Winter's coming. And with it comes time to perform needed maintenance chores at the asphalt plant. When setting up a plan of attack some people find it helpful to use a checklist to determine which items need special attention and which ones need only regular servicing. What follows is the author's inspection protocol, used to formulate a schedule for a plant maintenance and repair regimen.
   In Part 1 we talked about drum-mix plants, and the equipment common to both drum and batch plants. This article will concentrate on the batching tower and its specific requirements. As you perform these inspections keep a clipboard handy to note any needed repairs. Marking paint is also useful to highlight units in need of attention.
As stated in Part 1, the author is not familiar with every conceivable combination of equipment in our rapidly changing industry. Omissions are bound to happen. Keep in mind that this article is intended as a guideline and in no way professes to be the last word on the subject. Feel free to delete or add sections as needed to suit your particular application.
    NOTE: Remember to follow OSHA lock-out, tag-out regulations during these inspections.

     HOT STONE ELEVATOR:
   Hot elevator chains operate in a very hostile environment. Heat, dust and stress all work together to promote wear. Unless we track that wear and address it at the appropriate time, things can come shuddering to a halt. As we all know, when the bucket-line breaks digging out the 300 degree rock, untangling the piled up chain and making the needed repairs is a time consuming chore. One most of us would rather avoid, if at all possible. An inspection, using a critical eye, can go a long way towards dodging the proverbial bullet. Begin by removing all access and inspection doors.
  1- An external inspection is first. Look for thin metal, dust leaks and obvious structural damage.
  2- Remove the guard and inspect the hot stone elevator's drive system. Chain drives are common. If your unit use this method of propulsion it is important that you inspect the chain for excessive wear and both sprockets for any signs of damage. Look for a condition called 'fish eyeing', or cupping on the load side of the teeth. Mark any abnormalities you find.
If your unit is belt driven you need to look for cracked or glazed belts, excessively worn sheaves and a loose condition which requires adjustment.
  3- Examine the motor and gearbox for any abnormalities. Mark for attention anything that raises doubts.
  4- Inspect both upper bearings. Use a bar to pry the shaft around. With the weight of the chain on the shaft, this operation will require considerable effort.
  5- Look through the access door and examine the top sheave. On friction drives look for irregular wear patterns, breakage or looseness. For toothed sprockets look for excessive wear, fish-eyeing and obvious damage.
  6- Some elevators use idlers. It is essential that each one of these receive an exacting examination. Idler shafts have been known to break, and under certain conditions stall the elevator. On sprocket driven units the bucket chains have broken as a result of the impact with the idler shaft. NOT A PRETTY SIGHT! Look at each idler. Bent or grooved shafts and worn wheels must be scheduled for repairs.
  7- Look at the tail shaft and traction wheel. Is the wheel egg shaped? Is it worn out or loose? Are the bearings in good condition? Do the adjusters work? How about the shaft's dust seals, are they working? Mark anything that needs attention.
  8- Look at the individual chain links. If they are worn to the pins and the side bars abraded to knife edges do you really want to head into a paving season with them?
   Note: When ordering hot stone elevator chain, get the best you can afford. I know of no other situation where the adage 'you get what you pay for' applies as well.
  9- Lastly, examine each and every bucket. Look for excessive wear, cracks or missing bolts. Any severely distorted bucket is a liability and should be replaced.

    See the GENERAL MAINTENANCE section near the end of this article.

    SCREENS:
   The screens are very important to the operation of a batch plant. In fact, they are the only reason to own a batch plant as opposed to a drummer. Any examination of this unit should be done with an eye toward efficiency.
  1- Start with an external inspection. Look for missing parts, such as lid hold-downs, and for signs of dust leaks. Look at the skirting seals under the units. Are they there or long gone? Examine the top covers. Are they worn out where they rest on the frame or other lids? All these conditions should be addressed. Check to see that the screens move freely. If they don't, look for a buildup of aggregate under the drive end. Check the size of the material. If it's a useable size that normally goes into a hot bin the accumulation could point to leaks in the oversize discharge, exacerbated by screen flooding and carry-over.
   Note: Often, this condition can be addressed by repairing the leaks and reversing the screens so that they throw the material back toward the hot elevator. This makes the material stay on the screens a bit longer, giving it time to work through the screen cloth. This same approach can sometimes solve sampling problems which are caused by carry-over at higher production rates. It's not uncommon for a plant operator who has his machine calibrated and in 'spec' at a certain tons-per-hour rate to find he is being forced to run his plant ever faster in order to meet production demands. As production rates increase, so does the carry-over rate. Under the right conditions the finished product can go out of 'spec', leading to hair pulling and a midnight recalibration session. If you've experienced this scenario, try reversing the screens. This should produce a more consistent mix through a wider range of feed rates.
  2- Next, remove all the lids and side covers. It's a good idea to take a fire hose and clean the accumulated fines off the screen frames and springs. It makes it much easier to find damaged and cracked components. Caution: Remember to open all the gates below and provide a way for the water to escape from under the plant. Once the unit is clean you should inspect it for broken springs and cracks in the framework.
  3- Closely inspect each screen cloth. Look for excessive wear and broken wire. Check the screen trays. Are they all tight? Is the screen cloth secure? It's good insurance to stock a complete change of screens. When stored in a custom built rack on the screen deck, they are ready for installation with a minimum of fuss. Don't forget to keep a supply of the appropriate bolts handy.
  4- Examine the screen drive and eccentric. Follow the manufacturer's recommendations as to periodic maintenance. In general, you should look for damaged components, worn sheaves or belts and broken motor mounts.

