What makes trains screech

When you get to a corner, the wheel wants to go straight but the rail forces it to turn. You get a stick-slipping motion and the wheel starts vibrating. He adds: "A circular rail wheel is actually a pretty good radiator of sound. It's like a big loudspeaker. So once it gets vibrating, the sound radiates and you get this high-pitched squeal. The easiest way to lessen the noise is to put lubrication like water or grease, or friction modifiers, like graphite, on the rail so that the wheel can slide, Zapfe explains.

Another approach, he says, is to add rubber dampers or tuned metal frequency absorbers to train wheels to damp the sound. So what is the MBTA doing? Kimberly Woollard, the deputy director of light rail vehicle maintenance and engineering, says the T has put greasers on the lines over the years. These pump a little grease on the wheels and rail as the trains go by. There are currently 13 greaser units on the Green Line, says Woollard, with an additional six being added throughout the line.

But they're also trying something new. Just last week T engineers finished putting flange stick lubricators on all the newer model Green Line trains. Imagine a giant Pez dispenser that holds a stick of graphite pressed against a train wheel. The flange stick lubricators add the graphite to the wheels as they turn, aiming to reduce wear to the wheels and rails, and, at the same time, to reduce that squealing noise the wheels make. Engineers will monitor the flange stick lubricators for the next few months to evaluate whether they should be put on the rest of the Green Line fleet.

While Woollard says it's too early to say how successful the new lubricators will be at reducing the squealing, she says other transit agencies have had success with them.

As for that tight turn at Boylston station, that's not going to change. It was made for trains that don't exist anymore. So next time you're at Boylston, perhaps it will be a little quieter than it was earlier this year.

Please let us know if you think you've noticed an improvement. Skip to main content. The relationship between the lateral creep ratio and friction coefficient based on contact between a train and the railway track is shown in Fig. The characteristics of this friction change based on the material and geometry of the wheel and the rail, and the material between the wheel and contact surface.

The friction coefficient gradually increases with creep rate; when the former reaches its maximum value, it decreases as the creep rate increases. When the gradient of the friction coefficient against the increase in the creep rate becomes negative, the damping coefficient becomes negative at the specific natural vibration frequency of the wheel, which generates a self-excited vibration.

Therefore, if the damping coefficient of the wheel increases, or if the gradient of the friction coefficient with respect to the creep ratio is non-negative, the curve squeal can be reduced. Sound-insulating wheels and friction-adjusting materials have been developed to address this cause of noise.

Noise and abrasion due to wheel and rail head contact between the wheel and railhead occurs on the outer rail when the wheel passing passes over through a curve. Flange noise from the wheel depends on the its flange force of the wheel acting on the side of the rail head. Moreover, reducing the flange force not only prevents flange noise, but also is very effective in preventing wear on the wheel flange and the side of the rail side head.

Flange contact force F is shown illustrated in Fig. The lateral pressure is determined by driving conditions and the structure of the vehicle, and the lateral creep force of the inner and outer rails is determined by the creep ratio and friction characteristics. Therefore, when a friction-adjusting material is used, the lateral creep force is reduced relative to the creep ratio. The flange force is also reduced, thereby decreasing flange noise and wear.

The natural frequencies particularly the main frequency characteristics and eigenmodes of the wheel, which is the main source of squeal and flange noise, was analyzed using experimental and analytical methods. Since the curve noise occurs at the natural frequency of the wheel, we measured its frequency response function with an impact hammer as shown in Fig. The wheel shaft used in this measurement, which was mm in diameter, was made of wrought steel.

The distance between its faces was mm and it weighed kg in total. The wheel-set was suspended by a crane, so that the thread and flange of the wheel were not in contact with the rail. The impact hammer was used to excite the thread and flange of the wheel vertically, and the response characteristic was confirmed. Acceleration sensors capable of measuring in three directions were attached to the thread and flange of the wheel to measure the response characteristics in the axial direction and the radial direction simultaneously.

Sampling frequency was set to 20, Hz, and frequency resolution was set to a 2. The impact hammer and vibration acceleration sensor were used in exponential windowing. In addition, the value was analyzed using a linear average of five times. The response functions in the axial and radial directions on the side face of the wheel are shown in Fig. Various natural frequencies can be observed in the axial and radial directions.

Especially, periodic natural frequencies are observed at frequencies of Hz or greater. This shows that high noise in the wheel can be generated by the external stimulus at a high frequency range. To examine the mode of the wheel more specifically, a numerical analysis was performed using a finite element model of the wheel [ 9 ]. The wheel-set model was the same as the model used in the experiment, and the meshes were uniformly created using the sweep method so as to be optimal for a circular shape using 54, nodes and 11, elements.

The analysis was conducted from 2. The boundary conditions were set as free from all directions, and the axial direction was divided into axial and radial directions by a force of 1 N.

The frequency response function and mode shape of the wheel with respect to each natural frequency are shown in Fig. Although there was a slight difference between the analysis and test results in the radial direction, they show good agreement in the axial direction.

At a frequency of Hz or more, the radial mode was close to the resonant frequency in the axial direction mode. This seems to be due to the concentration of multiple modes in the high frequency region. It is difficult to distinguish the characteristics of wheel squeal and flange noises, as the two noises occur simultaneously when passing through a curved section.

