GB2198236A - An arrangement for pneumatically ascertaining the inside diameter of a cross-sectionally circular passage - Google Patents

Abstract

The invention is used in connection with a test piece 1, for example an injection line for diesel engines. By means of a rolled up thin wire 6, a displacement ball 7 is passed along the passage in the test piece and measuring air from a constant pressure source 2 is then forced through the passage in the test piece. The throttling effect of the measuring throttle formed at the displacement ball 7 in the test piece is evaluated by means of a gauge 13 placed between a fixed throttle 12 and the measuring throttle. The location of the displacement ball in the longitudinal direction of the test piece passage is varied by the wire 6 to ascertain the inside diameter at positions along the test piece passage. Various ways are indicated for compensating for a systematic measurement error when assessing very narrow passages. The arrangement is controlled by a computerised evaluating unit 41. <IMAGE>

Description

An Arrangement for ascertaining the internal diameter of a circular cross-section Passage The invention relates to an arrangement for pneumatically ascertaining the inside diameter of a circular cross-section passage in a test piece, in which the passage is at least indirectly connectable to a source of constant either negative or preferably above-atmospheric pressure, comprising a variable-length pressure-tightly encased thread, ire or the like having at its free end which projects into the passage in the test piece a cross-sectionally likewise circular displacement body which almost completely fills the cross-section of the passage, it being possible to hold the displacement body stationary at any desired position along the length of the passage, and also comprising a device for ascertaining the -throttling effect of the throttle formed by the displacemellt body and the passage, as a measuremen; of the inside diameter of the passage at that location.

Such an arrangement is know, for instance, from German Laid-Open Patent Specification DE-OS 19 53 316. In the case of the prior art measuring arrangement, the intention is to test small hollow cylindrical parts of an injection jet for diesel engines for dimensional stability.

Where these substantially hollow cylindrical parts are concerned, a mechanically accurately machined bore is provided, the inside diameter of which is in the range of about 4mm. The length of this bore, in other words the length of the passage in the test piece is of the order of 6 to 10 times the inside diameter; therefore, the passage in the test piece is itself relatively short and is, furthermore, absolutely straight. In the case of the known measuring arrangement, the test piece is fitted into the measuring arrangement with its passage set vertically, being sealed at both the inlet and outlet ends. From above a conical or double-conical displacement body was let into the bore to be measured by being suspended from a wire.The fixed top end of the wire was provided on the inside of a metal bellows and its height could be varied so that the displacement body could be altered in its longitudinal position in the passage in the test piece. Via the wire inlet end, air at a constant pressure could be fed into the passage. Together with the bore to be measured, the displacement body constitutes a throttle which, according to the size of the inside diameter of the bore, constitutes a more or less considerable throttle effect for the air flowing through the passage. The air flowing past the test piece was trapped and fed to an air quantity meter which in turn operated on the principle of a meter measuring quantities of bodies in suspension.After an appropriate calibration of thp air meter. the measured quantity of air passing through it could be assessed directly as a measurement for the inside diameter of the test piece. A disadvantage of this prior art measuring instrument is that by reason of various structural circumstances, it is limited to relatively short and straight passages in test pieces, which can be incorporated into the measuring system in pressure-tight fashion on both the inlet side and the outlet side.

For the rest, measurement of the quantity of air for determining diameters is dependent upon the viscosity of the air, in other words upon the environmental air pressure and the air temperature. Therefore, these influences must be excluded from the reckoning, which is a troublesome process.

The present invention seeks to provide a pneumatic measuring arrangement by means of which it is possible to use it for checking very long tubular and above all curved test pieces, a relatively speedy ascertainment of diameters over the entire length of the test piece being feasible, variations in viscosity of the air being largely irrelevant.

According to the invention, there is provided an arrangement for pneumatically ascertaining the inside diameter of a cross-sectionally circular passage in a test piece, the passage being at least indirectly connectable to a source of constant either negative or preferably above-atmospheric pressure, comprising a variable-length pressure-tightly encased thread, wire or the like having at its free end which projects into the passage in the test piece a cross-sectionally likewise circular displacement body which almost completely fills the cross-section of the passage, it being possible to hold the displacement body stationary at any desired position along the length of the passage, and also comprising a device for ascertaining the throttling effect of the throttle formed by the displacement body and the passage, as a measurement of the inside diameter of the passage at that location, wherein a) the displacement body is constructed as a small displacement ball; b) a row of wire pressure-tightly encases the smallest possible amount of dead space and with a definitely controllable adjusting drive and a rotary position transmitter is provided, the circumference of the reel being tangential to the beginning of the passage in the test piece; c) between the constant pressure source and the connection of the passage in the test piece there is a fixed throttle, a pressure gauge connected to the portion between the fixed throttle and the passage connection to measure the pressure in this zone to provide a measurement for the inside diameter of the passage.

