Down and Out

There are many reasons for shredder downtime.

January 16, 2012

Unplanned shedding system downtime can be caused by mechanical failures and by poor process control that does not allow your plant to reach it optimal production capacity.

Tracking Downtime Events

Downtime events can be tracked manually or automatically.

Shredder operators can manually track downtime events using a notebook, pen and clock. This method is dependent upon the discipline of the operator, and only major events tend to make to the list.

A computer database system that is tied into the electrical control equipment of the shredding system allows downtime incidents to be recorded automatically.

When a downtime event is detected, determined by the idle state of the shredder's main drive motor, for a predetermined time frame, these programs prompt a delay input screen. The operator selects a reason for the delay, and the program automatically tracks the amount of downtime until the system is operating again.

At the end of the shift, these programs record production totals and add up the unplanned downtime incidents. Users can then create reports on the plant's performance and downtime incidences.

The biggest benefit of automatic, computer-based tracking systems is that they have the ability to track even the shortest of delays, such as gaps in the material feed. This allows the manager to track the performance of the shredding system and also of the feeding equipment.

Studying Downtime Incidents
The downtime information presented here is a compilation of the top 10 downtime reasons from shredding plants with capacities from 80 tons per hour to 350 tons per hour for January through the end of September 2011. In these nine months at the 22 plants that we have information for, a total of 40,371 downtime events were recorded. These 40,371 events resulted in 3,395 hours of downtime, or 141.4 days.

These 22 shredding plants produce an average of 140 tons per hour; therefore, these 3,395 hours of downtime cost a total of 475,300 tons of production. If we say that shredded steel could be sold for $350 per ton, these companies lost a total of $166.4 million in gross revenue on shredded steel, not to mention the loss of revenue from the sale of nonferrous metals, as a result of lost production.

Breaking Down Downtime
The top 10 downtime incidents from the least to greatest in terms of hours are:

  • No. 10, miscellaneous issues, 278 events, 17.68 hours;
  • No. 9, main motor issues, 300 events, 122.54 hours;
  • No. 8, electrical issues, 268 events, 125.33 hours;
  • No. 7, downstream mechanical, 282 events, 254.53 hours;
  • No. 6, cranes and gaps in feed, 15,164 events, 286.34 hours;
  • No. 5, shredder issues, 330 events, 301.44 hours;
  • No. 4, material issues, 2,162 events, 386.16 hours;
  • No. 3, downstream jammed equipment, 1,985 events, 400.93 hours;
  • No. 2, feed chute issues, 19,591 events, 636.47; and
  • No. 1, hydraulic issues, 11 events, 863.05 hours.

No. 10, Miscellaneous. The 278 downtime events attributed to miscellaneous issues resulted in 2,475 tons of lost steel scrap production at a rate of 140 tons per hour, equal to $866,320 in lost revenue at $350 per ton. Because no detailed descriptions were offered, there is no way to know what to fix.

I suggest avoiding the use of such a category in your analysis, instead pinpointing the reason for the delay. It's hard to explain to your boss why you lost 17.68 hours of production, or more than $866,000 in revenue, if you don't know the reason why.

No. 9, Motor Issues. These 300 events resulted in 122.54 hours of lost time, or 17,155.6 tons of lost production and $6 million in lost revenue. The main cause of motor issues was overloading the main motor. Other issues that cause downtime in this category are exceeding the thermal capacity of the motor and dealing with what happens when the main motor shuts down unexpectedly.

Installing an automated feed system, such as Shredder Cruise Control, to feed the shredder more consistently and to keep the thermal capacity of the main motor in check at all times can help to avoid downtime related to motor issues.

No. 8, Electrical Issues. These 268 events resulted in 125.33 hours of downtime, 17,546.2 tons in lost production and $6.1 million in lost revenue. Overloading the motors in downstream equipment most often led to the downtime. Most of these issues were on the UMO (under-mill oscillator), first conveyor, magnet feed vibrators and magnet drums.

These issues can be addressed by installing an automated feed system to feed the shredder more consistently and to keep the flow of material consistent throughout the downstream system. Also, checking the amps that each of these motors uses and properly sizing drives for this equipment will help to keep these smaller motors in your shredder's downstream from overloading.

No. 7, Downstream Mechanical. These 282 events equaled 254.53 hours of downtime, 35,634.2 lost tons and $12.5 million in lost revenue. Most of issues in this category include ripped conveyor belts, broken gear boxes on conveyor drives, component failures on UMOs and failure of conveyor idlers.

