Gear Drives: Repairing/Rebuilding VS. Replacing
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Housing bores must be inspected for parallelism and
concentricity or the repair work could be futile. |
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After more than two decades of demanding use, that large gear drive unit at the heart of your operation needs serious attention. You are concerned that the drive may be failing and, if it does, you lose this vital part of your process.
It is clear you have a problem, but what is the magnitude? How do you find out what it will take to remedy the situation? Where do you start?
Start at the beginning, which is typically the cost factor. In order to make a decision on the gear drive - repair it, rebuild it, or replace it - there must be an understanding of what it will cost to accomplish each of these alternatives. If this information is unobtainable, a sound, well-informed decision cannot be made.
Along with price, other factors enter into the decision. Such questions to ask include:
- How quickly can a repair/rebuild job be done,
compared to replacing the entire unit?
- If it is rebuilt, what is the likelihood that
the unit will perform for a length of time that will recoup the
investment?
- Can a replacement unit be installed in place of
the current application without major realignment?
- Will a replacement unit require the
substitution of other system components such as motors and couplings?
- Who can do the job?
Professional Evaluation
Taking into consideration the many alternatives, it is obvious that any evaluation of the situation should not be done on the spur of the moment.
In the majority of cases, it cannot be decided whether a unit is repairable or not with a simple visual inspection, and even the trained maintenance person cannot fully evaluate what may be wrong with a gear box.
Normally, it is not until a trained technician closely inspects the unit with some measuring equipment, such as bore micrometers and the like, to determine whether or not there is housing, gear tooth or bearing damage - or other problems that the naked eye cannot see.
The opinion of someone totally familiar with gear boxes is necessary, which means that unless a facility has maintenance people who are capable with gear drives, an outside source must be consulted in order to obtain the appropriate information. The most logical place to obtain that expertise is through a gear drive manufacturer with repair/rebuild service capability. This presents a company with its first monetary decision.
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By following the proper rebuild procedures, pounded-out bearing bores can be built up, stress relieved, and remachined.
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Since, properly, there is an up-front cost involved in getting a thorough evaluation, it must be decided: Is it worth the cost to get as much information about the repair alternatives as possible? Will the repair/rebuild service provide an on?site evaluation? Will the additional information make a difference in the final decision to repair or replace?
If this evaluation is in regards to an expensive gear drive, it is highly recommended to turn the unit over to the repair/rebuild experts for an evaluation. With a less expensive gear box, other affordable alternatives could be considered before proceeding with a rebuild. Typically, if the unit is worth USD$100,000 or more, the investment of time, effort and money for a professional evaluation is absolutely worth it to get the best answers. A decision on repair/rebuild or replacement can then be made from a position of strength.
Repair/Rebuild Advantages
Generally, the advantages to repairing/rebuilding a unit far outweigh replacement. It normally is much less expensive to repair/rebuild and it affords the opportunity, while repairing the gear drive, to upgrade the unit, including design changes or improvements to increase its rating. In addition, if a reputable manufacturer's repair/rebuild department does the job, the unit will practically be "brand?new"- in some cases with a new unit warranty. This warranty factor cannot be overlooked when evaluating whether to repair/rebuild or replace a gear drive.
Another advantage to repairing/rebuilding versus replacing is time. More often than not, turnaround time to repair a unit is much faster than it is to build a new one. That may not be true for a manufacturer's standard products, but it certainly is the case when specifying modifications of those standards, custom design work, or obsolete units no longer manufactured for stock. In most cases, turnaround time is an essential part of the equation.
One final advantage to repairing/rebuilding is accommodating the uniqueness of a system application. In many cases, the gear drive is installed in a special foundation, connected to a motor and the driven equipment in such a way that duplication with a new unit would be difficult, time consuming and cost?prohibitive. In this situation, the gear drive is not only affected, but an entire application would need to be replaced. Theoretically, a rebuilt unit will fit right in.
