7. Fault clearance, maintenance and lubrication
7.1 Trouble removing .1 Mill running roughly
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7.1 Trouble removing 7.1.1 Mill running roughly
Heavy rumbling (extremely strong vibrations) means imminent danger to the mill reducer. Therefore the mill has to be shut down immediately. Rough run may be caused by too little material or by overloading the mill.
7.1.2 Fill level of feed bin too low
In case the mill has to be stopped because of the feed bin containing too little material there is a danger that the conveyors feeding the mill also run out of material.
In this case these supply units up to the mill feeder must be fed with material without delay. Otherwise the grinding bed will become thin very fast and the mill will start rumbling. This might damage the reducer very badly.
7.1.3 Start-up of mill without grinding bed
after repair work or if grinding bed due to lack of material supply was as less as 0-30mm prior to shutdown.
The mill has to be filled with material as follows:
a. Gear oil pump on.
b. Adapt overrunning clutch.
c. Seal air fan on.
d. Auxiliary drive on.
e. Rotary lock or three –flap gate lock on.
f. Switch on supply devices.
7.1.4 Filter outage
In case high dust emission is shown on the indicator, mill output must be reduced or the mill must be shut down.
7.1.5 Outage of feeding device
In case one feeding device is out of order the grinding plant may still be operated for a few minutes. However, mill supply by the remaining feeding devices must be increased to such an extent that the mill does not run out of material.
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In case the supply units from the pre-bin to the mill are out of order the grinding plant must be switched off before differential pressure and power consumption of mill motor reach the minimum value.
7.1.6 Outage of mill fan
In case of outage of discharge devices of cyclone or filter the plant has to be switched off immediately. In no case must the mill be kept running because the cyclones would be clogged very fast and it is difficult to empty them.
7.1.7 Fine material feeding unsmooth
The mill must be stopped when segregator and filter discharge port break off.
7.1.8 Outage of hot gas supply from kiln
In case the exhaust has fans of the kiln are out of order the grinding plant should be shut down since otherwise the mill will be overcharged and start rumbling.
7.1.9 Differential pressure too high
Feed to be reduced very much immediately. Watch differential possible causes:
a) Material feed too high due to malfunction of control of feeding devices.
b) Nozzle ring located at the periphery of the grinding bowl is clogged.
c) Air volume in mill too low or inconstant.
d) Setting of classifier much too fine.
7.1.10 Differential pressure too low Possible causes:
a) Outage of material feed.
b) Wrong indication.
This is the case if the remaining pressure indications ahead and downstream of the mill are o.k. Mill operation may be continued.
However, feed has to be changed from automation to manual operation.
7.1.11 Tension pressure dropping Possible causes:
a) Leakage in conduct pipe.
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b) Pressure relief valve out of adjustment.
c) Outage of tension pump.
d) Pressure switch defective.
With an intact plant, operating pressure may be reestablished by switching on the oil pump. It must be possible to maintain pressure without the pump being in operation. If not, the mill must be shut down in order to check the hydraulic system.
7.1.12 Seal air dropping
Check on the machine if pressure remains constant. In that case you needn’t shut down the mill. The cause of the pressure drop must be eliminated during the next down-time at the latest. The mill is switched off automatically if pressure drop is such that the amperage of the fan exceeds a predetermined value due to low counter-pressure. (This value is set during commissioning and should not be less than 5000Pa.)
7.1.13 The oil pressure or oil flow in reducer min. The mill is switched off automatically
7.1.14 Bearing temperature in reducer too high
If the bearing temperature exceeds a specific value, malfunction is signaled and the mill is shut down. Check cooling water.
7.1.15 Material spillage at flap of mill plinth Possible causes:
a. Mill is overloaded.
b. Air volume in mill is too low.
c. Nozzle ring cove is damaged.
d. Nozzle ring is worn.
Any temporary reduction of the air volume in the mill may also cause material spillage.
7.2 Maintenance
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7.2.1 Inspection
Regular inspection of all grinding plant avoids unforeseen down-time due to insufficient maintenance and lubrication. By inspecting the wear parts regularly replacement can easily be scheduled.
(1) Normal inspection a. Daily inspection
During operation general inspection of the mill should take place once every day watching for strange noises and vibrations, loose screws and oil leakage.
Check oil level of hydraulic units every day.
Make sure to also lubricate the classifier bearings, rotary lock and cyclone discharge locks every day.
b. Weekly inspection
1. Check the sealing of pull rod.
2. Check oil level in reducer and hydraulic station. Watch for oil leakage.
3. Clean seal air filters or replace them if necessary.
4. Check seal air pressure.
(2) Major inspection after long down-time
Some checks are the same as the weekly inspection.
In addition:
a. Check oil level in grinding rollers (once a month).
b. Check fastening of wearing parts on grinding track.
c. Roller tires, grinding track, liner, roller bracket, nozzle ring, housing walls and any other internals of the mill to be checked for wear.
d. Adjustment of grinding bed indication (necessary with progressing wear on grinding elements).
e. Check shoe plate of pull rods and sealing.
f. Check all flanges in the mill area for tightness, re-tighten them if necessary.