In this study, curve noises were reproduced using a full-scale test rig, as shown in Fig. In the large-scale derailment tester, when the railing is driven, the axle mounted thereon is also driven. The horizontal actuator attached to the axle frame allows the lateral displacement to be adjusted, and the two vertical actuators allow the right and left axles to be weighted.

The yaw angle and axle position can be adjusted using three actuators attached to the rail motor that control the speed of the vehicle. As described above, the yaw angle, vertical load, and lateral displacement of the wheel-sets can be adjusted so that the rig can simulate running the vehicle through a curve.

To measure the noise generated when the axle is driven, a microphone was installed 1 m from the left and right sides of the axle at a height of 0. The wheel squeal and flange noise of the test rig—under controlled rail speed, yaw axis, vertical load, and lateral displacement conditions—are summarized in Table 2.

The flange noise varies depending on the contact between the wheel and rail. The wheel squeal was measured at dB A at Hz—a slight difference from the natural frequency Hz obtained in the axial direction of the wheel—as shown in Fig. The flange noise was much quieter than that of the wheel squeal and was the frequency was similar to the radial natural frequency of the wheel flange within a range of approximately — Hz. We thus concluded that wheel noise is closely related to the natural frequency in the axial direction of the wheel and that flange noise is closely related to the natural frequency in the radial direction of the wheel flange.

Generally speaking, it is difficult to reproduce curve noise by using a large-scale test machine because the wheel condition may vary depending on the wear of the contact surfaces of the wheel and rail; the flange noise is more difficult to reproduce than wheel squeal.

To analyze the characteristics of wheel squeal and flange noises, noise and vibration were measured at the curve of electric multiple unit EMU driving. The radius of the curve was m, and its cant was 50 mm, as shown in Fig. As shown in Fig. For our measurements, four microphones were installed 0. The Hann function was used for the fast Fourier transform. To analyze the characteristics of curve noise, we analyzed noise and vibration for different vehicle speeds and directions of travel in a sharp curved section.

Figure 10 shows the average noise level measured 0. Curve squeal is constant regardless of vehicle speed, but the noise level is approximately 10 dB A depending on the driving direction.

When the vehicle travels from A to B, the noise level on the low rail is higher than that on the high rail by at least 5 dB A. However, when traveling in the opposite direction from B to A , the noise level of the high rail side is approximately 1—3 dB A higher than that on the low rail. The noise generated in the outer rail may be caused by the flange due to contact between the wheel and the rail, and the radiation noise could be due to the vibration of the rail.

On the downward slope, the contact force due to the flange increased, and the vibration of the outer rail also increased. In this case, the overall noise could increase [ 10 ].

However, there is a limit to understanding the exact mechanism of noise generation based on the measurements from this study.

Additional research is needed to analyze more precise causes. To confirm that the characteristics of the squeal noise curve along the running direction and manifest differently, as shown in Fig.

Peaks are present at Hz, Hz, Hz, Hz, and Hz from the inner rail when running from A to B, and noise at frequencies higher than Hz is displayed on the outer rail. In addition, when traveling in the opposite direction, the noise level of the inner rail decreases by approximately 10 dB A. Moreover, a high level of noise—in the —10, Hz region—is observed on the outer rail.

The resonance frequency in the axial direction of the wheel substantially coincides with the wheel squeal frequency as shown in Fig. Therefore, it is possible to confirm that the noise caused by the bending modes of the wheels generated from the inner rail is wheel squeal and that the noise generated from the outer rail is flange noise. A frequency analysis of the noise measured at a point 0. Although there are some peaks at frequencies of Hz and Hz, the wheel squeal noise is remarkably reduced.

The decrease in the curve squeal noise occurring in the A-to-B direction in the field test is influenced by the gradient as shown in Fig. Furthermore, it was confirmed that the generation of noise in the curved portion increased with the increase in speed as shown in Fig. The vibration level of the outer rail is larger than that of the inner rail, and the vibration level of the inner rail is similar in the vertical and horizontal directions; however, in the outer rail, the vibration level in the left and right directions is larger than that in the vertical direction, as shown in Fig.

This is a result of indirectly showing that the generation of the squeal noise and the vertical rail vibration are related to each other. In this section, the characteristics of noise generated in the actual curved section are analyzed based on various factors such as driving direction and speed.

This study analyzed the causes of noise in a curved section through various experimental approaches. First, we investigated the noise characteristics of the railway vehicle through a structural analysis of the wheel. The results confirmed that the wheel has various natural frequencies and eigenmodes in a high frequency range.

To investigate the characteristics of the noise generated when the wheel and rail are in contact with each other, a roller rig test was performed to measure the noise generated when the curved section runs through the actual wheel. Particularly, in this experiment, the squeal and flange noises were reproduced respectively by adjusting the lateral angle and vertical force.

The results confirmed that squeal noise occurs in the high frequency region and the flange noise occurs in many modes.

Furthermore, a study was conducted to measure and analyze noise in an actual urban railway curve section. Through a comparison of the frequency and natural frequency analyses of the actually measured noise, the wheel mode causing squeal noise was confirmed.

In addition, the influence of the noise generated inside and outside the curved part was investigated by velocity and the influence on the curved part noise was examined. This study provides information on the squeal and flange noises generated by a railway vehicle when passing through a curved section through various experimental approaches. Vincent, N. Curve squeal of urban rolling stock-Part 1: State of the art and field measurements. Journal of Sound and Vibration, , — Article Google Scholar.

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