SEED Unit Design - Cam Mechanisms < >

Once the preferred solution for a new design has been selected you should work out the operation of the machine as a timing chart to test the feasibility of the proposal. The extract from a chart drawn in Figure 3 shows the actuation of one operating member as a function of time or the angular rotation of a principal shaft. Similar diagrams are drawn to the same base for every operating unit of the machine to complete the chart. In most cases it is adequate to represent motions as straight lines, the slope indicating the direction of velocity. In this way operations can be synchronised and unknown times determined, noting the implications of equation (4). Inner Rise Outer Fall Dwell Dwell 0 90 180 270 360 Cam Rotation (degrees)

As an example, the complete timing chart for the cam-operated tape feed and punch mechanism shown in Figure 4 is drawn to scale in Figure 5. Two consecutive cycles are shown to emphasize the differences between active and idle operation. The tape feed is driven continuously at 18 000 cycles/min. but only indexes the tape on demand. The camshafts F and L, Figure 4, are synchronised to rotate at 2250rev/min. The 4-lobed cams T and M mounted on these shafts drive the feed frames G and J alternately through the feed movement of 2.54mm whilst the other frame returns, so balancing these assemblies. Although the feed frames are driven continuously the tape is only indexed when the command signal is routed to solenoid U or P as appropriate to release the grip lever S or 0 respectively. Then the lever is free to follow the profile of cam V or N so the tape is clamped and fed. If the solenoid is not energised feed is inhibited as the grip lever is held at the outer dwell position whilst the cams rotate to the next operating cycle for that mechanism. (The punch mechanism is shown on the left side of Figure 4).