Intensive mixer preventative maintenance program.

An intensive mixer is a hardy piece of equipment designed to very precise tolerances for maximum life. The types of materials processed using a mixer are typically very abrasive and corrosive. Life expectancy of a mixer varies anywhere from two to 20 years, depending on the type of materials processed. During this life span, rotor to tip clearance begins to increase and the tips also become rounded, reducing the shear rate. This causes the fill factor to increase. Throat wear allows stock to stick around the weight. As wear increases, so does the batch size and cycle times. Also, dispersion is adversely affected.

In order to determine when a mixer is worn out, monitor the quality of the material and production rates. When quality begins to drop, it will continue at an increasing rate due to the diminishing of the hard surfacing as the base metal is approached. If quality should start to drop, and the clearance has increased, consideration should be given to replacing the mixer body prior to the quality of the product becoming completely unacceptable.

These are the reasons why a preventative and scheduled maintenance program is important. It will help guarantee that the mixer will operate with a minimum of down time and maintenance expense for its useful life.

Another aspect of increasing concern is raw material and process control for consistent product quality. Mixer variables such as rotor speed, rotor, side and door top temperatures, coolant flow rates and ram pressures have to be monitored and checked on a periodic basis to be sure they are, in fact, what your production instrumentation indicates.

By establishing and following a comprehensive inspection procedure, a preventive maintenance program and lubrication schedule, you can maximize the useful life of the mixer and prevent costly unscheduled down time and loss of production.

On site inspections

The purpose of on site inspections is to determine the mechanical condition, predict mechanical life, locate areas requiring attention and identify replacement parts required.

Inspection involves a walk around visual check and an actual hands on measuring of clearances.

One of the best tools available for planned maintenance is an on site inspection of your mixer by a qualified technician from the manufacturer. It is recommended this take place twice a year, or at the very least, once a year. The machines should be inspected at four month intervals. Among the items to be inspected are:

* While in operation, the movement of rotors axially and radially can be measured for excessive movement, as dust stops are affected by this.

* While in operation, the movement of the hopper floating weight can be observed for excessive side movement, which indicates throat wear and piston rod gland wear. Also, the action of the floating weight on the piston rod can indicate if the rod is bound up at the point it attaches to the floating weight due to lack of lubrication. Loose hold down bolts can also be observed, along with loading door operation (wiper bars, air cylinders, etc.). This is also a good time to check with the operator for any problems he may have been having.

* While in operation, the discharge door operation can be checked for proper operation, such as the latch/door timing, contact plate condition, time it takes to open, stock sticking, water leaks, leakage around the seal, door shaft movement, rotary actuator operation, hydraulic leaks, check for safety hooks, and also the hydraulic unit itself.

* While in operation, the dust stops can be checked for lubrication, if any fittings are leaking, if the rings are pulsating excessively, if excessive stock is leaking out, and if the hydraulic pressure used is correct (hydraulic type) per the manufacturer's recommendation. Also, check the hydraulic power unit while the door is being opened to see if the pressure drops below the original setting for the dust stops. Dust stops work well if maintained properly (see the separate section on installing dust stops) and installed properly.

* While in operation, the dust stop and grease lubricating system should be checked for over-heating of the drive motors, any excessive vibration, and that the flow rates are properly set.

* While in operation, check the gearing, listen under both load and no load conditions for vibration and oil levels.

* While in operation, check the couplings for leakage of the seals.

* While in operation, check for water leaks, hot spots and the delta T if gauges are installed.

* With the machine shut down and properly locked out for safety (including the related equipment), the internal inspection can begin. Items that will be checked from within the chamber are as follows:

a) Rotor tip to side clearance, six places each rotor.

b) Rotor end to end late clearance, eight places. Also, check condition of rotor end plates.

c) Rotor type and overall condition, rotate by hand from the main motor shaft and observe.

d) Measure throat to floating weight clearance and condition, along with floating weight height in the throat.

e) Measure door seal area on the chamber sides and observe the bore condition.

f) Measure the door latch position (should be flush with the housing), and check for cracks and overall condition.

g) Measure the door to the support clearance and check overall condition.

h) Check the chute area for housekeeping and areas that can cause cross contamination.

i) Check thermocouple protrusion and condition. Also, if equipped with oil injector, check for leakage.

