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Gyratory Crushers: Misunderstood Maintenance
Posted By admin On March 1, 2013 @ 6:00 am In Articles,Equipment,Equipment Management,Featured Articles | No Comments
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
A common situation is operation of the gyratory crusher with contaminated lubricating oil. A circulating lubrication system keeps the bearing surfaces within the crusher lubricated and cool. When lubrication system maintenance is neglected, rock dust can infiltrate the lube system and contaminate the oil. As the dirty oil flows past the bronze bushings within the crusher, it tends to sand them down, resulting in severely worn components and excessive bushing clearances. At minimum, lube system maintenance checks, inspections, or services must include the following:
Daily (8 hours)
• Check tank oil level
• Check oil temperature
• Check flow back to tank
• Check oil flow indicators
• Inspect the oil tank vent
• Inspect the oil tank return screen
• Check air blower filter condition
Weekly (40 hours)
• Check for leaks
• Check Y strainers
Monthly (200 hours)
• Perform oil analysis
• Check safety devices
Yearly (2,000 hours)
• Change oil
6. Spider installation
The spider assembly of the gyratory crusher will be removed periodically, primarily for normal maintenance services such as mantle and/or concave replacement. It is always a good idea to match mark the spider and top shell flanges prior to disassembly; this assures proper orientation during the reassembly process.
When the spider assembly is reinstalled, it is very important that it is done correctly. All contacting surfaces must be clean and free of burrs, corrosion, or fretting; the contacting surfaces facing down, including the tapers, should be coated with light oil. It is important to note that you MUST NOT use a moly-based grease or anti-seize compound on these surfaces, as this type of lubricant could result in movement between the surfaces during operation and premature wear. Make certain that the contacting surfaces facing up remain dry; this prevents rock dust from sticking to this surface during the assembly process, which might prevent proper contact between the flanges. Orient the spider so that the spider arms line up correctly, as they must split the incoming feed. If the crusher is fitted with a “straight fit” spider bushing, the spider is reinstalled onto the top shell with the spider bushing already installed in the spider. If the crusher is newer, or has been updated, and is fitted with a “tapered fit” spider bushing, the spider bushing should be installed AFTER the spider has been mounted onto the top shell. When installing the spider flange bolts, tighten the fasteners in diagonal sequence to pull the sections together evenly so that a 0.004-inch feeler gauge cannot be inserted between the flanges at any point around the circumference.
7. Pinionshaft assembly maintenance
The gyratory crusher is equipped with a cartridge-type, anti-friction bearing, pinionshaft assembly. Pinionshaft assembly lubrication is a separate system; meaning oil from the circulating lubrication system never enters the pinionshaft housing. However, the same grade of oil, in most cases an ISO Grade 68, is used in both systems. The bearings are pool lubricated through a filler hole on the upper front surface of the housing. An oil-level sight gauge is provided for monitoring the oil level. Pinionshaft oil level should be checked every day (8 hours) and changed every six months (1,000 hours). Pinionshaft housing lubrication oil temperature should also be monitored and should not exceed 90 degrees F above ambient temperature. A lip seal at the drive end of the pinionshaft housing prevents dust infiltration. It should be lubricated every 100 hours of operation with a multi-purpose NLGI #2 grease.
8. Adjusting system primary relief valves
Two primary relief valves are located in the adjusting system cylinder control pipeline. Their function is to relieve the pressure in the hydraulic adjusting system if it exceeds the relief valve setting. The relief valves normally require little maintenance. If one, or both, is believed to be inoperative, it could be checked by attaching it to a high-pressure device, such as a hydraulic jack, or high-pressure grease gun with a gauge and noting the pressure at which the valve operates. This should be within 10 percent of that stamped on the valve. When installing or replacing these valves, be sure the pressure side of the valve, the male port, is attached to the high-pressure pipe, and the relief side of the valve, the female port, is away from the pressure and is open to atmosphere. The discharge port of these valves MUST NOT be restricted in any way.
9. Misunderstanding the mantle design
Misunderstanding the philosophy of mantle design can be another big problem. Most gyratory crushers can be supplied with mantles of different diameters, such as the standard shape mantle, an oversize mantle, and possibly even a double-oversize mantle.
When performing mantle or concave replacement maintenance, it is critically important to understand when each mantle should or should not be used. The gyratory crusher is designed to operate at the same discharge setting throughout the life of the mantle and concaves; it is NOT designed to operate at a tight setting one day and a larger setting the next. Please reread this and let it sink in. The bottom row concaves and the standard shape mantle have been designed, whereas when both are new and the pre-selected crusher setting has been established, where there will be 1 inch of oil underneath the adjusting piston. This is considered the starting point, or the lowest position at which the mainshaft should be allowed to operate. When the mainshaft is raised periodically to compensate for mantle and concave wear, the mainshaft head nut will become closer to the underside of the spider. When the head nut is 1 inch away from contacting the underside of the spider, the mainshaft should no longer be raised. This is considered the end point, or the highest position at which the mainshaft should be allowed to operate. If, at this point, it is determined that there is about 1 inch of wear to the bottom row concaves, an oversize mantle can be used.
After the oversize mantle is installed and the pre-selected crusher setting is once again established, there will be about 1 inch of oil underneath the adjusting cylinder. Once the oversize mantle is worn out, an inspection of the concaves will determine if another oversize mantle should be used or if a double-oversize mantle could be used. It is important to note that you may be required to use two or three standard shape mantles before experiencing enough wear to the bottom row concaves to justify installing an oversize mantle; the same holds true when switching from an oversize mantle to a double-oversize mantle. Many producers DO NOT understand this concept and use an oversize mantle, or a double-oversize mantle, when new concaves are installed or when there is minimal wear on the bottom row concaves. This can result in the adjusting piston resting against the bottom of the cylinder and the crusher setting being much tighter than the crusher is designed to operate. This situation can lead to crusher stall outs and premature failure of the eccentric bushing.
Manufacturer design engineers are incredibly intelligent individuals. Their many assignments include, but are not limited to, calculating bearing loads and bearing clearances; establishing the most efficient eccentric speed and throw combinations; designing mantle and concave profiles to give optimal nip on the material undergoing reduction and maximum throughput; and selecting proper interference fits and correct oil flows. This is done after careful calculations, prototype testing, and long-term field experience. When aggregate plant personnel make modifications that alter the intended design, they actually defeat the purpose of the original design.
The inevitable result is:
• A less reliable crusher;
• A non-productive crusher;
• A crusher operating frequently in an over-loaded condition;
• Damage of internal crusher components due to the alteration; or
• Catastrophic failure of the crusher due to the alteration.
No one wants the crusher to work any better than the manufacturer. So, please, prior to making any unapproved modifications, contact the manufacturer for advice and approval. It just makes sense.
In most cases, gyratory crusher maintenance problems are related to inadequate personnel training which, unfortunately, exists at many aggregate plants, big or small. Many times, the failure to understand the importance of proper maintenance procedures is based on misinformation which is frequently passed on from one employee to another in a guise of on-the-job training. In a few extreme cases, poor maintenance habits and/or incorrect maintenance procedures become entrenched in an organization to the point they are vehemently defended, and correct methods and proper procedures are heavily resisted or even laughed at.
Depending upon the severity of a training gap in a given organization, the solution can be relatively simple, or extremely difficult, but in no case is it easy. In order to be effective, the education which is provided to the gyratory crusher maintenance team must be kept current and must be an on-going continuous activity.
Mark Kennedy is the senior technical training instructor at Metso Minerals Industries, Inc. He can be reached at firstname.lastname@example.org .
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