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Perennial Problems with Engine Rubber Mountings

written by: Mike Aguilar • edited by: Lamar Stonecypher • updated: 10/12/2011

Engine foundation design norms for rubber engine mount bushings have not been adequately considered in the most important and harshest service condition parameters. This article by an automotive engineer looks at the current state of engine mounts and offers some suggestions for real improvement.

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    Common Problems with Rubber Engine Mounts

    Engine mounting rubber mounts, once fitted in a heavy-duty commercial vehicle, are never replaced throughout the vehicle’s service life. The engine mounts serve as a foundation for the engine. Quality parameters for the most important functional properties of dynamic operating conditions are ignored. The engine foundation design norms have not been considered in these most important service condition parameters. The bad effects are reflected in excessive engine vibrations and eventual damage to the gear box components. The rear rubber mounts are subjected to greater variable compressive loads due to the gear box added weight. This can be considered as an eccentric loading pattern.

    The engine mounts have to sustain variable compressive stresses. Fatigue testing of rubber mounts in two dimensional planes does not represent actual loading conditions. A gasoline run engine temperature is around 190 degrees Farenheit, whereas a diesel engine working temperature is 200 degrees Farenheit. Fatigue tests are not conducted at these temperatures.

    The engine mounts are made from natural rubber. It has excellent shock resistance. However, natural rubber is vulnerable to oil and ozone. Aggravation of engine noise is a sure indication of engine mounts failure. Epoxy blended natural rubber has shock absorbing properties and it has fair degree of oil and ozone resistance.

    Remedial Action

    I suggest use epoxy blended natural rubber reinforced by carbon composite fibers in the matrix. This is an ideal choice meeting all the stringent quality requirements for highly stressed engine mounts.

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    Natural Rubber Engine Mounts

    Although natural rubber possesses very good shock absorbing characteristics, it has limitations. Excellence in a single property is not a decisive choice. Rubber mountings are subjected to ozone deterioration, which is a factor ignored in test standards. Natural rubber is not oil resistant; it softens rapidly and loses its natural resilience. Even though oil resistance can be taken care by using epoxy blended natural rubber, the use of deteriorated, weak rubber engine mounts can damage the engine. NAtural rubber also suffers severely from temperature extremes, such as those found in the typical engine compartment or your car.

    Diesel engines generate more vibrational stress than gasoline engines. Rubber mounts are always in a vertical position to share the distributed engine weight. The static load on the engine mounts is a compressive force, which is a potential energy. Normally, when the engine runs, it sways laterally left and right within designed limits. This sway increases when the rubber loses some of its basic properties. If any one of the engine mounts becomes weaker, the other mounts will have to absorb excess stress, and this is the turning point for cumulative damages to the gearbox, as well as the engine moving parts within.

    When a speeding vehicle is brought to a grinding halt, which happens occasionally, the lateral or fore and aft pitching action of the engine due to the sudden change in momentum is detrimental to the engine mounts. A sports racing car in its start action is another example: the high intensity kinetic energy of the engine exerts a high shear stress in a rotational direction. The engine tends to swing forward (in a front wheel drive vehicle) or to one side (in a rear wheel drive vehicle) momentarily. This is why many high performance cars with high torque/high horsepower engines will normally be fitted with torsional chains to restrict this rotation. A tensile tear at the back face of the rubber mount is a distinct possibility, especially so if the mounts are aged or deteriorated due to ozone. This instantaneous pitch forward or sideways action is a deadly blow for the clutch and the gear box drive elements.

    The aforementioned serious implications are of more considerable magnitude in trucks than in cars. There is an imperative need for a thoughtful design for the engine suspension system. This requires a Six Sigma design approach to guarantee total requirements to save the precious engine for its long service life. No attention is given to engine mounts during periodic servicing such as oil changes and tune ups. The rubber mounts are not marked for its quality identification.

    As already pointed out, natural rubber is not oil and ozone resistant. Rubber mounts are not tested for fatigue tests at operating temperature. Engine heat will affect the rubber fatigue properties as will exremes of cold. Malaysian natural rubber is epoxy blended, which enhances its properties. When there are no identification markings, a mix up of two different quality products is certainly possible.

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    Images

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    Related Links

    www.tatamotors.com - reference : photos of rubber mounts

    www.gkndriveline.com/ -reference: Carbon composite fibres

References

  • Mike Aguilar has over 30 years of experience in the automotive field.

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