Increase the efficiency of your rotating machines by optimizing your bearing lubrication

Transcription

1 Increase the efficiency of your rotating machines by optimizing your bearing lubrication I. Introduction When designing oil lubrication systems for bearings, the classical criteria are the necessity for: - lubrication of all contacts, - providing cooling function if needed - and for ensuring wear particles removal. The increasing awareness about climate change and depletion of fossil energy sources results in increasing energy legislation and higher end-user energy prices. This trend results in a market demand of energy-efficient vehicles and machines. Because bearing losses are an important aspect of vehicle and machine drivetrains, it is becoming more and more important to take the energy efficiency of the bearing lubrication into account. On industrial machines, poorly lubricated rolling bearings can seriously impact the energy efficiency and bearing lifetime because over-lubrication leads to an increase of friction losses while under-lubrication accelerates the wear process. It s therefore a need for designers of machine and vehicle drivetrains to rethink the way bearing lubrication is organized. A smart oil lubrication system must consider several factors including the bearing temperature, the oil temperature (viscosity), the environmental temperature to set the correct oil quantity for certain operating conditions (load and speed). Losses in a rolling bearing can be classified according to four friction sources: rolling friction, sliding friction, seal friction and churning losses. The latter, which mainly depends on oil quantity and properties ( viscosity), is the lever on which a smart lubrication should act. Churning losses are caused by motion and displacement of surplus lubricant. These losses occur when the bearing is rotating in an oil bath. They are not only influenced by bearing speed, oil viscosity and oil level, but also by the geometry of the bearing housing. Losses in bearings can represent 20% to 30% of the total energy losses of a transmission and up to 50% for some weaving looms. Figure 1 shows the power loss for oil lubricated cylindrical bearing (NUP212) rotating at 1500 rpm, radially loaded with 5kN and operating at 60 C. For this example, 60% of the total loss can be attributed to churning.

2 Figure1. Friction for NUP212 (Data obtained from simulation with Schaeffler calculation tools [2]) One can notice from this figure that losses can be reduced by 20 % only by setting the oil quantity at the centre of the lowermost rolling element (1/2RE) instead of full rolling element (RE). It can be assumed that 1/2RE level is sufficient to guarantee the presence of sufficient lubricant in the contact zone for good lubrication where the bearing is rotating stationary. However, modifications to bearing lubrication should be implemented carefully to guarantee that lifetime is not affected. Therefore, Flanders Make has built a dedicated setup to study the effect of modifications of lubrication on bearing losses. This research is carried out in cooperation with Schaeffler. II. Bearing friction setup (Flanders Make) Figure 2 shows a photograph of the bearing friction setup at Flanders Make. The bearing under test (1) is driven by a speed controlled motor. Its outer ring is supported by a hub. A radial force can be applied on the tested bearing via the hub by means of hydrostatic pad (2) and pneumatic muscle (3) with minimal parasitic torque. The frictional force reacts on the outer ring and this reaction force is measured by means of the force cell (4). This force is a measure for the energy consumption of the bearing. The bearing temperature is also measured by mean temperature sensors (5).

3 Figure2. Bearing friction setup (at Flanders Make) The test setup allows testing bearings with an outer diameter up to 150mm and an inner diameter from 17mm. Several parameters can be controlled to study their influence on the measured friction torque: - Oil temperature: the oil entering the bearing can have its temperature set in the range : C - Room temperature: can be set between 17 and 30 C - Speed: the setup allows to test bearing rotation up to 3000 RPM. - Acceleration: up to 2200 rad/s 2 This allows the testing of bearings at fast oscillating speeds like those encountered in cam follower mechanisms. - Load: the bearing can radially be loaded up to 15kN (static) - Oil flow rate: up to 2000cc/min During the test, the friction torque, the bearing temperature and the amount of oil in the housing are monitored. The bearing temperature is measure at four points on its outer ring. The condition of the separating oil film in the contacts is also monitored with an electrical circuit. III. What can we do for companies? - We can compare different lubrication methods theoretically and experimentally. For example, grease lubrication can be compared to oil lubrication in controlled situations (bearing working temperature, room temperature, ).

4 - We can measure the bearing friction loss for a specific bearing and compare different bearings. - We can propose a more optimal flow rate of the lubricant to minimise the power loss while keeping lubricant and bearing within acceptable temperature limits. - We can evaluate the effect of modifications of the bearing housing geometry on efficiency, cooling and lifetime. IV. Some illustrative results Figure 3 shows how an energy saving of 20% (30W) is realized by reducing 10 times the oil flow rate for a NUP412 bearing. The bearing working temperature increases by 8 C, but it is still in the acceptable region for this case. It s common that a machine constructor uses a high flow to avoid any risk on bearings. This example illustrates a potential for reduction of power losses by decreasing the flow rate. Figure 3. Illustration of potential for energy loss reduction Figure 4 illustrates the correlation between measured friction torque with simulation results for a cylindrical bearing NUP212 rotating at 1500 rpm. The inputs for simulation (bearing temperature and oil level) are obtained from the experimental setup. This comparison between measurement and simulation shows a relatively good correspondence for the absolute friction torque as well as for its trend in function of oil quantity.

5 V. Conclusion Figure 4. Experimental measurements compared to simulation results The bearing friction setup allows testing bearings in different operating conditions (speed, load, oil flow rate, environment temperature, oil supply temperature). The friction torque as well as the bearing temperature are measured during the tests allowing determining optimal flow rate for low energy consumption without impacting the lifetime. The impact on lifetime is checked by means of the bearing temperature level and by measuring the condition of the separating oil film in the contact region. VI. References 1. Kenneth Holmberg, Peter Andersson, Ali Erdemir. Global energy consumption due to friction in passenger cars. Tribology International 47(2012), Schaeffler. Lubrication of rolling bearings. Principles, Lubrication methods, Lubricant selection and testing, storage and handling. TPI176, p