GB2378229A

GB2378229A - A vibration damper for a motor vehicle body

Info

Publication number: GB2378229A
Application number: GB0214740A
Authority: GB
Inventors: Anthony David Burton; Robert Alan Hargreaves
Original assignee: SOFTSHOCK Ltd
Current assignee: SOFTSHOCK Ltd
Priority date: 2001-06-26
Filing date: 2002-06-26
Publication date: 2003-02-05
Anticipated expiration: 2022-06-26
Also published as: GB0115551D0; GB2378229B; GB0214740D0

Abstract

A vibration damping device is mountable between a vehicle body shell and an interior floor of the vehicle. The device comprises a hollow frusto-conical elastomeric spring 2, adapted to deform progressively under shear. The spring is provided with a substantially rigid stop 10 which maintains a predetermined minimum spacing between the internal floor and the body shell. The stop may have a rigid core 11 encased in elastomer. The vibration damping device is particularly suitable for use as a mounting for the floor of a railway carriage, it effectively damping out both audible vibrations and infrasonic frequencies associated with motion sickness.

Description

VIBRATION DAMPER The present invention relates to devices to dampen structural vibrations, particularly those causing or transmitting noise in the audible frequency range. More particularly, but not exclusively, it relates to devices to reduce the noise transmitted through the floors of vehicles such as railway carriages.

Although the invention has uses in many other fields, it will be described with reference to its use in railway carriages.

Railway carriages are nowadays usually constructed with a monocoque body shell of steel or aluminium. An internal floor is mounted to the base of the body shell, and internal fittings of the carriage, such as chairs, tables, carpets and so on, are supported on the internal floor.

Conventionally, the internal floor is of plywood or similar light, rigid material, and it is mounted to the body shell by means of screws or bolts, or in the situation where sound insulation is required, a plurality of blocks or strips of solid rubber, in an attempt to isolate the interior of the carriage from track noise, engine vibration and the like.

Rubber in compression is significantly more rigid than it is in extension. As a result, these rubber blocks or strips, when loaded under the weight of carriage fittings and passengers, transmit a high proportion of the audible vibrations from the base of the carriage's body shell.

These rubber blocks or strips are slightly more successful at their other function, which is to keep the internal floor of the carriage on a relatively even keel, despite uneven loadings on the floor, or jolting or swaying of the carriage itself. Such solid rubber components in compression thus act as relatively"hard"springs.

A system has been proposed which employs rubber in shear mode, rather than in compression mode. Rubber in shear mode is, in general, far softer and more compliant than in compression, acting as a relatively"soft"spring. This will dampen out vibrations, more effectively than would rubber in compression. The mountings used between the internal floor and the body shell in such a system typically comprise a hollow frustoconical element, moulded from rubber. The load from the carriage fittings, passengers, etc. , is exerted generally axially downwardly on the frustoconical element, compressing it, and putting its angled walls under shear. Such a mounting is relatively effective at damping vibrations.

Since such mountings are relatively"soft", however, the frustoconical elements will continue to deform under increased loading, for example in the case of a crowded carriage. The resultant depression of the internal floor of the carriage may be unacceptably high.

In practice, therefore, such mountings have proved useable only in conjunction with a plurality of accompanying buffer units, also mounted between the body shell and the internal floor of the carriage. The buffer units take the form of plungers, mounted to the underside of

the internal floor, which are dimensioned to contact the body shell once the deflection of the floor has reached a predetermined allowable limit. Wire wool is held between a plate on the lower end of the plunger and the body shell, in an attempt to cushion the impact of the plunger against the body shell. However, this is still not totally satisfactory, and such direct metal-to-metal contacts can provide a route for vibration to circumvent the damping system.

Further problems with such a system include weight, cost and convenience. A typical carriage requires around 250 such rubber mountings and correspondingly requires around 250 buffer units, too. Each of these must be manually positioned and fitted, which, because the base of the carriage is not completely flat, is an extremely time consuming process open to error, and incurs significant extra labour costs. The added weight of the buffer units, summed over all the carriages in a train, will almost certainly be sufficient to cause a significant increase in the operation costs of the train.

