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