محرك جنيف

محرك جنيف أو آلية جنيف هو آلية تروس تحوّل الحركة الدورانية المستمرة إلى حركة دورانية متقطعة.

عادةً ما تكون العجلة الدافعة الدوارة مزودة بـدبوس يدخل في فتحة موجودة في العجلة الأخرى (العجلة المُدارة)، مما يُقدمها خطوة واحدة في كل مرة. تحتوي العجلة الدافعة أيضًا على قرص دائري مرتفع (قرص حظر) "يقفل" العجلة المُدارة الدوارة في مكانها بين الخطوات.

تاريخ

The name, Geneva drive, is derived from the device's earliest application in mechanical watches, which were popularized in Geneva.^[1] The mechanism is frequently used in mechanical watches, since it can be made on a small scale and is able to withstand substantial mechanical stress.

The Geneva drive is also called a "Maltese cross mechanism" due to the visual resemblance when the rotating wheel has four spokes.

In the most common arrangement of the Geneva drive, the client wheel has four slots and thus advances the drive by one step at a time (each step being 90 degrees) for each full rotation of the master wheel. If the steered wheel has n slots, it advances by 360°/n per full rotation of the propeller wheel. The minimum number of slots in a practical Geneva drive is 3; it is rare to find a mechanism with more than 18 slots.^[2]

Because the mechanism needs to be well lubricated, it is often enclosed in an oil capsule.^{[citation needed]}

الاستخدامات والتطبيقات

هذه الآلية هي التي جعلت الأفلام ممكنة.

أحد تطبيقات ميكانيزم جنيف (Geneva drive) هو في عارضات الأفلام وكاميرات السينما، حيث يتم سحب الفيلم عبر بوابة التعريض بحركات بدء وتوقف دورية. يتقدم الفيلم إطارًا إطارًا، حيث يظل كل إطار ثابتًا أمام العدسة لجزء من دورة الإطار (عادةً بمعدل 24 دورة في الثانية)، ويتسارع ويتقدم ويتباطأ بسرعة خلال بقية الدورة. تُنفذ هذه الحركة المتقطعة بواسطة ميكانيزم جنيف، والذي بدوره يُحرك كلابًا يتشابك مع ثقوب التروس في الفيلم. يوفر ميكانيزم جنيف أيضًا موضع توقف قابل للتكرار بدقة، وهو أمر بالغ الأهمية لتقليل الاهتزاز في الصور المتتالية. (قد تستخدم عارضات الأفلام الحديثة أيضًا آلية فهرسة يتم التحكم فيها إلكترونيًا أو محركًا متدرجًا، مما يسمح بالتقديم السريع للفيلم.) يعود تاريخ الاستخدامات الأولى لميكانيزم جنيف في عارضات الأفلام إلى عام 1896، مع عارضات أوسكار ميستر وماكس غليوي و"تيتروغراف" لـروبرت ويليام بول. أما العارضات السابقة، بما في ذلك عارضة توماس أرمات التي سوقها إديسون تحت اسم "فيتاسكوب"، فقد استخدمت "آلية المطرقة" التي اخترعها جورج ديميني عام 1893 لتحقيق نقل الفيلم المتقطع.^[3]^:138

Geneva stop with five spokes, allowing four rotations of the driving wheel

Geneva wheels having the form of the driven wheel were also used in mechanical watches, but not in a drive, rather to limit the tension of the spring, such that it would operate only in the range where its elastic force is nearly linear. If one of the slots of the driven wheel is occluded, the number of rotations the drive wheel can make is limited. In watches, the "drive" wheel is the one that winds up the spring, and the Geneva wheel with four or five spokes and one closed slot prevents overwinding (and also complete unwinding) of the spring. This so-called Geneva stop or "Geneva stop work" was the invention of 17th or 18th century watchmakers.

Other applications of the Geneva drive include the pen change mechanism in plotters, automated sampling devices, banknote counting machines, and many forms of indexable equipment used in manufacturing (such as the tool changers in CNC machines; the turrets of turret lathes, screw machines, and turret drills; some kinds of indexing heads and rotary tables; and so on). The Iron Ring Clock uses a Geneva mechanism to provide intermittent motion to one of its rings.

