MAINTENANCE

In an effort to increase production, engineers and maintenance personnel are many times called on to reconfigure and upgrade a mill’s existing drive systems

Design, Maintenance Qualification Essential For Drive System Upgrades

By LARRY THIBAULT

Production goals are ever changing and usually are pushed 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 goals don’t realize how wide-ranging the ramifications of those decisions can be on the plant engineers and maintenance personnel responsible for designing and maintaining the machinery driving production. Because production changes can affect the entire drive system, a plant engineer must be prepared to handle those changes.

The first step is to create a plant engineering/maintenance staff team to design and implement the drive system upgrade because the process has two distinct phases including design and maintenance evaluations. The design phase is used to determine the power and mechanical needs of the upgraded drive train system and is usually undertaken by plant engineering. System designers must develop an overall scheme for powering the new higher loads.

Subsequently, the maintenance department must undertake a careful, thorough review of the current drive system components that may be used in the upgraded system. It is essential that these steps 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.

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 this article will focus on describing the upgrading of one drive station connected to the driven equipment on the production line. The principles and practices described apply to one or more drive units.

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?

Horsepower, torque, and rpm must all be taken into account in qualifying the design phase of the upgrade. Once the new loads on the driven equipment are determined, the best thing to do is to call in a gear drive manufacturer’s representative to analyze whether the existing power transmission components rate both mechanically and thermally and can be utilized 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 evaluation of Service Factors (SF). An SF of 1.5 on the nameplate does not mean that the drive can handle a 50% increase in horsepower (assuming it was running at an SF of 1.0). The upper range of the SF 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 another component in the drive system. For example, changing the motor rpm will affect the power rating requirements of the gear drive. It is 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 slow-down on the low-speed shaft increases the drive’s output torque, and the drive system may end up with a high-torque requirement.

THERMAL RATINGS. 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 not be enough to handle the requirements of the upgraded drive system.

Larger-sized drives, auxiliary cooling devices such as fans, or another type of heat exchanger may be required. A metering oil pan may need to be added to the drive. It is more cost effective to determine these requirements in the qualifying stage rather than 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.

One of several scenarios is likely following design phase findings. The old equipment may not rate and new equipment may be required, some machinery may rate and can be utilized, or perhaps all the old equipment rates and can be configured to power the new loads.

MAINTENANCE QUALIFICATION. "Maintenance” is step two in the upgrade qualification process and applies to those components that checked out okay in the design phase. However, just because the "old” gear drive and couplings are rated appropriately for the new system, it isn’t simply a matter of hooking up a motor to the gear drive and calling the upgrade process complete. The drive components must be carefully inspected to make sure they are in good condition to assume new loading. If problems are discovered, repairs can be made or replacement parts ordered before system startup.

QUALIFYING BEARINGS. 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 will dictate the condition of the bearings.

Inspect the bearings and gear drive housing bearing bores. If the bearing bore is worn, it can lead to premature bearing failure, which may result in a catastrophic failure of the gear drive.

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 racers may reveal shallow holes in the surface (pitting), scuff marks, pealing of metal, spalling, or scoring.

Rust on the rollers is an indication of tiny 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 all 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 to replace a gear drive later that failed catastrophically due to a bad bearing. It could cost thousands of dollars to replace the gear drive alone, not to mention the cost of lost production time.

QUALIFY GEARING. It is important to qualify the gearing to make sure it is in good condition before asking more out of it in the commitment to higher loads. Prior to the upgrade, the drive gearing had been meshing over the history of the application in a certain load zone (Figure 1). When the application is changed, causing the drive to work harder, the load zone will also change. As a result, the relative position of the components may change, causing non-worn gear teeth to mesh with worn gear teeth. A change from a previous "match” of worn-on-worn to non-worn on worn, may result in broken gear teeth.

As with bearings, a complete visual inspection of the gearing should be done in the field. It doesn’t take a gear technician with a lot of sophisticated equipment to identify serious wear conditions. If it is serious, it can be seen. For example, broken teeth, holes from pitting and spalling (indicated by areas where a large amount of surface material has broken away) will be evident, as will 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. Seriously worn gearing should be replaced.

