To understand the ins and outs of gear drive cooling, it’s important to know why gear drives get hot in the first place.
To get a basic idea, think of a gear in a gearbox like a paddle wheel in the water. The more the paddle wheel is submerged in the water and the faster it turns, the more water it will displace. The act of pushing the water (or oil, in our case) aside generates heat, which is commonly referred to as churning.
Inside your gear drive, the heat from churning is compounded by the heat generated from friction in each gear mesh. Helical gear meshes are generally around 99% efficient. The remaining 1% power loss comes primarily in the form of sound and heat.
Additionally, bearings lose a small amount of power to heat as well, due to both churning and friction. The contact on the lip seal also creates heat from friction. When combined (along with external application factors), your gear drive naturally becomes hot.
In the best-case scenario, excessive heat will shorten the life of your oil. In the worst case, heat can damage components and cause early gear drive failure. Therefore, it’s critical to choose a gear drive with the right thermal rating for your application.
What to Look for in a Gear Drive Thermal Rating
When engineers assign a thermal rating to a gear drive, there are certain assumptions made about the operating conditions. It’s important to understand these assumptions and how they contribute to the thermal rating of your gear drive.
Ambient Operating Temperature
Ambient surrounding environmental factors are important to consider when you select a gear drive. What is the maximum ambient operating temperature for a prolonged period of use? A gear drive for an outdoor project in the desert will need to accommodate a higher ambient temperature than a gear drive in icy Northern Canada.
Is your gear drive sitting atop a line at 15,000 feet? When a gear drive is used at high altitudes, its natural ability to dissipate heat is diminished due to thinner air.
Ambient Air Velocity
It’s important to consider the ambient air velocity as well. A gear drive in a confined area like a factory space may run hotter than a gear drive in the open air. On a conveyer in the outdoors, your gearbox can benefit from natural wind fields (if they’re present) to cool the drive.
Another factor to consider is the duty cycle of your drive. In other words, how long does the drive run? If a drive runs continuously, the duty cycle is 100%. On the other hand, if you only run your drive for ten-minute increments, the likelihood of heating (or overheating) is greatly reduced. Generally, larger gearboxes benefit from shorter duty cycles, compared to smaller gearboxes. It takes much longer for a larger gearbox to reach a steady state temperature.
The orientation and position of your gear drive may also affect the thermal rating requirements. Depending on your drives position, oil may sit differently inside the drive. The position of the drive may also affect the way it runs.
Understanding the Thermal Rating
As you can see, there are number of factors to consider when you look at the thermal rating of a gear drive. Every drive has two different ratings:
- The mechanical rating, or the ability to transmit mechanical power.
- The thermal rating, or how much power can be transmitted continually through the drive without overheating.
Both ratings are extremely important. If the gear drive’s capacity to dissipate thermal energy is insufficient, your drive will overheat, causing severe damage and failure. Most manufacturers’ catalog thermal ratings are set based on the following assumed conditions (although these may vary by manufacturer and product line):
- Ambient temperature is 68°F (20°C).
- Altitude is between sea level and 2460 feet.
- Ambient air velocity is between 1.6 feet per second and 4.6 feet per second (typical of a large indoor room).
- Duty cycle is continuous.
- Orientation is floor-mounted with shafts in same horizontal plane.
As you look at the thermal rating, you’ll need to adjust based on your application. If the local thermal conditions differ from the assumed conditions above, you will need to factor them into your decision. Many adjustment factors are listed in the catalogue, or your Rexnord representative can assist you in factoring in the application adjustments required for your situation.
Options for Cooling Your Gear Drive
For many applications, you may need more than just Mother Nature to cool your gear drive. Fortunately, there are several options to bring down the operating temperature. As you compare the thermal ratings of various drives, you’ll see ratings to factor in fan cooling, water cooling, and without cooling.
If we take a step back in time, we learn that the heat generation of gear drives has only been a major consideration in the industry since around the 1990s. Before that time, gears were softer and to transmit mechanical power they were larger—and therefore gearboxes were built larger to house them. The larger gearboxes generally had enough surface area to dissipate the heat naturally.
With the mainstream use of case-hardened gears, mechanical power was transferred in a smaller package. As the surface area of the housing continues to shrink, so does the ability to dissipate heat. Think of it like a cup of hot coffee—if you have a very narrow cup, blowing across the top will still take a long time to cool. If you pour your coffee into a bowl or shallow pan, you have a much wider surface area that cools faster.
1. Shaft Fans
In many drive components, there are shaft-attached fans available. These fans work simply and effectively. A fan is affixed to the high-speed shaft, as the motor spins the shaft the fan also spins. The fan blows air over the drive, cooling it down during operation (and improving the thermal rating of the drive).
The man drawback of shaft fan cooling is it’s only so effective. The fan is only capable of blowing so much air to bring down the temperature. Its effectiveness is also tied to the motor speed, so gearboxes with lower input speeds will not benefit as much.
2. Water Cooling
Another option for cooling your gearbox is to add tubes inside the gear drive and pump water through the tubes. As oil heats up, the cool water inside the tubes pulls heat out of the oil sump. While this is a very effective solution, it requires careful engineering to ensure constant access to cool water and to prevent corrosion of the tubes, among other issues. Any leaks in the tubes could quickly contaminate the gearbox oil with water.
Another options for cooling down 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 the water directly through the gearbox.
The drawback of this cooling method is that you need to ensure constant access to cool water, similar to direct water-cooling. You also need to run oil through a pump to the external heat exchanger, which means adding a pump to your gear drive.
The pump requires using a separate motor, which creates another point where the gear drive may fail. You also must run electricity to the pump motor—often at a different voltage than the main gearbox motor.
Depending on your application, water accessibility, and external thermal factors, any of these gear drive cooling solutions may be effective. The objective is always to find the most efficient cooling solution with the fewest points of failure, requiring the lowest maintenance.
Why DuraPlate is a True High-Performance Solution
To answer the need for a durable, high-performance gear drive cooling solution, Rexnord engineered the patented DuraPlate cooling system, available as an option with our Falk V-Class Drives.
Instead of requiring a separate gear drive cooling system with an external heat exchanger, fan, or water pump, DuraPlate is a heat exchanger built for the Falk V-Class drive. It integrates with the gearbox, minimizing the physical footprint requirements. The DuraPlate system never needs coolant, water, or electricity, making it an easy, cost-saving, and low-maintenance solution.
DuraPlate consists of a plate-style heat exchanger made from stainless steel mounted on the gearbox itself. When the input shaft of the gearbox spins, the built-in shaft fan generates air flow. Hot oil flows through the low-restriction heat exchanger surrounding the shaft fan, cooling directly and efficiently, without the need for an external motor.
Instead of an external pump, a shaft driven, integral pump circulates the oil. This option offers you the cooling abilities of air and water-cooling, but in a self-contained packing within the drive. The added benefit is a fool-proof operation because the pump and fan are shaft-driven; if the gearbox is running, so is the cooling system. Although, there is no more need to ensure the cooling system is turned on when the gearbox is running.
DuraPlate is an ideal solution and much more cost-effective than external air-to-water cooling. For a no-maintenance, low cost solution, the DuraPlate system is an ideal option with the Falk V-Class Drive. The V-Class Drive is already engineered to operate cooler than the competition, and with the optional DuraPlate cooling system, you can rest assured your drive life is maximised.
Find out more information on the Falk V-Class drive with optional DuraPlate cooling system on the Rexnord website.