The strain gauge: an aid to the development of marine transport

THE STRAIN GAUGE:

AN AID TO THE DEVELOPMENT OF MARINE

TRANSPORT

by H. B. BOYLE

National Physical Laboratory, Hovercraft Unit, Hythe

Strain gauges measure directly strains in models and marine components and are parts oftorque and thrust

transducers and of force and moment balances.

Six-component balances of strut and shear-plate design and special sliprings are described.

Introduction

In 1957, the Ship Division of the National Physical

Laboratory (NPL) started to use strain gauges in a force measuring transducer. As an innovation, it was

treated by many with suspicion and by some with

strong disbelief. Since that time the strain gauge has

ousted all other forms of electro-mechanical transducer

for measurement of force, and it gradually replaces

other systems used in the Division. Force transducers measuring simultaneously up to six components have been produced. Devices measuring force and moment span the ranges from 0 ito 12,000 lbf (05 N to 60 kN)

and from 0 1 to 15,000 lbf ft (0 15 Nm to 20 kNm). The ranges cover both model and full-scale research on a variety of vessels including conventional ships,

high-speed boats, hydrofoil and hovercraft.

The advantages of small size,

low mass and

accuracy, the ability to convert any loaded structure into a force transducer, and the convenience of direct strain measurement made possible experiments that

would have been difficult, if not impossible, with other

transducer systems. It would be difficult to think of any single more important aid used for research into hydrodynamics in the Division than strain gauges.

The Gauges

The first gauges used at the Ship Division were

manufactured by NPL and were bonded with Bakelite

cement, whereas current practice employs

epoxy-backed self-temperature-compensated foil gauges for nearly all applications. Various glues are used, hot

setting adhesives being preferred wherever possible.

In 1960 a brief note in an American magazine

recorded the development of a strain gauge having a gauge factor of about 120. A set of four gauges were ordered and the first semi-conductor strain gauges arrived in the Division. They were used to measure

propeller shaft thrust fluctuations on a large passenger liner travelling across the North Atlantic. This type of gauge is now considered as a useful and normal exten-sion of strain gauge techniques and is normally applied to the measurement of low skin strains (not more than

10 pin/in) in stable temperature environments.

Energisation, Amplifiers and Slip Rings

The majority of strain gauge systems are energised

by a 5 V a.c. carrier system operating in the range from

1 to 3 kHz. On occasion, d.c. energisation is used for

high-frequency dynamic applications or where supply

and signal leads are extremely long.

In a carrier

system, the a.c. amplifier and phase sensitive

demodu-lator are incorporated in one unit or strain indicator; with d.c. units, separate high-stability amplifiers are

employed together with some form of d.c. calibration. Slip rings, as signal transfer devices, have been fully described (1) and only a simple very successful

'home-made' type of slip ring may be mentioned which is

constructed by bonding strips or silver 1/32in thick by in wide (0.8 x 6mm) to a rubber sheet. The assembly

is wrapped around the shaft and attached to it by

means of suitable straps or "hose clips". The details of the ring joint are shown in Fig. 1. Silver-graphite brushes are used in the normal way. This type of ring

has performed admirably on shafts from 3 to 24in

(75 to 600mm) diameter.

O'OIO'' JRRASS STRIP TO SOLDER FILLED JOIN WHICH RING 5 SOLDERED

GAP WIDE SILVER SLIP RING

Fig. 1. Details of ring joint of a silver slip ring. LEAD OUT WIRES

Strain Measurement

On the model scale it is often necessary to measure

strain on dynamically similar models. A model of a

fully cavitating propeller made of glassfibre filled resin

(Fig. 2) was gauged on its blade surfaces for a

deter-Fig. 2. Glassfibre filled resin model of car itating

propeller.

mination of the skin strain distribution. The difficulty

with this type of experiment is to attach the gauges

and complete the wiring and waterproofing of the

installation without either significantly increasing the blade thickness or causing significant irregularities on the surface. Extremely small epoxy-backed foil gauges were used, 1200 and 90° rosettes together with single

gauges. Larger gauges were bonded with their metal

foil side to the blade surface so as to increase the

effective thickness of the waterproofing layer. Tinned copper wire rolled into flat strip 000l5 by

0060 in (004 x 1

5 mm) was used for lead-out

wires. The strip was bonded to the surface and allowed

to follow the natural contour of the blades. Any

change of direction was accommodated by overlapping

a second strip and soldering. All leads were led to

grooves in the propeller boss and joined to

con-ventional insulated lead-out wires passing through the

hollow drive shaft. For waterproofing, two thin coats of epoxy resin were applied over the whole surface.

