凸轮机构的主要作用是（ ）。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='24894' and id < 2124503 and id<>'2124503' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='24894' and id > 2124503 and id<>'2124503' order by id asc limit 5