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 tell any difference in shape between the two flanks. However, corrective measures should be taken if there is a difference in shape. For example, the gearing could be "flopped,” or a new gear set could be installed.

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 are round. Shafts must be in good condition to ensure proper coupling connections. Fretting is one problem to look for. Fretting disrupts the shaft surface (like a plastic flow condition) and can lead to stress and eventual shaft failure.

Fretting typically is caused by misalignment or the coupling being too loose on the shaft. Therefore, shaft surfaces adjacent to coupling fits and keyways should be qualified for the proper size and for distress and should be repaired or replaced if needed.

COUPLINGS QUALIFIED IN TWO AREAS. Couplings should be inspected for evidence of fretting and for correctness of 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. Coupling/ shaft keyways and keys should also be visually inspected for wear.

The connecting elements of the couplings should also be inspected for wear and correct meshing. For example, the gear teeth on a gear-type coupling should be inspected in a fashion similar to the drive gearing. Grid-type couplings should be inspected to make sure the grids are slotting correctly and for signs of cracking, breakage, or other wear on the steel grid elements. Disc couplings are easily inspected for cracking on the diaphragm. Rubber element or elastomer couplings should be inspected for element cracking and distortion from stretching.

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 that 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. Foundation repairs should be made before upgrade startup.

SOLVING THE PROBLEMS. Up to this point, this article has reviewed the steps involved in qualifying a drive system for new loads. Hopefully, after going through the qualification process, the drive system will have the mechanical and thermal ratings to meet the new loads and the maintenance inspection will show everything to be in good operational order.

If that is the case, it’s time to proceed. However, that is probably too much to hope for. Chances are that some components will need to be replaced and/or renewed. Costs and lead times are factors to consider in determining whether to buy new components vs the repair of existing components. For many components, it may be possible to repair at less cost than buying new equipment. For others, it is more cost effective and quicker to buy new components.

There are a few options to consider regarding refurbishing or repairing equipment. There are certainly some repairs and refurbishing that a maintenance staff can undertake. 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, etc.

However, other repairs are more problematic—e.g., "flopping” gearing, repairing fretted shafts, returning bearing bores to specifications, repairing broken gear teeth, etc. In those cases it may make more sense to send the machinery out for repair, using a local repair shop or a gear drive manufacturer.

Some gear drive manufacturers will 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 assure that uptime will be maximized following the upgrade.

TORSIONAL CRITICAL ISSUES. The issue of "torsional critical” (due to torsional windup forces) comes into play in the upgrading of the drive system. Every drive train system has a torsional critical zone when 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, if the low-speed side shaft of the drive system is turning at a normal operational speed of 50 rpm, the torsional critical point may be 15 rpm. In this scenario, the operation quickly passes right 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 is still there and at some point the forces react together, in combination with the "perfect” load, and then it becomes critical. A typical symptom is a rattling gear drive.

Changes in the system parameters, such as increasing or decreasing rpm, increasing loads, or both, affect the torsional critical point’s location in the upgraded system. Predicting ahead of time, 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 startup. If vibrations are evident during startup, the upgraded drive system may be operating in the torsional critical zone.

This also reinforces the necessity to qualify 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. Time will be wasted in trying to find the problem. Is it torsional critical, a bearing, misalignment due to foundation stress, shaft fretting, gear stress, etc?

However, if both design and maintenance considerations were qualified, there was no history of vibration problems with the "old” drive configuration, and a vibration problem occurred at the startup of the upgrade, chances are it is attributable to torsional critical.

How is a torsional critical situation fixed? There are several options, but the most common, and a simple fix, is to change out 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 and temperature and noise levels once the new system is up and running. Then continue to monitor the system at regular intervals and compare measurements to the benchmark. Troubleshoot any changes from the benchmark should they occur. Finding any possible problems and conducting preventive maintenance will reduce chances for catastrophic failures.

LARRY THIBAULT is manager, Field Service and Renew, The Falk Corp., Milwaukee, Wis.