The degree of displacement for a 2 tube system and based on a constant elastomer can be designed-in by adjusting the basic parameters of length plus inner and outer tube diameters as follows:
This relates to displacement such that the inner tube is moved towards the outer tube whilst remaining axially parallel to it such that the thickness of rubber is reduced uniformly on one side. The radial displacement can be decreased by increasing the projected area (diameter of inner tube x length of outer tube). This can be facilitated by increasing the overall length of the bush or by increasing the outside diameter of the inner tube. When only radial loading is anticipated, these bushes will withstand significant shock loads in excess of normal rated load for the size (up to 5 times rated loading) However, if conical loading is anticipated in addition to radial loading, the nominal rated loading must be reduced. Also if cyclic loading is anticipated, the load catered for should be at least halved.
When one tube is rotated relative to the other whilst remaining axially parallel, this implies a torsional load. Reducing the thickness of the elastomer will reduce the angular displacement for a given torque (increasing the inner tube’s outer diameter or decreasing the outer tube’s inner diameter) or by increasing the overall length of the bush. Typically the maximum frequency of torsion loading accommodated is 8Hz (480 cycles per minute) and this would be for small angular movements up to +/-5 ° For lower frequencies +/-40° can be accommodated for some bushes where the rubber thickness allows.
If an angle is developed between the axes of the two tubes such that they are no longer parallel, this is a conical load. The displacement from such loads can be minimised by reducing the thickness of the elastomer or by increasing the length of the bush as above. Applications should limit the angular movement such that the thickness of elastomer at the end of the outer sleeve is not reduced by more than 30%. Clearly, greater conical movement is provided by a greater thickness of elastomer, which in turn can be determined by changes in tube diameters.
When the ends of the tubes are displaced relative to their no-load position whilst the axes remain parallel, this is an axial load. The displacement can be reduced by reducing the thickness of the elastomer or by increasing the length of the bush as above. Design loads should be kept to a maximum of 10% of the rated radial load.
The deflection parameters can also be modified by altering the hardness of the elastomer (harder reduces deflection) or by inserting an intermediate tube which reduces deflection in all directions as for example in our TB (triple bonded) bush type.
If you cannot find a suitable product by searching through our website, please send an enquiry with as much information as you have. We may already have a solution.
We specialise in all aspects of vibration and shock isolation using different rubber based materials, metals, springs, wire cable etc.