Worm gearboxes produce high torque at low cost

Worm gearboxes are low-cost options for producing high torque from relatively small electric motors. Their output shaft is at a 90 degree angle to the motor input shaft and higher reduction ratios are obtained compared with conventional gearbox designs. Tony Young, director of electrical and automation maintenance company, CP Automation, explains the benefits of worm gearboxes.

It would be hard to imagine how violin or guitar strings could be tuned without the use of worm gears, where each thumb screw is effectively a worm that turns a gear, the shaft from which in turn tightens a string. A complete turn of the thumb screw results in a much smaller turn of only a few degrees at the string shaft, and therefore much higher torque. More importantly, it provides precise tuning of the guitar string.

If another gear were to be used on the same plane instead of a worm screw, it would be almost minuscule by comparison. Another advantage, in the context of guitar strings, is that the thumb screws can be spaced out at right angles to the strings themselves, where their close spacing would make it impossible to have gears placed side by side. This simple example serves by way of introduction to the application of worm gearboxes in an industrial context.

Higher reduction ratios up to 300:1 are a key feature of worm gearboxes, whereas planetary gearboxes, for example, will typically have ratios up to 50:1. The 90 degree shaft angle is an advantage in the fact that an engineer can fit a gearbox into a smaller space than that required for a planetary gearbox.

Smaller dimensions mean better portability for mobile applications, for example a Hiab crane on the back of a lorry. The worm gear slewing mechanism is very reliable and thus keeps maintenance cost at a minimum. Worm gears are also suitable for winches and hoists on heavy haul tractors, digger derricks and aerial work trucks to provide durability, reliability and safe operation.

CP Automation is now distributing Chiaravalli's failsafe spring-applied motor gearboxes. The devices are suitable for heavy duty applications such as rolling mills, lifts, conveyors, printing presses and paper converting, as well as portable lifting equipment when coupled to a battery-operated DC motor. Worm gear boxes are applicable to various industries and they are used in anything from dairies to car wash equipment.

Safety advantages

The high torque has safety advantages as well, in applications such as lifts and hoists for instance. The worm drives the gear but it is almost impossible for the gear to drive the worm, like where the load is excessive. In other words, there is no backward drive, which could be dangerous in conventional gearing arrangements.

Available from stock within a week, with optional 24-hour despatch service, the Chiaravalli CH and CHM worm gearboxes could represent a rapid option for industries where downtime can prove expensive. Engineers can fit both AC and DC motors to a CH or CHM gearbox whereas most gearboxes are only suited to AC motors.

This allows OEMs and machine builders to expand into new markets where portable equipment may be required. By fitting a 12V, 24V, 48V or 90V DC motor, they are suitable for portable battery-operated lifting devices up to 5 tonnes. Because worm gearing has such a high ratio, it will not back-drive. However, because the motor's brake is applied by a spring, it is failsafe. Once the equipment you are lifting is in position, the brake safely holds the motor.

The CH gearbox has a motor mounting flange that can separate from the housing which incorporates the oil seal. This avoids the risk of damaging the oil seal in case of replacement of the input flange and allows the O-Ring to be eliminated.

The worm screw has a ZI involute profile providing a gear ratio up to 1:300 as well as better performance at reduced temperature. The gearboxes and motors are painted with RAL 9022 aluminium colour epoxy powder to protect the parts from oxidation and the micro-blowholes that can come about from die casting pressure.

The CHM gearbox has a square shape, making it considerably more versatile for mounting. The machining of the components, carried out using numeric control machines, guarantees maximum precision for restricted tolerances, resulting in a product that will remain reliable over time. The groups are constructed with aluminium casings from sizes 025 to 090, while the sizes 110 and 130 are made from cast iron.

The gears are supplied with at least one filling plug that is also used during testing to check for possible leaks. A connection flange allows two gearboxes to be combined in order to obtain high gearing ratios.

For more information about worm gearboxes please go to www.cpaltd.net.

CP Automation Ltd

Unit 8, Ashley Industrial Estate
Exmoor Avenue
DN15 8NJ

+44 (0)1724 851515



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