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What is the maximum surface temperature of a gear drive?

There are many external factors that affect the surface temperature of the gear drive.  Examples of these are air velocity, air temperature, humidity, and radiant heating from exposure to sunlight.  Because these variables are outside of our control, we do not set limits on the surface temperature of the gear drive.

The lubricant film strength is the most important factor for assuring the longest operational life of the gear drive.  Proper film strength assures separation of metal surfaces and prevents premature wear.  Since the film strength is determined by the oil temperature, it is the parameter that should be monitored during operation of the gear drive.  The lubricant used in our gear drives is selected to provide adequate film strength at 200°F (93.3°C).

Again, many external factors can affect the oil temperature.  There is no maximum value, only different solutions based on the actual operating oil temperature.  For example, if the oil temperature is between 180°F (82.2°C) and 200°F (93.3°C), a mineral oil has adequate film strength, but it oxidizes at a faster rate.  In this case, the oil must be changed more frequently.  (Many users will not do this, so our O&M Manual cautiously recommends synthetic oil above 180°F (82.2°C).)  At temperatures above 200°F (93.3°C), we recommend using a synthetic product that provides greater film strength than a mineral oil.  Only in extreme circumstances should the oil temperature reach or exceed 200°F (93.3°C). For oil temperatures exceeding 200°F (93.3°C), please contact the factory.

What should I do with my gearbox if it is going to be several months before it is put into operation?

Gearboxes that are not in use for extended periods of time should be properly cared for to ensure their trouble-free operation once they are put in to service. A great reference document is also provided by CTI’s Engineering Standard Guideline ESG 138 (Long Term Storage Procedures for Cooling Towers). There are two different considerations when preparing your gearbox for long-term storage. They are when the gearbox is out of the cooling tower and when it is in the cooling tower (in-situ).

Gearbox Long-Term Storage Not in a Cooling Tower:  

It is imperative that your fan drive gearbox is stored in a warehouse, out of the weather. A climate-controlled environment is recommended, but not required. If the gearbox is not new you should drain the old oil and replace it with new Amarillo Gear approved lubricant. It is best to fill the gearbox with oil to the top of the gear case within about 1 inch (25.4mm) to allow for expansion and contraction with temperature variations. The best spot for filling in this case is the vent hole located on the top gear case cover. This ensures all bearings, gears, and internal surfaces are being coated and protected. Once completely filled, reinstall the vent plug. Note on the new GEN II Series of fan drives, they cannot be filled above operating level due to leakage through the non-lip bearing isolators. In this case, the gearboxes should be ordered from the factory with a vapor phase rust inhibitor in the oil that will coat and protect the inside components. If you have questions prior to order, please contact the factory. Now is the time to attach a warning tag that states the gearbox is completely full of lubricant and must be drained to the proper operating level prior to operation in a cooling tower. All external shaft extensions should be coated with a rust inhibitor grease such as CORTEC VCI-368 or equivalent.

The input shaft does not need to be rotated in this situation since there is no loading on the bearings and false brinelling will not occur unless the gearbox is exposed to external floor vibration. In this case, contact the factory for recommended enhanced long-term storage procedures.

Long-Term Storage “In-Situ” in the Cooling Tower:

An in-situ gearbox should have the old lubricant removed (warning: imperative that the external sight glass on the fan deck must be removed and plugged prior to overfilling to prevent oil spillage on the fan deck outside the fan stack). Next, completely fill the gearbox with new Amarillo Gear approved oil through the vent hole in the gear case cover to within 1” (25.4mm) of the top. Reinstall the vent plug and apply warning tags as needed stating the gearbox has been overfilled and should be drained down to operating level prior to starting. If the gearbox is the new Amarillo GEN II, then after draining the old oil and installing new Amarillo approve oil, an Amarillo factory-approved vapor phase rust inhibitor should be installed with the oil and the gearbox allowed to run at full speed for at least 5 minutes so all internal components can be coated with the rust inhibitor. After the 5-minute run, plug all ports into the atmosphere so the vapor phase rust inhibitor cannot escape.