A crude method of damping out vibrations passing through the floor is to make the floor more rigid or heavy, but this is clearly worse than the systems described above, in view of the increased total weight of the carriage, and the consequences thereof.

The effectiveness of such mountings may be measured against the"natural frequency"of the system of which they form a part. Vibrations at a frequency close to such a natural frequency will tend to be transmitted through the mountings and the floor to the passengers, while vibrations at frequencies further away will be more effectively damped out.

In a railway context, there is a prevalent vibration at around 25Hz, generated by traction motors under load. There are a number of other unwelcome vibrations at higher audible frequencies. The lower limit of human hearing is usually taken as 20Hz, and at frequencies below this, vibrations are not heard but only felt, if sufficiently strong. Ideally, therefore, the natural frequency should be significantly below 25Hz, and preferably below 20Hz.

Conversely, if a system has too low a natural frequency, approaching around 5Hz, it will transmit vibrations at around this frequency, which are associated with rocking or swaying of the carriage, and can contribute to travel sickness.

The conventional systems described above, using solid rubber blocks or strips, generally have natural frequencies well into the audible range, so transmit many track or motor noises with little attenuation. The"soft"frustoconical mountings, also described above, have a natural frequency that depends on the loading imposed thereon. Under low loadings, they generally have natural frequencies around 10-15Hz, out of the audible range and well away from any of the frequencies of concern. However, under loadings towards the higher end of those experienced in practice, they begin to"wallow", transmitting the low frequency vibrations associated with motion sickness.

It is therefore an object of the present invention to provide a simple and effective mounting to support a vehicle floor which may act to damp out audible frequency vibrations, without permitting excessive deflection of the floor under high loads or transmitting nausea-inducing vibrations, and allowing the use of lightweight flooring.

According to a first aspect of the present invention, there is provided a vibration damping device, mountable between a vehicle body shell and an internal floor of the vehicle, comprising a spring means of elastomeric material adapted to deform progressively under shear, and provided with a substantially rigid stop means adapted to maintain a predetermined minimum spacing between said internal floor and body shell.

Preferably, said spring means comprises a generally frustoconical hollow body.

Advantageously, said spring means comprises rubber, such as filled natural rubber.

The vibration damping device may then comprise a single rubber moulding.

The stop means may comprise a core of metal, such as steel, preferably substantially encased in elastomeric material and may be substantially enclosed within said single rubber moulding.

The stop means and the metal core thereof may both be of a generally cylindrical configuration, preferably coaxial with the generally frustoconical hollow body, and within a central void thereof.

The vibration damping device may act as a sole form of mounting device between the vehicle body shell and the internal floor.

The vibration damping device may be so disposable in use that a wide end of the generally frustoconical hollow body is mountable to the vehicle body shell and a narrow end thereof is mountable to the internal floor.

The vibration damping device may comprise a metal attachment plate, mounted thereto adjacent the wide end of the generally frustoconical hollow body and provided with means to attach the device to the body shell.

The attachment plate may be provided with an aperture generally corresponding to said wide end of said hollow body and may then be embedded into the walls of said hollow body, a circumference of said aperture being provided with means to anchor it into said walls.

The vibration damping device may comprise a mounting fixture, disposed adjacent the narrow end of said hollow body, provided with means to mount the device to the internal floor.

Said mounting fixture may conveniently be connected to, or integral with, the core of the stop means.

The stop means is preferably so dimensioned that an end thereof comes into contact with the vehicle body shell when the internal floor has deflected by a predetermined maximum allowable distance.

The elastomeric material encasing the metal core of the stop means may advantageously comprise a resilient pad disposed between the vehicle body shell and the metal core to cushion said contact.

The stop means may be detachably mounted to the device, so as to enable differently dimensioned and configured stop means to be attached to the device to provide for alternative maximum allowable deflection distances.

