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Reverse Self-locking of Gearbox and Method to Prevent Reverse Rotation

Reverse self-locking of worm gear

The reverse self-locking of the worm gear is essentially a phenomenon in which the component of the force exerted by the worm gear on the worm in the helical direction of the worm is insufficient to overcome the friction, making the worm unable to rotate.

As shown in the figure above, when the lead angle is smaller than the friction angle, Fα is always smaller than the friction force Ff. At this time, the worm is in a self-locking state and cannot rotate.
When the friction angle remains unchanged, the smaller the lead angle, the easier it is to produce self-locking. In general, the larger the reduction ratio of the worm gearbox, the smaller the lead angle of the worm gear. Therefore, a worm gearbox with a large reduction ratio will usually self-lock.
Our company's worm gearboxes with a reduction ratio of 40:1 or above have self-locking capabilities. Please refer to this article. Self-locking of Worm Gear

In addition, because the planetary gearbox uses a planetary gear train for transmission and there is no friction to hinder the rotation, the planetary gearbox generally does not have the ability to reverse self-locking.

Other ways to prevent reversal

Since the efficiency of worm gearboxes with self-locking capabilities is generally low, if the load causing reverse rotation of the gearbox is not very large, other methods can be considered to prevent reverse rotation.

a. Using the positioning torque of the stepper motor

When the stepper motor is not energized, there will be a small positioning torque due to the magnetic field of the permanent magnet on the rotor. This torque is amplified by the gearbox, and combined with the mechanical friction resistance of the equipment, can prevent the worm gearbox from rotating reversely to a certain extent. However, because the positioning torque is very small, even if it is amplified by the gearbox, the reverse load it can withstand is also very small.

b. Utilize the holding torque of the stepper motor

When the stepper motor is energized, the coils on the stator generate a magnetic field, which attracts and locks the permanent magnets on the rotor to prevent the rotor from rotating. This torque that locks the rotor is called the holding torque, which is generally the torque marked on the stepper motor's data sheet.
The holding torque passes through the gearbox, which can prevent the gearbox from rotating reversely with a larger torque and can withstand larger loads. However, this torque is limited. Once the load exceeds the upper limit that the motor can withstand, the gearbox will still rotate in the opposite direction.

To sum up, in addition to using the characteristics of the gearbox itself to prevent reversal, the characteristics of the motor can also be used to prevent the gearbox from reversing. However, the ability to use the characteristics of the motor to prevent reversal is limited. Sorted according to the ability to prevent reversal, self-locking to prevent reversal > Maintaining torque to prevent reversal Positioning torque to prevent reversal.

Updated on: 08/02/2024

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