All exposed machined services such as shaft extensions should be coated with a rust inhibitor such as CORTEC VCI-368 or equivalent.

Allow the gearbox to rotate freely from windmilling. This allows loaded bearings to rotate and prevent possible false brinelling of the bearings between the rollers and race. If the gearbox has a non-reverse backstop, contact the factory. Disengaging may be required but varies by applications.

Commissioning a Gearbox into Service:

Prior to starting the gearbox for operation, the oil needs to be drained back to the operating level. If it has been overfilled in an “in-situ” condition, after lowering the oil level to operating level, the external fan deck sight gauge needs to be reinstalled. If your gearbox is the new Amarillo GEN II that had vapor phase rust inhibitor added and plugged, unplug the vent hole and drain a portion of oil out of the drain port to remove any water build-up from condensation.  Replace makeup oil back to the proper operating level. Inspect vent lines for any debris blockage and check all fill and drain lines & connections for leakage. Rotate the gearbox by hand making sure it rotates freely and smoothly before energizing the electric motor.

On a pump drive with spherical roller thrust bearing, why would the gear drive be difficult to turn by hand?

The spherical roller bearings are spring-loaded which increases the torque required to turn the shaft.  In addition, the thrust bearing outer race can be knocked off-center during shipment, so the gear drive would be very hard to turn.  The thrust bearing is self-aligning once it rotates with an applied load.

What is the maximum vibration level for Amarillo Gear drives?

The maximum value for lateral vibration is 0.3 in/sec (8 mm/sec) peak, filtered reading.  This means that on a vibration spectrum, with frequency on the X-axis, none of the peaks should be above the 0.3 in/sec (8 mm/sec) level.  Reference AGMA 6000-B96 and Cooling Technology Institute STD-163 for more information.

What is the procedure for removing the rust preventative coating from exposed shafts?

Cortec VCI-368 is applied to exposed shafts prior to shipment.  This product may be removed from the shafts using a cloth soaked in mineral spirits, alkaline cleaners, aliphatic solvents, or vapor degreasing solvents. Proper disposal of cleaning materials is required.

What are acceptable values for used oil analysis?

Typical acceptable values for continued use of a gear oil are:


Viscosity: AGMA 4 (ISO 150) – greater than 135 cSt and less than 165 cSt at 40°C

AGMA 5 (ISO 220) – greater than 198 cSt and less than 242 cSt at 40°C

AGMA 6 (ISO 320) – greater than 288 cSt and less than 352 cSt at 40°C

Water Content:         less than 400 ppm

Total Acid Number (TAN):  less than 2.0

Wear Metals:    Iron                           less than 300 ppm

Lead                         less than 75 ppm

Copper                     less than 275 ppm

Chromium                less than 30 ppm

Aluminum                 less than 40 ppm

Tin                            less than 30 ppm

Silver                        less than 30 ppm

Silicon                      less than 60 ppm

In addition, if the viscosity increases 10% or more between samples and the TAN increases, the oil should be changed.  This is an indication that the oil is oxidizing at a rapid rate.

Note:  TAN is the amount of KOH required to neutralize one gram of oil and is a measure of the oxidation.  This is sometimes called the neutralization number (NN).  Another measure, the oxidation number, is a product-specific value and is performed on an infrared spectrometer.  The units for oxidation number are absorbance/cm. To use oxidation number, the oil manufacturer must specify the acceptance limits for the specific product.  As an example, Mobil Gear 630 has an upper limit of 8.0.

Can the breather be installed inside the fan stack?

Amarillo Gear & CTI STD-111 does not allow placing the breather inside the fan stack. This placement could potentially draw humid air into the gear case and contribute to oil contamination. The breather should be installed at the end of a vent line running from the gearbox’s designated breather hole to outside the fan stack. The vent line should slope downward at a minimum of 1/2” per foot (25.4mm per 0.31m) to the outside of the fan stack with the breather at the lowest point.  Piping that forms “traps” should be avoided to prevent the vent line from filling with oil or water and preventing the gear drive from breathing.

