ADVICE 10. FACTS ABOUT RUBBER TO METAL ANTI VIBRATION MOUNTS. DID YOU KNOW?

The purpose of an anti-vibration mount is to attenuate vibrations. The design of the vibration isolator is key to achieve the targeted stiffness and load capacity.

This article aims to answer some of the recurrent questions about the design of anti-vibration mounts.

How do you know the max rubber to metal bond Stress?

An anti-vibration mount is expected to last for many years, environmental conditions such as exposure to direct sunlight, oils or other deteriorating agents can cause the bond to weaken prematurely. In absence of these parameters certain maximum allowable bond stresses are used and should not be exceeded.

Compression 750 psi

Tension 150 psi

Shear 150 psi

A vibration isolator should have no region of high localized stress, particularly at the interfaces of the metal and rubber. Sharp changes in profile can cause high stress points, this might be caused by boltheads, boltholes, or indentations. Below are some examples:

For obvious reasons AMC MECANOCAUCHO prefers designs of anti-vibration mounts that incorporate a flat surface for bonding, as they provide lower regions of high localised stress.

How does the temperature affect the creeping of the anti-vibration mount?

At 60º C (140ºF) the rubber creep is from two to nine times greater than at 25ºC (80ºF) depending on the compound.

How does the shape of the stiffness curve change according the type of loading on the rubber?

Anti-vibration mounts use rubber bonded to the metal to achieve an elasticity. However, depending on the type of loading the mounts receive they can exhibit different stiffness values.

Compression Loading

The word compression is used to indicate a reduction in the dimension (thickness) of an elastomeric element in the line of the externally applied force. The stiffness characteristic of elastomers stressed in compression exhibit a nonlinearity (hardening) which becomes especially pronounced for strains above 30 percent. Compression loading is frequently employed to provide a low initial stiffness for vibration isolation and a relatively high final stiffness to limit the dynamic deflection under shock excitations. Because of the nonlinear hardening characteristics of compression loading, it is the least effective type of loading for energy storage and therefore is not recommended where the attenuation of force or acceleration transmission is the primary concern. (The energy stored by any spring is the area under the load-deflection curve.)

Shear Loading

Shear loading, illustrated in the above figure, refers to the force applied to an elastomeric element so as to slide adjacent parts in opposite directions. An almost linear spring constant up to about 200 percent shear strain is characteristic of elastomer stress in shear. Because of this linear spring constant, shear loading is the preferred type of loading for vibration isolators because it provides a constant frequency response for both small and large dynamic shear strains in a simple spring­mass system. Shear loading is also useful for shock isolators where attenuating force or acceleration transmission is important, because of its more efficient energy storage capacity when compared to compression loading.

Tension Loading

Tension loading refers to an increase in the dimension (thickness) of an elastomeric element in the line of the externally applied force. Elastomers stressed in tension exhibit a nonlinear (softening) spring constant. For a given deflection, tension loading stores energy more efficiently than either shear or compression loading. Because of this, tension loading has been occasionally used for shock isolation systems. However, in general, tension loading is not recommended because of the resulting loads on the elastomer-to-metal bond, which may cause premature failure of the material.

Buckling Loading

Buckling loading, occurs when the externally applied load causes an elastomeric element to wrap or bend perpendicular to the applied load. Buckling stiffness characteristics may be used to derive the benefits of both softening stiffness characteristics (for the initial part of the load­deflection curve) and hardening characteristics (for the later part of the load­deflection curve). The buckling mode thus provides high energy-storage capacity and is useful for shock isolators where force or acceleration transmission is important and where snubbing (i.e., motion limiting) is required under excessively high transient dynamic loads. This type of stiffness characteristic is exhibited by certain elastomeric cushioning foam materials and by specially designed elastomeric isolators. However, it is important to note that even simple compressive elements will buckle when the slenderness ratio (the unloaded length/width ratio) exceeds 1.6.

Combinations of the types of loading described above are commonly used, which result in combined load-deflection characteristics. Consider, for example a compression type isolator which is installed at an angle instead of in the usual vertical position. Under these conditions, it acts as a compression-shear type of isolator when loaded in the vertical downward direction. When loaded in the vertical upward direction, it acts as a shear-tension combination type of isolator.

How does the rubber profile shape affect the stiffness of the anti-vibration mount?

The moulded shape of the rubber section of the vibration isolator has a great impact on the load deflection curve of the mount. Below are some examples of how the shape and design of the mount affects the stiffness of the anti-vibration mount.

Should you be interested in this topic do not hesitate to contact our applications engineers. They will be able to help and provide input. Contact form AMC-MECANOCAUCHO® Applications engineers.