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 new production goals do not realize how wide-ranging the ramifications of those decisions can be on the plant engineers and maintenance team responsible for designing and maintaining the machinery driving production. Production change can affect the entire drive system, so what steps should a plant engineer take?

Design/Maintenance Teams

The first step is to create a plant engineering/maintenance staff team to design and implement the necessary drive system upgrade. Representatives from the engineering and maintenance departments are essential, because the upgrade process has two distinct phases, including design and maintenance evaluations. The design phase determines 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 and thorough review of the current drive system components that may be utilized in the upgraded system.

It is essential that these steps be carried out in tandem, as doing one without the other is likely to result in downtime at a later stage when the upgraded system is required to drive the new loads.

Drive Upgrade Design Considerations

Many drive systems, such as conveyor belts or paper machines, may have several drive "stations" along the production line. However, for simplicity and clarity, this article will focus on describing the upgrade 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 overall power requirements of the new production line, plant engineers must qualify the motor and gear drive rpm, horsepower, torque and thermal rating requirements of the drive train. Questions that should be asked 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, RPM and Torque

Horsepower, rpm and torque 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 step to take is calling 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 will not 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 sufficient to handle the requirements of the upgraded drive system. Larger-sized drives or 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 than learn of them after system start-up and discover the need to plumb water, run electrical lines for cooling devices, or disassemble the drive to add an oil pan.

Following design phase findings, one of several scenarios is likely. The old equipment does not rate and new equipment is 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.

Drive Upgrade Maintenance Qualification Considerations

Maintenance is the second step in the upgrade qualification process and applies to those existing drive components that meet the new requirements established in the design phase. However, just because the "old" gear drive and couplings are rated appropriately for the new system, it is not simply a matter of connecting a motor to the gear drive and calling the upgrade process complete. The drive components must be carefully inspected to ensure they are in good condition to assume new loading. If problems are discovered, repairs can be made or replacement parts ordered before system start-up.

Qualifying Bearings

A complete inspection of the gear drive bearings is one of the most important maintenance steps in the qualifying process, and the gear drive's operational history will dictate the condition of the bearings.

It is essential to inspect the bearings and gear drive housing bearing bores, as if it is worn, it can lead to premature bearing failure, which may result in a catastrophic failure of the gear drive.

It does not 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 currently be a marginal problem, 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 later replacing a gear drive 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.

Worn bearing cage

Cracked or worn bearing cages must be replaced during the upgrade qualification stage.

Qualify Gearing

It is important to qualify the gearing, ensuring it is in good condition before demanding more of it in the commitment to higher loads.

Prior to the upgrade, the drive gearing has been meshing over the history of the application in a certain load zone. 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 teeth may result in broken gear teeth.

As with bearings, a complete visual inspection of the gearing should be done in the field, and again it does not require a gear technician with sophisticated equipment to identify serious wear conditions. If it is serious, it will be obvious. 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 is not 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 significant wear if there is a noticeable difference in shape between the two flanks. Take corrective measures if there is a difference in shape.

Dedendum pitted gear tooth

The photo illustrates a dedendum pitted gear tooth. Dedendum pitting may result when loads are at or close to maximum allowable surface loading values. The gear or pinion should be replaced, if found in this condition, during the upgrade qualification process.

Excessive bearing and housing-bore wear, soft-foot, and shaft misalignment may result in improper gear tooth face contact and subsequent possible tooth failure.

Qualify Drive Housing and Shafts

The most important aspect when 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. Fretting is one problem to look for, as this disrupts the shaft surface (like a plastic flow condition) and can lead to stress and eventual shaft failure. Fretting is typically caused by misalignment or by 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 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 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.

Drive Systems Rooted In Strong Foundations

It is important for the new drive system to have 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 the 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 start-up.

Solving the Problems

Once the above steps to qualify a drive system for new loads have been carried out, it is hoped that the drive system will have the required mechanical and thermal ratings to meet the new loads, and the maintenance inspection will show everything to be in good operational order, so the drive system is prepared to handle the increased production demand.

However, that may be too much to hope for. Chances are 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 rather than repair existing components. For many components, repair may cost less than buying new equipment, whereas for others, it is more cost-effective and faster to buy new components.

There are a few options to consider, regarding refurbishing or repairing equipment. Some repairs and refurbishment can be undertaken by a facility's maintenance staff. This is especially true in those areas that fall into regular or preventive maintenance normally carried out by the maintenance staff, such as simple foundation repairs, adding cooling devices and coupling maintenance. However, other repairs are more problematic, including flopping gearing, repairing fretted shafts, returning bearing bores to specifications and repairing broken gear teeth. In those cases, it would be sensible to send the machinery out for repair to a gear drive manufacturer or a qualified local repair shop.

A number of gear drive manufacturers will renew a 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.

Before and after

Before and after -- Some gear drive manufacturers will renew a 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.

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, during 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, 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 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 present 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 the torsional critical zone ahead of time, such as during the design phase, 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 also reinforces the necessity to qualify the upgraded drive system for design and maintenance considerations. This is because it will be extremely difficult to diagnose the source of a vibration problem discovered at start-up, if only the design ratings were qualified and not the physical condition of the machinery. However, if a vibration problem occurs during the start-up of the upgrade, and there was no history of vibration problems with the previous drive configuration, it is likely that it is attributable to the torsional critical zone.

There are several options of fixing torsional critical, but the most common and simple fix is to change out one or more couplings to decrease or increase stiffness. Replacing a "softer" grid coupling with a "rigid" disc coupling, or vice versa, may solve the problem.

Benchmarks and Monitoring

Once the new drive system is up and running, it is recommended to take a benchmark measurement of vibratory loads, and temperature and noise levels. Continue to monitor the system at regular intervals and compare measurements to the benchmark. Troubleshoot any change from the benchmark, should they occur. Finding any possible problems and promptly taking preventive maintenance measures will reduce the chance for catastrophic failure.

The next time production management dictates faster speed and higher production loads, use these recommendations as a guideline to a successful drive system upgrade.

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