FITMENT OF MR MOUNTS ONBOARD NAVAL SHIP

LT CDR TARUN, 42814-B ACOUSTIC TRIALS OFFICER, MTU(VZG)

1. Introduction. Magneto Rheological (MR) mounts is based on magneto rheological smart fluid which when subjected to a magnetic field increases the viscosity of the fluid substantially to a point that it becomes a visco-elastic solid. The yield stress of the fluid can be controlled by varying the magnetic field with the help of its closed loop feedback control system. These mounts are known to be very efficient in attenuating noise and vibration at low frequency range as compared to conventional rubber mounts. MR mounts operate in both active and passive modes. Shock and Vibration (SV) mounts predominantly used in marine application are the conventional rubber resilient mounts which are passive mounts. Vibration transmissibility tests and Resonance tests conducted on rubber mounts and MR mounts reveal that the later have better damping at resonance and in low frequency range (10 Hz - 50 Hz). 2. Magneto Rheological Fluid. MR fluid used in these mounts is a smart fluid which changes its viscosity from liquid state to a semi solid state within a short duration of time on application of a magnetic field. The fluid consists of surfactant coated magnetic particles suspended in carrier fluid medium. On application of magnetic field, the magnetic particles align themselves with the direction of applied magnetic flux and induce change in viscosity. The magneto rheological response of the fluid is due to polarisation of the suspended magnetic particles by the applied magnetic field. Induced diploes cause the magnetic particles to form columnar structures parallel to the magnetic field. These columnar structures form a chain which restricts the motion of the fluid thereby increasing the viscosity of the fluid. Research and studies have revealed that these fluids are non toxic, non reactive to majority of engineering materials and environment friendly. 3. Depending on the strength of the applied magnetic field, the extent of formation of columnar structures and their alignment takes place. With the increase in magnetic field, the yield strength required to break this alignment increases almost linearly. MR fluid can be used for applications where power density, accuracy and dynamic performance are key features. Excellent features like fast response, simple interface between electrical power input and mechanical power output and precise controllability make MR fluid attractive for many applications.

4. Magneto Rheological Mount. MR mounts are closed loop feedback controlled low frequency vibration isolation mounts which are designed and developed by NPOL. MR mounts are smart mounts consisting of load bearing elastomers, MR fluid chamber, MEMS based vibration sensor, electronic controller unit and a DC power supply unit. MR mount with all its accessories is shown in Fig 1. These mounts consist of a load bearing rubber element (for passive mode operation) in parallel to a dynamically varying semi active MR fluid element (for active mode operation). The load bearing rubber element provides a failsafe mechanism in case of failure of active element components. An MR mount is depicted in Fig 2.

Fig. 1 – MR Mounts with Accessories

Fig. 2 - MR Mount

5. MEMS based vibration sensor senses the real time vibration of the machinery and a current equivalent to the sensed vibration is sent to the MR mounts through an electronic controller. The electronic controller unit regulates the solenoid current in the MR fluid chamber, which in turn regulates the flow of MR fluid through the valve and its viscosity in real time. Inside the mount, the viscosity of the fluid changes and corresponding damping force is generated. Rubber isolator fitted on the top of the MR mount can attenuate vibrations generated by the equipment in passive mode. Rubber isolator is made of nitrile rubber with an estimated life of 8 – 10 years. These rubber

isolators can be detached from these mounts and replaced if found deteriorated. MR mounts require minimal maintenance and only continuity of cable and resistance across leads is to be checked as part of maintenance.