At some soldered joints local extra coatings were

needed because the resin tended to "fall away" from

high spots.

The system was installed in a water tunnel and slip

rings were fitted at the drive end of the propeller shaft. All gauges had one terminal connected at the propeller

end to a common lead fed to one slipring. The other gauge connections were fed to the remaining rings. Although this method of slipring connection is con-sidered to be bad practice, the technique was chosen

to provide a maximum number of signals with a

reduced number of leads within the shaft. The records

were very good and noise free with both a.c. and d.c. energisation. With time the insulation broke down, but regular drying out allowed the tests to be

com-pleted.

A further application is the determination of the propeller shaft torque on full-scale ships. A strain

gauge torsionmeter compares favourably with other

types of ship torsionmeter and has the unique

advantage that values of torque can be determined without prior calibration, provided the modulus of

rigidity of the shaft material is known. Strain gauge

torsionmeters have been used over a wide range of

shaft sizes. Fig. 3 shows a shaft section complete with

commercial and home-made rings. The order of

Fig. 3. Shaft section with commercial and 'home-made'

slip rings.

accuracy of a strain gauge torsionmeter depends on

many factors; with care accuracies within ±1 % of

full scale can be achieved. Research has shown that the torsionmeter is stable over long periods and that the signal output exhibits good linear properties.

Force Balances

Many types have been developed, from single-force

devices to complete six-component balances. A few

of them are described here. Cantilever balance

This type of balance has a wide range of application

for ship model experiments and a selection is shown in Fig. 4. For single force measurements, e.g. with a drag balance for hovercraft experiments, a hinge or

pivot is incorporated at the loading point and the

gauges are situated as near as possible to the position of maximum bending moment. Where a pivot is not practicable, as when holding a submerged body in a rigid attitude, two sets of gauges are placed a reason-able distance apart along the cantilever and the sets

are so connected that their individual outputs subtract.

The resultant signal is directly proportional to force and independent of the point of application as long as

the line of action of the force is not between the gauge stations.

Fig. 4. Several types of cantilever balance.

Thrust and torque dynamometer

Measurement of propeller thrust and torque in a ship model forms one of the most important basic experiments performed in any ship model towing

tank. In the past this measurement has been done by

means of mechanical dynamometers (Fig. 5) which have been developed into extremely accurate dynamometers

but show a number of inherent disadvantages. They

Fig. 5. Mechanical dynamometer for ship model towing tank.

are heavy.

are suitable only for calm water conditions and can-not be used in experiments involving waves,

measure the bearing friction of the transmission

shaft to the propeller,

do not lend themselves to electrical recording.

To overcome these difficulties,

_{a strain}

gauge dynamometer as been developed. Obviously it is difficult to achieve the same order of accuracy but by making a range of units, all easily interchangeable, a

satisfactory accuracy can be obtained. Two

pos-sibilities may be considered:

direct substitution of a strain gauge dynamometer

for the mechanical type, or

measurement of thrust and torque directly at the

propeller boss.

Method (b) provides a simpler and cheaper

dynamo-meter and also eliminates the problems posed by

measurement of bearing friction and of inertia forces

acting upon the shaft when the model is run in waves. Typical designs are shown in Fig. 6.

Fig. 6. Strain gauge dynamometers measuring both

thrust and torque.

The transducer element forms part of the propeller

shaft and takes the shape of a rectangular bar, the

long axis being the axis of rotation. Torque is

measured by foil gauges bonded at 45° to the rota-tional axis. The maximum strain is of the order of 65

p. in/in. Thrust is measured by semi-conductor gauges placed on opposite parallel faces, along and transverse

to the rotational axis: the maximum strain is of the order of 5p.in/in. The low compressive strain and the relatively high torsional strain make it impossible to align the thrust gauges so that they do not respond to

torsional strain, and due to unavoidable mismatch they

also respond to a bending moment.

In order to eliminate these unwanted signals, two

techniques have been developed. Torque interaction is almost completely eliminated by applying a portion of

a torque signal derived from a separate torque bridge

(2). The bending moment interaction is eliminated by resistively shunting that half of the thrust bridge which

gives the higher output for a given bending moment. This can be achieved satisfactorily only if a

parallel-sided transducer element is used, since the gauges must

be situated exactly on opposite sides of the shaft axis to avoid any phase shift between the output signals from the two half-bridges as the shaft rotates.

