Application Note

Solving a Critical Propulsion Problem at Penta using Two Torsional Vibration Meters Type 2523

by Kevin Gatzwiller, Brüel & Kjær and Anders Rylander,

Abstract Any larger ship is a sophisticated and complex construction, and, concomi- tantly, building such ships involves many different aspects of engineering. Among the aspects of primary concern is the design of an efficient propulsion system. In order to achieve a high degree of customer satisfaction, reliability, fuel efficiency and long overhaul intervals are of paramount importance for a high quality marine engine/propulsion sys- tem. For a naval ship, notably in a pa- trol or combat situation, the question of reliability is, for obvious reasons, of vital importance. This Application Note describes and examines the use of two Torsional Vi- bration Meters Type 2523 in a critical, severely time-limited, trouble-shooting situation where the performance of a torsional vibration elastic coupling, fit- ted to the propulsion system of a new series of smaller navy vessels, had to be thoroughly investigated in order for the propulsion system to be cleared for operational service. Preparing for a differential torsional vibration measurement at the Volvo Penta Marine Test Center, in ,

Introduction

When a shaft transmits mechanical are generated in the engine and in A number of different counter meas- power through rotation, a torque acts the propulsion system. These har- ures are available to minimize the on it. The crankshaft of a marine die- monic torque components will mani- concomitant problems [1]. To prevent sel engine is subjected to both a stat- fest themselves as torsional vibra- the built-up of large torsional vibra- ic, or constant, torque and to a tions that can endanger mechanical tion amplitudes and stresses at res- dynamic variable torque. The static components such as the crankshaft onance, a so-called detuning device is torque arises from friction and the and the entire propulsion system, often employed at the “free” end of a applied load. The dynamic torque possibly causing fatigue failures. The crankshaft. This device is typically arises primarily from the pressure phenomenon of critical engine speeds used in applications involving com- pulses due to the combustion process is closely related to this and occurs plex, interconnected mechanical sys- and secondarily from the unbalanced in marine diesel engines and propul- tems. For less complex and less inertia loads in the slider crank sion systems whenever one of the severe applications, a simple elastic mechanism used to achieve the recip- harmonic torque components, exerted damper is often employed as it pro- rocating motion. on the crankshaft system, coincides vides functional simplicity. The elas- As a result of these gas and inertia with a torsional natural frequency of tic coupling was an ideal choice for forces produced by the marine diesel the shaft system thereby creating a the directly driven propulsion system engine, harmonic torque components resonance. employed in the vessels to be de-

Brüel & Kjær B K scribed later in this Application Note and was hence chosen as a means of reducing the levels of torsional vibra- Rubber Elements tion in the propulsion system (tor- sional transmissibility is typically reduced by 20 dB per frequency dec- ade above resonance). Fig.1 shows a drawing of the torsional vibration elastic coupling that is employed in these vessels.

Torsional Vibration Meter Type 2523

Measuring torsional vibrations of ro- tating components is a notoriously difficult problem. Conventional tor- sional vibration transducers, such as torsiographs, shaft encoders, toothed wheels and magnetic transducers, have not only required excessive down-time of the machinery under investigation (due to a troublesome mounting and alignment procedure), 941231e but, equally important, they have Fig.1 The elastic coupling used in the G-boat “challenged” the skills of the vibra- tion engineer due to the fact that most of these conventional torsional vibration transducer types are sub- jected to “noise” problems if the ro- tating target or structure to which the transducer is attached, vibrate as a solid body. In a trouble-shooting situation the task of the vibration engineer is very often to establish the cause of exces- sive noise or vibration and subse- quently propose a possible cure for the problem at hand. In such a situ- ation the above-mentioned problems, when measuring torsional vibrations, are in most cases unacceptable to say the least, especially since most trou- ble-shooting tasks are carried out un- der a severe time pressure. Adding to this, the nature of trouble-shooting work often prevents the vibration en- gineer from knowing anything about the source of the excessive vibration. All together, this has meant that a more efficient means of measuring torsional vibrations has been re- quested — especially for proper tor- Fig.2 Torsional Vibration Meter Type 2523 sional vibration trouble-shooting work in-situ. Based on a low-power dual laser beam principle, the patented Torsion- order to ensure sufficient light back- ing principle of the Torsional Vibra- al Vibration Meter Type 2523 pro- scattered into the instrument) the tion Meter Type 2523 is outside the vides, without physical contact to the measurement object requires no pre- scope of this Application Note. specimen, an accurate and durable parations whatsoever. Due to innova- means of measuring the torsional vi- tive optical design, the instrument See references [2—4] for more infor- brations of a rotating object. Apart requires no on-site calibration or fo- mation on this subject. from attaching a piece of retro-reflec- cusing. Fig. 2 shows the instrument. tive tape around the shaft (needed in A thorough explanation on the work-

