INTRODUCTION

Elastomeric isolators may be designed in both bonded and semi bonded configurations. For the bonded isolator, metal elements are bonded to the elastomer on all load-carrying surfaces (Fig 1). For a semi bonded or unbonded isolator, the elastomeric load-bearing surface rests directly on the supporting structure (Fig. 2).

Bonded parts are typically more expensive due to the special chemical preparation required to achieve a bond with strength in excess of the elastomer itself. A bonded part is generally the preferred option since they can allow a higher level of stress for a given deflection, with this higher stress they provide higher spring constants and higher elastic energy storage capacity.

Bonded isolators can be designed to provide the correct load distribution in shear, compression, tension, or combination loading. More information on this topic can be found through this link. A more uniform stress distribution in the elastomer is obtained by bonding inserts on all the load-bearing elastomer surfaces, these inserts reduce the unit stress by distributing it more uniformly throughout the volume of the elastomer. In contrast, semi bonded parts usually fail to distribute the load uniformly, resulting in local areas of stress concentration in the elastomer body which can shorten the life of the isolator.

A significant difference between bonded and semi bonded elastomeric isolators relates to how elastomers behave under load. When an elastomer pad is compressed under load its volume remains constant, only its shape is changed. The rubber "bulges" under load. When this ability to bulge is controlled the load-deflection characteristics of the isolator are also controlled. In a bonded isolator, the load-carrying surfaces have a fixed degree of bulge because the elastomer cannot move along the bond line, and so it remains in a fixed position regardless of the load or environmental conditions.

In a semi bonded isolator this is not the case. The ability of the elastomer to bulge depends to a considerable degree the amount of friction present at the interface of the rubber and its supporting structure. When all surfaces are clean and dry, the difference between the ability of a bonded and a semi bonded isolator to bulge is negligible. However, if oil or sand can work its way into the elastomer-to-metal interface of the semi bonded isolator, the ability of the elastomer to bulge is greatly affected. Consequently its original load­deflection characteristics no longer exist. The isolator can exhibit load-deflection characteristics that are 50 percent less than when it was new, in many cases, this can cause the isolator to malfunction.

Thus, where consistent Load-deflection characteristics are required for the life of the equipment, bonded isolators should be used. Although the initial cost of a semi bonded isolators is lower, in many applications the cost of extra machining of the support structure and the reduced service life may well make semi bonded isolators a poor selection.

Status of a semi-bonded vibration isolator similar to our SCH mounts, on an application where a high ratio of Dynamic Load / Static load was present.