Embodiments of the present invention will now be more particularly described, by way of example and with reference to the accompanying drawings, in which :- Figure 1 is a plan view of a vibration damping device embodying the present invention; Figure 2 is a cross-sectional view of the device of Figure 1 taken along the line II-II ; Figure 3 is a cross-sectional view of a second embodiment of the present invention; Figure 4 is a graph of vertical load against deflection for the vibration damping devices of Figures 2 and 3 and for a comparison device; Figure 5 is a cross-sectional view of a third embodiment of the present invention; Figure 6 is a graph of vertical load against deflection for the device of Figure 5; and Figure 7 is a graph of natural frequency against deflection for the device of Figure 5.

Referring now to the drawings, a vibration damping device 1 comprises a substantially frustoconical hollow rubber moulding 2, a steel attachment plate 3 mounted to a wider end of

the frustoconical moulding 2 and a steel mounting disc 4, mounted to a narrower end of the frustoconical moulding 2.

The attachment plate 3 has two lugs 5, each provided with an aperture 6, through which bolts may be passed in order to attach the device to a lower portion of a railway carriage body shell. The mounting disc 4 is provided with a threaded central recess 7, adapted to accept a bolt to fasten the device 1 to an underside of an internal floor of the railway carriage.

As shown in cross-section in Figure 2, the attachment plate 3 is provided with a raised annular portion 8 encircling a central aperture, the portion 8 serving to anchor the attachment plate 3 securely into the frustoconical mounding 2. By virtue of being raised, the strength of the bond between metal and rubber is increased, thereby ensuring a long lasting mount and fulfilling the stringent high loading and crash loading requirements. The mounting disc 4 is also moulded securely into the frustoconical moulding 2, and has a diameter less than the internal diameter of the annular portion 8, so as to allow the rubber moulding to deform under shear.

The walls of the frustoconical moulding 2 are angled at approximately 45 to the plane of the attachment plate 3, when the device I is not under load. Said walls define a central void 9, within which is disposed a bump stop 10, extending downwardly in use from a narrower end of the frustoconical moulding 2. The bump stop 10 has a substantially cylindrical steel core 11, which is formed integrally with the mounting disc 4.

The rubber of the frustoconical moulding 2 extends to cover the steel core 11, forming a resilient pad 12 beneath a lower, in use, end of the core 11.

A second embodiment 13 of the vibration damping device, shown in Figure 3, is provided with a smaller bump stop 14, with a shorter steel core 15, compared to the device 1 of Figure 2.

A third embodiment 20 of the vibration damping device, shown in Figure 5, is particularly suitable for mounting to the railway carriage body with rivets. The device 20 is substantially identical to device 13, except that its attachment plate 3 has two lugs 5 each provided with a pair of apertures 21 adapted to receive rivets. The rubber comprising the frustoconical moulding 2 may extend to encapsulate both lugs 5 as well as the annular portion 8 of the attachment plate 3. This further improves the anchoring of the plate 3 to the moulding 2, and also provides a protective, corrosion-resistant coating around the plate 3. If desired, annular gaps 22 around each rivet aperture 21 may be left free of rubber, for example to receive a washer, or otherwise to provide a secure rivet-to-plate 3 contact.

In use, the attachment plate 3 is mounted to the carriage body shell and the internal floor of the carriage is mounted to the mounting disc 4. Internal fittings of the carriage, such as chairs and tables, are mounted on top of the internal floor. Under this load, the device 1 is compressed slightly, allowing a small downward deflection of the floor (in practice, a deflection of around 1 to 2 millimetres is acceptable at this stage). The angle of the walls of the frustocone decreases slightly, but the walls remain under shear, and so vibrations transmitted up from the carriage body shell through the attachment plate 3 are significantly

attenuated. Under a moderate load of passengers in the carriage, the device 1 compresses further, allowing further deflection of the floor. The device 1 still acts to attenuate the vibrations, and thus reduces the noise experienced within the carriage.

As the deflection due to the load on the floor approaches a predetermined limit, the resilient pad 12 of the bump stop 10 comes into contact with the carriage body shell. As it comes under compression, the rubber of the resilient pad 12 resists further deflection of the floor increasingly strongly, compared to the resistance of the rubber of the frustoconical moulding 2. Ultimately, higher loadings effectively compress the resilient pad 12 to its maximum extent, and the steel core 11 opposes any further deflection. The maximum possible deflection of the floor is thus mainly governed by the clearance between the lower in use end of the steel core 11 and the carriage body shell when the device 1 is in an unloaded condition.