    HOT HOLDING BINS:
  1- Inspect the inside of each bin. Look for thin metal, missing partitions and any structural damage. Check the overflow chutes and hats, if used, for leaks. Schedule maintenance on anything amiss. Look at the bin dividers. Do they go up close enough to the bottom of the screens to prevent cross-bin contamination?
  2- Check the gates and their pivots/rails. Do they work freely? Are they excessively loose?
  3- Examine the air cylinders. Check for leaks or loose rod and pivot bushings.
  4- Schedule the air solenoids for kits and cleaning. The same for the air oilers. This is cheap insurance and could possibly avert a problem at a later date.
    Note: Spare solenoids, air lines, a cylinder and an oiler might possibly shorten 'downtime' in the event of a component failure.

    AGGREGATE WEIGH HOPPER:
   The examination of this unit is essentially the same as for the hot bins with the exception of the weigh system.
  1- Check the basic iron for thin spots.
  2- Check the gate and its pivots/rails.
  3- Check the air system. Again, schedule the solenoid and oiler for kits and cleaning.

    Weigh system:
  1- Begin by cleaning all the knives/pivots. Use compressed air. Do not lubricate them once they are clean.
  2- Inspect all knives and pivots. Look for loose, missing or misaligned components. If you find anything wrong contact a reputable company that specializes in scales. Have the scales repaired and calibrated.
    Note: It's best to schedule this activity for a time when all other repairs to related equipment have been completed. The addition or deletion of metal in the weigh hopper can drastically affect calibration.

    ASPHALT OIL INJECTION SYSTEM:
   Generally, two types of oil injection systems are in use: Gravity feed and forced feed. The checklist for the weigh system and for the asphalt oil bucket is nearly the same for both systems. The gravity system has a clapper valve and some associated air controls. The forced feed system uses an injection pump which we will address last.

    Asphalt oil buckets:
  1- Examine the exterior of the unit. Is it covered in asphalt? If so, from where? Is it caused by overflows which could be pointing to a problem with the scale read-out and its signal to the blending computer, or possibly from a sticky fill valve that occasionally fails to shut off when told to do so?
  2- Check the heat transfer oil system for the bucket. Does it leak? Does it work? Check to see if the hot oil lines are binding the bucket, possibly resulting in inaccurate readings. If so, schedule repairs.
  3- Examine the clapper valve on the gravity system. Does it fit properly and seal? Are the pins worn out? How about the air cylinder, is it worn out? Air lines in good shape? As with all the other solenoids and oilers, it's a good idea to schedule those on the oil injection system for a cleaning session and then install tune-up kits.
  4- Check the spray bar where it enters the side of the pugmill. Is it free or does it bind? Under the right conditions a binding spray bar can lead to abnormally high oil contents. You should look for an accumulation of material between the bottom of the spray bar and the side the pugmill. This material can restrict the downward movement of the oil bucket as it fills and cause the scales to read lighter than the amount actually in the bucket. Any build-up here should be scheduled for removal.

    Oil bucket fill system:
  1- Check the overall condition of the valve. Is it leaking? How about its connections? Does the heat transfer oil system work? Does it leak?
  2- Examine the air actuation system. Use the same criteria to evaluate it as we've used throughout this inspection.

    Weigh system:
   This procedure is the same as the one for the aggregate weigh system.
  1- Clean all the knives/pivots. Use compressed air. Don't lubricate them.
  2- Inspect all knives and pivots. Again, look for loose, missing or misaligned components. If you find anything wrong contact a reputable company that specializes in scales. Have them repaired and calibrated.
    Note: As with the aggregate scales, it's best to schedule this activity for a time when all other repairs to related equipment have been completed.

