Active Outer Ring Cooling of High Loaded and High Speed Ball Bearings

Abstract

Bearings for aero engine applications are subjected to a high thermal impact because of the elevated speeds and loads. The high rate of heat generation in the bearing cannot be sustained by the materials used and in the absence of lubrication will fail within seconds. For this reason aero engine bearings have to be lubricated and cooled by a continuous oil stream. The heat which is generated in the bearings through friction is transferred into the oil. Oil itself has not unlimited capabilities and can only remove heat as long as its temperature does not reach critical limits. When the critical limits have been reached or even exceeded the oil will suffer chemical decomposition (coking) with loss of its properties and subsequently causing a detrimental impact on the rotating machinery. Oil is normally transferred into the bearings through holes in the inner ring thus taking advantage of the centrifugal forces due to the rotation. In its way through the bearing the oil continuously removes heat from the inner ring, the rolling elements and the bearing cage until it reaches the outer ring. Since the oil has already been heated up its capability to remove heat from the outer ring is considerably reduced. The idea to provide the bearing with an “unlimited” quantity of oil to ensure proper cooling cannot be considered since an increase in the oil quantity leads to higher parasitic losses (churning) in the bearing chamber and increased requirements on the engine’s lubrication system in terms of storage, scavenging, cooling, weight, etc, not mentioning the power needed to accomplish all these. In this sense, the authors have developed a method which would enable active cooling of the outer ring. Similar to fins which are used for cooling electronic devices, a spiral groove engraved in the outer ring material would function as a fin body with oil instead of air as the cooling medium. The number of “threads” and the size of the groove design characteristics were optimized in a way that enhanced heat transfer is achieved without excessive pressure losses. An experimental set up was created for this reason and a 167.5mm PCD (Pitch Circle Diameter) ball bearing was investigated. The bearing was tested with and without the outer ring cooling. A reduction of 50% of the lubricant flow through the inner ring associated with a 30% decrease in heat generation was achieved.

Effect of a Cooling Unit on High-speed Motorized Spindle Temperature With a Scaling Factor

“…In experimenting, we changed the coolant flux and working temperature by adjusting oil cooler (7), and the ambient temperature, shaft speed and oil/air flow rate for lubrication were regulated through the air conditioner, variable-frequency drive (6) and oil-air lubrication device (4) respectively. In order to monitor the spindle temperature change under different cooling parameters, 8 thermal resistance temperature sensors (12)(13)(14)(15)(16)(17)(18)(19) were evenly placed on the outer surface of spindle cooling sleeve and 2 built-in thermistors (10)(11) on the outer rings of front and rear spindle bearings as well. At the same time, the thermal resistances (8) and ( 9) are used to measure the temperature of discharged oil-air and coolant separately.…”

“…Further, cooling, as an efficient way to lower the temperature, is usually needed in two locations: bearings and motor for a high-speed motorized spindle [10]. The cooling for bearings is often accompanied by lubrication, whose corresponding impact on spindle temperature has been investigated by many scholars, such as Michael et al [11]. In contrast, the energy loss of motor armatures is much bigger than bearings and therefore is regarded as the most important thermal source of a spindle.…”