Mechanism and Countermeasures for Abnormal Noise Produced by Idler Pulley Bearings under Low Temperature Conditions

Transcription

1 Mechanism and Countermeasures for Abnormal Noise Produced by Idler Pulley Bearings under Low Temperature Conditions M. UEDA Idler bearings are used for automobile engine accessories. This bearing sometimes generates a noise when the engine starts at low temperature as in the winter. We found that this noise was caused by the resonance between axially elastic spring characteristics due to reduction of bearing internal clearance and passing frequency of rolling elements. In order to avoid the noise, it is necessary to specify the bearing clearance considering the raceway expansion when mounted. Besides the bearing clearance, belt wrap angle, belt tension, and grease are other important factors. Key Words: idler bearing, noise, clearance 1. Introduction Idler pulleys are one of the types of bearing application in automobile engine accessories. Idler pulley bearings sometimes produce an abnormal noise when the engine starts during the winter. In this paper, the noise produced by idler pulley bearings under low temperature conditions is analyzed and the mechanism of noise, and countermeasures for the noise are described. Sleeve (inner ring) Pulley (outer ring) Pulley Bearing Sleeve 2. Features of Idler Pulley Bearings Figure 1 shows a typical example of an idler pulley bearing. Idler pulley bearings are used to adjust tension or change the route of timing belts and belts that drive engine accessories. The former is usually referred to as a "tensioner bearing" and the latter an "idler bearing." Here we refer to both as "idler pulley bearings." Figure 2 shows a typical application for idler pulley bearings. A belt contacts the outer surface of idler pulley bearings, and the outer ring and the pulley are combined as a single part (see Fig. 1-a). Some pulleys are made of press-fitted metal and some are injection-molded plastic (see Fig. 1-b). Pulleys are often directly bolted to the engine block, and some are combined with the sleeve as a single part. If an idler pulley bearing has large axial run-out, belt misalignment increases, and the belt slides off the pulley. It is, therefore, necessary to minimize axial run-out. Radial clearance (hereinafter referred to as "bearing clearance") is designed to be small. Grease is sealed inside the bearing. Because of typical environment for this type of bearing, grease that maintains a lubrication film up to high temperatures is often used for highspeed rotation in high-temperature atmospheres. Timing belt idler (adjustment) a) Pulley and sleeve parts b) Injection molded plastic pulley, press-fitted sleeve Fig. 1 Examples of idler pulley bearings Crankshaft timing pulley Timing belt Camshaft timing pulley Timing belt idler (stationary) Idler unit (slide type) Accessory drive belt Fig. 2 Typical application of idler pulley bearings 3. Abnormal Noise under Low-Temperature Conditions As was described in the previous section, bearing clearance is minimized and high-temperature grease is used for idler pulley bearings. Thus, because the base oil viscosity of grease increases at low temperature, abnormal noise sometimes becomes a problem at engine start-up. This noise usually manifests a continuous whistling sound emanating from the area where the bearings are located Idler KOYO Engineering Journal English Edition No.16E (22) 37

2 independently of engine rotation speed and which sometimes dissipates after about one minute (here after referred to as "low-temperature noise"). The analysis results concerning the mechanism by which abnormal noise is produced from idler pulley bearings at low temperatures are described in the following section. Abnormal noise 3. 1 Abnormal Noise Reproduction Test A test to reproduce and confirm the cause of low temperature abnormal noise was conducted using idler pulley bearings that exhibited abnormal noise under actual usage conditions. Bearing vibration in axial direction Table 1 Abnormal noise reproduction test conditions Item Contents Engine rotational speed, min 1 (Bearing rotational speed) ~2 (sweep) (~3 5) Belt tension, N 2 Belt wrap angle, Ambient temperature, ; 2 (at beginning of test) Displacement of outer ring in axial direction Fig. 4 Power spectra Outer ring passing frequency Polyvinyl V-belt Gap sensor (For measuring displacement of outer ring in axial direction) Acceleration pickup (For measuring vibration of bearing in axial direction) Mounting bolt Test bearing Abnormal noise frequency, Hz 8 3fb 7 Abnormal noise produced 6 5 2fb Rotational speed, min 1 Fig. 5 Condition in which abnormal noise is produced Noise meter Fig. 3 Overview of measurement method Table 1 gives the conditions (actual vehicle specifications) and Fig. 3 shows the measurement method. When the frequency of abnormal noise, axial displacement of the outer ring and axial vibration of the bearing were analyzed, the frequency of the abnormal noise matched the frequencies of the outer ring's axial displacement and the bearing's axial vibration. The frequency also matched the outer ring passing frequency (n order component) of the rolling elements in outer ring. It is surmised that low-temperature noise is produced when the outer ring natural frequency in the axial direction resonates with the outer ring passing vibration of the rolling elements in outer ring. (see Figs. 4 and 5) Natural Frequency in the Axial Direction of the Outer Ring and Outer Ring Passing Frequency of the Rolling Elements in Outer Ring Figure 6 shows a model of bearing's vibration. K W Fig. 6 Model of vibration system 38 KOYO Engineering Journal English Edition No.16E (22)

