Engineering 1040: Mechanisms & Electric Circuits Fall 2011 Power Transmission Systems: Power Screws

Uses of Power Screws Obtain high mechanical advantage in order to move large loads with a minimum effort. e.g screw jack. Generate large forces e.g tensile testing machine, compactor press. Obtain precise axial movements e.g. camera calibration rigs.

Screw Jacks

Toolmakers Clamp

X-Y Precision Table

Advantages of power screws Compact design and takes less space Large load carrying capability Simple to design and easy to manufacture Can obtain a large mechanical advantage Precise and accurate linear motion Easy maintenance Self-locking feature

Thread Terminology Pitch is the distance from the crest of one thread to the next. When the screw rotates by one revolution the screw advances by its pitch.

Types of Thread

Ball Screws (1)

Ball Screws (2)

Advantages of a ball screw Have very high efficiency (Over 90%) Could be used in applications which require precise and repeatable movement Could be easily preloaded to eliminate backlash Smooth movement over full travel range Can use a smaller size for same load Has a longer life for thread

Disadvantages of a ball screw Requires higher levels of lubrication Possibility of the screw to contaminate Additional brakes have to be used if locking is required

Power Screw Selection