November 7, 2006
Glen Stapleton relies on his experience in troubleshooting for more than 30 years to discuss the most pressing pilger mill maintenance issues—causes, cures, and tips to prevent breakdowns and get the machines up and running again when they do fail.
Routine maintenance inspections are necessary for any cold pilger mill to perform optimally. The number, frequency, and quality of routine maintenance procedures have short-term and long-term effects on the machine's operations and capability.
Investigating more than 30 machines revealed that neglecting routine maintenance has a negative effect on tube quality and mill productivity. Some of the maintenance issues cited were severe.
During the past 10 years, many tube manufacturers have experienced work-related problems that have affected pilger machine operations and maintenance. Two pervasive problems are maintenance personnel changes and economic difficulties. Maintenance personnel changes involve job title and duty changes, retirements, and downsizing. Personnel changes tend to reduce the quality of maintenance a machine receives. Additionally, a lack of repair continuity affects the machine's overall operation. Difficult economic conditions may cause a company to reduce the amounts of time and money it is willing to spend on mill maintenance. Regrettably, reducing maintenance now generates more operational emergencies later.
Some tube producers have adjusted their mills to compensate for the lack of maintenance, which can cause questionable tube quality. In other cases, the adjustments limit productivity to compensate for unresolved quality problems.
Identifying the major maintenance issues and understanding their long-term role in the pilger mill's operation are the keys to a good maintenance program. Reviewing these problems, and their remedies, will allow a manufacturer to contemplate a different approach to restore product quality and machine longevity. Be aware that it may be more difficult than it seems. When a major subassembly creates a serious problem that lowers tube quality, the specific troublesome part may be difficult to identify and locate. The part can remain hidden or the flaw misdiagnosed as an isolated problem.
While many subassemblies can withstand slight misalignment, some cannot without affecting the process. For instance, the crankshaft and saddle are critical to machine alignment. If either of these becomes seriously misaligned, expensive failures and long-term tube defects result.
Pilger mill alignment encompasses two areas. First is the entry-to-exit system (including the saddle and tooling); second is the feed-and-turn gearbox. Because the tooling alignment has a direct impact on forming the product, saddle area alignment is critical. The feed-and-turn gearbox and feed bed are located farther from the tooling and are not as critical—they have more liberal alignment tolerances.
The machine manufacturer determines the alignment tolerances, which vary by machine size and are unaffected by the tube alloy or product tolerances. Acceptable misalignment depends on the kind of damage it can cause to the machine and the effect it has on the finished tube quality. If a tube OD requires a tolerance of ± 0.005 inch, the machine alignment within the saddle housing is approximately ± 0.002 in. The alignment must be tighter for closer tube tolerances. Permanent static alignment fixtures facilitate checking the machine alignment from time to time.
The saddle floats within the saddle housing. The saddle and tooling must float to accommodate incoming product size variations. Tooling tolerances must match the dynamic floating ability of the tooling to produce a finished product within the stated tolerances. The rigidity between the tube and the tooling determines the amount of floating.
Although many tube producers use the static alignment method, dynamic alignment is optimal for tube product tolerances of 0.0005 in. or less and for machines in excellent condition. Maintenance personnel should be aware that process parameters such as thermoexpansion, friction, material, and segmented load fluctuations have an impact on dynamic loading. Also, machine crashes and mill jams cause serious problems for dynamic loading and alignment.
When a building foundation shifts, settles, or cracks, the crankshaft can become misaligned (see Figure 1). Severe machine crashes or frequent, severe product jams also can cause crankshaft misalignment. A pilger mill is designed to withstand some misalignment, but it is impossible to compensate for foundation failure or severe abuse.
A misaligned crankshaft could cause abnormal bearing wear, reduced bearing life, premature bearing failure, and even crankshaft failure. It can also affect the bearings in various machine subassemblies connected to the crankshaft.
Saddle assembly misalignment causes the dies and mandrels to be misaligned because the saddle is connected to the crankshaft. The results are poor tube quality, poor tooling performance, counterweight misalignment, and abnormal wear or damage to counterweight ways and bearings. Mismatched or unpaired connecting arms misalign the saddle, create stress on the crankshaft, cause premature bearing failure, and lead to product problems.
Usually crankshaft misalignments are hard to identify with a simple inspection. To confirm crankshaft misalignment, use an alignment scope to site the machine's passline, crankshaft bearing housings, and other relevant assemblies. Combine the site readings (and their relationship) to quantify the extent of misalignment of the crankshaft and other related parts.
If the crankshaft misalignment is severe, you'll have to machine the crank housing bearing journals. This usually requires a portable milling machine, an alignment scope, and other precision instruments.
