May 1, 2012
Use these seven maintenance practices to keep mobile crushers dependable throughout their working life.
By Paul McLaren
Today’s mobile crushers are rugged and dependable machines, but crushing and processing material can result in excessive wear on certain components, excessive vibration throughout the plant, and excessive dust in the working environment. And a hard rock application is going to require more maintenance on top of standard maintenance, as it will include more vibration, more dust, and more wear than from a softer aggregate.
From the moment a mobile crusher starts, the machine is wearing itself out and breaking itself down. Without routine, regular maintenance and repair, a mobile crusher will not be reliable, nor will it provide the material customers demand.
1. Feed the beast properly
The first area of wear on any machine is the feed system. Whether it’s a feeder with an integrated grizzly or a feeder with an independent prescreen, how the machine is fed contributes to wear.
One of the major points to remember when setting up and maintaining a machine is that the machine must be level. If the machine is unlevel left to right, it will cause increased wear on all components from the feeder to the screens, crushing chambers, and conveyor belts. In addition, it reduces production and screening efficiency, as it is not effectively using the whole area of the machine. Having the machine sit high at the discharge end will have the effect of feeding the material uphill in the feeder, reducing its efficiency, and thus reducing production.
Another area for consideration is the equipment with which the machine is fed. If fed with a loader, the operator can’t see in the bucket and has no control over the feed size. With an excavator, the operator can see what’s inside and has more control over the feed into the hopper. Not feeding as much material all at once and controlling the size of the feed reduces wear in the impact zones of the feed hopper and eliminates material blockages due to feed size being too large to enter the chamber.
2. Grizzlies and prescreens
The standard integrated feeder is a grizzly section that is integrated into a vibrating feeder, so the feeder and the grizzly move at the same time. It’s a set of tapered fingers that allows material less than the size of the opening of the grizzly to drop through, bypassing the crusher. This reduces wear on the crusher, as material that does not need to be crushed never enters. That material is bypassed or sent to a side conveyor, but is not screened to a classified saleable product. These grizzly fingers must be rebuilt and hardfaced to maintain the sizing of material that is being removed.
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.
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.
5. Managing conveyors
Conveyor maintenance depends on the hardness of the aggregate being crushed. A customer who’s running a very soft limestone can get years from a conveyor belt that’s properly maintained, but only one or two months from one that’s not.
With a conveyor, it’s really important to ensure it is tracking in the correct path, and not all the way over to one side, rubbing on the chassis. Make sure there’s no build-up underneath that can drag against the bottom of the conveyor, or catch on it and be dragged into the machinery.
It’s important to keep the side-skirting maintained; that will keep rocks from catching at roller stations, then pushing down on the inside of the belt. No material should be caught in any part of the conveyor, whether it be the roller station, where material exits the crusher, or beneath the conveyor from spillage. Any of these will impact the longevity of the conveyor and belt.
A vibrating chute beneath the crusher where material exits onto a conveyor takes a lot of wear away from the belt; instead of hitting the belt, aggregate hits the vibrating chute with steel wear liners. The impact is absorbed by the chute, which then feeds aggregate gently onto the belt to discharge to the stockpile.
6. Wear of screens
Screens are obviously a vital part of the crushing and screening operation. Without the correct screen meshes, or with badly worn or maintained screen meshes, you will not get the product the customer specifies.
A standard woven wire mesh has a certain wire diameter, a square woven mesh with a crisscross pattern. As it wears, the openings become greater, going from a 3/4 inch opening to 1 inch in size. That will cause the aggregate to fall out of spec.
Also, as the wire diameter gets smaller and smaller, the mesh becomes weaker and more prone to breaking, causing the openings to become 1 3/4 inches by 3/4 inches, and again, the material is out of spec. Look for worn and broken wires and damaged crimps within the wire that are causing the screen mesh to move and allow larger material to pass through. Check the tension of the mesh. If the mesh is not tensioned properly, it will bounce around inside the screen deck and cause damage to itself and the surrounding screen box.
7. Engines and hydraulics
The engines and hydraulics systems should be maintained according to schedule. For example, greasing the bearings of the crusher, conveyor belts, screens, and feeders should be taken care of periodically to keep the machine in shape.
Frequency of service depends on the item. It could be weekly for greasing, 500 hours for an engine service, and 2,000 hours, depending on the manufacturer, for hydraulic service. These all should be done regularly, by schedule, as their need is not evident during visual inspection.
The manual will contain the specified intervals for lubrication or replacement of a certain part. Instead of having to hunt for a manual, many companies have their own maintenance logs, with their own servicing capabilities and trucks. If an interval checklist is posted inside a door to the machine, it can be reviewed on a daily basis, with a summary of what oil types are needed in what quantities and what the intervals are.
If the work is done by a dealer, the dealer will track the average number of hours on customer’s machines and visit the machines to carry out the service. Ultimately, the dealer should know more about what long-term maintenance is required, while the operator can keep track of the daily maintenance. There is no substitute for a dealer’s in-depth knowledge via a qualified, trained mechanic who can give the machine a complete health check.
If your manufacturer offers training classes, so much the better. Operational and maintenance training should be part of the initial commissioning of the plant, but the operator should attend a formal training program sponsored by the manufacturer to truly optimize maintenance and service.
Maintenance of mobile crushers for optimum production is not only about when the task is performed, but also how it is performed.
Unless the person making the checks has the training and knowledge to effectively take care of the machine, downtime will be increased and production decreased.
For optimum uptime, a thorough maintenance program will ensure a dependable machine for years to come.
Paul McLaren is product support manager for Kleemann.