The mounting element – a key component in vibration technology

The mounting element –a key component in vibration technology

HELMUT TRÖBINGER, IFE MATERIAL HANDLING, WAIDHOFEN/YBBS, AUSTRIA

www.ife-bulk.com

IFE Aufbereitungstechnik GmbH, Patertal 20, 3340 Waidhofen/Ybbs, AUSTRIA, www.ife-bulk.com

The following list of mounting elements includes most of the variants currently in use. Specific elements for niche applications are not considered.

2.1. COMPRESSION SPRING

Compression springs have been used for vibrating units from the beginning and are one of the earliest variants of mounting elements.

Their main advantage is the good vibration isolation and its stabilizing effect on the installation position of the machine. Thanks to their simpli-city and ruggedness, compression springs are ideally suited for rough operation.

The spring constant does not deviate over a very wide temperature range. Corrosive environmental influences, however, must be reduced by suitable means (e.g. paint or coating). These properties have resulted in compression springs being used as the standard mounting system for vibrating machines.

2.2. SPECIAL CASE: ISOLATING FRAME Especially for large, freely vibrating machines (typically for vibrating masses greater than 10 t), additional vibration isolation is installed by means of an iso-lating frame, which is sometimes also referred to as a counter-vibration frame. With respect to this, however, it must be made clear that the freely vibrating machine

vertical dyn. force Fv = a sin α x cv horizontal dyn. force Fh = a cos α x ch

a = vibration amplitude ( = ½ stroke)cv = vertical spring stiffnessch = horizontal spring stiffness

Figure 1: Single-mass vibratorFigure 2: Compression spring

1. INITIAL SITUATION

Vibrating machines are installed in steel structures or on concrete foundations. Their load acts on the sub-structure statically by their own weight and dynami-cally by a force alternating with the vibration frequency.

In all freely vibrating systems and in resonance sys-tems (with the exception of a few designs), the dyna-mic forces are defined by the vibration amplitude a and the spring stiffness c of the support springs.

The vibration amplitude is defined by the screening task required by the process. This material likewise defines the weight of the vibrating mass (sometimes referred to as machine weight). The dynamic forces are influenced by the stiffness of the spring support only and/or any damping elements that may be present.

The function of mounting elements is to separate the vibrating mass from the stationary structure and to absorb the vibration in such a way that as little vibra-tion as possible is induced into the substructure. The occurring static and dynamic forces must be taken into account when designing the structure under-neath the vibrating machine. In addition to the natural fre quencies of the structure, they must be taken into account when a steel support structure is being calcu-lated, for instance.

In the field, various components have been tried and tested and evolved into a “standard“ that all manu-facturers make use of. However, even such standard elements are subject to further development, which will be discussed in detail below.

Figure 3: Mounting element with compression springs and lift-off protection

2. STANDARD TYPES OF MOUNTING ELEMENTS


2.3. RUBBER BUFFERS

Rubber buffers are suitable for low vibration amplitudes only (up to max. 5 mm). They consist of a solid rubber body rigidly installed between the buffer support (on the top) and the centering plate (on the bottom).

The advantage of the rubber buffer is its very good position stabilization and its inherent lift-off protection, which has to be implemented externally in the case of spiral springs. When used in machines with low vibration amplitude – typically machines with low vibrating mass – the lower vibration isolation ( approx. factor ½ compared to the compression spring) hardly matters. However, the damping effect of the rubber buffer is used as an advantage in many applications, for example in hopper discharge chutes, since the im-pact energy must be dissipated when the bunker is filled firstly. The temperature range in which the rubber buffer may be used is inherently lower than for the compression spring (typically -40 to +60 °C); on the other hand, the rubber buffer is not subject to corro-sion.

in itself does not require a counter-vibrating frame, but the specific conditions on-site may require it. This is almost always the case with large machines.

Thus there is one more mass added to the system of the “single-mass vibrator“ to form a “double-mass vibrator“.

The isolating frame is typically dimensioned to 30-50 % of the weight of the vibrating mass. The disadvantage of the additional weight (static load) factors in far less than the reduction of the dynamic forces (typically damped to approx. 30 %) that are achieved. For installation in steel structures in particular, but also when the ground tends to transmit vibrations, the isolating frame represents a very eco-nomical solution.