Repair/Rebuild Considerations
With age, sooner or later, comes obsolescence. Rebuilding the unit might seem to be the best alternative, but how difficult will it be to get parts five years from now if it breaks down again? The older the unit, the more questions that need to be asked. A repair/rebuild vendor can help in the evaluation.
If it is decided to rebuild a gear drive, keep in mind that there are parts that should always be replaced. They include bearings, seals and shim gaskets. Time and usage - and the relative cost of those parts - make it more cost-effective to replace them.
The two areas where a good repair/rebuild service will look to evaluate a unit are the rotating elements and the gear housing. If there is wear in a rotating element, a reputable manufacturer can recut it to its original form.
Most improper repairs take place on the gear housing. This is a direct result of an inspection process that fails to detect, for example, a worn or pounded?out bore that causes improper seating of the bearings, which results in gears not making full contact and, consequently, the gear teeth are subjected to higher loads than they are designed for. Inspection of the housing is the most important aspect of the overall evaluation, yet it is probably the most overlooked.
Finally, rebuilding a gear drive is the ideal time to incorporate important design upgrades in the unit. In many cases, equipment that is more than 20 years old is a good candidate for design upgrades. Upgrades to current design philosophy or technology should be accomplished whenever possible and within the limitations of the unit's capabilities. Common design upgrades include improved oil sealing capabilities, improved bearings for longer life or hardened gear elements to improve wear rating.
Once the gear drive is rebuilt to the manufacturer's recommendations, the unit should be tested to its original specifications and then warranted as a new unit. This normally includes a spin test where the unit is coupled to a prime mover, turned over at its rated speed and checked for leaks and noise. The operating temperature is also monitored during the spin test.
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Bearing failures (left) are
often caused by lack of sufficent lubricant, lubricant
contamination, operating overloads, wrong lubricant, or oil
breakdown under high temperature. Gear teeth failure (right) is
typically caused by impact or shock load, insufficient lubricant,
contaminated lubricant, uneven load distribution, or operating
overloads.
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Summary
When contemplating gear drive repair/rebuild or complete unit replacement, eliminate as much risk as possible by evaluating time and cost factors, and consulting a reputable repair/rebuild facility for a proper evaluation and repair job. If the job is done properly, the repaired unit can last as long as the original gear.
A repaired/rebuilt unit from a reputable manufacturer provides the confidence and comfort of knowing the gear drive has a new unit warranty, experts have inspected it, and it is going to fit the existing application.
Is Your Gear Box Healthy?
Four testing methods should be used to determine the health of a gear box: vibration, sound, temperature, and oil analysis.
Vibration
The initial vibration level tends to remain unchanged for a long period of time. As components start to wear or fail, the vibration level rises. When this occurs, it is time to investigate the cause.
Sound
Sound can be accurately diagnosed through the use of frequency measurement devices. For example, if a gear mesh is starting to wear, the amplitude of the sound at the mesh frequency tends to increase. If the measurements indicate an increase in gear mesh frequency, look for a wear problem. A bearing, on the other hand, will generate different frequency and have its own signals to indicate wear.
An operator who is familiar with a piece of equipment may be able to detect a subtle change in the level or type of noise coming from the gear box. The use of instruments, however, quantifies noise characteristics and their variance over time.
Temperature
It is a common practice to monitor the operating temperature of gear drives. Variations in operating temperature can indicate that trouble is brewing. The easiest use of this measurement is to set a maximum toleration temperature for a given piece of equipment and have an alarm sound when the temperature reaches the threshold. Quite often, the alarm occurs because a bearing or the lubricant itself is failing.
Oil Analysis
Oil samples are an excellent way to detect the source of trouble before it affects the performance of the equipment. Samples should be taken in accordance with the equipment manufacturers' recommendations and test results compared to the lube manufacturer's specifications.
A common procedure is to collect a sample from the top central area of the oil reservoir immediately after shutting down the machine. This results in freshly circulated oil that has been evenly distributed.
Oil analysis targets three principal factors: changes in viscosity, the amount of water in the oil and the presence of foreign particles.
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