Can I replace my Brand X cooling tower gear drive with an Amarillo?

Yes.  Amarillo Gear Drives are designed specifically for the demanding environment of cooling towers. Therefore, the drives may be physically larger than many of the “industrial” gear drives that have been adapted for use in cooling towers.  Some tower modifications may be required to replace the existing gear drive with the Amarillo model.  Some of the items that must be considered are shaft sizes, location of the fan in the fan stack, height of the input shaft from the base, required coupling length, and bolt hole pattern. Please provide the nameplate HP and speed of the motor being used as well as the desired reduction ratio (or fan speed) when requesting a recommendation for a replacement Amarillo gear drive. For tower modifications, Amarillo Gear recommends you engage a qualified cooling tower manufacturer to assist with these changes.

Can I use ‘Brand X’ oil in my fan drive?

Only if “Brand X” oil contains a rust and oxidation inhibitor (R&O) and meets specifications in the Amarillo Gear fan drive Operation & Maintenance Manual which also meets AGMA 9005-F16 lubrication requirements. In addition, lubricants must be compatible with nitrile and Viton seals, Amarillo’s lacquer-based internal gear case sealer, and sprag-type backstops. Extreme pressure lubricants are not recommended. A list of Amarillo Gear approved lubricants are included in the operating and maintenance instructions available at

During cold weather operation, when are heaters and/or synthetic oil recommended for fan drives?

Oil heaters with thermostats or Synthetic oil are recommended whenever the fan drive will be started with an ambient temperature around the gear drive of 20°F (-6.7°C) and below.  If hot water flowing through the tower maintains the ambient temperature above this limit for a period of time prior to start-up, then heaters are not required.

It is important that the oil is above its pour point temperature prior to start-up. Starting the fan drive when the oil temperature is below the pour point will result in no oil flow to the upper bearings and possible damage to internal components.   Very viscous oil can cause internal damage to the gearbox.

Synthetic oil has a much lower pour point than mineral oil.  The synthetic oils listed in our O&M Manual (viewable at may be used in our fan drives, without a heater, when the gear drive ambient temperature is   -20°F (28.9°C)  and above. Additional advantages are listed in our O&M Manual.

Below -20°F (-28.9°C) contact factory for recommendations.

Ambient Air Temperature               Recommendation

>20°F (6.7°C)…….mineral or synthetic oil

20°F (6.7°C) to -20°F (-28.9°C)….mineral oil with a heater/thermostat or synthetic oil

< -20°F (-28.9°C)…..Contact Factory

What modifications does Amarillo Gear Company recommend based on ambient temperatures around our cooling tower gear drives?

 Ambient Air Temperature        Modification Required

< 120°F                                    None

120°F – 150°F                          Synthetic oil and Viton seals

> 150°F                     Synthetic oil, Viton seals, external cooling

What are the pros and cons of adding an oil filter to a gear drive?

The main benefit of a properly designed and selected oil filter is that it removes most of the particles that cause abrasive wear on the gears and bearings.  For the oil to also be properly filtered, it must pass through the filter under pressure.  The use of an optional oil filter will extend the service life of the gear drive under most circumstances, but regular oil sampling should be done to ensure adequate lubrication is in place. Although the main causes of premature failure in cooling tower fan drives are low oil level or poor oil quality, not abrasive wear.  Oil filter elements require periodic replacement and the external plumbing increases the potential of an oil leak. On the other hand, the Amarillo Gear Service Unit (GSU) oil filtration cart is unique and will extend the interval between oil changes.  Its special oil filter media does an excellent job at capturing particulate down to 3 microns and water. Visit for more information on this product.

Describe how the oil filter system works on a double reduction fan drive?

The filtering system used on a double reduction fan drive utilizes a positive displacement pump that is driven by the intermediate shaft.  The pump forces oil through the filter mounted on the outside of the gear case.  The filter contains a 75 micron, acrylic fiber element that has a low-pressure drop.  The filter housing has a built-in bypass valve to ensure oil flow with a clogged filter.  After filtering, the oil is routed inside the gear case where it is distributed to the bearings and gears using a manifold system.

