Imagine you are driving down the highway. Scrolling through the dashboard options, you see that the oil temperature is stable, the coolant temperature is safe, and the vehicle speed is set to an ideal level. During such a journey, these conditions remain largely constant. You have achieved steady state. All systems' ability to generate and dissipate heat is in balance.
But what happens when external conditions change-the grade becomes very steep; you are pulling an oversized trailer; the air conditioning is running and it is 98 degrees Fahrenheit outside? Unless you adjust your expectations for the vehicle or the design team tests to ensure sufficient cooling capacity to maintain thermal balance consistent with the physical limitations of all components, you may experience thermal imbalance.
In most bearing applications, normal operating conditions will change to some degree. However, in some sectors, particularly those related to electrification, aerospace and even automotive and heavy equipment, these conditions are being pushed to their limits. Companies want to reduce weight, run at higher speeds and loads, and reduce power losses from lubricant churning, resulting in potentially higher heat output from bearings.
"They want to minimize lubrication, push bearings faster and faster, and shrink the size of everything to reduce weight," noted Chris Napoleon, president and chief engineer at Napoleon Engineering Services in Olean, New York. "So we may have heavier loads on smaller bearings with less lubrication, and now we are in danger of thermal imbalance.
"We are generating so much in this smaller package that it stresses the entire system."
It is important to understand the complexity of thermal imbalance failure. According to the Hertz Atlas of Contacting Machine Component Failures, thermal imbalance failure is caused by more heat being generated than can be removed at the same time. It is different from other contacting component failure modes because it is a system failure; the effects may be primarily manifested in one or a few components, but the failure mechanism is the loss of thermal balance of the machine elements as a system.
It consists of three failure events:
1. A steady state of thermal equilibrium within the volume enclosing the machine element results in heat generation exceeding heat removal.
2. The temperature of some or all of the contacting parts of the machine element (including the lubricant) and often some other components exceeds the maximum design level.
3. Depending on the size, location and duration of the temperature excursion, several different intermediate failures may occur, including lubrication failure and loss of operating clearance.
The resulting lubricant degradation, loss of operating clearance and wear typically result in large parasitic loads on the Hertzian contacts. The increased load further increases the heat generation, leading to rapid, often catastrophic failure. But the ultimate failure of thermal imbalance is a runaway temperature excursion, commonly referred to as a "burnout".
Severe thermal imbalance can lead to the destruction of an application through one of several failure modes. These include wear (with seizure), thermoplastic deformation, fracture and melting of plastic parts. Smaller thermal imbalance events, if contained, may leave no observable effects or may result in long-term effects leading to potential future failures. These include loss of lubrication supply, lubrication breakdown and loss of hardness.
Is it all really the bearing's fault?
"If the bearing is designed and manufactured correctly, then probably not," Napoleon said. "It's a slippery slope. The bearing industry has significantly improved material quality, combined with design optimizations to distribute stresses and reduce frictional characteristics through design or manufacturing techniques. As a result, theoretical L10 bearing life has increased. This suggests that you can downsize the bearing or increase load and speed conditions and show acceptable design life.
But will you still be in a stable thermal state? Such actions place greater demands on the OEM's design engineering team to attract adequate heat dissipation, because when design targets are pushed beyond the norm, there may be much less margin for thermal stability.
"We are asking more of our grease and dry film lubricated bearing solutions to eliminate the oil delivery system," Napoleon said. "So, OEM design engineers need to consider how to remove heat that has historically not been removed to this level, which is likely to be removed by the oil delivery and cooling systems.
"There is no magic bearing component solution to solve the heat dissipation problem. Bearing engineers can combine design controls that reduce friction, self-lubricating cages, and wear-resistant heat treatments. The system or OEM designer has a unique responsibility to provide sufficient cooling to compensate for operations that are not typically coexistent with standardized formulas for bearing life and heat calculations.
This means investing sufficient time and effort in experimental design and physical testing to determine the level of heat generated and the necessary cooling capacity for component-level solutions."
"People don't usually think about it too much," Napoleon concluded. "But that's what you need to do to achieve your original goal, which is stability; the kind of stability you might enjoy while driving on the highway."