The action of an independent prescreen is completely independent of the feeder. It also has the grizzly fingers at the top, or punch plate, but underneath, material falling through goes through a woven wire mesh sized to any product the operator wants. Material not passing the mesh is bypassed around the crusher and discharged with the crushed product, thus reducing wear inside the crushing chamber.

In addition to maintenance on the grizzly fingers, the screen mesh must be inspected for worn, broken, or bent wires and the seating face and tensioning devices inspected for the mesh. To obtain a screened product, it is critical to maintain the mesh so not to allow material to fall outside of the required specification.

Thus, with an independent prescreen, the operator avoids unnecessary wear inside the crusher and boosts production of material that actually needs to be crushed.

3.  Fighting dust

Dust is a problem in its own right, especially for the power plant of the mobile crusher. In a very dusty application, a radiator can become plugged up, causing the engine to overheat. High dust levels can cause increased maintenance intervals on air filters and, if not controlled properly, dust can enter the diesel tank and cause problems with the fuel system. Also, dust inside the crusher increases wear. But if systems are put in place to remove the dust, it should not enter the machine in the first place.

The wear plate of this crusher box has almost completely worn through (oxidized metal) and must be replaced, or the crusher box will be permanently damaged.

Dust also is a hazard on walkways and a problem for conveyors. If maintained, side-skirting and sealing of conveyors keeps dust from spilling out, building up underneath the conveyor, or building up in rollers, pulleys, bearings, and causing wear on shafts. It’s important to maintain the sealing rubbers on the conveyor belts to avoid those issues. Routine maintenance calls for removal of accumulated dust from inside the machine and under it.

Dust also causes problems for circuit boards and programmable controllers used with modern machinery. Electrical systems under positive air pressure don’t permit dust to penetrate into the control system. Dust causes electrical switches to malfunction because it stops the contacts from correctly seating. In control panels with a correctly maintained positive pressure system, filters remove dust from air that is being pumped into the cabinets. If the filters are plugged, the system will not pull as much air through, allowing dust, moisture, and heat to build in the cabinet.

4. Inside the crusher

The major wear part for a mobile crushing plant is the crusher itself. In a jaw crusher, the major wear parts are very limited — only a fixed jaw and a swing jaw, and two or four side or “cheek” plates.

An impact crusher has more wear items. The blow bars — which hit the rock and make it explode inside the machine — take a majority of the wear. In addition, there are impact aprons against which the rock is thrown that also see high wear. There are side plates or “wear sheets” on the sides of the machine. The highest wear area is around the impact crusher itself, around the circumference of the rotor. If wear items are not maintained, they will wear through and compromise the structure of the crusher box.

A daily visual check of the machine is recommended. The jaw is simple; stand up on the walkway, and take a look down inside. A jaw plate can be flipped to allow two sides of wear. Once half the jaw is worn out, flip it; once that side is worn, change it.

The impactor will have an inspection hatch to allow the operator to see inside. How much material is left on the blow bars? How much is left on the wear sheets on the side of the crusher box? If after one week half the bar is worn out, anticipate changing the blow bars in another week. The frequency of changes depends entirely on the application and the rock being crushed.