if it says something like "the cartridge is meant to 円弧を描くように動く, rather than rotate".

It depends on the context.

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Note added at 20時間 (2012-05-02 23:28:03 GMT)
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=arcuate motion
a steep, inclined (i.e., generally vertical), arcuate motion
http://www.us-pat.com/pat/6499826.html

or: in the form of a segment of a circle

文脈次第ですね。

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Note added at 2 days1 hr (2012-05-04 04:35:52 GMT)
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3. The ink cartridge as claimed in claim 2, wherein the movement restricting part is shaped like and disposed so as to form an arc of a circle centered at a joint of the first plate and the second plate of the deformation restricting part.
http://www.faqs.org/patents/app/20110063382#b

9. The ink cartridge as claimed in claim 1 or 3, wherein the partition wall has a circular arc shape in cross section.
http://www.google.com/patents/US6802602

The print mechanism is a row of dot hammers that is almost as wide as the page. The hammers are mounted on a shuttle that oscillates back and forth approximately two inches in a track. The hammers are magnetically released at the appropriate time and bang into a ribbon and onto the paper.
http://www.answers.com/topic/line-matrix-printer

Several days old, so not sure if you still need an answer, but since no answer so far can be considered a part/object...

Parts of constructs that *move* in an arc are often called "booms" (as in the boom on a crane, the boom on a sailboat, etc). Maybe even "arc boom."

Hope this helps.