Ihave bought a second hand electric saw aqnd while all seems in order it won't drive the chain . ON investigation the "brake" drum on the end of the shaft was turning loosely.After dismantling the drive gear itself was spinning on the shaft. There does'nt appear anything to lock it to the armature shaft other than a vigourous tightening of the nut that holds the whole lot together. Is this the norm? perhaps safety feature to stop burn out if the saw jambs.