2 Volvo Penta

Volvo was founded in 1927 by the two of combustion engines and transmis- its field, bringing a number of inno- Swedish engineers Gustav Larsson sions, it was only natural to expand vative products to the market over and Assar Gabrielsson who had come the activities of the Volvo company the years. The headquarters of Volvo up with new and intelligent ideas for into designing and producing engines Penta, situated at Gothenburg in an automobile. Established in for marine vehicles. This part of the Sweden, employs at present more Gothenburg in the south of Sweden, Volvo Group is known as Volvo Penta. than 500 people. the company soon grew to be a suc- Specializing in marine engines and cessful manufacturer of private vehi- aquamatic stern drives for smaller cles and trucks. With an accumulated and mid-range boats, Volvo Penta has high level of knowledge in the field become a world-leading company in

The G-Boat and the Waterjet Propulsion System

Volvo Penta TAMD 42 WJ

941230e Fig.3 The Waterjet propulsion system with the TAMD 42WJ engine

For highly specialized sea transpor- yard. Volvo Penta, being chosen as let tubes. In order to minimize vibra- tation purposes, the elite unit of the the sub-supplier for the engines, de- tional problems, the propulsion Swedish Navy employs smaller ves- livered a 6-cylinder/170 kW engine, system was finally designed with an sels know as G-boats. To improve the known as the Type TAMD 42WJ, for elastomer rubber coupling for tor- performance, handling, ruggedness this demanding application. sional vibration damping and four and manoeuverability of this type of The layout of the engine and the elastomer engine mounts functioning naval craft, an advanced waterjet propulsion system in the G-boat is as translational vibration elastic cou- propulsion system — instead of a tra- shown in Fig.3. The waterjet propul- plings. ditional propulsion system with a sion system is directly driven by the propeller — was proposed when the in-line 6 cylinder, 170 kW, engine. All Swedish Navy ordered a new series sea manoeuvring is undertaken by of these G-boats at a Finnish ship- pivoting and tilting the waterjet out-

Introducing the Problem

Optimum Noise-and-Vibration-Hars- could be approved. A number of these gine/propulsion system, thereby chal- ness (NVH) characteristics of the elastic couplings were ordered and lenging the design team with two power unit/propulsion system can ob- built into the first series of the new obvious questions: “Are new torsional viously only be obtained when choos- G-boats. vibration elastic couplings needed for ing a correct (matching) elastic Based on more than four hundred the up-coming series of G-boats?” and coupling for the specific application. hours of extensive sea trials, some even more importantly: “Are new tor- The Reich coupling Type 26998A3 changes were proposed to the water- sional vibration elastic couplings were suggested by the shipyard (boat jet system (and soon implemented) to needed for the series of G-boats all builder). Calculations performed by further improve the performance of ready in operational service?” the calculation department at Volvo the vessel. This, however, lead to new Penta showed that the couplings dynamic characteristics of the en-

3 Using In-situ Measurements to Solve the Problem

The Volvo Penta Calculation Depart- series of the G-boat, two choices were extremely cumbersome at best, leav- ment performed a new series of cal- basically feasible for the shipyard. ing the first alternative (although ex- culations in order to provide an They could — at high cost — replace pensive) as a best choice. answer to these vital questions. The the couplings with new heavy-duty Nevertheless, the Torsional Vibration result of the calculations raised doubt couplings, or, they could as a second Meter Type 2523, employed in a dual regarding the capability of the chosen alternative make use of our offer to configuration, i.e. with two instru- coupling to absorb the vibration en- perform a full-scale verification test ments measuring differentially in ergy at a critical speed — heating up of the existing couplings. The latter real-time, enabled Volvo Penta to of- the coupling in the absorption proc- could be performed by measuring, as fer the cheaper, faster and much sim- ess. However, the extensive sea trial a function of the engine RPM, the pler second alternative to the had indicated that the chosen cou- maximum angular deflection be- shipyard. pling was serving flawlessly without tween the input shaft and the output any deterioration of performance. shaft of the chosen elastic coupling”. Volvo Penta explains: “In connec- Employing traditional torsional vi- tion with the chosen torsional vibra- bration transducers, the full-scale, in- tion elastic couplings for the first situ verification test would have been