The configuration of the resilient pad 12 governs, to an extent, how smoothly the maximum possible deflection is approached.

The graph of Figure 4 shows the imposed vertical load and the resulting vertical deflection for the devices of Figure 2 and Figure 3, and as a comparative example, for a similar device, not in accordance with the invention, which is provided only with a small rubber bump stop, without a rigid core.

Line 17, marked with hollow triangles, represents the response of the device 1 of Figure 2.

Line 18, marked with solid squares, represents the response of the device 13 of Figure 3. Line 19. marked with solid diamonds, represents the response of the comparison device described above.

All three lines 17,18, 19 are approximately straight lines for small loads and deflections. However, the gradient of line 17 (device 1) begins to rise appreciably at around 3mm deflection, representing the point at which the resilient pad 12 has contacted the carriage body shell and is starting to come under compression. At a total deflection of 5 to 5.5mm, the line 17 is almost vertical. At this point, the device 1 has reached the effective limit of the deflection it will permit. (The device I fits a specification under which the internal carriage floor would deflect by about 2mm under the load of the internal carriage fittings, with a maximum allowable extra deflection due to the weight of passengers of a further 3mm.) The device 13 fits a specification under which the maximum allowable extra deflection, beyond that of an empty carriage, is around 6mm. Line 18 therefore has a generally constant gradient for deflections below about 5mm and only becomes particularly steep beyond a total deflection of about 7mm.

The need for a substantially rigid core for the bump stop is demonstrated by line 19 (comparison device), which is effectively of constant gradient throughout the deflection range measured. While a line of such shallow gradient indicates a generally satisfactory vibration damping performance, such a device would allow the floor to sink ever further under increased loading, which would not be acceptable to carriage manufacturers, operators or passengers.

The devices have a substantially linear response to horizontal loading, so will damp out the horizontal component of vibrations from the carriage body shell relatively effectively, at all loadings experienced in practice.

The graphs of Figures 6 and 7 show how the loading and the natural frequency of the device of Figure 5 varies with the imposed load. In Figure 6, line 20 represents the relationship between the loading on each mount and its deflection. Dashed line 21 represents a loading corresponding to a carriage fitted out with a floor, seats, etc. , but empty of passengers. It intersects line 20 at a deflection of around 1.5mm, shown at line 23. Solid line 22 represents a loading corresponding to a carriage with every seat filled, i. e. a maximum designed static load. Line 22 intersects with line 20 at a deflection of around 4. 0mm, shown at line 24. The range 25 thus represents the normal difference in deflection of the floor, between an empty and a full carriage, of around 2. 5 millimetres Line 20 rises sharply beyond about 6.0 millimetre deflection, indicating a rapid increase in stiffness of the mount beyond that deflection. The mount is designed to have a maximum allowable total deflection of around 8.5 millimetres, whatever the loading, static or transient, imposed thereon.

Turning to Figure 7, line 26 represents the relationship between the deflection of the mount and the natural frequency of the system. The natural frequency of most simple systems falls with increasing loading. However, the natural frequency also rises with increasing stiffness. Hence, for the mounts of the invention, which become stiffer at high deflections, it is found that the line 26 rises for higher deflections.

At the deflection corresponding to an empty carriage (line 23), the natural frequency of the mount is around 14Hz, shown by dotted line 27. At the deflection corresponding to a full

carriage (line 24), the natural frequency of the mount is around 8Hz, shown by dotted line 28.

The range 29 thus represents the usual range of natural frequencies of the system for loadings between an empty and a full carriage. Both low frequency audible vibrations and those associated with motion sickness will be well damped, whatever the passenger load.

If the deflection exceeds the"full carriage"deflection, either due to overloading or due to transient loads, the natural frequency should never fall much below 8Hz. At very high loadings, it even begins to rise, although under loads/at deflections that are likely to be experienced in practice, it should never reach the audible range. The damping properties of the mounts are thus relatively immune to overloading.

