Abstract    In practice, clearances in the joints are inevitable due to tolerances, and defects arising from design and manufacturing. In the presence of clearance at a joint, a mechanism gains some additional, uncontrollable degrees of freedom which are the source of error. Moreover, joints undergo wear and backlashes and so cannot be used in precision mechanisms. In this study, the dynamic behaviour of a planar mechanism with revolute joints, in the presence of clearances is investigated. A continuous contact force model, based on the elastic Hertz theory together with a dissipative term, is used to evaluate the contact forces. Moreover, using a new contact model the dynamic characteristics of planar mechanical system with multiple revolute joints in the presence of clearance are analyzed. Numerical results for four-bar linkage with one, two and three revolute clearance joints are presented and compared.

Keywords    Dynamic, Four-bar linkage, Clearance, revolute joint

چکیده    در عمل، بواسطه وجود تلرانس­ها و معایب موجود در فرایند ساخت، وجود لقی در مفاصل امری اجتناب­ناپذیر است. باید توجه نمود که در صورت وجود لقی در مفصل، چند درجه آزادی غیرقابل کنترل که می­تواند منبع ایجاد خطا باشد، به مکانیزم افزوده خواهد شد. همچنین، وجود سایش و پس­زنی در مفصل، دقت مکانیزم را کاهش می­دهد. در این مقاله، رفتار دینامیکی یک مکانیزم صفحه­ای با در نظر گرفتن لقی در مفاصل لولایی آن مورد بررسی قرار می­گیرد. برای مدل­سازی نیروی تماسی، از تئوری جدیدی که ترکیبی از تئوری هرتز و جمله میرایی است، استفاده شده است. به عنوان مثال عددی، مشخصه­های دینامیکی مکانیزم چهارمیله­ای دارای یک، دو و سه مفصل لق مورد اریابی و مقایسه قرار می­گیرد. نتایج نشان می­دهند که با افزایش تعداد مفاصل لق، کارایی دینامیکی مکانیز کاهش و تولید شوک و ارتعاش درآن افزایش می­یابد.

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