The advantages of the isolating frame become par-ticularly evident when the machine is operated for longer periods, i.e., run in continuous operation. When a two-mass system runs down, several natural frequencies are overlapping each other in such a way that, for the duration of 2 to 3 vibrations, a massive amplitude increase occurs on the counter-isolating frame (up to 10 times the vibration amplitude of the oscillating mass). This is compensated by a springy connection from isolating frame to the

Figure 4: Feeder on isolating frame

Figure 5: Waste screen on isolating frame

Figure 6: Rubber buffers

2.4. VIBRATING FEEDER ELEMENTS

Vibrating feeder elements are sometimes also called “Rosta elements“ after the name of the market leader. These elements also include rubber buffers, but they are arranged between the external housing and the internal square tube in the corners. As a result, the r ubber buffers do not react directly to the machine weight, but to the torsional forces that are generated in the elements.

substructure, using either rubber buffers or springs (Figure 4: Feeder on isolating frame, Figure 5: Waste screen on isolating frame).


Unlike all other mounting elements mentioned here, the use of vibrating feeder elements rigidly connects the vibrating machine to the substructure. This is a significant advantage in all mobile and semi-mobile installations and also for applications with adjustable inclination angles and applications suspended from above (without complex design). The vibrating feeder elements can be used at ambient temperatures from -40 to +80 °C and the technology is wear-free. It should be noted, that in the event of a shock load (similar as previous with the rubber buffers) the impact energy is converted into heat, so that repeated impact shocks can change the damping characteristics of the rubber.

2.5. AIR SPRINGS

All mounting elements mentioned so far are selected according to the machine’s specific requirements and cannot be altered later. Air springs, which are known from vehicle engineering (the so-called „shock absorbers“), can circumvent this restriction, as they take advantage of the compressibility of air. Air springs are rubber bellows filled with compressed air, similar to a vehicle tire. Due to this, they can dam-pen vibrations and shocks very well. Added external compensating tanks enhance the damping effect so that dynamic forces emanating from the screening

3. COMPARISON OF THE MOUNTING ELEMENTS

In order to be able to select the best mounting element for a specific application, both operating states – start-up and run-down on the one hand and continuous operation on the other – must be taken into account.

3.1. MEASUREMENT RESULTS

Measurement campaigns were carried out on a real screening machine (SMV 1600*6000 F-UW30; vibrating mass 12,500 kg). The results with regard to the effecti-ve dynamic forces can be summarized as follows. Rub-ber buffers are not considered here, as these mounting elements are unsuitable for this machine weight.

Figure 8: Vibrating feeder element

Figure 9: Air spring

Figure 10: Compensation tank in the control cabinet

Figure 7: Rotation torsion element

machine are reduced to a minimum. The robustness of the compression spring can of course not be achie-ved with rubber bellows.


Figure 11: Comparison – start-up and run-down

Figure 12: Comparison – continuous operation

The numerical values specified in the two graphics must be understood as relative magnitudes. It´s clear that the mounting values that can actually be achie-ved depend on the respective task. The measured dynamic forces are indexed to the vertical force com-ponent of the compression spring to ensure direct comparison.

3.2. INTERPRETATION AND FINDINGS

Regardless of the requirements of the individual application and the costs, the result is that air springs show the best damping effect among the elements considered, with specific regard to the start-up and run-down phase. Thus, when an application involves frequent starts and stops, air springs may be the first choice among mounting elements. However, if the application is in continuous operation, the counter- vibrating frame is unbeatable despite its greater weight.

Copyrights:Fig. 1 © Schenckprocess, Fig. 7 © Rosta,other: IFE Material Handling

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Vertical Horizontal Comparison vertical forces during operation

Compression spring

Compression spring

Isolation frame

Isolation frame

Forc

e [%

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ROSTA-element

ROSTA-element

Air spring

Air spring

Air spring incl.compensation tank

Air spring incl.compensation tank

Forc

e [%

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Vertical Horizontal

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The mounting element – a key component in vibration technology

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