Designing with heat
dissipation in mind
Imagine you’re cruising along the highway. Scrolling through the dashboard options, you see that your oil temperature is steady, your coolant temperature is secure and your speed is set to a desired level. On a trip like this, those conditions will largely remain the same. You’ve achieved a stabilized situation. The heat generation and heat dissipation capacity of all systems is in balance.
But what happens when outside conditions change – the grade becomes extremely steep; you’re pulling an oversized trailer; the air conditioning is running and it’s 98 degrees outside? You might experience a thermal imbalance unless you adjust your expectations of the vehicle or the design team tested to ensure sufficient cooling capacity to maintain thermal equilibrium consistent with all component physical limitations.
In most bearing applications, normal operating conditions are going to shift to some degree. However, in some sectors, particularly those related to electrification, space and even automotive and heavy equipment, those conditions are being pushed to the limit. Companies desire weight reduction, operation at higher speed and load and reduced power loss from lubricant churning, resulting in potentially higher thermal output from the bearing.
“They want to have the least amount of lubrication, push the bearing faster and faster and downsize everything to reduce weight,” noted Chris Napoleon, president and chief engineer at Napoleon Engineering Services in Olean, New York. “So, we might have heavier loads on a smaller bearing with less lubrication, and now we’re in danger of experiencing a heat imbalance.
“We’re generating so much in this smaller package, straining the overall system.”
UNDERSTANDING the complexity of heat imbalance failure is important. According to the Failure Atlas for Hertz Contact Machine Elements, heat imbalance failure results from heat generated in excess of the heat simultaneously removed. It differs from other contact component failure modes in that it’s a systems failure; the effects may manifest themselves primarily on one or a few components, but the failure mechanism is the loss of thermal equilibrium of the machine element as a system.
It is comprised of three failure events:
1. The steady state of thermal equilibrium in a volume enclosing the machine element gives way to an excess of heat generation over heat removal.
2. The temperature of some or all contact components and often that of some other components of the machine element (including the lubricant) rises in an excursion above the maximum design level.
3. Depending on the magnitude, location and duration of the temperature excursion, several different intermediate failures might occur, including lubrication failure and loss of operating clearance.
THE RESULTING lubricant degradation, loss of operating clearance and wear generally leads to large parasitic loads on the Hertz contacts. The increased load further increases heat generation, creating rapid, often catastrophic failure. But the ultimate failure of heat imbalance is a runaway temperature excursion, often referred to as a “burn-up.”
Major heat imbalance could cause the destruction of an application through one of many failure modes. These include galling (with seizure), hot plastic deformation, fracture and melting of plastic components. Lesser heat imbalance events, if arrested, may leave no observable effects or result in a long-term effect that causes potential future failure. Among those are loss of lubrication supply, lubrication decomposition and loss of hardness.
Is all this really the fault of the bearing?
“If the bearing is designed and manufactured correctly then perhaps not,” Napoleon said. “It’s a slippery slope. The bearing industry has significantly improved material quality, incorporated design optimization to distribute stress and reduced 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 the load and speed conditions and show acceptable design life.
But will you still be in a stabilized thermal condition? Such actions put a greater demand on the OEM’s design engineering team to draw in sufficient heat dissipation capability as the margin for thermal stability can be far less when design goals push beyond the norm.
“We’re asking more of our grease and dry film lubricated bearing solutions to eliminate oil delivery systems,” Napoleon said. “Therefore, the OEM design engineer needs to consider how one will remove the heat that’s historically not been at this level and likely removed by oil delivery and cooling systems.
“There’s no magic bearing component solution to solve heat dissipation. The bearing engineer can incorporate friction-reducing design controls, self-lubricating cages and wear resistant thermal treatments. The system or OEM designer has the distinct responsibility of providing sufficient cooling capacity to compensate for operations that often do not co-exist with standardized formulas for calculation of bearing life and heat generation.
This means that sufficient time and effort needs to be put into performing experimental design and physical testing to determine what level of heat generation is created by component level solutions and the necessary cooling capacity.”
“People often don’t think a lot about that.” Napoleon concluded. “But that’s what you need to do to achieve your original goal, which is stability; the same stability you might enjoy while driving down the highway.”
We invite you to reach out to an NES application engineer to see if we can help assess the risk of heat imbalance in your application. To do so, call at 877-870-3200 or email at [email protected].
By J.P. Butler