Upon completion of the inspection, a complete report will be generated, including recommendations.

Note: All safety and lock out procedures must be used.

Replacement of self sealing (SSA) dust stops

* Clean the dust stop socket area prior to any disassembly of the dust ring.

* Loosen the spring tension adjusting screws on the container ring (use caution not to lose spring and buttons). Springs and buttons may require a small hook or screwdriver to pry them out of the retaining hole due to stock buildup.

* Remove the lube and process lines, then the wear ring retaining bolts. This is a split ring, however, and it may not be removed easily due to stock buildup.

* Remove the floating ring by pulling it straight back using the pull holes provided on the O.D. (Caution should be used not to lose the two keys that drive the ring.) This ring is a split type and also has an o-ring groove on the inside which will have to be replaced.

* Clean and inspect all the removed parts, along with the rotor collar and the dust stop socket for wear and damage to determine if they are reusable. If in doubt, replace them.

* The entire socket area should be cleaned thoroughly prior to reassembly.

* Install the new or reused parts in the reverse order, making sure the floating ring is free to move (fit floating ring with keys on the rotor collar without the o-ring installed to check free movement; if it hangs up, there is a good chance it will hang up during operation) axially.

* It is strongly recommended that the dust stops be lapped in. Lapping in most cases can be completed in four to six hours with the use of a lapping compound called Time Saver. This type of lapping compound does not require the dust stops be removed and cleaned, as does the carborundum type. It can be flushed out by running the machine with the oil flowing.

Replacement procedure for hydraulic yoke dust stops

* Clean dust stop socket area prior to any disassembly of the dust ring. Remove bearing guard covering the oil seals. This is split ring with two socket head cap screws which clamps it around the rotor shaft.

* Disconnect hydraulic line from yoke cylinder after bleeding the hydraulic power unit pressure to zero pressure.

* Remove yoke retaining nut and yoke from the socket area. Put aside for cleaning and inspection for proper set screw setting. Change the yoke cylinder at this time, if required.

* Remove all lube lines from gland assembly up to the lube feeder bolts. Check all flexible hoses and check valves. Replace if needed. Be sure the check valves are free flow in the correct direction, as well as being located as close to the lube feeder as possible when reinstalled.

* With the ring still in place, loosen the lube feeders and socket head cap screws. Pry the gland assembly out (if necessary) into the open for access and finish removing the lube feeders and socket head cap screws. This will separate the bronze gland ring facing from the gland ring backings. Next, remove the gland ring backing split line bolts. The ring will now fall apart into four pieces and can be removed from around the rotor shaft.

* Clean the dust ring cavity, socket area and all parts that will be reused, as well. Closely inspect these parts for possible damage.

* Inspect the dust ring cavity and the condition of the rotor collar (mating face for the bronze gland ring face) for possible replacement, if required. A field replacement split rotor collar is available from the mixer manufacturer if replacement is required.

* With all parts inspected, cleaned and found to be reusable, you are now ready to install the dust stop assembly.

* To install, the reverse order should be followed. The only exception is the installation of a new Teflon o-ring on the O.D. of the bronze facing (if equipped with o-ring). It is important that the ring is installed evenly to avoid cutting of the o-ring. In the event the o-ring is cut, it must be removed and a new one installed and tried again until success is obtained. (If the o-ring is cut, leakage will occur.) The o-ring material should be cut approximately .062" (1.574 mm) short when installing it in the ring groove. This gap will close up when installing the ring in place within the cavity.

* The yoke adjustment should include. 125" (3.175 mm) minimum free travel, measured at the hydraulic cylinder area, measured with no hydraulic pressure applied, which will allow for rotor axial movement while mixing.

* Assuming the machine is ready to run, we recommend the lapping in of the dust stops.

