To get a basic idea, think of the gears in a gearbox as a paddle wheel in water. The longer the paddle wheel is immersed in the water and the faster it spins, the more water it will move. The act of pushing the water (or oil, in our case) aside generates heat, which is commonly called stirring.
What to look for in a thermal evaluation of a reducer
When engineers assign a thermal rating to a transmission, some assumptions are made about operating conditions. It is important to understand these assumptions and how they contribute to the thermal evaluation of the gear drive.
Ambient operating temperature
It is important to consider surrounding environmental factors when selecting a gear transmission. What is the maximum ambient operating temperature for a long period of use? Gear for an outdoor project in the desert will need to adapt to a higher ambient temperature than gear in cold northern Canada.
Application altitude
Is your transmission at the top of a line at 15,000 feet? When a gear is used at high altitudes, its natural ability to dissipate heat is reduced due to the thinner air.
Ambient air speed
It is also important to consider the ambient air speed. A transmission in a confined area, such as a factory space, may become hotter than a transmission outdoors. On an external conveyor belt, your gearbox can benefit from natural wind fields (if present) to cool the drive.
Duty cycle
Another factor to consider is the duty cycle of the unit. In other words, how long is the trip? If a converter runs continuously, the duty cycle is 100%. On the other hand, if you only turn on the unit in ten-minute increments, the likelihood of overheating (or overheating) will be greatly reduced. Generally, larger gearboxes benefit from shorter duty cycles than smaller gearboxes. It takes much longer for a larger gear to reach a stable temperature.
Mounting position
Gear orientation and location can also affect thermal rating requirements. Depending on the location of the unit, the oil may be located differently inside the unit. The location of the unit may also affect its operation.
Understanding Thermal Rating
As you can see, there are several factors to consider when looking at a gear's thermal rating. Each unit has two different classifications:
- Mechanical assessment or ability to transmit mechanical energy.
- The thermal rating, i.e. how much power can be transmitted continuously through the inverter without overheating.
Both assessments are extremely important. If the gear's ability to dissipate thermal energy is insufficient, the unit will overheat, causing serious damage and failure. Most thermal ratings in the manufacturer's catalog are defined based on the following assumed conditions (although these may vary by manufacturer and product line):
- The ambient temperature is 68°F (20°C).
- The altitude is between sea level and 2,460 feet.
- Ambient air speed is between 1.6 feet per second and 4.6 feet per second (typical of a large indoor room).
- The work cycle is continuous.
- The guidance is mounted on the floor with axes in the same horizontal plane.
When looking at the thermal rating, you will need to adjust it based on your application. If local thermal conditions differ from the conditions assumed above, you will need to factor this into your decision. Many adjustment factors are listed in the catalog, or your Rexnord representative can help you consider the application adjustments needed for your situation.
Options for cooling the reducer
For many applications, you may need more than Mother Nature to cool your transmission. Fortunately, there are several options for reducing operating temperature. When comparing the thermal ratings of various units, you will see ratings that take fan cooling, water cooling, and no cooling into account.
If we go back a bit, we learn that heat generation from gears has only been an important consideration in the industry since around the 1990s. Before that, gears were softer and larger to transmit mechanical power and therefore , gearboxes were built larger to accommodate them. Larger gearboxes generally had enough surface area to dissipate heat naturally.
With the traditional use of hardened gears, mechanical power has been transferred to a smaller package. As the surface area of the case continues to decrease, the ability to dissipate heat also increases. Think of it like a hot cup of coffee: if you have a very narrow cup, blowing over the top will still take a long time to cool down. If you pour the coffee into a shallow bowl or pan, you have a much larger surface area that cools faster.
Shaft Fans
In many drive components there are fans connected to the shaft. These fans work simply and effectively. A fan is attached to the shaft at high speed, as the motor rotates the shaft, the fan also rotates. The fan blows air over the unit, cooling it during operation (and improving the unit's heat production).
The disadvantage of spindle fan cooling is that it is only effective to a certain extent. The fan is only capable of blowing a certain amount of air to lower the temperature. Its effectiveness is also linked to engine speed, so gearboxes with lower input speeds will not benefit as much.
Water cooling
Another option for cooling the gearbox is to add tubes inside the gear and pump water through the tubes. As the oil heats, the cold water inside the tubes takes heat away from the crankcase. While this is a very effective solution, it requires careful planning to ensure constant access to cold water and prevent pipe corrosion, among other things. Any leaks in the hoses can quickly contaminate the gear oil with water.
Oil-water cooling
Another option for cooling a gearbox is to use an external shell and tube heat exchanger. This oil-to-water cooling option offers similar benefits to direct water cooling, without the need to run water directly through the gearbox.
The disadvantage of this cooling method is that you need to ensure constant access to cold water, similar to direct water cooling. You also need to run the oil through a pump to the external heat exchanger, which means adding a pump to the transmission. The pump requires the use of a separate motor, which creates another point where the transmission can fail. It is also necessary to supply electricity to the pump motor, often at a different voltage than the main gear motor voltage.
Depending on the application, water accessibility, and external thermal factors, any of these gear cooling solutions may be effective. The objective is always to find the most efficient refrigeration solution, with the fewest points of failure and requiring the least maintenance.