6. MR Mount Operation. MR mounts can operate both in active and passive modes. When the vibration level is below a threshold value it will operate in passive mode. Whenever the vibration level crosses the threshold value, it will automatically switch from passive to active mode. These smart mounts can attenuate vibration up to 10 Hz, which is much lower than the commercially available passive rubber mounts. The structure of MR mounts is similar to that of a conventional hydraulic mount, except for the fluid being MR fluid and its properties being altered in real time to achieve the desired attenuation with changes in vibrations and damping force. 7. MR Mounts Specifications. Specifications of MR mounts are enumerated below:-

S No Parameter Value

(a) Weight Carrying Capacity 60 +/- 10 kg per mount (b) Acceleration Level 70g for 8 ms transient

> 2g for continuous operation

(c) Isolation Initiating Frequency 15 Hz (d) Static Displacement 4 mm (under full load) (e) Dynamic Deflection 2 mm (f) Height of Mount 135 mm (g) Base Footprint 100 mm X 55 mm (h) Maximum Current Consumption 2A per mount

(j) Operating Voltage 12 V DC (k) Operating Mode Active and Passive

8. Load Carrying Capacity. Presently, MR mounts are available for a load carrying capacity of 60+10 kg per mount. Higher load capacity mounts are under development. Therefore, heavy weight equipment presently could not be fitted with MR mounts. Secondly, low weight equipment will also have higher number of mounts per equipment. In case of the Indian Naval (IN) ship, 08 AKCC mounts were replaced by 12 MR mounts. Fitment of MR Mounts Ver ll Onboard Naval Ship 9. Fitment of MR Mounts Ver ll onboard nominated IN ship followed by STW and commissioning of mounts was undertaken Jan 18. The challenges experienced during installation and STW of these mounts are described in succeeding paragraphs. 10. Fabrication of Control Unit Racks. The dimensions/drawings of control unit racks were generated by IN in-house. The same were fabricated in the shop floor as per the dimensions and fitted as shown in Fig 3.

Fig. 3 – Three in no. Control Unit Racks 11. Matching of Equipment with Foundation. The equipment was removed by Naval Dockyard and landed at shop floor for matching of stool and foundation. For achieving this, holes were built up by welding and ground flush as shown in Fig 4.

Fig. 4 - Equipment Stool Holes Metal Built up for MR Mount Fitment

12. Modification of Primary Foundation. The modification was a primary challenge as it changes the entire characteristics of equipment operation if not designed properly. The top plate of the primary foundation was found not matching the foot prints of MR Mounts. Therefore, the existing plate (170 mm wide) was cut and a new plate with 190 mm width was welded. Pre modification and post modification pictures are shown at Fig 5.

Fig. 5 – Modification on Primary Foundation

13. Fitment of MR Mounts. The holes were drilled in-situ by Naval Dockyard in the foundation plate and equipment stool for mounts. The mounts were fitted and the equipment was installed (Fig. 6).

Fig. 6 – MR Mounts Post Fitment

14. Modification on Pipes. The height of MR Mounts being more than existing AKCC mounts (by 89 mm), the position of equipment also got raised by the same height. This led to requirement of modification on discharge pipe. Therefore, the pipe was modified as shown in Fig 7.

Fig. 7 – Modification on Discharge & Suction Pipes 15. Analysis of Equipment Foundations. In order to assess the behaviour of primary foundation of the equipment due to the fitment of MR Mounts, Bump tests and FEM analysis for equipment foundation were conducted both with and without the equipment. For a comparative assessment, the Bump tests and FEM analysis of same equipment of a sister class of ship (fitted with AKCC mounts) was also conducted. 16. Bump test is conducted by recording data from the actual foundation available onboard. FEM is done by making foundation modal in ANSYS software (as per measured dimensions of the foundation) and loading conditions are then simulated on the model. Onboard the ship, bump test was conducted by “Impact Hammer” method using FFT analyser, impact hammer and accelerometer. For FEM, foundation dimensions were measured and solid models were generated using CATIA software. The model was then imported to ANSYS. Fig. 8 shows the foundation models with and without equipment imported into ANSYS for modal analysis for both the ships.

Fig. 8 Model of Primary Foundation with and Without Equipment for Bump test and FEM

17. Mode Shapes and Frequencies on the Two Ships. From the modal analyses and FEM results it was established that the modes of vibration of test structure are far away from the rotational frequency, hence it is safe for operation with MR Mounts fitted for nominated ship and AKCC mounts for the sister ship. The mode shapes of 1st mode are shown in Fig. 9. The bump test and FEM displayed following frequencies:-

Mode No.