Procedure for lapping hydraulic yoke type dust stops and SSA type

* Remove one lubrication line from a lube feeder (located on the gland ring backing, and for SSA type on the bronze wear ring). Install a grease fitting in the lube feeder. A 90 degree fitting and a short hose may be required to allow easy access when using a grease gun to inject the Time Saver lapping compound, which will be required for each ring.

* a) Set dust stop hydraulic pressure at 200 psi.

b) Start the lube system and adjust to minimum flow.

c) To fill the grease gun, use yellow label Time Saver lapping compound, mixed with a light hydraulic oil to the consistency of a 50 weight oil.

d) Start mixer at slow speed, if equipped with two speeds.

e) Connect the grease gun to one of the installed grease fittings. Approximately ten initial pumps will ensure the filling of the line to the face of the ring. This has to be done for each ring that requires lapping. Proceed with two pumps per fitting approximately every five minutes, while running at low speed, and gradually increase the hydraulic pressure to the recommended setting for the first hour.

f) After the first hour, switch the mixer to high speed, if so equipped, and repeat the five minute intervals with two pumps of the grease gun each time for a period of approximately eight hours. During this time, heat should begin to build up as the rings become seated. After the eight hours have passed and no heat is evident, there is a good possibility the ring or rings will leak under load. Further lapping may be required if a ring should remain cold. Also, the ring may have to be disassembled and inspected to determine if it has been installed properly.

Procedure for hardsurface lapping is the same with the exception that green label Time Saver is used and the lapping period is approximately 24 hours.

Preventive maintenance schedule

A basic preventive maintenance schedule is outlined below. It is recommended that plant management establish procedures to assure that this schedule is followed.

A properly maintained mixer will operate safely and productively for an extended period. The time and money required to accomplish these recommended steps are minimal when compared to possible production losses or replacement resulting from inadequate maintenance.

Caution: Always follow prescribed safety procedures when working on or around the mixer.

Each shift

* Check pressure gauges and supply sources for the following (refill tanks where required): Dust stop lubrication system; grease lubrication system; hydraulic system (pump pressure); air system (lubricator, filter, pressure); and bearing flood lube system (if equipped).

* Verify correct operation of the following (test operate if required): Floating weight; drop door and latch; hopper loading door; and dust stops.

* Make sure all instruments are functioning.

* Make sure the hopper is free of foreign objects.

Daily

* Grease: All selas and packing that are not covered by an automatic grease system; floating weight rod pin; and check grease reservoir.

* Check dust stops for indications of lube flow and correct operation.

* Check lubrication of discharge door and latch, hopper charging door and weight rod seals.

Weekly

* Check mixer drive lubrication system filters and the condition of oil.

* Check hydraulic system pressure - verify correct accumulator pressure setting.

* Clean dust stop area, but take care not to pull any lube lines loose with a hook. Do not disturb setting if dust stops are operating satisfactorily.

Monthly

* Inspect discharge door, latch and door top. Examine for evidence of misalignment (chipping or one sided wear).

* Inspect floating weight assembly. Examine for evidence of air leakage or worn packing.

* Inspect cooling system strainers (if so equipped).

* Grease drive couplings, taking care not to over-grease.

* Clean accumulated grease away from fittings and overflows.

Every six months

* Drain and flush all oil reservoirs. Clean filters. Refill with clean oil.

* Examine condition of drive gear teeth.

* Flush water/steam system.

* Examine all pipe and hose connections and fittings.

* Inspect mixing chamber and rotors.

* Repack main bearings if not on an automatic system.

Annually

* Have the complete mixer installation inspected by a qualified technical factory representative.

Rotac installation and setting

* Have the door hanging in the free position (approximately six o'clock).

* Set the timing mark on the spline shaft of the rotac at the bottom position (approximately six o'clock).

* Now install the rotac. Rocking of the door frame and rotac may help when trying to get coupling engagement. However, use caution not to lose the setting.

* With the rotac mounted and piped, you can now begin to adjust the spool for the proper slow down (deceleration).

