The conical rubber ring vibration mounts

A form of the conical rubber vibration mounts design is given again in Figure 1. It is described as a machine suspension and is used to isolate vibrations of heavy machine tools, e.g. eccentric presses becuase the specific energy storage of rubber vibration mounts is greater than that of steel ring . The machine support absorbs vertical force very softly but at the same time is sufficiently stiff to cope with horizontal forces. It thus meet the requirements it will prevent any undersirable floating of the suspended machine that would arise with compression loaded elastic rubber vibration mounts. Furthermore the low overall height and full protection of the rubber from oil leaks are worth emphasising as well as the complete freedom from maintenance and the great durability, which is where a machine support derives most benefit from the favourable combination of shear and compressive stresses in the rubber vibration mounts.

Figure 1 conical rubber ring vibration mount

Sound isolated piping

The example of sound attenuation from water pipes and the consequent reduction in sound reflection off the walls shows the extent to which rubber can reduce the volume of sound. The situation is reproduced schematically in Figure 1 . An open tap causes a loud noise. The sound of the following water is conducted by the piping and then by wall brackets to the wall, from where it emanates into a neighbouring room. If the pipe is isolated from the brackets by rubber bands, the sound volume is considerably diminished, as Table 1 shows. This depends on the force exerted in the tightening of the rubber strip. The measurements show that rubber is more effective than cork. Light to average tightening of the bracket clip screws give an optimum noise reduction.

Figure 1 Sound-deadened water pipes

No.	Sound proofing material	Tightening of isolating material	Sound volume (phon)	Improvement with insulating material (phon)
1	without	-	55.5	0
2	rubber	light	39	16.5
3	rubber	average	39	16.5
4	rubber	strong	42	13.5
5	cork	light	42.5	13
6	cork	average	46	9.5
7	cork	strong	48.5	7

Table 1. Reduction of sound volume with rubber and cork in water pipe

Sound damping

Sound describes those mechanical oscillations in acoustics which lie within the audible range of man’s ear. It is the range between abount 10 Hz and 16 kHz. These oscillation spread in solid liquid and gaseous bodies. Noise according to BS661 is sound undesired by recipient and DIN 1320 defines it as disturbing sound.

In modern sound technology a sound baffle is understood as something that ia an obstacle to the spreading out of sound by a reflecting structure. Sound damping material is sound absorption, i.e. its convert into heat. The first case deals with damming, the second with attenuation.

Rubber materials act as effective sound barriers. The aim of sound damping material is to prevent the sound produced by a machine from spreading further, i.e. sound isolation. Also, transmitted reflected sound originating from orther bodies in thus obstructed. The grade of sound damping material is mostly judged with the help of sound meters. Thus the noise strength is measured before and after isolation. The sound strength is plotted for the individual frequencies of which the noise consists.

The processes involed in the damming of the sound which is being propagated through a body are truly complicated. To this day there are no extensive general references which would enable the designer to calculate the effectiveness of a given rubber material barrier. The problems occurring in this field up to the present have had to be handle purely empirically. It has been shown that each dynamically solved rubber damping also carries with it appreciable sound damming. A survey of the incidence of loudness is given in table 1 . For the inportance of sound damming in the framework of sound damping material. The ability of rubber to dam sound is proved by the slowness with which the sound is propagated. (Sound propagation in rubber is approximately 1/70 that in steel.)

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Sound isolated piping

Table 1 Order of magnitude of sound volumes

Phon	Volume of sound
10	very soft whisper
20	slight rustle of leaves
30	ticking of a clock
50	normal conversation
60	vacuum cleaner noise
70	telephone ring, desk apparatus at 1 m distance
80	very busy road
90	machine shop with lathe and automatics
100	noise in a cotton and silk weaving shop
110	pneumatic riveters in a boiler shop
120	peening of welds with pneumatic hammers at 2 m distance
130	pain threshold, sound effect at the operator’s head during peening with pneumatic hammers

The flat rubber mounts

The flat rubber sandwich mounts both the loaded face and the rubber cross section are rectangular. As with circular sandwich rubber pads it is possible to have a pure compressive stress or a pure shear stress or a combination of both. Two flat rubber mounts shear springs would be used which are preloaded in compression to avoid possible tensile strain. The rubber layer is intentionally narrowed by 5 to 10% using compression. In this way it is possible to choose a shear loading three times higher than that for a flat rubber mounts without compression.

Flat rubber mounts of greater length are called rubber strips or mats. They are avilable in lengths up to 2,000 mm and are specially suitable for isolating the vibration of heavy machines.

Rubber strips are delivered in stock lengths, but can also be cut to any required length. The metal parts are sufficiently thick for holes to be drilled and tapped later for mounting purposes.

Installation example of flat rubber mounts are shown in Figure 1.

Figure 1 Example of flat rubber mounts sub-assemblies

Rubber properties : Creep and Set

The deformation of rubber is influenced by the length of time under stress. If the rubber is statically loaded to a give amount, as occurs for example by the support of a machine, then, in accordance with Figure 1, an elastic deformation takes place superseded after a longer period of time by creep. Creep behavior follows an exponential law and is conclude some time afterwards. If the spring is released then its shape return elastically and by creep to a new position at witch recovery is arrested. This represent an inelastic after-effect for a given rubber mix. High elastic grades show a small residual strain and minimal creep. With good elastic qualities, the residual strain (Permanent Set) after prolonged loading and unloading lies between 2 and 5 %, and creep lies between 5 and 10 % of the total elastic strain.

Figure 1. Creep Diagram for soft rubber

Creep and residual strain are proportional to the total elastic strain under a static load. Because of this they are expressed as a percentage of the elastic strain. Thus both effects are independent of the magnitude of load, but they are temperature dependent as show in Figure 2 . The diagram represents a butadiene mix; natural rubber mixes behave more favourably.

Figuer 2 . Temperature dependent of creep , permanent set and modulus for rubber cylinder 20 mm in diameter x 20 mm under stress of 1 MPa

When an oscillating (dynamic) load is superimposed on a static load a further undesirable deformation results from the oscillating load. This is described as Dynamic Set. The dynamic set is dependent on the number of stress reversals and the magnitude of the oscillating load. It also reaches and end value quickly. For vehicle springs the set may take up the same magnitude as creep at full static loading of the vehicle. The spring constants are not influenced by creep and set. The sole effect is a parallel displacement of the spring curve by an amount of the set. In rubber spring design it is useful to allow from 8 to 10 % of the elastic deflection of the spring for permanent deformation through creep and set.

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1. What is the standard test for oil seal.