Gyratory Crushers: Misunderstood Maintenance
Make sure maintenance personnel pay close attention to these 10 important areas to keep gyratory crushers in good condition.
By Mark Kennedy
Many producers will experience decreased profit margins, mainly due to the fact that their maintenance team does not fully understand the maintenance requirements of their gyratory crusher. Crushers that are maintained by personnel who do not adequately understand them will eventually suffer from escalating operating costs due to reoccurring problems such as poor productivity, premature component wear, unscheduled repairs, or catastrophic failure.
Cost-effective maintenance techniques begin with workers who are knowledgeable about the maintenance requirements of the equipment to which they are assigned. In the real world, however, the level of equipment knowledge demonstrated by many aggregate plant employees is far too often found to be inadequate.
The following is a list that outlines 10 critically important areas of gyratory crusher maintenance which are misunderstood, improperly performed, or just flat out neglected.
1. Spider bushing clearance
Failure to monitor or maintain proper spider bushing clearance is one of the most neglected maintenance areas. When spider bushing clearances are excessive, the lower journal (mainshaft) will have misalignment with the eccentric bushing, resulting in a cross-corner contact pattern and improper load distribution. This misalignment can lead to a catastrophic failure of the crusher.
Spider bushing clearance should be checked every month, every 500 hours, or at every mantle change, whichever comes first. It is important to note that the spider bushing clearance must be measured at the fulcrum point of the spider bushing bore and while the mainshaft is at its working position. This assures that the most worn area of the mainshaft sleeve is aligned with the fulcrum point of the spider bushing bore. If clearances are found to be out-of-tolerance (excessive), the spider bushing, mainshaft sleeve, or both will need to be replaced.
2. Spider bushing lubrication level
Another common maintenance mistake is the failure to maintain proper spider bushing lubricant level. The crusher mainshaft has both rotary and gyratory motion at slow speed and exerts fairly high pressures on both bearing surfaces, the mainshaft sleeve, and the spider bushing. A lubrication pool is used to lubricate these surfaces. The lubricant is retained in the bearing cavity by a spider bushing seal arrangement. It is important to understand that all spider bushing seals will leak a bit. Since ferrous metals are used in both the mainshaft sleeve and the spider bushing, the lubricant should contain an “extreme-pressure” additive. Proper spider bushing lubricant level is maintained at approximately 1.4 inches above the spider bushing flange. A low lubricant level will lead to rapid spider bushing wear, mainshaft sleeve wear, and, eventually, an eccentric bushing failure.
The spider bushing lubricant level should be checked every day on a new crusher to determine the normal leakage trend. After a leakage trend has been established, check the lubricant level periodically to assure that the proper level is being maintained.
3. Balance cylinder nitrogen pressure
Neglecting the balance cylinder is a somewhat common error. Irregularly shaped feed material, large chunks, or tramp in the crushing chamber of the gyratory crusher sometimes force the mainshaft upward in a quick “jumping” action. A single balance cylinder on some units, or multiple balance cylinders on others, is furnished to make the piston follow the mainshaft up when it is raised by jumping and eases the mainshaft down to its original position after the jump ends. The balance cylinder contains both oil and nitrogen gas, which are separated by a sealed piston. After the adjusting system has been bled of air, and with the mainshaft raised hydraulically to its operating position, the balance cylinder is charged with nitrogen gas. It is important to understand that the nitrogen pressure must be measured after the adjusting cylinder system is filled with oil and the mainshaft assembly is supported on a column of oil underneath the piston. Under normal conditions, the nitrogen pre-charge pressure of the balance cylinder is less than the hydraulic oil pressure produced by the weight of the mainshaft assembly, but more than that produced by the weight of the adjusting piston.
Balance cylinder nitrogen pressure should be checked every 30 days of operation.
4. Concave installation
Whether manganese steel sectional concaves or alloyed steel sectional concaves are being used, improper installation is a common occurrence. When installing new concaves, certain steps MUST be accomplished. For instance, vertical spacing between the concaves must not exceed 3/8 inch to 5/8 inch, horizontal spacing between rows or tiers must not exceed 3/8 inch, the top row must be even with the top to 1/4 inch lower, and vertical spacing should alternate between tiers. When using epoxy as a backing material, the back of the concaves and the inside diameter of the top shell must be clean, dry, oil free, and sand blasted down to bare metal. All four landing pads cast into the back of each concave must be in contact with the top shell seating surface. Accomplishing each of these steps ensures that the concaves adhere tightly to the top shell, allowing very little, if any, chance of a concave coming loose during crushing operation.
5. Lubrication system oil cleanliness
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