Mechanical drives system

Mechanical drives system

When the various driving methods like hydraulic, pneumatic, electrical servo motors and stepping motors are used in robots, it is necessary to get the motion in linear or rotary fashion.

When motors are used, rotary motion is converted to linear motion through rack and pinion gearing, lead screws, worm gearing or bail screws.

Rack and Pinion Movement:

The pinion is in mesh with rack (gear of infinite radius). If the rack is fixed, the pinion will rotate. The rotary motion of the pinion will be converted to linear motion of the carriage.

Ball Screws:

Sometimes lead screws rotate to drive the nut along a track. But simple lead screws cause friction and wear, causing positional inaccuracy.

Therefore ball bearing screws are used in robots as they have low friction. The balls roll between the nut and the screw. A cage is provided for recirculation of the balls. The rolling friction of the ball enhances transmission efficiency to about 90%.

Gear Trains:

Gear trains use spur, helical and worm gearing. A reduction of speed, change of torque and angular velocity are possible. Positional errors are caused due to backlash in the gears.

Harmonic Drive:

For speed reduction, standard gear transmission gives sliding friction and backlash.

Moreover, it takes more space.

Harmonic drive due to its natural preloading eliminates backlash and greatly reduces tooth wear.

Harmonic drives are suitable for robot drives due to their smooth and efficient action. The circular spline is a rigid ring with gear teeth machined on the inside diameter.

The flex spline is a flexible ring with the teeth cut on its outside diameter.

The flex spline has fewer teeth (say 2 teeth less) than the circular spline. The wave generator is elliptical and is given input motion.

Wave generator and flex spline is placed into the circular spline such that the outer tooth of flex spline is in mesh with the internal teeth of circular spline.

If the circular spline has 100 teeth and the flex spline has 98 teeth, and if the wave generator makes one complete revolution, the flex spline will engage 98 teeth of the circular spline.

Since circular spline has 100 teeth and only 98 teeth have been in engagement for one complete rotation, the circular saline's position has been shifted by 2 teeth.

Thus after 50 revolutions of the wave generator, the circular spline will have made one full rotation. The ratio of harmonic drive is 2: 100 or 1: 50.

The gear ratio is influenced by the number of teeth cut into the circular spline and the flex spline. The harmonic drive has high torque capacity.