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g. All parts in the mill to be checked for wear, especially the seal air ducts to the grinding rollers.
h. Check all dampers and compensators in the piping for tightness and movement, respectively and replace them if necessary.
i. Check hydraulic cylinders for leakage and replace sealing element if necessary.
j. Check plant fan for caking or wear and tear.
k. Check classifier vanes for caking.
7.2.2 Repair work
(1)Disassemble grinding rollers See installation drawing.
a) Open maintenance doors on the frame.
b) Disassemble the sealing air pipes and temperature measurement on the grinding roller.
c) Lift the extendible support by means of the conic cover plates, and fix it by bolts (when assembling, the support is in the conic cover plates).
d) Three sets of supporting bases are mounted on the support by bolts, and the supporting bases can be turned to the required positions. The height of supporting bases is determined by the length of the adjustable bolts.
e) Mount six connecting bases on the support of the grinding rollers.
f) The vertical distance between the supporting base and connecting base is adjusted to about 10mm, and then fixed by the bolts. After these, the grinding rollers are fastened on the grinding plate by means of wooden wedges.
g) Dismantle the connecting chain plate between the grinding rollers and the pressure frame.
h) The pressure housing is lifted up by the tension system, and then is suspended on the classifier by three pieces of ropes.
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i) After dismantling two connecting bases on the grinding roller, the grinding roller is lifted up and removed out of the maintenance door by a crane.
j) The second grinding roller and the third grinding roller are, in turn, removed from the door according to above method.
k) Proceed in reverse order for reassembling.
l) When the mill is remounted, the grinding rollers are mounted into the housing before the pressure housing and classifier are mounted, so the assembling tool shall not be needed.
2) Disassembly of grinding plate segments
Once the rollers are dismantled it is very easy to remove the grinding plate segments. Even with the rollers in place, it is not difficult to replace the segments. In this case the segments are removed between the rollers and new ones are filled right away. By turning the mill with the auxiliary drive, it is possible to replace the entire grinding plate.
Process as follows (rollers being removed):
a) Remove grinding rollers (see above).
b) Remove cover plates of grinding track.
c) Loosen clamp bolts.
d) Remove clamp plates.
e) Dismantle segments.
Start with the inner segment. Since raw material gets pressed between the segments during grinding, such deposits need to be removed prior to dismantling the first segment. Moreover, the lateral wedges have to be removed. For ease of removing the segment, drive in two flat wedges on each side between the segment and the two adjacent ones to make the gap a bit wider by some lengths of millimeter. Then drive in another flat wedge between grinding bowl and segment and loosen it.
f) Once all wedges are removed again and the segment is loose, lift it up.
g) If the surface of the grinding bowl turns out to be uneven, smooth it by means
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of an electric hand grinding machine. To improve the seal of the segments, appropriate metal putty or any similar material may be spread underneath.
h) Before installing the new segments, check if the edges of the contact surfaces of the two cast noses are rounded or beveled so that they do not press onto the contact surface of the grinding track.
i) When mounting the segments, ensure to fit of the two cast noses tightly on the contact surface (outer edge of grinding bowl) and for proper seal of segment on the grinding track surface. Check by means of a feeler gauge. On the outer edge of the grinding track, 0.05 mm are allowed between the noses and contact surface, whereas the clearance between the segment base and the grinding track surface must not exceed 0.1 mm. In case these values are exceeded, the segment is required to be ground by means of a hand grinding machine. As already stated above, the seat of the segment on the grinding track surface may be improved by means of appropriate metal putty.
j) Once three segments each are in place, fix them by clamp plates.
k) Tighten clamp bolts (T-head bolts) with torque wrench. Observe torque indicated on drawing.
l) Also, place wedges between each segment and the two adjacent ones.
m) Make sure to provide for proper seating of the segments both on the outer edge of the grinding track and on its surface. If required the noses have to be machined as described under g) above.
3) Replacement of grinding roller bearing and grinding roller sealing
The bearings used in the grinding rollers are such that extremely long lifetime can be achieved. However, it may nonetheless be necessary as is the case with any mechanically moveable parts to replace the bearings. In the following the various steps as to how to replace the bearings and sealing are described. For this purpose, the grinding rollers have to be removed as described above.
a) Disassembly of bearing
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Once the two bearing covers are removed two connections on the bearing housing are exposed. Through these holes, oil can be pumped by means of a hydraulic hand pump in between the outer rings of the bearing and hearing housing allowing for the bearing to be removed.
b) Fitting of bearing and sealing
1. Fasten bearing cover on housing without using the O ring or radial shaft sealing rings.
2. Heat up bearing housing and bearing cover to about 110℃.
3. Push outer ring of cylindrical roller bearing into housing until it rests against bearing cover.
4. Distance ring to be pressed into bearing housing until it rests against the outer ring of the cylindrical roller bearing.