Drive-and-flywheel misalignment normally occurs when the building foundation cracks. It can also develop after replacing a troublesome part that caused the flywheel to shift off-center (see Figure 2). This type of misalignment often is seen
in the clutch or brake assembly. Shoes wear prematurely, or in severe cases, the brake or clutch malfunctions or slips intermittently. If the drive-and-flywheel misalignment and crankshaft misalignment occur together, the crankshaft bearings may wear quickly. Additionally, bearing holders or journals may become oblong and out-of-tolerance. In some cases, this may stress the crankshaft enough to cause it to fail.
Identifying drive-and-flywheel misalignment requires an alignment scope for two reasons. First, the distance between the drive, crank housing, and saddle housing is large. Second, the crank centerline is perpendicular to the passline.
You may need to use shims or machine the bearing holders to correct the misalignment. In some cases, you might have to modify the flywheel foundation.
Saddle misalignment can result from severe machine crashes, frequent mill jams, and long-term wear and stress (see Figure 3). Saddle assembly alignment, relative to the machine passline, is critical to machine performance and tube quality.
Saddle alignment has a direct effect on product quality. Dynamic alignment is important for machines that produce close-tolerance products. When a machine runs at high speeds, tube defects may decrease because the dynamic alignment, or saddle passline, shifts. The saddle hydroplanes, and the modulation effect of the crankshaft changes relative to the zonal saddle condition&—that is, high machine speeds change the saddle alignment.
Severe saddle misalignment causes the dies and mandrel to be misaligned relative to the tubing, creating ID and OD defects and spiral tubes. Components such as the dies, mandrel, saddle housing wear plates, rack-and-pinion gears, and entry and exit grips wear prematurely. Entry and exit systems become misaligned from the stress of a misaligned saddle. The saddle housing adapts to the saddle misalignment and becomes bowed or bent. Tube dimensions and surface quality deteriorate.
Saddle misalignment is revealed by checking the saddle alignment from entry to exit position, relative to the machine passline. Use a wire for a rough check from entry to exit position only. On a level saddle-housing base, check the saddle alignment from side to side—again, this is a rough evaluation. Use an alignment scope to measure precisely the relative misalignment position and amount of misalignment (from entry to exit and side to side) relative to the machine passline.
Saddle misalignment can be corrected, depending on the severity and amount of damage. Check all the parts related to the saddle and saddle housing for alignment. These parts include the saddle housing base, rack gears relative to the pinion gears, and the saddle housing sides relative to the saddle assembly.
You may need to machine any or all of the subassemblies to correct the misalignment. Shimming the wear plates also may correct it.
Severe machine crashes, frequent mill jams, saddle misalignment, and long-term stress and wear can cause entry and exit system misalignment. Entry grip misalignment can often cause tube misalignment relative to the tooling and cause bearings and gears to wear prematurely. The entry and exit system can be misaligned from the passline horizontally, vertically, or both.
Tubing misalignment effects normally are small, depending on the severity of the entry and exit system misalignment. In severe cases of tube misalignment, abnormal pressure is directed to the dies and mandrel. This causes ID and OD defects and can cause mandrels to break. Grips wear prematurely, and gripping the tube may become difficult. On reactive tube materials, entry and exit misalignment can cause galling and surface defects. The tube's dimensions and surface quality are likely to deteriorate. Use a wire to check the hollow shaft's rough position or an alignment scope to determine its precise position.
Use shims to correct a slight misalignment, or machine the base of the entry and exit system if the misalignment is severe relative to the passline and the crankshaft plane.
Gearbox misalignment can stem from severe machine crashes, frequent jams, severe bearing wear, gear failures, and long-term wear and stress. Two types of misalignment are internal and external misalignment.
Internal misalignment usually does not affect the product. On the other hand, it can have a serious impact on productivity. Frequent gearbox breakdowns interrupt production for many hours or days.
Gearbox misalignment is difficult to locate unless it is severe. In severe cases, various parts will show the effects of misalignment—damaged parts and burned or scored bearing housings. Gears, shafts, bushings, and bearings wear quickly or unevenly and are likely to break prematurely. Part failure can be caused by poor lubricant filtration and impact stress from product jams. The best way to determine gearbox misalignment is through an annual inspection. Disassemble the gearbox and inspect the parts and subassemblies to determine which parts exhibit abnormal wear, including bearing holders in this inspection.
External gearbox misalignment is difficult to isolate, yet can severely damage some machines.
Long feed-bed machines (those with one feed carriage) are not as likely to show the impact of misalignment on products. The long distance from the gearbox to the saddle distributes most of the misalignment. Abnormal wear does not show up on the feed carriage.