Performing the Full-scale Measurement

The set-up used by Volvo Penta dur- ing the full-scale test on one of the Tacho Output G-boats is shown schematically in Fig.4. The TEAC RD200T digital tape recorder is used to record the AC out- put from the two Torsional Vibration Rubber Element Meters Type 2523 during the run-up Volvo Penta measurement. The output signal TAMD 42 WJ from a tacho probe is recorded as well to allow for subsequent order analy- sis on a dual-channel FFT analyzer. Note that, as indicated on the figure,

L a se r O L n o ck

L a O s n e /O r Both surfaces are ff

Laser On

Lock

Laser the phase of one of the Torsional Vi- On/Off prepared with bration Meters Type 2523 has to be retro-reflective tape Torsional Vibration ° inverted 180 in order to measure dif- Meter 2523 AC output with ferential values of torsional vibration active 180¡ correctly [1]. Depending on the con- phase inversion ditioning possibilities of the connect- ed analyzing equipment, this phase Torsional Vibration RPM inversion can either be achieved in Meter 2523 output this equipment, or it can be achieved AC output with by means of a simple 180° phase in- inactive 180¡ version feature, built-in to the Tor- TEAC RD 200T RPM phase inversion sional Vibration Meter Type 2523 as Digital Tape Recorder output SONY a standard option. Tacho During the measurement, the two Input Torsional Vibration Meters Type 941267e 2523 were mounted on their tripods, positioned directly on the engine Fig.4 A schematical set-up used for the in-situ measurement of differential torsional room platform and loosely secured by vibrations across the elastic coupling means of tape. The two laser trans- ducers were aimed at the coupling ously transmit vibrations (primarily Secondly, although no calibration of input shaft and output shaft respec- translational vibrations) to the two the Torsional Vibration Meter Type tively. Prior to the measurement, the Laser Transducers. However, transla- 2523 is required, a scaling factor has shaft measurement positions were tional vibrations (in any degree-of- to be applied if the laser transducer prepared with two pieces of retro-re- freedom) of the measurement object is tilted or pivoted relative to the flective tape. The actual set-up and/or the Laser Transducer will measurement object. Normal position (though on a different type of boat) have no influence on the torsional vi- (i.e. the position where no scaling fac- can be seen in Fig.5. bration measurement. This is due to tor is needed) is shown on Fig.6, At this point, a number of impor- the fact that the dual-laser beam whereas Fig.7 shows the situation tant comments should be made. principle of the Torsional Vibration where a scaling factor would be need- Firstly, with the engine running, Meter Type 2523 is inherently insen- ed. The scaling factor can be calcu- the engine room platform will obvi- sitive to translational vibrations [4]. lated as the ratio of the actual speed

4 (for example, output from a tacho probe) to the apparent speed (rota- tional speed output from the Torsion- al Vibration Meter Type 2523) or it can be calculated as: 1 1 ------⋅ ------(1) cosθ cosφ when the pivoting angle θ and the tilting angle φ are known [2] . Thirdly, in a number of applica- tions, the measurement object is not necessarily ring-shaped. Other ge- ometries might well occur. However, the output of the Torsional Vibration Meter Type 2523 is totally independ- ent of the cross-section geometry of the measurement object [2].

Fig.5 The actual set-up used. Along with the two Torsional Vibration Meters Type 2523, a TEAC RD 200T digital tape re- corder is used to record the AC outputs of the Torsional Vibration Meters Type 2523 and a tacho signal. The latter was record- ed to allow for subsequent order analysis on a dual-channel FFT analyzer

Result of the In-situ Differential Torsional Vibration Measurement

Having recorded the two AC output Side of Shaft Measurements signals and the tacho signal on the Plane normal to digital tape recorder and analyzed axis of shaft rotation them using order analysis, the meas- Shaft urement is finally transferred to a Axis of PC. With the aid of dedicated soft- shaft rotation ware, the PC then calculates the pow- er generated in the elastic coupling

Laser On

Lock as function of a chosen order. This Laser On/Off calculation is based on the well- known formula

P = M ⋅ ω (2) Laser Transducer absorbed MM 0071

where: Pabsorbed is the total ab- sorbed power in the elastic soupling, d= beam separation distance

MM 0071 Torsional Vibration Transducer

INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM. B M is the exerted torque on ditto and 7/6-'89 K 1.2 mW, 780 nm CLASS 3B LASER PRODUCT.