Ship with MR mounts Ver ll Ship with AKCC Mounts

Freq. observed from Bump

Test (Hz)

Freq. observed from FEM (Hz)

Freq. observed from Bump Test

(Hz)

Freq. observed from FEM

(Hz) 1 68 77 186 178 2 164 156 315 313

3 197 180 346 338 4 209 206 - - 5 249 235 - -

Fig. 9 - 1st Mode shapes of Foundation With and Without MR Mounts Fitted

Equipment Trials Post Fitment of MR Mounts 18. STW, Commissioning and Trials. Post fitment of the MR Mounts, cable connectorisation were undertaken for STW, commissioning and Trials (Fig. 10 & Fig. 11). Post commissioning, vibration trials of the equipment were conducted by the IN vibrations trial agency. Further, in order to have a comparative analysis, vibration trials were also conducted on same equipment of the sister ship, which is fitted with eight AKCC mounts.

Fig. 10 – STW/ Connectorisation of MR Mounts

Fig. 11 – Connectorisation of Mounts with Control Units and Power Supply

19. Vibrations at Bearing Points. On comparison of vibration values obtained from both the ships in passive mode, reduction of vibrations at the bearing points has been observed. Further, the comparison of vibration values of the nominated ship, with MR Mounts in passive and active modes, has also displayed reduction of vibrations at the

bearing points. The recorded vibration values at the bearing points are placed at Table 3.

Table 1. Vibration Values at Bearing Points

Ser Measuring Points↓ AKCC Mounts

(vib. in mm/s) MR Mounts

Passive Mode (vib. in mm/s)

Active Mode (vib. in mm/s)

(a) Driver Free End Axial 6.0 1.6 1.0

(b) Driver Free End Radial 6.3 2.3 1.9 (c) Driver Drive End Radial 7.4 1.4 1.2 (d) Driver Drive End Axial 2.7 1.2 0.9 (e) Driven Drive End Axial 2.7 2.1 1.0 (f) Driven Drive End Radial 5.3 4.8 4.0 (g) Driven Free End Radial 6.4 4.2 3.9 (h) Driven Free End Axial 4.2 2.0 1.2

20. Attenuation. The attenuation across mounts was measured and found to be satisfactory. Through FEM it was established that in order to evaluate the attenuation correctly, vibration readings are to be taken at top of MR mounts and near the base of primary foundation at 2-3 points. The attenuation readings evaluated thus are as follows:-

Mount No 1 2 3 4 5 6 7 8 9 10 11 12

Top (mm/s) 1.4 1.7 1.4 1.5 1.3 1.7 1.6 1.7 1.8 1.9 2.4 2.6

Btm (mm/s) 0.2 0.3 0.2 02 0.2 0.3 0.2 0.2 0.3 02 0.3 0.3

Attenuation (%) 86 82 86 87 85 82 88 88 83 89 88 88

21. Structure Borne Noise Across Mounts. The comparative analysis of both the ship’s vibrations reveals greater Structure Borne Noise reduction by MR Mounts in the lower frequency region. The SBN reduction values of both MR Mounts and AKCC mounts are placed Table 2.

Table 2. – Reduction Achieved in SBN Values (in dB) at Lower Freq. Region

1/3 Octave Centre Freq. (Hz) →

10 12.5 16 20 25 31.5 40 50

MR Mount 1 10 13.3 18.4 14.7 23.3 17.2 18.9 22.5

AKCC Mount 1 8.2 5.2 7.6 7.9 13.9 11.1 15 5.8 MR Mount 2 11.2 16.1 18 14.1 22.7 18.8 12.4 19.6 AKCC Mount 2 7.3 5.8 6.6 8 12 7.5 10.1 4.6 MR Mount 3 13.2 17.3 18.4 14.2 24.3 17.5 11.1 20.5 AKCC Mount 3 12.6 8.2 6.1 8 11 6.5 10 7.6

22. Conclusion. The MR Mounts fitted onboard IN ship have displayed greater SBN reduction as compared to AKCC mounts. The long term performance of these mounts in marine environment is under assessment at present. Certain design related issues are being addressed by the development agency. Further, MR mounts for load capacities higher than 60 kg are being developed for use on heavy equipment.