Principle :
• It works on the principle of compression. It reduces size by compressive force.
Construction :
• A schematic diagram of the Blake jaw crusher is shown in Fig. It has a fixed jaw and a movable jaw. The movable jaw is pivoted at the top. The jaws are set to form a V open at the top. The swinging jaw (movable jaw) which reciprocates in a horizontal plane usually makes an angle of 20 to 30o with the fixed jaw (which is nearly vertical).
• The jaws are usually made of manganese steel or some other material that will withstand abrasion. The faces of the crushing jaws are usually corrugated for concentrating the pressure on relatively small areas.
• In addition to the jaws, crusher consists of a pitman, toggles, flywheel, eccentric shaft, drawback rod and springs and frame. In this machine, an eccentric causes the pitman to oscillate in a vertical direction, and this vertical movement is communicated horizontally (reciprocating motion) to the movable jaw by the toggles.
• The speed of operation should not be high or otherwise a large quantity of fines is produced as the material cannot escape quickly and gets repeatedly crushed. Since the crushing action is intermittent, the loading on the machine is uneven and due to this the crusher incorporates a heavy flywheel.
• Since the maximum movement of the jaw is at the bottom (discharge), there will be little tendency for the crusher to choke.
Protection of Machine :
• The machine is usually protected so that it is not damaged if accidental pieces of iron such as hammer heads, stray bolts, etc. enter into the crusher, by making one of the toggles in the driving mechanism relatively weak. That is, one particular toggle is made into two pieces which are held together with bolts that are purposely made the weakest part in the crusher so that, if stresses are set up, these bolts shear first.
• Thus, the failure is made at a point that can be easily and quickly repaired, instead of breaking some vital part of the machine.
Working :
• The material to be crushed is admitted between two jaws from the top. The material caught between the upper part of the jaws is crushed to a smaller size during forward motion by compression. The crushed material then drops/falls into the narrower space below during the backward motion and is recrushed as the jaws close next time. After sufficient reduction, the crushed material drops out the bottom of the machine.
• The jaws usually open and close 250 to 400 times per minute.