Machine Design Essay

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2.72 Elements of Mechanical Design
Spring 2009

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2.72
Elements of
Mechanical Design
‘Lecture’ 11:
Screw drives

Schedule and reading assignment
Quiz
‰

Bolted joint qualifying Thursday March 19th

Topics
‰

Screw drive group activity - 90% hands on

Reading assignment
• Read:
13.1 – 13.7
17.1 & 17.3

• Skim:
Rest of Ch. 17

© Martin Culpepper, All rights reserved

2

Screws
Convert rotary motion into linear motion:

Types of “lead screws”:
‰
‰
‰
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Sliding contact thread lead screws
Ball screws
Hydrostatic lead screws
Others

© Martin Culpepper, All rights reserved

3

Typical screw types

Image by jgelens on Flickr.
Images removed due to copyright restrictions. Please see
http://news.thomasnet.com/images/large/455/455175.jpg
http://www.danahermotion.com/website/com/eng/img/product/LeadScrew2Nut.jpg

© Martin Culpepper, All rights reserved

4

Errors
Some error sources
‰

Misalignment
• Bearings, Carriage

‰

Geometry
• Straightness
• Varying pitch diameter (periodic error and backlash)
• Errors in thread-contacting elements

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Loads
• Elastic
• Vibration

Active error management
‰
‰

Periodic errors can be mapped
Linear position sensors

© Martin Culpepper, All rights reserved

5

Preload – why…
Nut-screw:
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‰
‰
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Backlash
Use two nuts that are preloaded against each other
Use oversize rolling elements
Use a split-circumferentially clamped nut

Screw-machine:
‰
‰
‰
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Backlash

Buckling

Straightness

Vibration

© Martin Culpepper, All rights reserved

6

Cross feed design exercise
Kinematic (trade off)

Loads and power (limits)

Constraints (bearing, flexure?)

Preload (Nut-screw and screw-machine)

Stress/fatigue

Errors (Causes, systematic, random)

© Martin Culpepper, All rights...