Electricity + Control July 2017

Electricity

Control

JULY 2017

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

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.

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.

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 4: Worm gearbox.

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.

Take Note!