Production goals are ever changing and usually in an upward direction. For example, production management may dictate that belt conveyors run faster and with an increase in loads. Sometimes the production managers who set the goals don't realize how wide ranging the ramifications of those decisions can be on the plant engineers and maintenance people responsible for designing and maintaining the machinery driving production. Since production changes affect the entire drive system, what's a plant engineer to do?
Design and maintenance teams
Any upgrade to a process has two distinct phases that include design and maintenance evaluations.
The first thing to do is create a team from the plant engineering and maintenance staff to design and implement the drive system upgrade. The design phase, usually the responsibility of plant engineering, determines the power and mechanical needs of the upgraded drive train system. System designers must develop an overall scheme for powering the new higher loads.
Subsequently, the maintenance department must undertake a careful and thorough review of the current drive system components that may be used in the upgraded system. It is essential that these two phases be done in tandem. Doing one without the other is asking for trouble and probable downtime later when the upgraded system is asked to drive the new loads.
Drive upgrade design considerations
Many drive systems, such as paper machines or conveyor belts, may have several drive "stations" along the production line. However, for simplicity and clarity, let's focus on upgrading one drive station on the production line. The principles and practices described here apply to every drive unit.
After determining the new production line's overall power requirements, plant engineers must qualify the drive train's motor and gear drive rpm, horsepower, torque, and thermal rating requirements. Questions to ask include:
- Is a higher horsepower motor needed?
- Does the motor rpm need to change?
- Will the existing motor do the job?
- Do the gear drive and coupling horsepower, output torque, and mechanical and thermal ratings meet the new system's requirements?
You must take into account horsepower, torque, and rpm in qualifying the design phase of the upgrade. After determining the new loads on the driven equipment, the best thing to do next is to call a gear drive manufacturer's representative to analyze whether the existing power transmission components rate both mechanically and thermally and can be used in the upgrade.
It is not enough to look only at the gear drive's nameplate to determine if it will meet the new mechanical requirements. For example, a common mistake involves the Service Factors. A Service Factor of 1.5 on the nameplate does not mean that the drive can handle a 50 percent increase in horsepower (assuming it was running at a SF of 1.0). The upper range of the Service Factor number indicates the drive's ability to handle momentary peaks, not continuous duty operations.
Another common mistake is assuming that changing one factor in the equation won't affect the rating requirements of other componenst in the drive system. For example, changing the motor rpm affects the power rating requirements of the gear drive. It's also not true, as some people assume, that an upgrade designed to slow the system will result in easier operations for the gear drive. A slower low speed shaft increases the drive's output torque and the drive system may then face a high torque requirement.
Qualifying the system for thermal rating is as important as doing so for rpm, horsepower, and torque. The upgrade may cause a change in the drive system that alters the thermal requirements. The old thermal ratings may be insufficient to handle the requirements of the upgraded drive system. The upgrade may require larger drives or auxiliary cooling devices such as fans or another type of heat exchanger. A metering oil pan on the drive may be needed. It is more cost effective to determine these requirements in the qualifying stage. If you don't learning of them after system startup and discovering the need to plumb water, run electrical lines for cooling devices, or disassemble the drive to add an oil pan is the only option.
One of several scenarios is likely following design phase findings. New equipment is required because the old equipment does not rate, some machinery may rate and can be used, or perhaps the old equipment rates and can be configured to power the new loads.
Maintenance qualification of those components that checked out okay in the design phase is the second step in the upgrade process. However, just because the "old" gear drive and couplings are rated appropriately for the new system does not mean that you can hook up a motor to the gear drive and call the upgrade complete. You must carefully inspect the drive components to make sure they are in condition to assume the new loading. If maintenance discovers problems, then you can make repairs or order replacement parts before system start up.
A complete inspection of the gear drive bearings is one of the most important maintenance steps in the qualifying process. The gear drive's operational history dictates the condition of the bearings.
Inspect the bearings and gear drive housing bearing bores. Worn bearing bores can lead to premature bearing failure that may result in a catastrophic gear drive failure.
It doesn't take an expert to determine that a bearing is in bad shape. First, remove the drive housing cover and inspect the bearing cage for wear or cracks. Next, inspect the bearings, turn the rollers, and make sure there is no surface distress. Visual inspection of the rollers and races may reveal shallow holes in the surface (pitting), scuff marks, peeling of metal, spalling, or scoring.
Rust on the rollers indicates microscopic holes. It may be a marginal problem now but it would be unwise to start an upgrade on a bearing that may become a major problem in six months.
Replace bearings that show wear distress or that have been in operation for a significant amount of time. It is more cost effective to replace the bearings during the qualification process than replacing a gear drive later that failed catastrophically from a bad bearing. It could cost thousands of dollars to replace the gear drive alone not to mention the cost of lost production time.
It is important to qualify the gearing to make sure it's in good condition before asking it to accept higher loads. Before the upgrade, the drive gearing had been meshing over the history of the application in a certain load zone. Changing the application causes the drive to work harder. The load zone will change.
As a result, the relative position of the components may change. This causes non-worn gear teeth to mesh with worn gear teeth. Changing from a previous match of worn-on-worn to non-worn on worn may result in broken gear teeth.
As with bearings, make a complete visual inspection of the gearing in the field. It doesn't take a gear technician with a lot of sophisticated equipment to identify serious wear conditions. If it's serious, you'll see it. For example, look for:
- broken teeth
- holes from pitting and spalling (indicated by areas where a large amount of surface material has broken away),
- wear ridges
- rippling from plastic flow (a condition where the metal isn't removed but is moved around on the gearing surface), and
- other wear conditions.
Replace seriously worn gearing (see Troubleshooting gear drives, Plant Services, May 1996, page 23, for related information on indication of gear wear).
