Patent ID: 7004297

Claim:
A drive power transmission device comprising first and second cylindrical rotary members coaxially arranged to be rotatable relative to each other; a main clutch mechanism composed of plural main outer plates rotatable bodily with said first cylindrical rotary member and plural main inner plates rotatable bodily with said second cylindrical rotary member, said main outer plates being arranged in alternate fashion with said main inner plates each for friction contact with said main inner plates adjacent thereto; an electromagnetic pilot clutch mechanism comprising an armature and an electromagnet for attracting said armature thereto; and a cam mechanism operable upon operation of said electromagnetic pilot clutch mechanism for bringing said main outer and inner plates into friction contact, wherein said electromagnetic pilot clutch mechanism further comprises means for generating a clutch magnetic path whose magnetic flux passes to reciprocate plural times across said electromagnetic pilot clutch mechanism, wherein the device further comprises a magnetic flux isolation member made of a non-magnetic material and formed with a circular groove on a surface thereof directed to said armature, wherein said electromagnetic pilot clutch mechanism further comprises a pilot inner plate, and arc slits are formed in said pilot inner plate on at least triple circles whose outermost and innermost circles respectively correspond in diameter to said magnetic flux isolation member, and wherein circular slits are formed in said armature.