FORM-2
THE PATENTS ACT, 1970 (39 OF 1970)
COMPLETE SPECIFICATION (See Section 10)
1. TITLE OF INVENTION
"A method of reducing ringing noise in gear assemblies".
2. a) Mahindra & Mahindra Ltd., b) having administrative office at Mahindra Towers, Worli, Mumbai - 400 018., c) an Indian company
The following specification particularly describes the nature of the invention and the manner in which it is to be performed.

Field of the Invention
This invention relates to a method of reducing undesirable ringing noise characteristics in a system comprising of gear assemblies.
Background and Prior art
Ringing is generated in the gear assemblies during impact excitation between gear teeth while transmitting the motion from one gear to the other. The problem of ringing becomes acute in systems such as timing gear trains in engines.
As the engine operates in a wide range of Ram’s the frequency at which the ringing and rattle is generated varies considerably and hence the solution required to absorb / attenuate the sound has to be a broadband solution and cannot be a narrow band solution. The constrained layer damping provides this kind of a broadband damping.
The load driven by timing gear trains like camshaft, fuel injection pump etc. is highly fluctuating in nature, generating a make and break of gear flank contact within the limits of the backlash. These problems have generally been overcome by high cost solutions by shifting to timing belt drive systems or chain drive systems.
The variation of sound pressure with time corresponds to damped natural vibrations, which are excited periodically by impulses. Following frequencies can appear in gear noise:
a) Rotational frequency fn = n/60 Hz.
b) Tooth Frequency fz = z.n/60
c) Machine error frequency fm = n. zm/60
d) Gear natural frequency
The impulsive frequencies and their integral multiples produce an increase in loudness when they coincide with a natural frequency. The resonance frequency can correspondingly be recognised by characteristic increase in loudness at this point.
There has been a long felt need to eliminate or reduce the undesirable ringing under conditions of low idle rpm of the engine without making any significant modification in the design or material of the gear system.
Attempts in the past have been made to reduce the rattle in such systems by:
• Reducing backlash in gear
• Changing module of gears
• Changing helix angle of gears
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• Using helical gears in place of spur gears.
• Putting one soft gear in gear train to reduce response of gear train.
• Introducing backlash reducer to maintain continuous contact of gears.
US Patent 4825712 discloses a solution utilising multiple viscoelastic sheets sandwiched between damping plates and side plates of particular dimensions by bolts. This technique although useful for large and fabricated gears, it has geometrical features which are difficult to manufacture by mass production-methods like forging. In the case of automotive timing gear assemblies this method also becomes operationally unsuitable as it increases the number of subassemblies and components. It also calls for use of many bolts, which could loosen by vibrations and drop into the assembly of gears causing problems.
US Patent 5067364 relates to a device to eliminate mechanical rattle between a pair of inter-engaging gears comprising a gear element co-operating with one gear, the said gear element being rotatably mounted on a hub portion joined to said one gear, having the same tooth profile and number of teeth as the gear and being connected to the gear via rotationally resilient elements, which are pretensioned in the engagement position of gears.
This solution needs several precision parts, increases part count and the cost of the gear assembly. Also the elastic rubber elements, which are crucial to the function of this system, are constantly subjected to relative motion and wear and tear, and can degrade the performance of the mechanism over a period of time.
US Patent 4317388 provides a gear wheel with a split-damping ring on each side that is assembled on gear wheel by means of bolt nuts and elastomer type ring washers. This suffers from limited damping capabilities and therefore does not provide appropriate solutions to the elimination of problems associated with ringing noise reduction.
US Patent Application no US2002/0112556A1 discloses constructions in which vibration-damping mass made from heat resistant and oil resistant metallic material is attached to the gearwheel with surface contact pressure. This solution is expected to shift the natural frequency I.e. the tone of the ringing noise and not reduce or eliminate it. As the engine operates in a wide range of RPM's the frequency at which the ringing or rattle is generated varies considerably and therefore this solution disclosed in this patent does not provide adequate broadband damping.
US patent 5473122 discloses a dual constrained viscoelastic-damping mechanism for structural vibrational control is used for aerospace applications. The material used for bonding is aromatic polyamide or aramid fibre and the constraining layer used is aluminum reinforced with graphite fibre,
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US Patent 4447493 discloses Vibration-damping Constrained layer constructions wherein the damping material is sandwiched between the panel to be damped and a relatively stiff layer.
Both of these constructions do not provide a comprehensive and robust solution to the gear train ringing noise reduction, as they are prone to deterioration of the elastomer at the edges where it can come in contact with oil.
In US patent 5705769 a continuous damping layer is bonded to surface of structure and then segmented constraining layer bonded to a portion of continuous damping layer. However this is not an solution to the problem of ringing involving gear trains as these solutions for underwater applications the constrained layer is segmented and is used to damp the shell of vibrating structure and not to improve the quality and level of sound radiated as in the case of the gear trains as is also seen from fig.4 it reduces the efficacy of the damping at some frequencies.