Calibration results have been excellent, the

calibra-tion factor for thrust being constant within ± 05

per cent, that for torque within ± 025 per cent. The sensitivity of both bridges changes with temperature

linearly over the usual range of application (15 to

25°C). the scatter about the mean line being less than

±025 per cent for both bridges. Calibrations are

therefore taken over a range of temperatures. During

tests the constancy of the water temperature ensures a high degree of zero stability.

Development of the six-component

balance

The earliest form of a multi-component strain gauge

balance consisted of three cantilevers and measured three components of force. Attempts were made to

isolate each flexure so that each cantilever carried only

one component of load. This technique is often used

but, in the author's opinion, only complicates the

balance design without achieving the desired effect.

Results of equal merit are obtained by placing gauges on neutral axes or by relying on the cancellation of the

bridge network to eliminate unwanted signals.

Coupled with the techniques described already, this can produce balances of simple design.

This philosophy also led to the design of the first six-component balance (Fig. 7). Three struts separate two plates, the "force plate" carrying the test vehicle

Fig. 7. Six-component balance formed by three measur-ing struts.

and the "earth plate" bolted to a support structure.

All loads are transmitted from one plate to the other

through the three struts which in encastré bending

give two force directions and in direct axial loading the third. Algebraic sums of the independent outputs give the three corresponding moments (Fig. 8). Since

nine signals are produced, electrical summing circuits

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PITcH PQCE

ROIL UOM(N1

THREE STRUT - SIX COMPONENT FORCE BALANCE

Fig. 8. Output circuits for six-component balance

formed by three struts.

are provided to give the six components of loading. Conventional gauges are used in bending and

semi-conductor gauges for the axial loading. To increase the

semi-conductor strain level, the struts are shaped as double tapers reducing at the centre where the

semi-conductor gauges are located and where the maximum

direct strain and the minimum bending strain are found. The struts are encased in flexible oilfilled bellows giving a high degree of temperature stability

for the semi-conductor gauges.

The advantages of this balance are that

the construction is simple,

the strut design is controlled by the force system

alone,

the physical position of the struts controls the

sensitivity to moments.

The original balance was developed for a hydrofoil project and the prototype was built, gauged and cali-brated in about four weeks. Since then, a number of similar balances have been built to carry forces up to

Nm). For a given set of forces and moments the

inter-action values are low, not normally exceeding 5 per cent for full load in all planes; the maximum

inter-action occurs on the semi-conductor gauges.

In tests the measured values of interaction are

higher since test conditions may not be similar to the original design condition. Interaction, within reason, does not prohibit satisfactory measurement but com-plicates balance calibration because (I) calibration is

time consuming and expensive, (2) precise

multi-directional loading is difficult to achieve for large

loads where deadweight calibration is not practicable,

(3) the balance must have a very high degree of

stability so that frequent calibrations are not required.

For the type of experiment performed in the Ship Division of NPL, a wide range of force measurement

is required and many multi-component balances have to conform to certain hydrodynamic requirements, for instance to fit into a given shape. Hence many different

balances are constructed and sometimes have a life

limited to one set of experiments only. A balance

therefore should be cheap to produce and have little or no interaction.

From these considerations a new type of balance

has been built, the shear plate balance (3) which

Fig. 9. Simple flexure

unit based on shear plate

principle.

measures the linear strain resulting from shear. Its advantage is that the end fixing conditions are not

critical. The simple type of flexure unit shown in Fig. 9

has excellent characteristics and is insensitive in all

directions other than the measuring direction. A

combination of such individual units may measure up

to six components of loading (Fig. 10).

Fig. 10. Prototype six-component balance made up

from simple flexure units.

Acknowledgment

The work described has been carried out at the

National Physical Laboratory and the author is

indebted for help to the staff of the Ship Division of

NFL, in particular to members of the Equipment

Group.

References

D. A. DREW, "SlipringsA Review", Strain,

Vol. 2, No. 4, 1966, p.35-41.

H. B. BOYLE, "Method of reducing signal

inter-action of a strain gauge balance", Strain, Vol. 4,

No. 2, 1968, p.39-41.

H. B. BOYLE, "The shear plate as a

multi-com-ponent balance", Strain, Vol. 4, No. 2, 1968,