Electricity + Control July 2017

DRIVES, MOTORS + SWITCHGEAR

Rotary speed reducers Gearboxes perform the same functions of speed and inertia matching. Common gearbox types, in- clude worm and wheel, spur gears and (for robots) harmonic. Belt (tooth type) and chains have their place as well. Figure 3 shows a spur gearbox. The gears could be straight cut or helical and typically run in an oil bath.

Take Note!

A normal threaded bar, for instance, has one start. Thus reductions up to 100:1 in a single stage are possible. All of the above suffer from one major disad- vantage, i.e. lost motion, also known as backlash. This often results in a serious disadvantage as the output shaft position is indeterminate. There will be more on this subject. The following two gear- box types address the backlash problem very well. Figure 5 shows a planetary gearbox. This is the generic term but this system is also known as Epi- cyclic Gearing. Construction consists of four parts: • The inner or driven gear, also known as the sun gear. It is connected to the input shaft • A fixed outer internal gear (teeth on the inside). Also known as a ring gear • In between the ring gear and the sun gear are the three planetary gears which mesh with both ring and sun gears • The three planetary gears are carried by the carri- er or spider.The planetary gears are free to rotate and the carrier is connected to the output shaft

Mechatronics is a mix- ture of different disci- plines. System parameters need to be defined be- fore components can be chosen. Component choice is an iterative process.

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Figure 3: Spur gearbox.

Figure 4 shows a worm gear box. The reduction ratio is the number of worm teeth divided by the number of starts on the worm gear. The term of starts refers to the number of threads on the screw.

Figure 5: Planetary gearbox.

Multiple gearsets can be driven in tandem and ratio per stage is generally in the range of 2:1 to 5:1. Backlash is often as low as 10 arc minutes and output torque for a given volume is high due to the multiple meshed gears. The output shaft is co-axial with the input shaft i.e. in the same straight line.

Figure 4: Worm gearbox.

Electricity + Control

JULY 2017

11