Manual.pdf

Soundcard Based Dynamic Balancing Machine for Model Gas

Turbine

Miklós T. KONCZ, HUNGARY, (kmiklos@vnet.hu)

ABSTRACT

Balancing of the rotor is one of the most important crucial key elements of the gas

turbine construction. In this article the author will present a new method for

balancing model gas turbine. The main aim was to implement a sophisticated

balancing machine for homebuilders without large investment, complicated

electronics and mechanics. Every balancing machine consists of two parts: mechanics

and electronics and/or software. The design concept of the mechanical part is to

provide easy balancing of complete turbine. This in situ, soft suspension, balancing

system (balancing in own support) helps to correct the unbalance problems coming

from bearings and shaft misalignments (angular and parallel). The other advantage

of this system is implementing the balancing instrumentation virtually without

electronics. Only an electronic switch provides the selection between left and right

signal of the sensors. All measurement functions are implemented by software with

digital signal processing, average workshop personal computer and low cost

soundcard. This project has not completely finished yet, but it can probably help for

other gas turbine enthusiasts all over the world.

Keywords : dynamic balancing, soft suspension, in situ balancing, gas turbine, jet

engine, soundcard, RC, digital signal processing, dynamic unbalance, static

unbalance, couple unbalance

INTRODUCTION

Balancing of the rotor is one of the most important key elements of the gas turbine

construction. If the rotor unbalance grade is G0.4 (ISO 1940/1 the strictest grade for

gyroscopes) and the rotor weight is 0.5kg, it can causes 0.017gmm unbalance (the

grade relative to the weight of rotor and to the revolution speed).

[

gmm

]

9549

[

]

Equation 1 Maximum allowable unbalance calculation from grade

It means that there is a 0.017g weight at 1mm radius (but its RPM out of the

maximum service speed range of the standard). This unbalance makes the following

force:

( n

)

Equation 2 Unbalance caused force

The value of the force generated by eccentricity in the example it is 2.5N at

117000RPM. This unbalance should be split into two planes according to the mass

distribution of the shaft. This relatively high force effects on the bearings and the

bearing reacts to the main shaft. It can cause vibration, shorter lifetime (fatigue),

deformation, power degradation, fraction and can be dangerous for life, despite of the

fact that this is the lowest grade from the standard. There is no exception the precise

balancing has to be done for every gas turbine.

In the case of rotating shaft the unbalance causes periodical forces to the suspension

and the periodicity corresponds to the rotational speed or with other words it is

synchronous with rotational speed (first order). In order to balance the rotor the

vibration of shaft revolution frequency should be selectable in the balancing

instrument. This reduces the disturbance caused by the noise, harmonics, bearings and

blade frequencies etc.

The unbalance is radial in their line of action and it is a vector quantity. It has both

size and direction. The direction can be characterised by the phase between the

unbalance vector (from the centre of the shaft) and a vector to the reference point at

the shaft (from the centre of the shaft).

The general dynamic unbalance consists of the static (single plane unbalance) and

couple unbalance. The former is when the mass centre line is not the same but parallel

with the rotational axis. Only this kind of balance exists in disk shape structures. It

can be eliminated by only one compensating weight.

Figure 1 Static unbalance

The later is when a pair of weight is at the two ends of the shaft but opposite side of

one to other (180°). The rotor is in static balance, but the centrifugal forces will

produce a moment about the centre of mass when the rotor turns. In the case when

only couple unbalance exist the mass centre line cross the shaft axes at the mass

centre point.

Figure 2 Couple unbalance 1

1 http://www.balancemaster.com/Balancing_explained.htm

The couple unbalance can be compensated by two weights, which were put to

counteract the couple unbalance at two planes.

The ideal balancing task is to reduce the inhomogeneous mass distribution caused

forces by putting or removing weights along the shaft, but the most cases are enough

to use two or more correction planes. In the balancing system the vibration produced

by unbalance are measured at two planes and after the compensation weight and place

are calculated to the correction plane from the measurement. Unfortunately the

unbalance at the one side can cause shaft movement around the mass centre point of

the rotor and it makes movement at the other side. These phenomena called cross

effect, because the unbalance at one side can not be treated independently from the

other side.

