Pneumatically powered surgical cutting and fastening instrument with a variable control of the actuating rate of firing with mechanical power assist

A surgical instrument comprising:

• a handle assembly;
• a distal member distally coupled to said handle assembly and configured to support a surgical tool;
• a pneumatically powered rotary motor configured to generate at least one actuation motion upon receipt of at least one pneumatic signal from a source of pneumatic power fluidically coupled to said pneumatically powered rotary motor, said pneumatically powered rotary motor having an output shaft coupled to a drive shaft assembly that operably interfaces with the surgical tool to transmit linear actuation motions to the surgical tool in response to said actuation motions from said pneumatically powered rotary motor;
• a power assist trigger operably supported by said handle assembly and mechanically linked to said drive shaft assembly independent from said source of pneumatic power by a linkage assembly comprising:
• a first gear operably attached to said output shaft of said pneumatically powered rotary motor;
• a threaded shaft in threaded engagement with a portion of said power assist trigger such that when said power assist trigger is manually actuated, said threaded shaft is rotated;
• and
• a second gear operably coupled to said threaded shaft and in threaded engagement with said first gear, such that upon manually actuating said power assist trigger, at least one non-pneumatically generated actuation motion is applied to said drive shaft assembly.