$$ F_{a1} = \frac{2 T_2}{d_2} $$
where d2 is the pitch diameter of the worm wheel. The friction components are related to the normal force. Through detailed equilibrium analysis, we derive the required torque T1 as a function of T2:
$$ T_1 > T_2 \cdot \frac{(\tan \Psi – \tan \gamma) \cdot d_1}{(1 + \tan \Psi \tan \gamma) \cdot d_2} $$
Since tan Ψ tan γ is small compared to 1, we can approximate:
$$ T_1 > T_2 \cdot K \quad \text{with} \quad K = \frac{(\tan \Psi – \tan \gamma) \cdot d_1}{d_2} $$
This linear relationship indicates that the release torque T1 is proportional to the load torque T2 on the worm wheel. The factor K depends on the worm geometry and friction properties. For our design, with a worm pitch diameter of 40 mm, worm wheel pitch diameter of 200 mm, lead angle γ = 3°, and equivalent friction angle Ψ = 6°, we calculate K ≈ 0.0105. Thus, for a maximum expected reverse torque T2 of 3500 N·m, the required release torque T1 is only about 36.8 N·m, which is easily supplied by a small auxiliary motor.
We conducted laboratory experiments on a prototype to verify the theoretical relationship. The worm wheel was loaded with different torques using a hydraulic brake, and the torque required on the worm to initiate rotation was measured. The results are summarized in the table below.
| Load Torque T2 (N·m) | 0 | 100 | 200 | 400 | 800 | 1500 | 2000 | 2500 | 3500 |
|---|---|---|---|---|---|---|---|---|---|
| Release Torque T1 (N·m) | 1.8 | 2.9 | 4.1 | 6.2 | 10.7 | 18.5 | 24.1 | 29.6 | 40.9 |
The data clearly show a linear trend. A linear regression of T1 versus T2 yields a slope of approximately 0.0112, which is close to the theoretical K value of 0.0105. The small discrepancy can be attributed to additional friction in bearings and the clutch. This validation confirms that our worm gear design can reliably release the reverse torque with a proportionally small input torque.
The auxiliary motor control circuit is designed to enhance safety. It includes a time delay to ensure the main shaft has fully stopped and the brake is engaged before attempting release. The current detection controller monitors the motor load; when the reverse torque is fully released, the motor current drops, and the controller shuts off the motor automatically. This prevents any unnecessary wear and ensures the system is ready for maintenance. The indicator light provides a visual confirmation that the release is complete. The entire process is fully automatic, eliminating the need for manual intervention and reducing the risk of accidents.
In comparison with the traditional ratchet-pawl brake system, our worm gear brake offers several advantages: (1) automatic engagement without moving parts like pawls and springs, which are prone to fatigue failure; (2) automatic release with controlled slow rotation, avoiding sudden shocks; (3) higher safety factor due to the inherent self-locking of worm gear pairs; (4) reduced maintenance because worm gear pairs are enclosed and lubricated; (5) ability to handle high reverse torques with a small auxiliary motor. The table below summarizes the key differences.
| Feature | Ratchet-Pawl Brake | Worm Gear Brake (Our Design) |
|---|---|---|
| Engagement mechanism | Pawl engages ratchet teeth | Overrunning clutch + worm gear self-locking |
| Release method | Manual release by operator | Automatic release via auxiliary motor |
| Safety | Low – operator exposed to high torque | High – enclosed, automatic control |
| Reliability | Moderate – pawl springs can break | High – no springs, wear-resistant materials |
| Torque capacity | Limited by tooth strength | High – worm gear can handle large loads |
| Noise during operation | Loud clicking | Quiet operation |
In conclusion, the application of a worm gear pair combined with an overrunning clutch provides an innovative and reliable braking solution for screw pump drive systems. The self-locking property of the worm gear ensures that the drive shaft is securely locked when reverse torque occurs, preventing uncontrolled rotation. The automatic release mechanism, controlled by an auxiliary motor and a simple circuit, allows safe and gradual dissipation of stored energy, reducing labor intensity and eliminating safety hazards. The linear relationship between release torque and load torque was verified experimentally, confirming the feasibility of this design. This worm gear brake system significantly improves the safety and operational efficiency of screw pump installations, and it can be readily adapted to other applications requiring controlled braking and energy release.