Comparing the working flank and trailing flank of gears and pinions is sound field qualification. Generally speaking, there is no significant wear if you can't see any difference in shape between the two flanks. However, take corrective measures if there is a difference in shape. For example, you can consider flopping the gearing end for end or installing a new gear set.
Qualify the drive housing and shafts
The most important aspect to qualifying the drive housing is making sure the bearing bores are sized correctly and round.
Shafts must be in good condition to ensure proper coupling connections. Look for fretting that disrupts the shaft surface (like a plastic flow condition) and can lead to stress and eventual shaft failure. Misalignment or a too loose coupling on the shaft causes fretting. Therefore, qualifiy shaft surfaces adjacent to coupling fits and keyways the proper size and for distress. Repair or replace them if needed.
Couplings qualified in two areas
Inspect couplings for evidence of fretting and for correct bore diameters. Wear on the inside perimeter of the coupling may lead to a loose fit and eventual failure under the new higher load requirements. Visually inspect coupling and shaft keyways and keys for wear.
Inspect the connecting elements of the couplings for wear and correct meshing. For example, inspect the gear teeth on a gear-type coupling in a fashion similar to the drive gearing. Inspect grid type couplings to make sure the grids are slotting correctly and for signs of cracking, breakage, or other wear on the steel grid elements. Inspect disc couplings for cracking on the diaphragm. Inspect the rubber element or elastomer couplings for cracking and distortion from stretching.
Drive systems rooted in strong foundations
The new drive system needs a sound and rigid foundation to ensure it can handle the new load assignments. Problems in the foundation may lead to drive shaft and gearing distortions. The distortions, in turn, may result in misalignment and eventual system failure. Inspect the foundation for physical deterioration such as cracks in concrete or distortion in the steel. Look for looseness between the base plate and the foundation. Movement in this area can lead to unwanted vibrations in the system. Repair the foundation before start-up.
Solving the problems
Up to this point, we've reviewed the steps involved in qualifying a drive system for new loads. Hopefully, after going through the qualification process, you'll find that the drive system has the mechanical and thermal ratings to meet the new loads. If you are lucky, the maintenance inspection shows everything to be in good operational order. If that's the case, you're ready to go.
However, that's probably too much to hope for. Chances are good that some components need to be replaced or renewed. Costs and lead times are factors to consider in determining whether to buy new components or to repair existing components. You may be able to repair many components at less cost than buying new equipment. For others, it is more cost-effective and quicker to buy new parts.
There are a few options to consider when refurbishing or repairing equipment. There are certainly some repairs and refurbishing that your maintenance staff can undertake in-house. This is especially true in those areas that fall into regular or preventive maintenance normally done by the maintenance staff such as simple foundation repairs, adding cooling devices, coupling maintenance, and so forth. However, other repairs are more problematic including flopping gearing, repairing fretted shafts, returning bearing bores to specifications, repairing broken gear teeth, and the like. In those cases it may make more sense to send the machinery out for repair. You can send the equipment to either a local repair shop or to a gear drive manufacturer.
Some gear drive manufacturers renew the gear drive to "like new" conditions at a cost guaranteed to be less than that of a new drive and offer a one-year new drive warranty. A renewed gear drive enhances the reliability of the new drive system and helps ensure maximized uptime following the upgrade.
Torsional critical issues
Torsional windup forces bring the issue of "torsional critical" into play in upgrading the drive system. Every drive train system has a torsional critical zone in which the system is out of balance at some point in its operating range. It is manifested by a serious vibration problem that must be addressed and solved before a catastrophic failure occurs. Most of the time, the torsional critical point is outside the normal operating range of the drive system and does not represent an operational problem. For example, the low speed shaft of the drive system may turn at a normal operational speed of 50 rpm but the torsional critical point may be 15 rpm. In this scenario, the operation quickly passes through the torsional critical zone on its way to normal operating speeds.
Every component in the drive system, from the motor to driven equipment components, has torsional windup. Regardless of how little windup each component may have, it's still present. If, at some point, the forces interact in combination with the "perfect" load, then it becomes critical. A typical symptom is a rattling gear drive.
Changes in system parameters such as increasing or decreasing rpm or loads affects the location of the torsional critical point in the upgraded system. Predicting, in the design phase, where the torsional critical zone will be is very difficult and probably not worth the time and effort to do so. However, it is an important factor to monitor during start up. If vibrations are evident during start up, the upgraded drive system may be operating in the torsional critical zone.
This reinforces the necessity of qualifying the upgraded drive system for design and maintenance considerations. Why? It will be extremely difficult to diagnose the source of a vibration problem discovered at startup if only the design ratings were qualified and not the physical condition of the machinery. You'll be chasing your tail trying to find the problem--is it torsional critical, a bearing, misalignment due to foundation stress, shaft fretting, gear stress, or what?
At that point, management isn't going to like the idea of lost production attributable to "shot in the dark" troubleshooting. However, with both design and maintenance considerations qualified and absent a history of vibration problems with the old configuration, a vibration problem at start up probably is attributable to torsional critical.
How is torsional critical mitigated? There are several options, but the most common and a simple fix is to change one or more couplings to decrease or increase stiffness. The problem may be solved by replacing a "softer" grid coupling with a "rigid" disc coupling, or vice versa.
Benchmarks and monitoring
Take a benchmark measurement of vibratory loads, operating temperature, and noise levels once the new system is running. Then, continue to monitor the system at regular intervals and compare measurements to the benchmark. Troubleshoot any change from the benchmark. Finding possible problems and conducting preventive maintenance reduces chances for catastrophic failures.
Hopefully, the drive system upgrade steps described in this article will be of help next time production management dictates faster speed and higher production loads.