* Rotate door into the closed position.

a) Loosen drive screw with 3/8" 12 point socket, three full turns.

b) Tap drive screw with hammer towards rotac to free drive washer wedge.

c) Looking at the rotac spool cam with door closed, position spool so that top is on top dead center. (Rotac mounted right of water, at water end). Top is at 135 [degrees] counterclockwise from to dead center if mounted right of water at drive end. Warning: Do not attempt to turn spool without freeing drive wedge.

d) Retighten drive screw securely when adjustment is complete.

* The door should decelerate and close completely with a slight bang. If not, adjust the spool following the fifth step.

* The latch should not be required to close the door completely. It is absolutely required when mixing.

Summary

The purpose of this column was to touch on the most troubled areas that are experienced within the industry, and help reduce unscheduled downtime. However, it did not cover all aspects of all mixers. There are so many different configurations of the same style machine in the field that in order to cover them all it would take weeks, or perhaps months to compile.

We have touched on the inspection of your mixer, dust stop removal and installation, lapping, troubleshooting and the preventive maintenance schedule.

Table 1 - troubleshoooting guide

Problem                       Cause

Ram will not move up          1. Leaking packing around the
or raises slowly,                cylinder or piston,
                              2. No air pressure.
                              3. Faulty controls.
                              4. Material sticking in
                                 hopper neck.

Material sticking in          1. Accumulation of material.
the hopper neck.
                              2. Oversize batch.
                              3. Improper loading of liquids.

Material sticking to          1. Machine temperature incorrect.
rotors or discharge door.
                              2. Indicated batch temperature
                                 incorrect.

                              3. Improper loading sequence.
                              4. Ram pressure too high.
                              5. Door open to close time too short,
                                 trapping material.

Stock temperature             1. Ram pressure too high.
rises too quickly.            2. Improper batch size.
                              3. Improper loading sequence.

                              4. Cooling system not working
                                 properly.

Stock temperature does        1. Ram pressure too low.
not rise as required.         2. Improper batch size.
                              3. Improper loading sequence.

Discharged batch not          1. Improper batch size.
satisfactorily mixed.         2. Improper loading sequence.
                              3. Basic ingredients not of
                                 sufficient quality,

Excessive leakage             1. Incorrect lubrication.
through dust stops,
                              2. Incorrect adjustment for wear.

                              3. Worn pads.
                              4. Poor face contact.

                              Type SSA only

                              1. Adjusting screws not set
                                 correctly at installation.
                              2. Floating ring hung up.

                              Hydraulic yoke type
                              1. Improper hydraulic cylinder
                                 setting.

Problem                       Possible solution

Ram will not move up          1. Repack - see maintenance section
or raises slowly,                for instructions.
                              2. Correct condition.
                              3. Correct condition.
                              4. See next problem.

Material sticking in          1. Clean hopper neck. See maintenance
the hopper neck.                 section for instructions.
                              2. Reduce batch size.
                              3. Use more care in loading liquids,
                                 An oil injection system will
                                 help cure this problem.

Material sticking to          1. Vary machine temperature within
rotors or discharge door.        limits of material.
                              2. Check batch with handheld
                                 pyrometer and compare with
                                 indicated reading.
                                 Check thermocouple if
                                 discrepancy exists,
                              3. Vary sequence of loading.
                              4. Reduce pressure.
                              5. Allow door to remain open longer.

Stock temperature             1. Reduce pressure.
rises too quickly.            2. Resize batch.
                              3. Vary loading sequence or mixing
                                 technique.
                              4. If necessary, clean system. See
                                 maintenance section for
                                 instruction.

Stock temperature does        1. Increase ram pressure.
not rise as required.         2. Resize batch.
                              3. Vary loading sequence or mixing
                                 technique.

Discharged batch not          1. Resize batch.
satisfactorily mixed.         2. Vary loading sequence.
                              3. Verify with quality control
                                 department and/or supplier.

Excessive leakage             1. If required, fill lube pump.
through dust stops,              For further instructions,
                                 see maintenance section.
                              2. See appropriate maintenance
                                 drawings and instructions.
                              3. Replace worn pads.
                              4. Replace or lapp.

                              1. Adjust properly.
                              2. Remove and check for obstructions.
                              1. See appropriate maintenance
                                 drawings and instructions.