THE ROLE OF THERMAL RATING IN THE SELECTION OF AN ENCLOSED GEAR DRIVE

Maintaining an acceptable temperature in the oil sump of an enclosed gear drive is critical to the life of the gear drive. Therefore, the selection of an enclosed gear drive must consider not only the mechanical rating but also the thermal rating.

Thermal rating is defined as the maximum power that can be continuously transmitted through the drive without exceeding a specified oil sump temperature. The thermal rating must equal or exceed the actual service transmitted power. The magnitude of the thermal rating depends upon the specifics of the enclosed drive (type, size, ratio, speed, oil type and level, cooling devices, etc.), operating environment (ambient temperature and air velocity, duty cycle, etc.) and the maximum allowable oil sump temperature. The higher the maximum allowable oil sump temperature, the greater the thermal rating becomes.

The maximum allowable sump temperature generally used is 200ºF, as recommended by the American Gear Manufacturers Association (AGMA). Operating experience with the 200ºF maximum has shown it to provide the gear drive and lubricant life demanded for most applications. However, based on the gear manufacturers experience and/or customer request, selections are sometimes made for a maximum sump temperature below or above 200ºF. Generally, lower sump temperatures, e.g., 140 to 150ºF, are requested in special applications where the high reliability required offsets the additional cost incurred by requiring a larger drive size or additional cooling devices. Operating above 200ºF can provide acceptable gear drive performance in some applications. Some gear manufacturers have used a maximum allowable sump temperature as high as 250ºF. However, it must be recognized that operating above 200ºF may reduce lubricant and contact seal life and increase the surface deterioration on the gears and bearings with a subsequent increase in the frequency of maintenance.

The operating sump temperature of an enclosed drive depends upon the balance between the heat generated in the drive and the heat dissipated by the drive. The mechanical rating of enclosed drives has increased dramatically over the years due to stronger and harder gear materials, more accurate gear cutting and finishing methods, advanced bearing designs and improved lubricants. The consequence of the increased load intensity (mechanical rating per unit surface area of housing) is increased heat generation in the drive. Therefore, there is a need for increased heat dissipation or cooling capacity to maintain a given sump temperature for the same housing size. The cooling capacity depends upon the type of cooling employed.

There are a number of methods which can be used to cool an enclosed gear drive. They can be classified according to transfer of heat to the surrounding air (air cooling) or an external water supply (water cooling).

Air cooling methods consist of:

• Natural cooling
• Shaft driven fan (s)
• Oil-to-air heat exchanger
• Electric fan

Natural convection and radiation heat transfer to the surrounding air is quite often not adequate with the load intensity employed in today's drives. The addition of a shaft mounted fan or fans adds forced convection heat transfer to the surrounding air and can increase the cooling capacity significantly. The effectiveness of the shaft mounted fans depends upon the fan diameter and input shaft speed. Often with today's drives the shaft mounted fans do not provide enough cooling since the size of the fan is limited by the physical size of the drive and the speed is limited to that of the application. Also, the fans require straight radial blading so that they will work in either direction of rotation thus eliminating the use of blade pitch and curvature to increase their effectiveness.

Sufficient air cooling can be obtained with and external oil-to-air heat exchanger. A pump circulated the sump oil through a radiator that is cooled by an electric fan. The natural, shaft mounted fan and external oil-to-air methods represent those that have been traditionally used on enclosed drives. A new method of air cooling that can be an effective alternative to the shaft mounted fan or external oil-to-air methods is electric fan cooling.

Electric fan cooling is illustrated in Figure 1 where the fan assembly is attached directly to the enclosed drive housing. A baffle is employed to direct the air along the sides and top of the housing as well as support the fan assembly. Compared to shaft mounted fans the electric fan method has the following advantages:

• Input Shaft Access: The input shaft and extension of the enclosed drive are free for other devices (backstop, fluid coupling, etc.).
• Greater Cooling Capacity: An efficient fan design can be selected whose air flow performance exceeds that of shaft fans and is independent of the speed and direction of rotation of the enclosed drive input shaft.
• Controllability: The electric fan can be thermostatically controlled to provide additional cooling only when required.
• Maintenance: The electric fan can be removed for cleaning without disturbing the enclosed drive system.

Compared to an external oil-to-air heat exchanger the electric fan is more compact and less expensive and will have sufficient cooling capacity in many applications.

Figure 1: Electric fan cooling

Water cooling methods include:

• Oil-to-water heat exchanger
• Cooling tubes

Traditionally, water cooling has used an external oil-to-water heat exchanger and a pump to circulate the sump oil. Typically, the flow rates of both the water and oil through the external cooler range from approximately 5 to 25 gallons per minute. An effective alternative to this water cooling method is a new method that uses internal cooling tubes in the base of the enclosed drive.

Figure 2 shows the cooling tubes in the base of a parallel shaft speed reducer. The tubes are externally finned and heat is removed from the sump oil by contact with these fins which conduct the heat to the water flowing inside the tubes. The internal agitation of the sump oil caused by dipping the gearing in the oil aids this heat transfer process.

Figure 2: Cooling tubes in the base of a parallel shaft speed reducer

Compared to the external oil-to-water cooling method the cooling tubes have the following advantages:

• Lube pump eliminated.
• Reduced Risk of Oil and Water Mixing: The likelihood of a water leak developing and causing the water to mix with the sump oil is much reduced by locating all tube connections and water flow direction changes outside of the enclosed drive housing.
• Reduced Water Usage: Since the heat transfer process is controlled by the sump oil-side coefficient, the water-side coefficient can be reduced by lowering the flow rate without adversely affecting the overall cooling tube heat transfer coefficient. Normal tube flow rate is 2 to 5 gallons per minute.

The inside of the cooling tube can be cleaned without removal. Also, the cooling tube installation is designed such that the tubes can be replaced without disturbing the drive system. Alternative tube materials for highly corrosive water supplies are available. A number of cooling tube passes can be installed in a drive to provide sufficient cooling capacity in the majority of applications thus making them a viable alternative to the external oil-to-water heat exchanger.

Figure 3 illustrates the measured effectiveness of various cooling methods on the operating sump temperature of a double reduction enclosed gear drive. Note the sump temperature reduction achieved with the shaft fan as compared to natural cooling. The electric fan achieves an even greater reduction and the cooling tubes lower the sump temperature an additional 15 to 25ºF below the electric fan results. These curves illustrate that at a given maximum allowable sump temperature, e.g., 200ºF, increased cooling capacity results in allowing a greater transmitted load, i.e., thermal rating.

The increased load intensity of today's enclosed gear drives has challenged gear manufacturers to provide increased cooling capacity. This challenge has been met by providing a variety of cooling methods, both traditional and newly developed, from which the best method for a particular application can be selected. The consideration of the cooling method and its resulting thermal rating has become an integral part of the enclosed gear drive selection process.