In this article simple software based balancing system for model gas turbine including

mechanics will be described. The mechanics is as so important part as the electronics

and software. The most sophisticated signal processing method without reliable and

precise mechanics can not achieve proper result. Therefore the first part of the article

will be about the mechanics and the design rules for the sensors and the second part

will be about the PC software, its advantages and further possible enhancements.

LITTLE HISTORY

János Horváth initiated and supports this project. János has a very good machinery

factory and he is a "ancient” modeller since 1969. He started his hobby with

helicopters and he has seen jet engine for 3-4 years, and at the first sight he has fallen

in love with it. He has built by his own a KJ-66, and put into a small ALBATROS.

Currently, he is constructing a GRIPEN 1:5.5, and he wanted to build a 120-160N

trust jet engine for it. We started to work together to tune and enhance his Phoenix

Mk-4 and the FADEC. The first, should be solved thing, was the balancing of the

turbine (In my viewpoint, it is very important basic element of GT construction). I

built the electronics for the balancing machine, but we were not satisfied with the

result. It has ambiguous readings (power LED strobe) at the low level of unbalance.

(In my opinion the original mechanics works near the resonance frequency; it is nor a

hard and neither soft suspensions machine). I decided we should improve the

capabilities of the balancer. Currently we have an own in situ (complete turbine), DSP

based balancing machine, but we have to do lot of things to enhance the machine and

to try them.

János and his friends organise the World Masters in 2005. We believe in that we can

learn from our friend all over the world including members of GTBA. Maybe World

Master will increase the popularity of the GT technology in Hungary. Nowadays, only

few people build GT propelled aeroplane in Hungary and I think that János is a leader

in building own GT engine.

THE SENSORS

Two kinds of sensor are used in the balancing machine what convert the mechanical

movement and the position information to electrical signal. The first is the vibration

sensor and the second is the position reference sensor.

Vibration sensors

Sony Ò 80mm in diameter loudspeakers with 50mm magnet in diameter was selected

for the vibration sensors. The sensor rod should be glued with epoxy resin to the

membrane of speaker. It has more advantages than disadvantages for this application.

Advantages of using loudspeakers as vibration sensor in balancing machine:

• Low impedance, especially at low frequency range (where they are used in

balancing machine).

• Low sensitivity for the hum (50 and 60Hz), it is consequence of low impedance.

• Velocity sensitive sensor (U ~ v).

• Low high frequency sensitivity vs. accelerometer. It comes from acceleration is

the differential of velocity. This means that it suppress the high frequencies noise.

Figure 3 Using loudspeaker as a vibration velocity sensor

• It is relatively soft suspension; complete mechanics with turbine has low

resonance frequency. The balancer is used above its resonance frequency.

• It does not require signal conditioning and power supply; it has floating output

(independent from earth to avoid earth loops).

• Low cost, easily replaceable.

• Not fragile.

• It has not high temperature dependency (Piezo has).

• Free angular movement, about ±20°.

• Off-the-shelf available.

Disadvantages of loudspeakers:

• Low output voltage vs. piezo sensors.

• High sound (large surface, microphone) and ambient or base vibration sensitivity.

• Membrane is very volatile, and the rubber ring is an ageing component.

• Over the free angular range it can squeeze and cause phase problems if it is small

degree, or amplitude and phase problems if it is large.

Figure 4 Fixing of the speakers

Position reference sensor

A coil of disassembled relay (12V) was chosen for position reference sensor.

Advantages of using inductive revolution sensor for balancing machine:

• Relatively low impedance, especially at low frequency range (where it is used in

balancing machine)

• It does not require signal conditioning and power supply; it has floating output

(independent from earth to avoid earth loops).

• It has pure sine-wave signal output.

• It provides contactless measurement.

• It uses the built-in magnet ring of the rotor.

• It does not have hysteresis.

• It is not disturbed by ambient light.

• It is very simple and cheap.

• Changing the distance between the magnet and the sensor can control the signal

level.

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