A Geneva drive was used to change filters in the Dawn mission framing camera used to image the asteroid 4 Vesta in 2011. It was selected to ensure that should the mechanism fail at least one filter would be usable.^[4]^[5]

النسخة الداخلية

A variant exists where the drive wheel is inside the driven wheel. While an external Geneva drive advances the driven wheel one step in less than 180° rotation of the drive wheel, so (assuming a constant-speed drive wheel) the stop is always longer than the motion, in an internal wheel the motion always requires more than 180° rotation of the drive wheel, so the motion takes longer than the stop. The axis of the drive wheel can have a bearing only on one side.

Internal Geneva drive
Animation showing an internal Geneva drive in operation

The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses.^{[بحاجة لمصدر]} A shorter stop time can also be achieved by having more than one drive pin on an external drive wheel.

النسخة الكروية

Another variant is the spherical Geneva drive.^[3]^:128

Spherical Geneva drive

كينماتيكا

The figure shows the motion curves for an external four-slot Geneva drive, in arbitrary units. A discontinuity appears in the acceleration when the drive pin enters and leaves the slot, occurring at the instant the rigid bearing surfaces make contact or separate. This generates an "infinite" peak of jerk (Dirac peak), and therefore vibrations.^[3]^:127

انظر أيضا

Dwell cam

المصادر

^ Willis, William A. (April 1945). "Analysis of Geneva Mechanisms". Journal of the Society of Motion Picture Engineers. 44 (4): 275–284. doi:10.5594/J09782. ISSN 0097-5834. Archived from the original on March 25, 2018.
^ "Geneva mechanism | Cam-operated, Indexing, Ratcheting | Britannica". www.britannica.com (in الإنجليزية). 20 July 1998. Retrieved 2023-12-28.
^ ^أ ^ب ^ت Bickford, John H. (1972). "Geneva Mechanisms". Mechanisms for intermittent motion (PDF) (in الإنجليزية). New York: Industrial Press Inc. ISBN 0-8311-1091-0.
^ "Camera" (Mov), Multimedia, US: Jet propulsion laboratory, Nasa, http://dawn.jpl.nasa.gov/multimedia/movies/camera6.mov .
^ Christopher Russell; Carol Raymond (2012). The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres. Springer. ISBN 9781461449034.

وصلات خارجية

Geneva Mechanism: its history, function, and weaknesses, The University of Nebraska, http://emweb.unl.edu/Mechanics-Pages/em373honors-S2001/em373/geneva/geneva.htm .
External Geneva drive, Brock eng, http://www.brockeng.com/mechanism/Geneva.htm .
U.S. Patent 6٬183٬087 – Quickermittent. Modified starwheel for fast pulldown.
"LEGO Geneva Mechanism", Brick engineer, Oct 7, 2007, http://www.brickengineer.com/pages/2007/10/07/geneva-mechanism/

[1] Willis, William A. (April 1945). "Analysis of Geneva Mechanisms". Journal of the Society of Motion Picture Engineers. 44 (4): 275–284. doi:10.5594/J09782. ISSN 0097-5834. Archived from the original on March 25, 2018.

[2] "Geneva mechanism | Cam-operated, Indexing, Ratcheting | Britannica". www.britannica.com (in الإنجليزية). 20 July 1998. Retrieved 2023-12-28.

[:0-3] أ ^ب ^ت Bickford, John H. (1972). "Geneva Mechanisms". Mechanisms for intermittent motion (PDF) (in الإنجليزية). New York: Industrial Press Inc. ISBN 0-8311-1091-0.

[4] "Camera" (Mov), Multimedia, US: Jet propulsion laboratory, Nasa, http://dawn.jpl.nasa.gov/multimedia/movies/camera6.mov .

[5] Christopher Russell; Carol Raymond (2012). The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres. Springer. ISBN 9781461449034.

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