Preventive maintenance and routine inspections of each moving component in your shredding system can help to prevent these downtime incidents. Idlers that no longer turn should be replaced immediately, oil in gearboxes should be checked and changed regularly, the drive and toggles on the UMOs should be inspected and spares should be kept in stock for all of these items. Ripped belts can be caused by long, sharp pieces (pokers) in your shredded steel.

No. 6, Cranes and Gaps in Material. These 15,164 events resulted in 286.34 hours of lost production, 40,087.6 in lost tons and $14 million in lost revenue. The majority of issues in this category are too few cranes feeding the shredder and crane operators that are not able to fill in the gaps on the infeed conveyor.

Operators can address this issue by increasing the number of cranes and loaders that are feeding the shredder. Also, it would be beneficial to train crane and loader operators to fill in gaps on the infeed conveyor and to work more quickly.

No. 5, Shredder Issues. These 330 events equaled 301.44 hours of lost time, 42,201.6 in lost tons and $14.7 million in lost revenue. Broken wear parts (anvils, hammers, grates and side liners) inside the shredder are the major issues.

These issues can be prevented by inspecting your inbound scrap more diligently and training your people to understand what to look and feel for when unloading mixed and baled scrap. Massive unshreddables are the No. 1 cause of broken wear parts. It is always a good idea to have at least one complete set of liners for your shredder as well as two full sets of grates and at least 10 sets of hammers on the ground so that you have the parts necessary to operate until your next wear part order arrives.

No. 4, Material Issues. These 2,162 events resulted in 386.16 hours of downtime, 54,062.4 tons of lost production and $18.9 million in lost revenue.

The major issue in this category is unshreddable and explosive items inside the feedstock. This category sounds similar to the previous one, but this one deals with the actual handling of an unshreddable or with the damage that an explosion has caused.

A pivoting first conveyor can help to lessen the amount of time spent handling unshreddable materials after they have been rejected from the shredder. With a push-button on the control screen, the operator can remotely move the first conveyor, run the unshreddable onto the ground, then move the conveyor in back into position and begin shredding without having to touch the material.

No. 3, Downstream Jammed Equipment. These 1,985 events resulted in 400.93 hours of downtime, 56,130 tons in lost production and $19.6 million in lost revenue. Issues in this category include downstream equipment, such as conveyors, vibrating tables, magnets and Z-boxes, being jammed by material.

Transition points in the downstream system should be free of material buildup. Long bars or pokers also can cause jam-ups downstream. Changing the configuration of the grates in your shredder and shredding more densely can help to prevent pokers. A poker picking device can be added to the start of your downstream system to remove these long bars.

No. 2, Feedshoot Issues. These 19,591 events totaled 636.47 hours of lost time, 89,105.8 lost tons and $31.2 million in lost revenue. Issues in this category include material blockages in the feed chute, feed rolls that cannot grip and feed material, material that is too large and broken chains on feed-roll drives.

Materials to be shredded should be inspected and should be sized to fit inside your shredder. The teeth and bars on your feed rolls should always be kept in like-new condition; sharp teeth and edges give the feed rolls the best opportunity to grip the material and feed it into your shredder. Feed chute floors and side walls also should be kept in like-new condition. Feed chutes that have worn areas and low spots should be repaired so that material will not hang up in these low areas. Lastly, build guards to protect scrap from getting into feed roll-drive chains; this is the main reason these chains break.

No. 1, Hydraulic Issues. These 11 events equaled 863.05 hours in downtime, 120,901.2 tons in lost production and $42.3 million in lost revenue. Issues in this category included failed hydraulic pumps on double-feed roll (DFR) units and failed cylinders on these units.

Preventive maintenance and routine inspections of each hydraulic unit in your plant can help avoid unplanned downtime. Oil and filters must be changed regularly. Dirty oil is the No. 1 killer of hydraulic pumps and motors that are used in our shredding systems. Cylinders must be inspected; oil leaks mean that seals and glands are worn and need to be replaced. Cylinder clevises and pins should be repaired as they wear.

Avoiding Future Instances
Unplanned downtime can be costly. It is imperative that you have a system to help you identify the causes of such instances. Once you understand what is causing these issues, corrective action can be taken to minimize the amount of unplanned downtime that you experience.


The author is director of new equipment sales for Riverside Engineering, San Antonio. He can be contacted at