A machine for crushing rocks is comprised of n main body, a bonnet which rests on the upper frame of the main body and a concave holder threadably inserted into the bonnet. The bonnet is maintained in intimate contact with the main body by means of a plurality of tramp iron cylinders while the concave holder is screwed into the bonnet by means of a adjusting gear and matching pinion. The concave holder is locked in position within the bonnet by means of a plurality of locking cylinders positively engaging a locking slide ring. A mantle covered male conical member is driven by a crown gear and pinion in an elliptical rotation within the confines of a concave thus crushing the raw rock on each near pass. The concave holder will automatically lift and/or tilt off of the bonnet in response to the individual or collective actions of a plurality of tramp iron cylinders and matching accumulators which are reactive to an excess loading between the male conical member and its mantle and the concave. A two part stepped slip seal provides containment of excess lubricant which excluding abrasive particles from contact with critical components. The seal is incorporated as part of a thrust bowl which rides against the underside of the male conical member and absorbs crushing loads transmitted via the male conical member and the concave.