3 In the vibration model of Fig. 6, W represents the outer ring mass and the spring represents the elastic contact between the raceway and rolling elements. Using this model, the natural frequency of the outer ring in the axial direction, f a, can be expressed as follows: fa = 1 2p Where: W : Outer ring mass K : Spring constant in axial direction g : Gravitational acceleration In the case of outer ring rotation, the frequency at which the rolling elements pass through one location of the outer ring f b is expressed as follows: fb = Z 2 K g W Dw N 1 cosa dm Where: D w : Ball diameter Z : Number of balls d m : Bearing P.C.D. a : Bearing contact angle N : Outer ring rotational speed When n-th component of natural frequency of the elastic vibration in the axial direction of the outer ring f a and the frequency at which rolling elements pass through the outer ring f b match, that is, when f a = n f b, it is possible that resonance causes abnormal noise Factors of Abnormal Noise at Low Temperature As explained in sections 3. 1 and 3. 2, possible factors of low-temperature noise are: A) An elastic spring effect is established in the axial direction in the contact point between the rolling elements and the raceway. In other words, the bearing clearance becomes negative. B) Factor to increase running vibration (here after referred to as "running vibration exciting force"). Figure 7 provides a more detailed breakdown of these factors. 4. Test to Confirm Influence of Factors Next, the effect of the factors shown in Fig. 7 was confirmed Bearing Clearance Decreasing in Operation Ordinarily, the bearing inner and outer rings are often pressfitted on the shaft and in the housing respectively. Bearing clearance is established by taking into account expansion and contraction of the raceway due to press fitting. However, because the inner ring of the idler pulley bearing is often directly bolted to the engine block, it is difficult to study what happens to the bearing clearance during operation. The bearing clearance during operation relative to bearing clearance before mounting was studied. First we confirmed that the inner ring raceway diameter increased when the inner ring is bolted to this engine block. (See Fig. 8) The bearing clearance decreases by exactly the same amount. Raceway diameter increase, lm Bolt clamping torque, N m Inner ring Fig. 8 Raceway diameter increase caused by bolt clamping Low-temperature noise Outer ring resonance Bearing clearing filled in (elastic spring in axial direction) Running vibration exciting force Grease oil base viscosity, penetration Unit radial clearance Increase in raceway diameter when bolt is tightened Raceway circularity Difference in temperature of outer and inner rings Types of grease Ambient temperature Belt tension Belt wrap angle Fig. 7 Factors of abnormal noise at low temperature KOYO Engineering Journal English Edition No.16E (22) 39

