By Andy Watterson, Regional Sales Manager, SAM Division, Philadelphia Mixing Solutions 
When a mixer gearbox fails, often an entire manufacturing process is halted; downtime of that process causes loss of production and revenue. The failure also frequently causes the need for additional labor spending or overtime. The plant’s labor force is then sidetracked from other scheduled, and potentially important, tasks.
Funds may additionally have to be diverted from a budgeted need to this unexpected failure, so getting the mixer gearbox repaired and back to work as quickly as possible becomes number one priority. In an effort to prevent these occurrences, let's address the mixer before the failure.
Within an uncertain economy, with more companies being restricted with their capital funds, the need for a dependable mixer gearbox is as prevalent as ever. Unscheduled downtime is just not an option, and maintenance professionals are being asked to squeeze as much life as possible from their gearboxes. They are furthermore being pressured to halt downtime associated with gearbox failures as processing plants need to maintain uptime to maximize their margins.
Maintenance professionals do have fundamental tools, or dos and don’ts, at their disposal for anticipating and preventing gearbox problems. The first of these tools is oil analysis: By analyzing the oil and monitoring for particulates, the condition of a mixer gearbox and its lubricant can be maintained.
Many gear and bearing failures also result directly from insufficient or interrupted lubrication. Scoring, galling and general fatigue of gearing are frequently a result of poor lubrication, which can cause metal-to-metal contact and elevated operating temperatures. When inspecting gearing that has been subjected to poor lubrication and elevated temperatures, the “bluing” of the gears is often a telltale sign. Proper inspection and repair by a comprehensive repair facility, or gear replacement will eventually be the only option.
Here are a few more quick dos and don’ts for maintaining proper operation of mixer gearboxes. While this is not an all-encompassing list, it provides some straightforward steps that can easily be accomplished by maintenance and operations professionals at the plant level:
- First, selection of the proper lubricant is key. The correct lubricant depends on factors such as the ambient temperature, the model of gearbox/type of gearbox, the process load, operating temperatures, etc. Philadelphia Mixing Solutions, for example, provides information on the proper lubricant with each of its mixer gearboxes.
- Proper installation of the gearbox and wetted parts (per manufacturer recommendations) is also crucial. Improper alignment, unsatisfactory leveling and generally incorrect installations can create an ongoing problem. For instance, misalignment may show itself in the form of frequent mechanical seal failures. The mechanical seal failures would be the symptom; however, the root cause would be the improper installation of the mixer.
- Proper oil levels are also critical to the life of the gearbox. As stated above, with an insufficient level of oil, metal-to-metal contact of the pinions and gears and an elevation of running temperatures occur, resulting in fatiguing of or damage to gearing, thereby reducing gearbox life. Complete gearbox failure can occur, too. A good oil analysis and lubrication program can prevent this from the start.
- Keeping the oil clean and free of foreign materials is important as well. This includes the most common infiltrator — water. The initial selection of a well-built gearbox can eliminate many future headaches, such as water infiltration, while gearboxes with plastic covers can be trouble waiting in the wings.
- Leakage of oil at the output shaft of the mixer is almost always a result of over-lubrication. More oil is not always better when it comes to some mixer gearbox designs. Gearboxes with a drywell surrounding the output shaft are often grease sealed. When the gearbox is overfilled with lubricating oil, the oil sits on top of the grease, penetrating it and breaking it down. A head of oil on top of these grease seals can push oil out at the shaft, often making a mess on top of sealed tanks. Even worse, the oil could leak into the process itself. Following the manufacturer’s recommendations for oil levels is crucial.
- Improper and incorrect installation of oil level site gauges can display an erroneous level of oil required. The lubrication technician may be accomplishing his or her job to perfection, according to the gauge, but, if the gauge level isn’t correct, too much oil is being added every time the lubricator does his or her job. Consultation with the mixer manufacturer and a properly trained lubrication staff can prevent this.
- When operating batch processes, never operate a mixer outside of the liquid process. Running the mixer in just air is a fast way to damage the mechanical seal or stuffing box. It can also damage bearings within the gearbox, and even resonate upward into the gearbox, damaging pinions and gearing. The shaft and impeller are designed to run in the liquid process. Without the process load, the agitator can rock back and forth as it spins. Once this lateral movement starts, it can grow exponentially and bend the agitator shaft itself.
- A good vibration analysis program can additionally help diagnose potential problems and predict failures. Once a potential vibration problem is identified, a process outage/gearbox repair or replacement can be scheduled along with other scheduled downtimes or during slow production times. This way, the cost of the failure relative to loss of production can be greatly reduced.
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