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Technical Articles

Q & A About DBS NSA-Series Aerators
By James Hu, PE, Senior Project Engineer
August 1, 2000

DBS has been actively promoting the NSA series low-speed surface aerator product for the past three years. The following are the common questions or comments from our clients:

Comment: The DBS NSA is less expensive than a model of X brand, but the NSA is also lighter and smaller than X brand. DBS must be cutting corners to bring the price down.
Answer:
The most noticeable difference is the gearbox size. The NSA aerator uses a planetary gearbox, which is highly compact and known for high torque to volume ratio. To simplify the discussion, let us just compare the biggest and the most important components--final output gear and bearing.

The final gear of a planetary gearbox is an internal gear driven by three pinions. To deliver the same amount of torque by a gear driven by a single pinion, a traditional double reduction parallel shaft (DRPS) gearbox would need a final gear 3 times the diameter. After adding the external pinion and the first stage gear set, the actual housing of a DRPS gearbox would be about 5 times as big as a torque equivalent planetary gearbox.

Let's look at the final output bearing. The DRPS gearbox final output bearing is big, but it is subjected to 300% to 1000% of the rotor radial load due to the long shaft leverage, which amplifies the load on the bearings. The planetary gearbox's final gearing is almost as big, but it is only subjected to 120% of the rotor radial load. Thus, the NSA aerator final bearing has a much higher safety factor.

It is clear that a well engineered planetary can carry the same load as its much larger DRPS gearbox counterpart. This intuitive approach serves well for relative size comparison; but remember, the actual rating of the DBS NSA gearbox is strictly based on AGMA and AFBMA standards.

Comment: The gearbox is down below; oil change is going to be a headache.
Answer:
Because of the gearbox location, gravity drain is not practical. However, getting oil out is still a breeze with our supplied oil change kit. The oil is pumped out and refilled through the drive shaft. Contact DBS for details.

Question: How often do I need to change the oil?
Answer:
Because the gearbox uses synthetic oil and is cooled by water, its operating temperature is only a few degrees above ambient water temperature. This is actually an ideal application for permanent lubrication. Just to be on the conservative side, DBS currently recommends oil change every six months. From the oil samples we have collected so far, oil after 6 months of operation looks like fresh oil added a few days ago. The oil change interval is likely to extend after we compile more extensive oil analysis data.

Question: What is the spare parts requirement for the first two years operation?
Answer:
None if oil is not considered as spare parts.

Question: How do I know the gearbox has oil?
Answer:
Our standard design does not provide an oil level indicator. A pre-measured amount of oil is placed in the gearbox, which has no drain and no seal below oil level. This simply means there is absolutely no chance for an oil leak. An oil level indication option is available.

Comment: The DBS price is low, the product must not be as good as X brand, which cost 30% more.
Answer:
The biggest saving is from the gearbox and the lubrication system. The planetary gearbox used on NSA is a mass-produced, off-the-shelf-item. It is less expensive than the specialty DRPS gearbox.

Because all moving parts of the NSA aerator are below oil level, it uses simple and reliable oil bath lubrication instead of an expensive forced lubrication system. Due to these advantages, DBS NSA aerators can beat our competitors not only on performance, but also on price.

The customer can expect more savings to maintain the equipment due to its simplicity and reliability.

Comment: I really like the DBS aerator; unfortunately, my platform is built for certain model of X brand.
This is not a problem. DBS can retrofit our aerator to your existing structure with little or no modification to your platform.

Question: I have been going through one aerator every 6 month to two years, how long will the NSA last?
Answer:
Aerator applications are very destructive to gearboxes. Many types of aerators fail rather early for various reasons. Below are some reasons for early failure and how DBS NSA aerators succeed.

Under rated gearbox There are two widely used standards to rate the gearboxesÑISO and AGMA. ISO is more recognized in the international market and AGMA is more popular in the US. When comparing gearboxes, it is important to know the service factor and the basis for the service factor. Make sure the service factor is based on the weakest bearing in the whole drive train and ask the manufacturer to translate the service factor to design life of the gearbox in hours for your aeration application. Most European aerator manufacturers provide gearboxes with a 1.4 Service Factor. This Service Factor is inadequate to provide the Specified Design Life of 100,000 hours.Ê

It also pays to look at the design of the aerator to examine how reliable the lubrication system is, and how the machine handles shock load. The Consulting Engineer or concerned contractor should request that the aerator manufacturer provide the make and model of the proposed aerator gearbox. The Consulting Engineer should also require that the gearbox manufacturer provide documentation that ALL bearings (Not just the output shaft bearing) have an L-10 life of 100,000 hours.

The NSA-series aerators are well designed to handle the vigorous aeration conditions. Depending on the model, service factor ranges from 1.8 to 6.5.

Rotor radial load As a rotor interacts with water, the resultant radial force on the rotor is never zero due to installation imperfection, rotor blade geometric variations, and water flow and wave action, etc. The radial load can be very high. On a DRPS style aerator, the radial load is transmitted back to the gearbox through the long output shaft which amplifies the load by 3 to 10 times through leverage. This load frequently causes output shaft bearing failures on a DRPS style aerator. The DBS NSA aerator final bearings are located right above the rotor. The loads on these large bearings are about equal to the radial load on the rotor.

When similar bearings are used, bearings on NSA will last 3 to 10 times longer.

Rotor radial vibration The long shaft of the DRPS style aerator is cantilever supported. In many cases, the shaft is extended with a flange coupling. The final radial run-out at the tip of the shaft is significant. As the shaft turns, the combination of shaft run-out and large mass of the rotor becomes a perfect vibration generator. The result is damage to both gears and bearings. The DBS rotor is directly supported by the bearings. The run-out is equal to the run-out of a precision bearing, which is infinitesimally small. As a result, the DBS NSA aerator operates very smoothly.

Output shaft torsional vibration The long shaft of the DRPS style aerator will twist by a small amount when under torque load. The stress level of the shaft may be acceptable, but it may vibrate torsionally and cause damage to the gearbox.Ê

The DBS NSA aerator does not use output shaft, which completely eliminate this problem.

Lubrication failure The DRPS style aerator has to use a forced lubrication system because it inevitably has bearings above oil level. A forced lubrication system involves pump, filtration, plumbing, etc. Any problem in the system can potentially cause catastrophic lubrication failure.

The DBS NSA aerator uses reliable oil bath lubrication. The oil runs cooler to provide the best lubrication for the gears and bearings.

As you can see, the NSA is a well-engineered product and designed specifically for the aeration application. We have actually conducted an accelerated life test on one gearbox by putting twice the rated horsepower of the gearbox. The gearbox lasted 9 months, well over its equivalent100,000 hours when under normal horsepower.

Comment: This aerator is great, but it would not meet the specification.
Answer:
Most of the specifications are written for the DRPS style aerators. The NSA aerators will meet performance and functionality portions of the specification, but most of the design detail specifications are not applicable due to uniqueness of the NSA design. We need your help to have this product specified for your next project.

Comment: The NSA aerator installation history is too short. I would like to select something proven.
Answer:
In the short history of NSA aerator, we have a short list of proven records:

All installations are successful.
Not a single incident of failure, major or minor.
All customers are happy with our equipment.

On a broader scope, our records are:

DBS has 30 years experience building quality equipment.
The NSA aerator is designed by a team of professional engineers.
The gearboxes are mass-produced by Fairfield and Brevini with installations and distribution centers all over the world.
DBS uses motors made by reputable motor manufacturers.

We also use motors chosen by customers. We are completely committed to serve our customer and back our NSA aerators with an exceptional two-year warranty.