5. Self aligning roller bearing to be pressed into bearing housing.
6. Inner ring of cylindrical roller bearing to be heated in oil bath up to about 120℃ and then pulled onto shaft.
7. Remove bearing cover as soon as bearing housing has cooled down to ambient temperature.
8. Inner distance ring to be put into housing so that it rests against the self-aligning roller bearing.
9. Shaft to be pressed carefully into housing until inner ring of cylindrical roller bearing rests against inner distance ring.
10. Mount locking washer.
11. Mount sealing rings into bearing cover.
12. Mount bearing cover by means of mounting ring.
13. Remove mounting ring.
14. Assemble grinding roller and pressure bracket.
15. Mount grinding roller into mill.
Grease and oil the rollers according to instructions (see lubricant chart) prior to
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putting the mill back into operation.
(4) Replacement of grinding roller segments
For replacing worn-out roller segments, if they are worn mostly on one side, for turning them around on the roller body, it is recommended to remove the roller from the mill. For ease of removing and refitting the segments, a tilting arrangement may be fitted on the inner wall of the mill housing allowing for the segments to be removed from the mill and new ones to be refitted by means of a tackle suspended from a crawl ( sort of wall mounted tilting crane).
One roller has twelve segments. Three of them are partly covered by the pressure yoke whereas the three opposite ones are facing the grinding plate. So start with the three segments on the right side or left side of the roller. After having removed at least two of three segments on one side, the new one will be fitted. Then turn the roller on the grinding plate with the auxiliary drive so that another segment can be replaced.
Since the roller turns anticlockwise, the second segment to be replaced is either that following the first one top right which has been fitted before on the roller body or that following the first one bottom left fitted before depending on whether you start working on the right side or left side of the roller.
Procedure for replacing or turning the segments is as follows:
a) Loosen clamp bolts.
b) Remove clamp plates.
c) Lift off segments and take them out of the mill.
d) Clean contact surface and check it for unevenness.
e) In case the contact surface on the roller body is not smooth, use metal putty to improve the seat of the segments or weld the contact surface. But make sure not to cause any torsion of the roller body as a result of heating it up too much during welding. It is also recommended to dismantle the bearings and sealing from the roller prior to welding.
f) Mount new segment or turn the old one around 180 degree. Make sure that
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the edge of the contact surface of the segment is rounded so as to avoid any concentrated force on the roller body.
g) When laying the segments, ensure for proper sealing of the lateral contact surfaces and base surface. Check by means of a feeler gauge. On the lateral contact surfaces, a 0.05mm clearance between the contact surface of the segment and that of the roller body is allowable, whereas the clearance between base surface of segment and roller body must not exceed 0.1mm. if these values are exceeded, the segment needs to be ground with a hand grinding machine As already described under e) above, the seat of the segments on the roller body may be improved by using metal putty if mechanical machining can’t be done.
h) Segment to be pressed tight onto roller body by means of a special pressing device or hydraulic pump. Pressing must be maintained until clamp plates are fixed and clamp bolts are tightened with a torque wrench.
i) Fix clamp plates
j) Tighten clamp bolts with torque wrench. (Torque to be observed is indicated on drawing of grinding roller.)
k) The segments are provided with cast noses. If new segments are mounted, these noses might have to be ground.
l) We wish to point out once again that proper seating of the lateral contact surfaces and base surface is imperative without exceeding the tolerances indicated above. If required the lateral contact surfaces are to be machined as described under g) above.
7.2.3 Disassembly and assembly the reducer in mill
In case there is damage at the reducer of mill making it necessary to remove the gear, this can be pulled out form below the mill without dismantling the latter. The grinding parts can be left in the mill. For any work to be carried out on the gear itself, the relevant manufacturer’s instructions are applicable.
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(1) The following preparations are required for removing the gear below the mill:
(a) Pump oil out of reducer. Follow gear manufacturer’s instructions.
(b) For safety reasons, all electric switches in the plant are to be off and secured (most suitably by padlock). Remove fuses for auxiliary drive, main drive and oil pump form fuse box in the central control room. Cut off all drives from power supply.
c) Once the oil has been pumped out of the reducer, remove all thermometers, thermocouples and manometers which hinder the removing of the gear.
d) Remove coupling protection and any other protective arrangements. Also disengage compressed air line from claw coupling, if any, and any other piping that are hindering.
e) Disengage all electric feed lines of all motors that are to be removed, i.e.
auxiliary drive and main drive of mill. Don’t forget to mark the different cables to reconnect them correctly later on.
f) Remove auxiliary drive complete with supporting stand as well as mill main drive with motor support. In case, for ease of alignment, shims had been placed under these supporting structures, these must be marked to facilitate reinstallation when the parts are remounted later on.
g) Start removing the reducer now.
(2) Removal of reducer below the mill
As already stated, for this purpose, the mill is not required to be disassembled nor do the grinding parts have to be removed Proceed as follows:
a) Unscrew and remove from below the three covers provided in the bottom of the
a) Unscrew and remove from below the three covers provided in the bottom of the