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Pergamon Mechanics Research Commumcations, Vol. 27, No. 5, pp. 529-538, 2000 Copyright 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0093-6413/00IS-see front matter Plh S0093-6413(00)00126-9 DESIGN OF A SPRING-ACTUATED HIGH-SPEED CAM MECHANISM WITH NON-CONSTANT ANGULAR VELOCITY K. Y. Ahn, J. H. Kim and S. H. Kim Department of Mechanical Engineering, KAIST, 373- I, Kusong-dong, Yusong-gu, Taejon 305-701, Korea (Received 11 January 2000; accepted for print 24 July 2000) Introduction A spring-actuated cam mechanism is a simple and reliable device for movement of heavy loads at a high acceleration and for the control of dynamic characteristics of followers. Such a mechanism is used to a vacuum circuit breaker (VCB) which completes switching action within several tens milliseconds 1. In this mechanism, the angular velocity of a cam is not constant because pre-loaded springs rotate the cam. For this reason, conventional design techniques assuming that the cam operates at a constant velocity are not applicable. The design of a spring-actuated or a variable speed cam has seldom been studied in the literature. Rothbart designed a variable speed cam actuated by a Whitworth quick-return mechanism using a graphical cam profile synthesis 2. Barkan proposed a cam design method using the energy equations of a simplified spring-actuated cam system for only simple longhand calculation 3. The latter author simply considered energy loss as empirical loss coefficients in the energy equations. This paper presents the design method for a cam with non-constant angular velocity based on the dynamic model of a complete spring-actuated cam system. In the derived dynamic model, the external forces and friction are incorporated. The follower motion, which is dependent on the requirements during the closing period of a VCB, is designed using a polynomial function. The cam motion and the cam shape are obtained according to the proposed design method. Analyzing the effects of the cam shape with different cam inertia, the initial cam is modified to satisfy the specific requirements of the cam rotation angle. The simulation results of the designed cam are compared with those of the original cam. Spring-actuated cam mechanism Figure 1 shows a VCB with a spring-actuated high-speed cam mechanism. The rated voltage and the interrupting current of the breaker are 12 kV and 40 kA respectively. The essential parts of the mechanism consist of closing 529 530 K.Y. AHN, J. H. KIM and S. H. KIM springs that drive the cam, a follower with opening springs, wipe springs that push electric contacts, and mechanical parts including linkages and latches. The closing procedure of VCB is as follows: the cam (0 in Fig. I) is initially constrained by the closing latch ( in Fig. 1). After releasing the closing latch, pre-loaded closing springs ( in Fig. 1) rotate the cam and the cam pushes the follower ( in Fig. 1) to a closed position of the electric contacts ( in Fig. 1). During the closing operation, the opening springs are extended and the wipe springs compressed after the closing of the electric contacts. Figure 2 shows the cam velocity during the rapid closing operation within 30 msec. It is obtained by differentiating the cam motion obtained according to the proposed design method. It can be seen that the cam velocity begins at 0 rad/s and reaches over -200 rad/s. m c a m O -supporter electric contacts : fixed and movable contact I /-, . cmsmg closing latch : -., )( cam w fixed openingspr,ng contact , . !v h,4 ) I , X ooenlng o t . i vacuum latch L -.- roller movable J I contact ff -._ / follower wipe spring (a) structure (b) representation Fig. 1 Vacuum circuit breaker / - -20 -40 -60 -80 3o- -50 0 50 X mm Fig. 11 Cam shapes with different cam inertia 538 K. Y. AHN, J. H. KIM and S. H. KIM 220 210 200 : 190 180 E 170 lX 104 , ?t 05 . o -o.5t .1, r 1 0 5 10 15 20 25 30 35 40 5 . -5 , . t i 0 5 10 15 20 25 30 35 40 8 160 15o I 14o t 13% 10 15 2o 2 so o Percentage variation of cam inertia % Fig. 12 Cam rotation angles with different cam inertia 5 10 15 20 25 30 35 40 Time msec Fig. 13 Dynamic characteristics of follower during closing operation 15000 original design I . final design I0000 o 5000 5 10 15 2