    Oil injection pump:
  1- Check the pump and all its lines. Schedule any leaks for repairs.
  2- Check the drive system. Some pumps use couplers and a direct drive motor, others use a belt drive while still others are driven by a chain off the pugmill itself. Whatever method yours uses examine it for the same flaws we've discussed on other units, such as fish-eyeing, worn sheaves/belts and couplers.
  3- On units that use a vacuum breaking valve to control emptying the weigh bucket you should check it and its operating system for any problems. For air operated systems start by checking the air cylinder. Is it worn out? Are the air lines in good shape? As with all the other solenoids and oilers, it's a good idea to schedule these for a cleaning session and then install tune-up kits.


    PUGMILL:
   Follow OSHA Lockout-Tag out regulations before inspecting this potentially lethal unit.
  1- Remove the inspection doors and examine the interior of the unit. Look for excessively worn shanks, tips and liners. Cracked or broken components should be slated for immediate replacement.
  2- Check the asphalt injection spray bar. Are all the spray nozzles in place? (see note 1) Is the bar worn thin on the top where the aggregate from the weigh hopper cascades over it? (see note 2)
  3- Examine the mainshaft bearings. Again, use a bar to pry the shafts around. Mark for replacement any bearing you are in doubt about.
  4- Inspect the drive assembly. With chains, look for 'fish-eyeing' and excessive wear. For belt drives look for glazed or loose belts and worn sheaves. Special attention should be given to those units that use shaft couplers. Check for any signs of damage or movement. Again, when in doubt replace them. Failure of one of these devices on certain pugmills can throw the timing off, resulting in catastrophic damage to the machine.

    Note 1: Several oil related problems plaguing hot mix manufacturers can be traced back to the spray bar. One, sluggish emptying of the weigh bucket, can result in slowed production rates. Which usually leads to the removal of some of the spray nozzles in an attempt to speed things up. This, in turn, leads to the second, potentially more damaging issue: Spray nozzles control the distribution of oil in the mixer. If adjusted and sized properly the oil is uniformly distributed throughout the pugmill. If set-up incorrectly, or removed altogether, the resulting spray pattern can produce lean/rich spots in a particular batch of mix. If the state's sample person gets mix from a lean spot and his results reflect a low oil content, the operator adjusts the oil upward to avoid going out of spec. If, on his next test, the inspector pulls the sample from a rich spot the resulting jump in oil percentage could easily put a plant out of spec and into penalty territory. If the swing is bad enough it could result in a mandatory shut-down and plant recalibration. Put quite simply, all of this could be due to an easily addressed oil distribution problem in the pugmill. Using a slat conveyor, batcher and holding silo usually eliminates this problem. However, for those plants that operate without a silo the problem can easily put you into a adversarial situation with the state DOT since all the data you have (tank stickings and quantities used over time) shows that you are, in fact, putting in the correct percentage of asphalt oil. Unfortunately, both you and the state would be right under these conditions. But the state has the final say, making it best to avoid the situation from the start.
A word of advice: Remove or modify spray bar nozzles only when you thoroughly understand what result that action will have. If you are experiencing distribution problems like those discussed earlier, analyze the issue. Once you see what's going on don't be afraid to experiment. In general, you want to be sure that oil comes out the farthest end of the spray bar from the weigh bucket. If you must plug nozzles to get the oil to the far side, start with the one closest to the weigh bucket since it gets oil first and has it last.
    Note 2: The addition of a trough on top of the oil spray bar to catch and hold a quantity of aggregate can go a long way toward eliminating wear on this item. Try a 6" x 1" x 1/8" channel iron. Have it welded to the top of the bar, the uprights pointing skyward.

    GENERAL MAINTENANCE:
   All gearboxes and speed reducers should be examined for oil leaks and noted for repair if needed. It is a good idea to schedule each box for an oil change during the winter months. Collect properly marked samples from each box and give them to a reputable oil analysis company, they will be able to detect any abnormalities and perhaps avert a surprise breakdown at a later date.
   A thorough greasing regimen should be completed prior to plant restart. For electric motors, follow the manufacturer's recommendations. Each year numerous motors fail because uninformed service personnel pump them full of grease like they would a troughing roller. Some motors vent excess grease internally, so once enough of the stuff is pumped in failure is bound to follow. Most motor manufacturers recommend a yearly greasing schedule consisting of one or maybe two pumps of the gun. Read and follow their guidelines.
   You should check all wiring and junction boxes for any condition that could render the workplace unsafe. Mark anything that is substandard.

    HOUSEKEEPING:
   This is a safety issue. A clean place to work, statistically, has fewer accidents than one covered in grease and oil, and the accumulated debris of years of maintenance.
   Another issue is that a clean, neat, squared-away facility is looked upon more favorably and less critically by state plant inspectors and officials from the DEQ than one which appears to have been resurrected from the briny depths of the ocean for the sole purpose of blundering through the upcoming job. Think about it.
   Good luck on your repairs, and in the coming season. Pave on men, turn 'er black and don't look back!
 

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