d Brüel & Kjær ω is the rotational speed. For order Laser Transducer MM 0071 analysis, the exerted torque is calcu- Side view Φ = 0¡ End view Axis of lated as: shaft rotation 941233e M =2 ⋅∑η ⋅⋅k ϕ (3) Fig.6 When the Laser Transducer MM0071 is neither tilted nor pivoted, a scaling factor i is not required where: i is the number of measured orders, η is the coupling efficiency, k differential measurement, as indicat- half of the generated power and, ac- is the rotational stiffness of ditto and ed on Fig.4, is carried out across the cordingly, by multiplying with two, ϕ is the differential torsional vibra- middle coupling section and the fly- the total power absorbtion in the cou- tion amplitude as function of engine wheel. Experience have shown that pling is achieved. speed. The factor 2 is used since the the two sections absorb very close to

5 ⋅ η ⋅⋅ϕω ⋅ Pabsorbed =2 ∑k (4) The measurement indicated that 9 W, whereas the measured value is i the orders of interest were the 3rd, approximately 4.5 W at 2700 RPM. to the 6th. Fig.8 shows the total gen- Volvo Penta was therefore able to ap- where ω is now obtained by mul- erated power in the elastic coupling prove the originally chosen torsional tip;ying the rotational speed with the at these orders. The maximum allow- vibration elastic coupling for the first i’th order. able value for the generated power is series of the G-boat.

The Torsional Vibration Meter Type 2523 as the Optimum Transducer for Torsional Vibration Measurements at Volvo Penta

One of the primary considerations for Axis of End view the design of Torsional Vibration Me- Side view shaft rotation ter Type 2523 was to ensure that the Φ d cos Φ dual laser beam optical principle MM 0071 Torsional Vibration Transducer

INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM. B 7/6-'89 K 1.2 mW, 780 nm CLASS 3B LASER PRODUCT. would operate successfully even in d Brüel & Kjær Laser Transducer troublesome industrial environments Φ ≠ 0¡ MM 0071 with a maintained high degree of con- fidence and accuracy. The outcome of Side of Shaft End of Shaft these considerations is an instrument Measurements Measurements that, amongst other things, provides Plane normal to features as introduced in the previ- axis of shaft rotation ous sections: No focusing and no cal- Shaft Shaft ibration needed, no sensitivity to Axis of translational vibrations of the laser shaft rotation θ=35¡ transducer or of the measurement ob- θ =60¡ ject, no sensitivity to the cross-sec-

Laser O L as n e L r O ock n Lo ck La O se Scaling Factor= n/O r La ff O se n/O r tional geometry of the measurement ff 1 RPMtrue object. Above φ θ = θ θ cos á cos RPM2523 For Volvo Penta these features are max=55¡ max=55¡ view Laser Transducer of special importance since many of Laser Transducer MM 0071 their torsional vibration measure- MM 0071 ment tasks are carried out under less than ideal laboratory environments.

The ruggedness of the Torsional Vi- 941232e bration Meter Type 2523 along with Fig.7 Applying a scaling factor to the calibrated output of the Torsional Vibration Meter its ease of operation therefore played Type 2523 an important role when Volvo Penta decided on the optimum torsional vi- bration transducer for their needs. 5

Conclusion 4.5 4 By employing two Torsional Vibra- 3.5 tion Meters Type 2523 in a dual con- figuration, it has been shown how 3 Volvo Penta were able to verify, in- situ, that the power absorption of tor- 2.5 sional vibration elastic couplings, mounted onto the propulsion system 2 of a series of Navy vessels, were with- Generated power [W] in specifications. This verification 1.5 avoided a costly and cumbersome coupling replacement. Despite diffi- 1 cult measurement conditions that in- 0.5 cluded severe difficulty of access to the measurement positions at both 0 sides of the coupling, the measure- 500 1000 1500 2000 2500 3000 3500 4000 ment was successfully performed, Engine speed RPM 941271e leaving no doubt that the chosen elas- tic couplings would operate flawless- Fig.8 The power absorption in the elastic coupling as function of engine RPM. The limit for approval was 9 W ly. Furthermore, practical aspects of employing two Torsional Vibration Meters Type 2523 in a dual configu- ration have been discussed.

6 References

[1] K.Kurr, B. Jörg: “Comparison of Vibration Damping Components for the Drive Line”. Paper presented during the First Colloquium on Vehicle and Engine Technology on October 27 - 29, 1987 in Aachen, West Germany.

[2] Torsional Vibration Meter Type 2523, Brüel & Kjær Instruction Manual BE 1111

[3] The Torsional Vibration Meter Type 2523, Brüel & Kjær Product Data BP 0958

[4] N.A. Halliwell, C.J.D. Pickering and P.G. Eastwood: “The Laser Torsional Vibrometer: A New Instrument” Journal of Sound & Vibration (1984), 93(4), 588-592

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