Replacing Mixer Drives Takes Full Understanding of Application
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A new mixer drive is often required when systems need to
be upgraded or equipment is outdated. |
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As with most things mechanical, mixer drives do not last forever. At some point in time, the signs of age or extensive use are going to be unavoidable: loud noise levels, unacceptable vibration in the gearbox, oil leaking from the unit into the application.
Or, the day of decision may come about through an entirely different set of circumstances. The present mixer drives are running fine, but the process is rapidly approaching obsolescence -- the time has come to upgrade the system with a new agitator and a prime mover with accompanying reducer capable of providing more power to the system.
In either case, the user is now at a critical juncture. In order to keep and maintain business, the mixer drive problem must be addressed. Now the question becomes: Is repairing the existing drives the best alternative, or is replacement the way to go?
When a Mixer Drive Fails...
How can you tell when your mixer drives are simply not working right anymore? By monitoring the vital signs of gearbox
performance. These are: vibration and noise, changes in operating temperature, leaking oil, and frequency of service.
Some plants will have a comprehensive predictive maintenance program which monitors important mechanical equipment on a regular schedule, maintaining reliable records which permit evaluation of "change" in performance. When evaluation is relegated to personal memory and ear measurement, such as -- "I do not think it sounded this bad the last time I listened to it" -- the more likely the gearbox will suffer a sudden failure.
Sometimes the warning signs are obvious. When the unit begins to noticeably shake on its foundation or pedestal, or when that quiet hum is replaced by a loud whine or worse, it is a good possibility that the gears or bearings are going bad.
Other times the signs are less obtrusive. A mixer drive that becomes extremely hot during normal operation typically means the unit needs attention. If allowed to continue, it will likely fail. A "change" in normal operating temperature usually signals one of two things: a change in load or power draw, which can be verified with an ammeter; or a failure of the lubricant, typically in quality or quantity, which includes viscosity breakdown.
Additionally, leaking lubricating oil is a sure sign that a seal has failed. It demands immediate attention for two reasons -- as the gear reducer loses oil, it runs hotter, causing additional internal damage and eventually failure; also, since
most liquid mixing in the chemical industry is performed by a drive mounted above a tank, leaking lubricating oil into many applications can have a catastrophic affect.
Ultimately, frequent service or continual parts replacement may serve as the best gauge of mixer drive health. Check the system's operation and maintenance records. Has a pattern of frequent maintenance developed? The unit is probably sending a message.
Upgrading the System
But perhaps the mixers themselves are operating just fine. The application, however, must be upgraded to accommodate more speed, more powerful or efficient blades, or a new mixing application. A serious change in the process probably means that consideration must be given to new drives to move the system.
So, users of mixers in the chemical processing industry will be faced with two options -- repairing or replacement -- at some time in the life of their operation. The distinctions between the two, and therefore the decision on which option to choose, are not always obvious.
The Repair-Rebuild Option
The initial reaction to a failing mixer drive, particularly one that has been reliable over the years, is to repair it. There is a certain degree of reluctance to replacing a system with something unknown and untried.
Now the relationship between user and mixer manufacturer --
and the reputation of the reducer manufacturer -- becomes important factors. Some mixer manufacturers offer their customers a factory rebuild program, providing the user a completely
rebuilt, refurbished gear reducer. However, these rebuilt units will often times include used parts and do not have the reliability of a new reducer.
Age is probably the single biggest factor against repairing mixer drives. When a unit reaches 10 to 15 years old, the chances are slim that it can be repaired and give good service at a low cost. Likewise, parts for old units are typically no longer in inventory and the expense for a manufacturer to tool-up to make a small quantity of parts for units considered obsolete makes it cost prohibitive.
This introduces the question of price. How much is the user willing to spend to rebuild an old mixer drive? Because of age, or due to the fact that the unit was custom built, will the price of the spare parts exceed the cost of a new unit? Or, is the reputation of the old unit so strong that rebuilding it, no matter what the cost, is a top priority. Remember that a new gear reducer will typically incorporate the latest advances in technology -- in design of the gearing, bearings, etc. -- as well as manufacturing accuracies.
The final factor to examine when considering the repair- rebuild option is downtime. The majority of mixing applications in the chemical processing industry are complicated, long-term
operations. The mixing cycles for some products may take up to a week of around-the-clock action -- a process taking place simultaneously in 50 tanks.
In applications such as these, downtime is costly. Since repairing or rebuilding existing mixer drives is more time consuming than replacement, the downtime must be carefully scheduled. And, a predictive maintenance program takes on great importance, since failing mixers in the middle of a mixing cycle can result in wasted product and re-starting the cycle.
It is generally accepted that when parts and labor to repair a gearbox approach 60 percent of the cost of a new reducer, serious consideration be given to replacement with a new reducer.
The Replacement Option
In the case of a system upgrade or outdated equipment, it is unlikely that rebuilding the mixer drives will do the job. The changes in the application -- output torque, revolutions per minute, ratio, etc. -- point to replacement as the best option.
What are the factors to consider when selecting a replacement mixer drive?
First, the replacement drive must be of a compatible size to fit the application. Horsepower, output speed, and start-up torque must all be matched or the mixer drive will be destroyed. If, for example, the application requires frequent stops and starts, a high start-up torque capability in the reducer is a
must to allow the mixing blades to resuspend any solids which may have settled.
Along with compatible capacity, the replacement drive must
be compatible in physical size, at least to the extent that it can be mounted within the space available to it. There is a definite advantage if the replacement unit can be dropped directly in as a replacement, with no structural modifications. This is known as having the "same footprint," and results in quick replacement with minimum downtime.
Next, was the existing mixer driven with a worm gear reducer? If it is more than 20 years old, the answer is probably yes. Advancements in the industry may dictate that the user specify a reducer with bevel gearing as the replacement. Bevel gearing is more energy efficient and more cost effective. For instance, imagine a mixer using a 25 hp motor with a bevel- gearing reducer. To do the same work with a worm gear reducer
would require a 40 hp motor. This extra power consumption is a continuing expense every hour that the mixer is operating. Energy savings alone may justify a new reducer.
The next consideration is availability. Depending on the supplier, a replacement reducer may have a lead time of 12 weeks or longer. When a breakdown occurs unexpectedly, replacement may be needed in days, not weeks.
Finally, ensure that replacement mixer drives are designed specifically for mixer applications. The mixer gearbox is
designed differently from a standard industrial gearbox -- larger output shaft to accommodate heavy mixing loads, larger bearings
for longer life under constant use, and a drywell type design to eliminate oil leaking into the application.
Remember, there are a lot of manufacturers who make mixers, and a lot of manufacturers who make gearboxes. But there are only a few gearbox manufacturers who have the engineering expertise to design and manufacture a gearbox specifically to meet the needs of a mixer.
The bottom line is that there is no substitute for finding a reducer manufacturer with a thorough knowledge of mixer applications and associated reducer loading. It will have a direct effect on the quality of the mixer reducer a user purchases.
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