Application of constrained layer damping to reduce the vibrations and noise in a gear train calls for very specific solution due to the harsh environment in which it has to perform. The elastomer to be selected depends upon the ambient temperatures that the assemblies are subjected to and the design of the constrained layer has been in this case used to isolate the elastomer from being exposed to the oily environment.
Constructions suggested in the prior art have either insignificant effect on the ringing characteristics of such systems or are costly, involve higher number of parts and complexity.
Thus the long felt need to eliminate or reduce the undesirable ringing under conditions of low idle rpm of an engine without making any significant modification in the design or material of the gear system continues to be challenging.
Summary of the invention:
This main object of the invention is to provide a method for reducing or eliminating undesirable ringing noise in a broad frequency range of 1000 Hz -5000 Hz with the substantial rattle reduction in a system comprising of gear assemblies and in direct injection diesel engines with timing gear train under condition of low idle rpm, by laminating selective gear surfaces with viscoelastic material followed by a rigid layer in a lamination structure without needing alterations of gear materials and design, operating environment and conditions of operation of the gear assembly.
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It is another object of the invention to reduce the ringing characteristics in engine comprising of timing gear train.
It is yet another object of this invention to reduce the ringing characteristics in direct injection diesel engine with timing gear train.
It is yet another object of this invention to achieve the reduction or elimination of the ringing characteristics without modifying the basic design features of the gear " train.
It is another object of this invention to achieve the reduction or elimination of the ringing characteristics without changing the gear material and operating conditions of gear train.
It is also another object of the invention to selectively treat minimum number of gears in the assembly to achieve the desired results
Another object of the invention is to provide a gear wheel that has elastomeric material with resilience and high damping properties on one or both the surfaces, covered with appropriate metallic or non-metallic rigid laminate, to act as a constrained layer and protection to the elastomer from the harsh operating environment of the gear system.
Thus in accordance with this invention the method of reducing ringing noise in gear assemblies comprise the following steps
• Assessment of the ringing characteristics of the gear train assembly
• Evaluation of the sensitivity of each gear in gear train assembly to generate ringing noise
• Selection of the set of gears that contribute most to ringing for treatment
• Bonding/fixing of the gear surface with appropriate material, fixing of the laminate and testing of the response
• Resembling of the gears in the gear train
Thereby obtaining a laminate structure of gear surface / viscoelastic material / rigid laminate.
Detailed description of the invention
The method described here is one of the preferred embodiments of this invention.
Sensitivity of each gear in gear train assembly to generate ringing noise is studied and gears are selected based on their noise profile for applying the treatment.
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A typical gear is disc shaped and has gear teeth on the cylindrical circumference and a hub for guiding and supporting it on the shaft. In between the hub and the cylindrical surface there is generally a recess. This recess is used in this invention for the purpose of applying the noise reduction treatment. The viscoelastic material is bonded / held in contact under pressure in this recess and the constrained layer of metallic or non-metallic material is moulded / fastened over the viscoelastic material.
A typical construction involving a gear (1) from the gear assembly is shown in figure 1 comprising
a. The basic gear £1), which has been treated to reduce ringing and rattle.
b. The viscoelastic material as a damping layer (2).
c. The rigid layer (3), metallic or non-metallic as a constrained layer.
d. Rivet (4)
Bosses are also provided on the gear to fix the two layers and riveting the viscoelastic and rigid layer together. In case the rigid layer is clamped using rivet, placement of bosses on gear is done such that the rigid layer exerts uniform pressure on the viscoelastic layer.
In another embodiment, the viscoelastic material (2) and the rigid Jayer (3) are compression moulded at high temperature and pressure. Alternatively the viscoelastic material (2) and the rigid layer (3) are adhesively bonded to each other and also to gear surface.
The gear surface is applied with a variety of materials selected from butyl rubber, nitrile rubber, neoprene and their like. Selection of the elastomer is based on the temperatures that the assemblies would be subjected to in operating environment of the gear assembly such as lubricants like oil / grease, etc.
The preferable viscoelastic dampening materials, (2) are selected from butyl rubber and nitrile rubber. The most preferable materials for high temperature application upto 130° C are selected from high nitrile butadiene rubbers with Shore hardness of about 40 to about 65 and glass transition temperatures of about -30° C. to -50° C.
Various processes may be achieve the bonding/fixing of these materials of the gear surface. One of the ways is compression moulding by thermal treatment under pressure and subjecting the system to an appropriate curing period. Another way is mechanically clamping a pre-moulded sheet of the viscoelastic material g) and the rigid layer (3) with rivets (4) or bolts. Yet another way could be bonding by application of adhesives.
In another embodiment of this invention, a rigid layer (3) is provided on the exposed surface of the rubber thereby enhancing the sound quality of the system
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and simultaneously providing a protective barrier to the exposed surface of the bonded material. The rigid layer is selected from a range of metallic or non-metallic sheets such as mild steel, aluminium, plastics, composites etc.
The ratio of the thickness of the rigid layer: viscoelastic layer may vary from 1:2 to 1:5.
