## Vibration isolators

Fans and other rotating machines not only generate airborne noise, but also vibrations which need to be dampened. Otherwise these vibrations can lead to structural damages due to material fatigue. Also increased wear and tear and structural noise is increased. The vibration isolators offered by **Witt & Sohn** can prevent or at least minimize the effects of these kind of vibrations. They are selected to fit with the weights and frequencies involved. Just as is the case with silencers to minimize airborne noise it is recommended to purchase the fan and the vibration isolators together to minimize co-ordination problems.

### Isolator program

For smaller fans rubber isolators are used, while larger fans normally are mounted with totally enclosed metal vibration isolators. For special applications we offer a range of open, metal spring vibration mounts. In addition we can supply vibration isolation matts etc.

**Rubber mounts**

For the elastic mounting of light to medium heavy devices we offer our ST-series of rubber mounts. The series consists of 6 types with a maximum load of max. 350 kg/mount and a static deflection of 11 mm. The unique design with the totaly encapsuled steel mounting plates provide the mounts with a very high stability and resistance to mechanical damage.

**Enclosed metal isolators**

For medium heavy to heavy devices up to 1600 kg per damper we can offer our SA-range. They encompass 5 completely enclosed metal spring mounts types with 11 variants each. The isolators consist of a machine made bottom cup spot welded to a base plate, pre-punched for holding down bolts, and fitted into the cup would be steel helical springs mounted on a neoprene high frequency vibration isolation pad. The springs would be held at the top by a pressure plate levelled by a set screw operating through a tapped insert in the machine made top cup. The top cup would be jig formed to its base to enclose a neoprene snubbing ring to ensure lateral stability.

**Open spring mounts**

For many applications, e. g. simple mounting of machines, ducts superstructures etc. we recommend an open metal spring isolator. Using different attachments weights up to 125 kg per spring with a maximum deflection of 25 mm can be isolated.

**Isolation mats**

Our 50 mm Corlam Isolation membranes are being used in foundations, sound proof rooms etc. with a maximum load of 300 kN/M2 and a deflection of 20 mm they can satisfy the most applications of isolation mats.

**Selection**

Compare the array of vertical loads per isolator with the static loading on the isolator position under consideration. If maximum isolation efficiency is required, selection of the nearest value below the maximum load and running down that vertical line will show at its base the ISOLATOR TYPE.

Read across horizontally from the shown (or interpolated) value, for vertical load, continuing across the deflection line, to read off the value of STATIC DEFLECTION. This line is continued until it meets the shown (or interpolated) value for the lowest speed acting upon the required isolators and reading vertically upwards, shows the ISOLATION EFFICIENCY (%). These selections are based upon vibrating systems supported on a high mass compared with that of the system. Selection below 800 rpm should include an inertia base for which we would be pleased to provide a fully detailed quotation. We are able to provide a computer selection for your vibration problem including a "hard copy" for your records.

**Example**

3600 kgs equally on 4 mounts at 500 RPM, using an SA 1000 DH giving 45 mm deflection and 90 % isolation.

#### **Basics**

The main function of a vibration isolator is to minimize the transfer of structur borne noise and vibrations from the fan to the rest of the installation, e. g. the ship, the building etc.

**Calculation of the isolator eigenfrequency**

For fans one can assume the fan speed(s) to be the main excitation frequency, i e.: Incination frequency

$ f_{vent} = \frac {\texttt{fan speed}} {60} = \frac {n_{vent}} {60} \texttt{ in Hz} $

The degree of isolation i and the structural noise damping value D are only dependent of the frequency ratio $ \lambda $ between the excitation frequency of the fan $ f_{vent} $ and the eigenfrequency $ f_{iso} $ of the vibration isolator.

#### Frequency ratio:

$ \lambda = \frac {f_{vent}} {f_{iso}} $

#### Degree of isolation:

$ i = \frac {\lambda^2 - 2} {\lambda^2 - 1} \cdot 100\% \Leftrightarrow \lambda = \sqrt{\frac {2-\frac{i}{100}} {1-\frac{i}{100}} } $

#### Structural noise damping value:

$ D = 20 \cdot log \frac {1} {1 - i} db(A) $

#### Vibration isolator eigenfrequency in a point A:

$ f_{iso_A} = \frac{1}{2 \pi} \cdot \sqrt{\frac{C_A} {F_A}} = \frac{1}{2 \pi} \cdot \sqrt{\frac{C_A} {M_A \cdot g}} $

$ C_A $ Spring constant

$ F_A $ Force in point A

$ M_A $ Weight

$ g $ Gravitation

Normally one intends to achieve a degree of isolation of more than 60 % and a structural noise damping value of minimum 8 dB. It follows from that, that the frequency ratio $ \lambda $ must be larger than 1,87:

$ i = 60\% \lambda =< \sqrt{ \frac{2 - \frac{60}{100}} {1 - \frac{60}{100}} } = 1,87 $

When solving the equations for various $ \lambda $ values or when illustrating it graphically one finds that for

$ \lambda \leq \sqrt{2} $

the degree of isolation becomes higher than 100 %, i. e. an increase in the energy transferred occurs. This is called operating in the subcritical area and must be avoided.

Picture 2 shows the relationship between the excitation frequency, the degree of isolation (and structural damping value) and the necessary degree of static deflection.

The static deflection $ S_{statA} $ in a point or an area A is derived from the spring constant and the forces that act at this point or area.

$ S_{statA} = \frac{F_A}{C_A} \texttt{ in m} $

The maximum deflection can be arrived at from the equation below:

$ S_{max} = \frac{S_{stat}}{1 - \lambda ^ 2} \texttt{ in m} $

and the maximum acceleration according to the following equation:

$ a_{max} = S_{max} * (2 \pi * f_{vent})^2 \texttt{ in m/s}^2 $

It is important that when operating vibration isolators in parallel, the resulting spring constant is the addition of the individual constants.

When mounting vibration isolators in series the resulting spring constant is arrived at by fraction addition.