Design of fully rotatable, roller-crank-driven, cam mechanisms for arbitrary motion specifications

The design of a non-traditional cam and roller-follower mechanism is descri bed here. In this mechanism, the roller-crank rather than the cam is used a s the continuous input member, while both complete a full rotation in each revolution and remain in contact throughout. It is noted that in order to h ave the cam fully rotate for every full rotation of the roller-crank, the c am cannot be a closed profile, rather the roller traverses the open cam pro file twice in each cycle. Using kinematic analysis, the angular velocity of the cam when the roller traverses the cam profile in one direction, is rel ated to the angular velocity of the cam when the roller retraces its path o n the cam in the other direction. Thus, one can specify any arbitrary funct ion relating the motion of the cam to the motion of the roller-crank for on ly 180 degrees of rotation in the angular velocity space. The motion of the cam in the remaining portion is then automatically determined. In specifyi ng the arbitrary motion, many desirable characteristics such as multiple dw ells, low acceleration and jerk, etc., can be obtained. Useful design equat ions are derived for this purpose. Using the kinematic inversion technique, the cam profile is readily obtained once the motion is specified in the an gular velocity space. The only limitation to the arbitrary motion specifica tion is making sure that the transmission angle never gets too low, so that the force will be transmitted efficiently from roller to cam. This is addr essed by incorporating a transmission index into the motion specification i n the synthesis process. Consequently, in this method we can specify any ar bitrary motion within a permissible rone, such that the transmission index is higher than the specified minimum value. Single-dwell, double-dwell and a long hesitation motion are used as examples to demonstrate the effectiven ess of the design method. Force closure using an optimally located spring a nd quasi-kinetostatic analysis are also discussed. (C) 2001 Elsevier Scienc e Ltd. All rights reserved.