“Differential screw”的意思、由来-开放百科全书

A differential screw is a mechanism used for making small, precise adjustments to the spacing between two objects (such as in focusing a microscope,^[1] moving the anvils of a micrometer,^[2]^[3] or positioning optics^[4]). A differential screw uses a spindle with two screw threads of differing leads (aka thread pitch), and possibly opposite handedness, on which two nuts move. As the spindle rotates, the space between the nuts changes based on the difference between the threads. These mechanisms allow extremely small adjustments using commonly available screws. A differential screw mechanism using two nuts incurs higher friction and therefore requires more torque to turn than a simple, single lead screw with an equivalent pitch.^[5]^[6]^[7]

Examples

Many differential screw configurations are possible. The micrometer adjuster pictured uses a nut sleeve with different inner and outer thread pitches to connect a screw on the adjusting rod end with threads inside the main barrel; as the thimble rotates the nut sleeve, the rod and barrel move relative to each other based on the differential between the threads.

Another arrangement holds the two "nuts" co-axially in a single fixture and has two separate screws with slightly different pitches (length between threads) entering from opposite ends. The "heads" of the screws are fixed to the two objects whose spacing is to be adjusted. Each rotation of the fixture holding the nuts moves one screw into its nut by a small amount and moves the other screw out of its nut by a slightly larger amount. The total spacing between the screws, and thus the objects, will be slightly changed based on the difference in travel between the two screws.

More arrangements are possible. Two nuts can be fixed to each of two objects to be adjusted and the two screw heads attached to each other in the middle. The combined screws would be turned to adjust the spacing in that case.^[8]

Calculating motion and effective thread pitch

For single start threads, each turn changes the distance between the nuts by the effective pitch, {{math|P{{ssub|eff}}}}. For a bolt with a given thread per inch, {{math|TPI{{ssub|1}} }} on one end and a second thread per inch, {{math|TPI{{ssub|2}} }} on the other, the change in distance (or {{math|P{{ssub|eff}}}}), and the effective thread per inch {{math|TPI{{ssub|eff}} }}, is calculated by:

For example, a bolt with coarse threads (16 tpi, {{convert|0.0625|in|mm|abbr=on|disp=sqbr}} per turn) on one end and fine threads (24 tpi, {{convert|0.0416|in|mm|abbr=on|disp=sqbr}} per turn) on the other changes the distance between the nuts by about {{convert|0.02|in|mm|abbr=out}} per revolution and is equivalent to a 48 tpi (0.53 mm/thread) thread:

References

1. ^"A ONE-DOLLAR COMPOUND MICROSCOPE"
2. ^{{US patent reference | number = 343478 | y = 1880 | m = 02 | d = 08 | inventor = McArthur, Duncan | title = Micrometer Calipers}}
3. ^{{cite web |url=http://www.mitutoyo.com/TerminalMerchandisingGroup.aspx?group=1560 |title=Micrometer Heads Series 110-Differential Screw Translator (extra-Fine Feeding) Type |publisher=Mitutoyo, U.S.A. |work=Product Catalog |accessdate=December 11, 2012 |deadurl=yes |archiveurl=https://web.archive.org/web/20111109164636/http://www.mitutoyo.com/TerminalMerchandisingGroup.aspx?group=1560 |archivedate=November 9, 2011 |df= }}
4. ^{{cite web|url=http://www.newport.com/servicesupport/Tutorials/default.aspx?id=91 |title=Tutorials: Opto-Mechanical Components, Translation Stages, Drive Options |accessdate=2012-12-14 |deadurl=yes |archiveurl=https://web.archive.org/web/20100115231910/http://newport.com/servicesupport/Tutorials/default.aspx?id=91 |archivedate=2010-01-15 |df= }}
5. ^{{cite web|title=Screw Threads - The Theory Of Machines - Engineering Reference with Worked Examples|url=http://www.codecogs.com/reference/engineering/the_theory__of__machines/screw_threads.php#sec8|accessdate=2012-12-024}}
6. ^{{cite journal|url=http://www.homemetalshopclub.org/news/10/newsletter1003.pdf#page=5|title=That Ol’ Differential Screw Clamp|date=March 2010|journal=Home Metal Shop Club Newsletter|volume=5|number=3|pages=5–6|author=Kostelnicek, Dick|accessdate=2012-12-24}}
7. ^{{cite journal | url=https://books.google.com/books?id=ZW8fAQAAMAAJ&dq=editions%3A30SAkOZ_SpkC&pg=PA32#v=onepage&q=clamp&f=false | title=A Toolmaker's Novel Clamp | author=Kuhrt, O.C. | journal=American Machinist | year=1916 | volume=44 | pages=32}}
8. ^¹{{cite book | url=http://ebooks.library.cornell.edu/cgi/t/text/text-idx?c=kmoddl;cc=kmoddl;view=toc;subview=short;idno=kmod019 | title=Analytical essay on the construction of machines | publisher=R. Ackermann | author=M.M. Lanz & Betancourt, translated from the original French | year=1817 | location=London | pages=14–15, 181 Plate 1 fig D3}}
9. ^{{cite book |last=Yoder |first=Paul R., Jr. |date=2005 |title=Opto-Mechanical System Design, Third Edition |url= |location= |publisher=CRC Press |page=247 |isbn=1420027239 |accessdate= }}