Why doesn’t Amarillo Gear recommend EP oils?

A customer may elect to use an Extreme Pressure (EP) lubricant; however, Amarillo Gear Drives do not need the EP additives to operate properly.  In fact, EP additives will prevent our optional sprag non-reverse devices from properly functioning.

In the past, data from bearing manufacturers had suggested that EP lubricants might extend bearing life. At that time, we recommended EP lubricants in gear drives without a sprag-type backstop.  However, current research indicates that some EP additives may reduce bearing life as evidenced in recent bearing manufacture’s testing when water and humidity are present. Most EP additives include sulfur, and when in contact with water molecules, will become an acidic pH that is corrosive and reduces bearing life. Because Amarillo Gear has a long history of gear drives in service using non-EP oils, we conservatively decided to take EP oils out of our O&M Manual.

Does my double reduction fan drive need a mechanical oil pump?

Often, mechanical oil pumps are specified because many of our competitor’s gear drives require the oil pump for lubrication.  This is because they sell multipurpose, “industrial”, gears that are adapted for use in cooling towers.  Amarillo Gear Drives are designed specifically for cooling towers and use a non-wear oil slinger system that operates in either direction.  A mechanical oil pump on our fan drive is an extra cost option but is not required (unless an oil filter option, a pressure switch option, or operation below 450 rpm input speed are selected). The oil slinger is installed and functional, even when the oil pump is added as a redundant lubrication system.

Our mechanical oil pump is truly bi-directional.  It will pump in either direction while using the same suction and discharge ports.  Many years ago, we worked with Viking Pump to design and manufacture this bi-directional oil pump. The reason other gear drive suppliers warn against running in the reverse direction is that they use a standard unidirectional oil pump and they do not have the oil slinger as a redundant system.

Will “windmilling” damage my cooling tower fan drive?

Typically, windmilling of the fan poses no problem for the gear drive.  During windmilling, the speeds are typically low and the load on the bearings is light, so the existing lube film is adequate.

Please note that a windmilling fan in the reverse direction should come to a stop prior to energizing the motor in the forward direction if the motor control is not either “soft start” or variable frequency drive (VFD) to prevent peak shock loading from being transmitted into the gearbox which could cause permanent damage.

Does my gearbox need to be aligned? If so, what are the tolerances?

Gearbox alignment is critical for a smooth and quiet running gearbox with long trouble-free service life.  If the gearbox is grossly misaligned, the driver (motor or engine) cannot transmit uniform rotation to the gearbox.  Consequently, the following problems may occur:

  • Vibration
  • Noisy running
  • Reduced bearing life
  • Premature gear tooth failure

The input shaft of the gearbox and the output shaft of the driver should be parallel, and the centerlines of both shafts should be in a common vertical plane.

Many different types and makes of driveshafts are being used.  The tolerances for the shaft angular misalignment vary.  Typically, a gearbox can tolerate larger misalignment than can a drive shaft. Rather, the tolerances should be obtained from the drive shaft suppliers.

What is a bearing’s “L10a” rated life?

L10a rating life is the life that 90% of a group of identical bearings will exceed before a fatigue spall develops, with adjustments for material, reliability, and operating conditions. Please consult your product catalog for your products bearing life disclosure.

Where should I put a vibration sensor on the gearbox?

It is recommended at a minimum the vibration transducers be placed in line with the input and output shaft. For Amarillo Gear’s recommended placement on our gearboxes to avoid erroneous vibration readings, request our Form 612A (fan drives). If specified at the time of order, we will drill/tap/spot-face vibration transducer locations on the gear drive. In the field, however, to avoid the possibility of drilling through the gear case and creating an oil leak, it is recommended to attach sensors using epoxy-mounted thread blocks in the specified locations.

What hardware and torque values are used to install my fan drive in the tower?

Hardware and torque are specified in Amarillo Gear Engineering Form 551.  For a copy of this form, please contact us at

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