4 Next, we studied what happens to bearing clearance variation due to bearing temperature rise during operation. After the engine starts, the bearing heats up to a certain temperature due to heat from the engine, internal heat caused by rotation, and heat produced by belt sliding. The variation in bearing clearance caused by the difference in thermal expansion resulting from a difference in temperature of the inner and outer rings due to heat conduction from the engine to the inner ring was studied. As a result of calculation study, it is predicted that the bearing clearance decreases when the temperature of the inner ring becomes higher than that of the outer ring. The temperature difference between the inner and outer rings was measured during a low-temperature noise reproduction test with a bearing of initial clearance 2 lm. Abnormal noise was produced when the temperature difference became large, according to the calculation study, enough to make the bearing clearance negative. (See Fig. 9) Bearing clearance when running, lm Heat Heat Engine area Test bearing ( : Noise, #: No noise) Temperature difference, : Fig. 9 Abnormal noise and temperature differenc of innere and outer rings under low temperature In actual fact the overall temperature of the bearing quickly rises due to internally generated heat and heat produced by belt sliding. Therefore this condition does not last for a long time. (Note: Low-temperature noise is not produced when the temperature rises and the grease becomes softened.) Thus it was confirmed that the clearance of idler pulley bearings becomes smaller during operation than that in the original condition. In other words, to prevent factor A described in section 3. 3 (bearing clearance becoming negative), this must be taken into account when designing the initial clearance of the bearing Running Vibration Exciting Force Next, the reproduction test for abnormal noise at low temperature was conducted to confirm the effect of items concerning running vibration exciting force. This was done to establish how low-temperature noise was produced when certain conditions of the test, as described in section 3. 1, were changed. The greases shown in Table 2 were used for the test. After being bolted down, negative clearance of bearing A and positive clearance of bearing B (could be turned smoothly by hand) were used to confirm the effect on bearing clearance. The results of the test are given in Table 3. As a result of the test, it was predicted that low-temperature noise was produced when the bearing clearance becomes negative. Even if bearing clearance becomes negative: q Abnormal noise does not tend to be produced for softer grease (low base oil viscosity) at low temperature. w The larger the belt wrap angle, the less likely abnormal noise is to be produced. e The higher the belt tension, the less likely abnormal noise is to be produced. With q, the softer the grease, the smaller the exciting force of running vibration is. With w and e, the larger belt wrap angle and the higher belt tension produce a larger load on the bearing, and consequently larger constraining force on the outer ring. Because this makes it harder for an elastic spring to be formed between the raceway and the rolling elements, abnormal noise is less likely to be produced. Table 2 Properties of greases Item Grease A Grease B Base oil Ether-type synthetic oil Poly-a-olefin Base oil ; viscosity (cst) 1; Worked penetration 25;, 6W Dropping point (;) Min. 27 Min. 26 Low temperature Start torque ( 3;) (N m) Rotation KOYO Engineering Journal English Edition No.16E (22)

5 Table 3 Test results ( : Noise, #: No noise) Factors Result No. Grease type Belt wrap angle Belt tension Bearing ( : Noise) (see Table 2) ( ) (N) 1 2 (see note) 2 (see note) Grease A 3 Bearing A 4 # ( negative 5 clearance ) 6 # Grease B 7 8 # 9 1 # Grease A 11 Bearing B 12 # ( positive 13 clearance ) 14 # Grease B 15 2 # 16 # Note: More noise was produced for No. 1 than No Conclusion Belt tension 2N Belt tension 2N Fig. 1 Power spectra w Clearance becomes negative when the inner ring expands due to engine heat transferring to the inner ring. Actually the overall temperature of the bearing quickly rises due to the internal heat of the bearing and heat produced by belt sliding. It is therefore unlikely that low-temperature noise would be produced in this condition. e The condition by which abnormal noise is produced changes according to the belt-wrap angle, belt tension and grease. The smaller the belt rap angle or the lower the belt tension is, or if hard grease is used at low temperature, the low-temperature noise tends to be produced. It is therefore of particular importance to establish clearance according to these conditions when using idler pulley bearings. As a result of the reproduction test, it was found that lowtemperature noise of idler pulley bearings is produced when the bearing clearance becomes negative during operation, causing an elastic spring effect to form in the axial direction, which in turn causes the outer ring to resonate with the outer ring passing vibration of the rolling elements. It is therefore necessary to note the following points in order to prevent low-temperature noise. q Bearing clearance should be designed by taking expansion of the raceway when nuts are tightened that bearing clearance does not become negative during operation. M. UEDA * * Bearing Engineering Experiment & Testing Department, Bearing Engineering Center KOYO Engineering Journal English Edition No.16E (22) 41