Self Locking Gearbox

What is Self Locking Gearbox?

Basically, a self locking gearbox consists of a number of declutch shift shafts that penetrate into the shell of the gear box and interlock the shifting fork shafts. This mechanism can be used for a variety of industrial applications.

A common example of a self locking gearbox is a windscreen wiper motor. This gearbox is self-locking because when the motor is switched off, the motor stops in place. This means the gearbox is under a vibration-free condition.

The self-locking feature of the gearbox will perform as expected in a vibration-free application. However, in a vibration-prone application, the self-locking feature may be eliminated. This can happen when shocks or external vibrations occur. The resulting back-driving can be undesirable.

The self-locking mechanism of a worm gearbox is helpful in lifting and holding loads. It prevents the worm gear from rotating backward when the input-side load is released. This is particularly useful for crane applications. In these situations, it can save a significant amount of money by avoiding the need for a braking system.

A worm gearbox can be either static or dynamically self-locking. This is dependent on several factors. These factors include helix angle, static tooth friction coefficient, and resistance to friction at the worm interface.

A functional self-locking drive unit has a low breaking torque. It is often achieved with a four-stage planetary gearhead. In addition, it can be complemented with a holding brake. Normally, a non-self-locking worm drive has a driving efficiency of 50% or higher.

What is the Self Locking Function？

Self-locking means that the screw nut and the screw cannot move without an external force. It is related to pitch and friction coefficient. Self-locking allows the user to eliminate expensive brakes in many applications. Single head trapezoidal screw drives are self-locking.

A self-locking worm gear is a type of worm gear that does not allow the interchangeability of the input and output gears. As you know, in spur gear trains you can interchange the driving gear and the driven gear but the same is not possible for the self-locking type of worm gears. For this type of gear, the worm always acts as a driving gear and the spur gear as a driven gear- vice versa is not possible. If you try to run it otherwise, it will lock automatically.

As a matter of fact, most of the worm gear trains used in industry are of the self-locking type. But you can of course design a non-self- locking type of worm gear. Approximately, if the tangent of the helix angle of the worm gear is less than the coefficient of friction between the worm and the gear, then the worm gear train should be a self-locking type.

Advantages of Self-Locking Worm Gears

You can achieve a large reduction ratio (as large as 200:1) from a self-locking worm gear without increasing the size of the gear box. How? A 360 degree rotation of a single start worm causes the meshing spur gear to rotate by one tooth. So, if a 10 teeth spur gear is meshed with a single start worm then you will get a reduction ratio of 10:1 straight away. Whereas, for achieving the same reduction ratio by using a spur gear train, you have to use another 100 teeth spur gear with that 10 teeth spur gear. So imagine the comparative size reduction.

Worm Gear Back-driving

Unlike a standard gear, a self locking gearbox with worm gear back-driving can transmit torque in only one direction. The back driving of the worm gear is achieved when the worm wheel and the worm screw lose contact. This condition is achieved when the friction angle is greater than the lead angle. However, it is difficult to predict the exact degree of back-driving.

To calculate the exciting force, the displacement of vibration is used. This is calculated by using the motion equation of the whole worm gear mechanism. The forces applied are then substituted into conditional expressions. The exciting force is then obtained, and the torque is measured.

When the worm gear starts to back-drive, the exciting force is increased. This is achieved through feedback control. This procedure is repeated at 10, 30, and 50 Hz. Some of the experimental values are matched with theoretical values, but the actual value may be lower.