Existing systems put under such loads would have natural frequencies tending towards the infrasonic frequencies associated with motion sickness, producing a very uncomfortable ride.

Although the devices described above each comprise an essentially frustoconical rubber moulding, it is envisaged that alternative configurations of the moulding may yield benefits in particular situations. The term"generally frustoconical"should therefore be understood also to comprise shapes with varying wall slopes, such as"bell"shapes or"bottlenecked" shapes, or even shapes which deviate from a completely circular axial symmetry.

While conventional carbon-filled natural rubber mouldings are suitable for such devices in most situations, other forms of rubber (such as neoprene rubber, isoprene rubber or styrene/butadiene rubber) may be more appropriate in some applications. Other elastomeric

polymers, such as polyurethane elastomers, may also be employed. The terms"rubber"and "elastomeric material"should therefore be interpreted accordingly.

The best performance of the devices described above is achieved when the diameter of the mounting disc 4 at the upper, narrower end of the device is significantly less than the diameter of the central aperture of the attachment plate 3 at the lower, wider end of the device. They may be used at an average density of approximately two hundred and fifty per

2 2 carriage, or between about one per 4m2 and one per 5m2.

The vibration damping device described thus acts to damp out a significant proportion of the noise that would otherwise be transmitted from the body shell of a railway carriage (or similar vehicle) to the interior thereof. Under low, normal or excess loadings, it has a natural frequency below the audible range and above the range which may induce motion sickness. Unlike existing devices, it can act to prevent excessive deflection of the carriage floor under heavy loads without requiring auxiliary fixtures or fittings, yet it is very little heavier or more complex than existing devices, and is considerably easier to install.

There is a weight saving resulting from the use of such devices which is considerable, offering benefits on operating costs. Since there is a need to install only half as many individual units as for systems using existing devices, cost and time savings during installation are also very significant.

Claims

CLAIMS 1. A vibration damping device, mountable between a vehicle body shell and an internal floor of the vehicle, comprising a spring means of elastomeric material adapted to deform progressively under shear, and provided with a substantially rigid stop means adapted to maintain a predetermined minimum spacing between said internal floor and body shell.
2. A vibration damping device as claimed in claim 1, wherein said spring means comprises a generally frustoconical hollow body.
3 A vibration damping device as claimed in claim 2, wherein a wide end of the generally frustoconical hollow body is mountable, in use, to the vehicle body shell and a narrow end thereof is mountable to the internal floor.
4. A vibration damping device as claimed in any one of the preceding claims, wherein said spring means comprises rubber, such as filled natural rubber.

5 A vibration damping device as claimed in any one of the preceding claims, wherein the stop means comprises a core of metal, such as steel, preferably substantially encased in elastomeric material 6 A vibration damping device as claimed in claim 5, wherein the core of metal is substantially enclosed within a single rubber moulding.

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7 A vibration damping device as claimed in either claim 5 or claim 6, wherein the stop means and the metal core thereof are both of a generally cylindrical configuration, disposed coaxially within the generally frustoconical hollow body.

8 A vibration damping device as claimed in any one of claims 3 to 7, wherein an attachment plate, mounted adjacent said wide end of said hollow body, has an aperture generally corresponding to said wide end.

9 A vibration damping device as claimed in claim 8, wherein the attachment plate is

embedded into the waiis of said hoiiow body, a peripheral zone of said aperture being provided with means to anchor the plate into said walls.

10 A vibration damping device as claimed in any one of claims 5 to 9, further comprising a mounting fixture connected to or integral with the core of the stop means and disposed adjacent the narrow end of said hollow body, said mounting fixture being provided with means to mount the device to the internal floor.

11 A vibration damping device as claimed in any one of the preceding claims, wherein the stop means is adapted to contact the vehicle body shell when the internal floor has deflected by a predetermined maximum allowable distance 12 A vibration damping device as claimed in claim 11, further comprising a resilient pad disposed between the vehicle body shell and a metal core of the stop means and comprising elastomeric material encasing said metal core.

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13 A vibration damping device substantially as described herein with reference to any one of Figures 1, 2,3 and 5 of the accompanying drawings.