Sunday, July 24, 2011

The rubber ring spring

The rubber ring spring, known as a low-frequency stud mounting, can be, as shown in Figure 1 (a), stress by three types of loading A, B, and C. The elasticity of rubber ring has different values in the separate directions. It is largest in direction B and smallest in direction A, as shown in the characteristic lines in Figure 1(b). In compressive loading (direction A), the compression may be so large that the hole is completely closed. The description ‘low-frequency mounting’, mean that this rubber ring is suitable for such cases where a low natural frequency is required. It can be loaded up to 100 N per mounting. Thus rubber ring uses lies in the field of precision instrument applications.

Rubber ring

Figure 1 Rubber ring spring

Sunday, July 17, 2011

Rubber product : Rubber supported rails

In general, rails used to be supported by wooden sleepers. Wood was preferred because of its elasticity and its damping action. With modern permanent way, however, preference is given to reinforced concrete sleepers which facilitate the use of continuous welded rails. Their use eliminates rail joints and reduces the maintenance expenses due to joint ends, fishplates and securing bolts. However, the concrete sleepers are rather inelastic and because of this use is made of rubber sheet rail mats of special design and characteristic. These are mats with trapeze shaped grooves provided at the top and bottom faces to ensure the required compressive and shear elasticity. Because of the wide temperature variation, which on European railways can rang from 243 to 333 K (-30 to 60 *C), use has to be made of synthetic rubber. The mats are made in two size, 166x123x4.5 mm and 141x125x4.5 mm. The end are flanged to prevent displacement. The Geman Federal Railways(DB) have successfully applied these pad and also at high speeds. Similar pads are also used experimentally by the French State Railways and the Stockholm Underground.

Friday, July 15, 2011

Rubber mounts : Relay boxes

Example of rubber mounts that presents in this article is relay boxes. The relay box of the numerically controlled machine tool in Figure 1 is fixed to the machine itself. To protect the sensitive relays from impact and vibrations of the machine, the box is insulated with the help of rubber product. With regard to load application, two different types of elements are used. A moment is produced by the weight of the box, which applies tension to the upper rubber mounting point. But tensile applications are unfavourable to rubber. Therefore a bell shape, such as in Figure 1, is chosen for the upper rubber mounting point, in which the rubber is then stressed in compression. The lower ‘W’ shape is likewise stressed in compression by the moment. The shear load is taken up in almost equal parts by both elements.

Rubber mounts

Figure 1 Example of rubber mounts : relay boxes

Wednesday, July 13, 2011

Rubber Properties : Damping Properties

Damping is the difference between deforming work and elastic recovery. The energy loss is caused by friction and is determined by the loading and unloading phase of a process. Damping due to surface friction between the rubber product and a mating surface may be present. But there is also internal friction within the elastic body itself, call material damping. With internal friction, unlike surface friction, no appreciable variations occur in the fraction forces. In both cases the work of damping is changed into heat, which thus account for the energy lost in vibration. The case of oscillating loading is presented by Figure 1. The area of the loop U1 – U2 is a measure of damping. It is equal to the energy loss per oscillation and is called absolute damping in joules but published values so far are given mainly in kgf cm . The ratio of the area of the strip U1-U2 to the area under the upward curve U1 is the percentage damping. The area U1 is the total strain energy, also called absorbed energy. The Roelig percentage damping is thus given by

damping[%] = 100x(U1-U2)/U1


Vibration Dampers

Figure 1 Damping Properties

A percentage damping of, for example, damping = 30% mean that 30% of the total energy imposed on the rubber material is absorbed by the rubber material, i.e. damped. The percentage damping of natural rubber grows from 6 to 30% with increase rubber hardness. With synthetic rubber, e.g. butadiene rubber, the value are higher for soft grades but are almost identical for harder grades. The damping capacity of rubber is considerably greater than that of steel. The percentage damping is not greatly used in the calculation of dynamic problems. The damping has no constant value. It depends on the grade of the rubber, temperature, strain rate(velocity) and acceleration, shape, and type of stress (compression, tensile, or shear). Generally valid damping values which would be approximately dependent on IRHD can't be stated. The magnitude of damping has to be obtained in given cases by enquiry from the rubber product manufacturer or ascertained dynamically with the help of a suitable apparatus. The determination of dampingand dynamic modulus is governed by ASTM D945-55. A preloaded probe is stressed dynamically. The damping and the dynamic modulus are determined from the area of the hysteresis loop and its position. The Yerzley oscillograph is particularly well suited for determination of damping under impact load when the oscillation curves are plotted. The behaviour of rubber product which are subject to forces caused by oscillating masses is tested on the Barry impact machines. If energy tires is designed , its damping properties is lower than general tires becuase energy tires must have lower energy lost.

Sunday, July 10, 2011

Why use rubber material in vibration dampers ?

Vibration dampers is mechanical part that absorb energy from working condition. The ability to absorb energy is dependent on the damping of material. Rubber material have high damping when compared with other material thus it has been used as material in vibration dampers . Its properties that indicate high damping is energy absorption. Which must be considered in the design of vibration dampers. Energy absorption must define with specific energy , some time referred to inadvisably as energy density, is the capacity to store elastically the spring energy U which is absorbed by 1 kg of spring mass. It is calculated from

Usp = U/m (J/kg)

where m is the mass of the spring material. Table 1 shows that the specific energy storage of a rubber spring is greater than that of steel. This mean that in practice the use of rubber material represents an appreciable saving in weight. Specific energy storage is greater in simple shear than in compression or tension. The best performance is shown by rubber material which are subjected simultaneously to shear and compression, yielding both the best exploitation of material and highest endurance strength.


Table 1 Specific energy Usp of rubber material in comparison with steel





Table 1 shown that vibration dampers that absorb energy depends on material and type of loading. In design vibration dampers must be consider type of loading which affect in performance of vibration dampers.