The effectiveness of invention is now illustrated with a few non-limiting examples: ■•
Example 1
A gear selected from the gear assembly is treated by the method disclosed in the invention in which viscoelastic material such as nitrile rubber of 6mm and rigid layer of 1.5mm thickness are compression moulded at 190°C and around T50 tonnes of pressuf&forabout 20 minutes. This treated gear is tested on a Modal testing rig for ringing frequency response characteristics. The frequency response spectrum is given in Figure 2.
It may be noted that the response at the natural frequency of the individual gear is marginally shifted as a result of the treatment. It may also be noted that the amplitude response of the individual gear has come down significantly in the range of 1000 to 5000 Hz and thereby ringing noise is dampened.
Similarly, results are obtained by thermal bonding on various rubbers such as butyl rubbers. One of the results of these is exhibited in Figure 2.
Example II
The engine comprising of gear train of five gears is mounted on a dynamometer and sound pressure levels are recorded at a distance of one meter in front of the engine in motoring condition to evaluate the mechanically generated noise. The gear train assembly is then dismantled from the engine. Out of five gears, only two gears, which have more surface area, are treated by the method disclosed in this invention in which the viscoelastic material pf=6riirnand^ihe rigid layer of J.5mmJthickness are compression moulded. The system is reassembled and tested on the dynamometer for recording of the sound pressure as earlier. The comparative results are given in Figure 3 which clearly indicates that the mechanical system noise is significantly reduced from 4 to 7 dB(A) in the entire frequency range of 100 tolOOOO Hz thereby demonstrating that the ringing characteristics are positively impacted.
When system flexes during vibration, the damping layer is forced into shape that shears adjacent material sections. This alternating shear strain in constrained damping layer material dissipates the vibration as low-grade frictional heat thereby enhancing sound Quality and reducing the ringing.
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Example III:
In this case the damping and constraining layer over the gear has been segmented.
The frequency response function is then measured to investigate the difference over a gear with full damping and constraining treatment.
In another construction only the constraining layer over the gear is segmented and the frequency response function is measured to investigate the difference over gear with full damping and constraining treatment.
It is observed that the gear without segmentation of the rigid layer gives the best results.
One of the results of these is exhibited in Figure 4, where frequency response function of segmented gears is more than gears without segmentation.
Selectively treating gears in the gear train by the method described in this invention exhibit significant decrease in ringing characteristics including substantial reduction in the rattle features of the assembly.
Appropriate treatment of the selected gears can also lead to the elimination of ringing characteristics of the gear assemblies.
The ringing characteristics and the rattle are reduced and/or eliminated without changing the gear material or modifying the conditions of the gear train. The basic design features of the gear train also remain unaltered.
The method of this invention is a cost-effective means of reducing / eliminating the ringing sound of the engine in a reliable manner with a wide variety of applications in the Automotive / Farm Equipment and Household equipment industries, etc.
The method disclosed in this invention therefore effectively & systematically
improves the sound quality of the engine to bring them to a high degree of
customer acceptability.
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We claim,

1. A method of reducing undesirable ringing noise with the substantial rattle reduction in the frequency range of 1000 Hz - 5000 Hz in a gear assembly wherein selective gear surfaces of the gears contributing to the noise are laminated with viscoelastic material as_ a dampingjayer followed by a rigid layer as a constrain layer in a lamination structure in which the ratio of the thickness of the rigid layer: viscoelastic layer is 1:2 to 1:5.
2. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the viscoelastic material selected from butyl rubber, nitrile rubber, neoprene and their like, the most preferably materials for applications in temperature range from 30° C to 130° C selected from high nitrile butadiene rubber with shore hardness of about 40 - 65 and glass transition temperature of about - 30Q C to - 50Q C.
3. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the rigjd_ constraining layer in the. laminatedstructure is selected from metallic or non-metaflic sheets such as mild steeT aluminum, plastics and composites.
4. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the lamination is achieved by compression moulding.
5. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the bonding of viscoelastic layer to the gear surface is optionally achieved with adhesives.
6. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the laminated structure is optionally achieved by mechanically clamping a pre moulded sheet of viscoelastic material and the rigid constraining layer with rivets or bolts.
7. A method of reducing undesirable ringing noise as claimed in claim 1 wherein the bosses are optionally provided on the gear to achieve in the laminated structure by riveting.

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Abstract
This invention relates to a method of reducing undesirable ringing noise in a system of gear assemblies comprising of steps such as assessing ringing characteristics of the gear train assembly, evaluating sensitivity of each gear in gear train assembly to generate ringing noise, selecting set of gears that contribute most to ringing for treatment, bonding/fixing of the gear surface with appropriate damping layer, fixing a constrained layer and testing of the response followed by reassembly of the gears in the gear train. The damping layer material are selected from butyl rubber, nitrile rubber, neoprene and their like based on the temperatures that the assemblies would be subjected to operating environmental of the gear assembly. The constrained layer is selected from a range of metallic or non-metallic sheets such as mild steel, aluminium, plastics, composites and the bonding is achieved by adhesives, compression moulding, rivets or bolts.