Short feed-bed machines (those with two feed carriages) are subject to larger amounts of damage, mainly in the carriages, feed beds, feed screws, gearboxes, and entry system. Tube products may be scored on the incoming shells. The incoming tube will be out of alignment with the passline, causing stress or damage to the entry system and saddle tooling. Tube dimensions and surface quality are likely to be difficult to maintain.
Using a wire is enough to determine gross misalignment of the gearbox and feed bed on these machines. However, using an alignment scope is necessary to determine precisely the extent of misalignment. Adjusting the feed-bed wear plates usually provides the necessary amount of correction. In severe cases, you may need to replace the entire feed bed.
Severe machine crashes, frequent jams, bearing or gear failure, and long-term wear or stress can cause saddle, die, and mandrel misalignment (see Figure 4).
Die and saddle misalignment have short-term and long-term effects. Short-term effects include intermittent tube defects and difficulty in maintaining tube dimensions. Long-term effects include tool and saddle cracking and failure.
Tooling misalignment also can result in abnormal wear in the saddle window wear plates, saddle wear plates, entry and exit grips, roller bearings, housing chucks, and the mandrel rod.
A wire alignment check can help determine if the dies are aligned with the machine passline. Make adjustment inside the saddle assembly to correct tooling misalignment. If the saddle is misaligned, you may need to shim the saddle housing. In severe cases, machine the saddle housing to correct misalignment.
Despite the vast amount of information on how various lubricants perform, many machines still have problems that stem from lubrication.
Selecting a good lubricant is not difficult. The difficulty is in maintaining the quality of the lubricant, which determines how effective it will be over time.
Aside from providing lubricity between the product and the tooling, OD lubricant provides necessary cooling of the product and tools. Too little lubricant leads to excessively high temperatures that lead to abnormal material flow. Too much lubricant inhibits the reduction process, applying abnormal pressure to the tooling.
Maintaining a proper concentration and quality of OD and ID lubricant is crucial to the mill's operation. Using a dirty OD lubricant results in poor product quality—the tube's finish will show signs of poor material flow (overlap, finning, and pits) and other surface defects. Mandrels will show signs of burning. If gearbox lubricants are not filtered properly, metal fines cause premature wear and failure.
Check the lubricant's concentration and cleanness daily. During shutdowns, look for signs of wear or impending failure when inspecting gearbox bearings, slides, bushings, and gears. Abnormal quantities of metal fines indicate that the gearbox filter is no longer functioning properly.
Companies are making a shift to store part assemblies, rather than only individual parts. This is driven, in part, by the limited time dedicated to remedying machine breakdowns. It is generally much faster to replace an assembly rather than one specific component. The savings can be tremendous from both financial and maintenance perspectives.
From a financial standpoint, increasing the machine's uptime is likely to justify the cost of keeping subassemblies rather than individual parts in inventory.
Subassemblies can be constructed or repaired while the machine is operating, so maintenance personnel don't have to rush or encounter overtime costs. Many spare subassemblies can be stored in shrink-wrap plastic to prevent corrosion and contamination.
Access to the right information is critical during a machine breakdown. Time often is wasted searching for machine blueprints or other machine information. Keep a set of blueprints at or near the machine for quick reference. Revised and relevant information, including important dimensional changes from the last maintenance repair, is critical for maintenance personnel. This information is especially valuable relative to electronic components or computer program changes. The cost of a set of machine prints is justified by how many maintenance personnel must wait for information during a breakdown. Readily accessible prints eliminate guesswork and help to ensure the machine will be assembled accurately and quickly.
Minimizing downtime is the key. Having the right information, at the right time, is as valuable as having the right parts available.
Setting up the mill with the proper adjustments is critical to producing quality tube. To manufacture precise tube dimensions, fine-tuning often is necessary after the mill has warmed up. It is necessary to adjust the rack-and-pinion gears, die gap, and sometimes the mandrel's position because these parts have a direct effect on tube quality. Check roll neck bearings, crankshaft bearings, cam and followers, saddle slides, miter gears, and other gearbox components annually, at minimum.
Several tube quality issues can result from poor tooling adjustments, such as problems with tube OD, ID, wall dimensions, straightness, and ovality. Gear backlash settings are crucial because rack-and-pinion gears coordinate the top and bottom dies.
Identifying incorrect tooling adjustments is relatively simple because they are easily accessible. Some mill adjustments are more challenging because of their locations within gearboxes and housings. Many bad adjustments cause timing delays and create abnormal noise. They also result in stress on mating components. Some examples are:
Don't wait until tube quality suffers or the mill stops running to check out critical areas. During maintenance shutdowns or tool changes, check and adjust the cams, followers, clutches, roll neck bearings, and other mill components.
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