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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