Motion System Design

Cooling for speed reducer life

Because today’s speed reducers must handle higher loads, their gears generate more heat, which reduces service life. Thus, it is has become essential to specify adequate cooling as part of the speed reducer selection process

Maintaining an acceptable oil temperature in an enclosed speed reducer has always been necessary to ensure satisfactory life of the gear drive. But temperature has taken on added importance over the years as the mechanical rating of speed reducers rose due to better designs, materials, and manufacturing methods. All these advancements have led to higher loads and, in turn, increased heat generation in the drive. Thus, selection of a speed reducer must consider not only the mechanical rating, but the need for cooling to maintain an adequate thermal rating as well.

Thermal rating

A geared speed reducer's thermal rating is the maximum power that the reducer can continuously transmit without exceeding a specified oil sump temperature. This means that for optimum life, thermal rating must equal or exceed the transmitted power.

A thermal rating depends on speed reducer parameters (type, size, ratio, speed, oil type and level, and cooling devices), operating environment (ambient temperature, air velocity, and duty cycle), and the maximum allowable oil sump temperature. The higher the allowable temperature, the greater the thermal rating.

Sump temperature

A maximum allowable oil sump temperature of 200 F is recommended by the American Gear Manufacturers Association (AGMA 6010-E88 “Standard for Spur, Helical, Herringbone, and Bevel Enclosed Drives,” plus other product standards). In most cases, a 200 F limit provides adequate gear drive and lubricant life. However, the gear manufacturer’s experience or customer requirement can lead to selection of a maximum temperature either below or above 200 F.

Generally, sump temperatures should be lower, such as 140 to 150 F, in special applications where a need for high reliability offsets the additional cost of a larger drive or additional cooling devices.

Though generally not recommended, temperatures above 200 F can provide acceptable gear drive performance in certain applications. For these, some gear manufacturers use a maximum allowable sump temperature of 220 F. But operating above 200 F (even for a short time) shortens the life of lubricants and contact seals and it increases surface deterioration on gears and bearings. Minimizing these detrimental effects requires a scrupulous maintenance routine.

Several methods can be used to cool an enclosed gear drive, thereby increasing its thermal rating. The two basic methods are air cooling and water cooling.

Air cooling

Methods that use air for cooling include:

• Natural cooling.
• Shaft-mounted fan.
• Oil-to-air heat exchanger.
• Electric fan (external).

The most basic method, natural cooling, uses natural convection plus radiation heat transfer to the surrounding air. But, this method is often inadequate for the loads imposed on today’s drives.

Shaft-mounted fans, on the other hand, add forced convection heat transfer to the surrounding air, which significantly increases the cooling capacity, Figure 1. The effectiveness of shaftmounted fans depends on the fan diameter and input shaft speed. In many cases, these fans do not provide enough cooling because the fan size is limited by the size of the drive and its speed is limited by the application.

Moreover, shaft-mounted fans require straight radial blades so they will work in either direction of rotation. This requirement eliminates blade pitch and curvature, which reduces effectiveness of these fans.

Another option, an external oil-to-air heat exchanger, uses a pump to circulate the sump oil through a radiator where it is cooled by an electric fan. These heat exchangers usually require additional floor space and external piping for oil flow.

Electric-fan cooling provides an effective alternative to the shaft-mounted fan and external oil-to-air heat exchanger methods. Here, a fan assembly is attached to the speed reducer housing, Figure 2. A baffle directs air along the sides and top of the housing.

Benefits of an electric fan include increased input shaft accessibility, cooling efficiency, and controllability. First, the input shaft of the speed reducer is free for attaching other devices such as belt pulleys, or backstops and fluid couplings as used in conveyor applications.

Second, a more efficient fan blade design can be selected because, unlike a shaft-mounted fan, an electric fan operates independently of the speed of the reducer and direction of shaft rotation.

Finally, an electric fan can be thermostatically controlled to provide additional cooling when needed, for example, during the summer when ambient temperatures are higher. And, it can be removed for cleaning without disturbing the speed reducer.

Water cooling

As an alternate approach to air cooling, water cooling methods include:

• Oil-to-water heat exchanger.
• Cooling tubes.

In the traditional method, a pump circulates sump oil through an external oilto- water heat exchanger where it is cooled by water flow. Typical flow rates of water and oil through the external cooler range from 5 to 25 gallons per minute (gpm).

A newer alternative to heat exchangers consists of finned cooling tubes in the sump of the enclosed speed reducer, Figure 3. As oil circulates within the sump, cooling tube fins conduct heat from the oil to water flowing inside the tubes.

Cooling tubes offer several advantages. First, they require no lubricant pump because the speed reducer gearing churns and circulates oil around the cooling fins. Additionally, all connections and changes in water-flow direction occur outside the speed reducer, which reduces the likelihood of a water leak inside the oil sump.

Most importantly, cooling tubes reduce water usage. The normal water flow rate for cooling tubes is 2 to 5 gpm, compared with up to 25 gpm for external oilto- water heat exchangers.

From a maintenance standpoint, the inside of the cooling tubes can be cleaned without removal. Also, the tubes can be replaced without disturbing the drive system. Tube material to withstand highly corrosive water supplies is available.

Performance and selection

Cooling ability varies considerably among different cooling methods, Figure 4. Compared to natural cooling, for example, a shaft fan reduces the temperature and an electric fan reduces it further. Cooling tubes lower the temperature an additional 15 to 25 F, depending on the load.

Selecting the best cooling method for a specific application requires analyzing the application, reliability, and cost. In many cases, the method selected depends on the available cooling medium. For example, oil-to-water heat exchangers require a constant water source.

Shaft-mounted fans are the most commonly used. Where shaft-mounted fans lack sufficient thermal rating or the shaft extension is required for mounting some other device, electric fans or cooling tubes are selected.

Oil-to-water heat exchangers are often applied where water cooling is required and the lubrication system requires a lubricant pump. Oil-to-air heat exchangers are generally used only where the required thermal rating exceeds that available with cooling fans (shaft or electric) and where cooling water is not available.

Richard Schunck is a senior research engineer for The Falk Corp., Milwaukee.

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