servo motor 30 simply moves the linear cam 32 outward by a greater amount conversely to apply lower pressure the servo motor 30 simply moves the linear cam 32 inward by a lesser amount the ability to apply variable amounts of pressure enables the servo motor 30 to easily accommodate springs 20 a 20 b having various spring constants the servo motor 30 can also be programmed to apply a time varying pressure for example if one wishes to smoothly increase the applied pressure from a first value to a second value it is only necessary to program the servo motor 30 to gradually move the linear cam 32 outwards from a corresponding first position to a second position in operation the servo motor 30 generally moves the linear cam 32 so that the cam followers 36 a 36 b remain within the inner section 35 b this ensures that in the open position the sealing bars 12 a 12 b remain fairly close together this enables the sealing bars 12 a 12 b to be quickly brought together on the other hand when operation of the packaging machine is halted it is desirable to move the sealing bars 12 a 12 b further apart to prevent stray heat from the sealing bars 12 a 12 b from melting the web in an uncontrolled manner to do this the servo motor 30 drives the linear cam 32 inward far enough so that the cam followers 36 a 36 b enter the outer section 35 a of the linear cam 32 since the path functions at the outer section 35 a have a.