By reason of the spherical shape of the displacement body, this is readily suitable also for curved passages. By virtue of the fact that the wire supporting the displacement ball is rolled up onto a pressure-tightly encased wire reel, the volume preceding the throttle point constituted by displacement ball and test piece passage can be kept very small despite the fact that the wire can travel a long way, so that the measuring pressure can be very rapidly adapted to the particular diameter and throttling conditions which are appropriate, a rapid and continuous measurement being possible. Consequently, also relatively long test piece passages can be assessed in an acceptable period of time.

By reason of the upstream provision of a fixed throttle in the connection to the constant pressure source, a pressure divider circuit is provided, there being in the portion of line between the fixed throttle and the measuring throttle formed by the displacement ball and the test piece passage a pressure proportional to the inside diameter of the test piece passage.

The volume of this portion of the line in which the wire reel is also incorporated should be as small as possible for a low-inertia and rapid measurement process, which means in other words that the dead space should be as small as possible. On the other hand, the cross-section within this portion of line should be sufficiently large that no throttling actions likely to falsify the measured value can occur within this portion of the line.

Viscosity influences of the air are largely avoided in that, as a measured signal, the pressure in this aforementioned portion of line is evaluated, so that it is possible to dispense with a viscosity-dependent measurement of quantity. Otherwise, a measurement of quantity would be very cumbersome in the case of very long and curved test pieces because with curved test pieces, the location of their other end is completely undefined.

Even better elimination of the effect of viscosity of the air can be achieved by applying a fluidic bridge circuit of throttles, one of which is a measurement location constituted by a displacement ball and the passage, the pressure gauge being a differential pressure gauge disposed in the bridge diagonal located transversely of the direction of flow.

Some embodiments of the invention will now be described by way of examples and with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic view of a first example of embodiment of a pneumatic measuring arrangement, partly in an oblique view and partly shown as a circuit diagram; Fig. 2 is a diagrammatic illustration of the embodiment shown in Fig. 1 in an extended arrangement with the relevant indication of the pressure pattern along the passage to which the air is applied; Fig. 3 is a similar view of another embodiment with a pneumatic bridge circuit of fixed throttles for complete compensation of viscosity fluctuations in the air; Fig. 4 is an example of an embodiment of the pneumatic measuring device modified in comparison with the embodiment shown in Fig. 3, and in which a compensating wire is disposed in the outlet part of the test piece;; Fig. 5 is a further modified example of embodiment in which a measurement is conducted on a master tube simultaneously with measurement of the test piece; Fig. 6 shows an axially parallel cross-section through a wire reel and its casing; Fig. 7 is an axially perpendicular cross-section through the wire reel according to Fig. 6 taken on the line VIT-VIT but without the casing; Fig. 8 is an axial view of the opening wire reel according to Fig. 6, taken on the line VIII-VIII; Fig. 9 is a detailed view enlarged some hundred times in comparison with the view in Fig. 6, showing the outer periphery of the wire reel (detail IX in Fig. 6);; Fig.10 shows an axially parallel sectional view and Fig. 11 shows an axial view of the clamping connection between the hollow shaft of the wire reel and the associated fit-in shaft of the adjusting drive; Fig.12 shows a measurement diagram with a plurality of diagram curves and a results curve obtained from repeated measurement with a reversed clamping position; and Fig.l3 is a measurement diagram of two different test pieces of the same configuration but of different inside diameter with the associated indication of the pattern of curvature over the length.

The diagrammatic view in Fig. 1 of a first and simple embodiment of measuring arrangement shows as a constant pressure source 2, an overpressure source with a compressed air system 3, a pressure regulating valve 4 and a pressure storage means 5. Connected to the constant pressure source is a fixed throttle 12 which is already a component part of the measuring arrangement. An essential mechanical constituent part of the measuring arrangement is a pressure-tightly encased wire reel 8 which has the smallest possible dead space and which will be dealt with in greater detail hereinafter.At this juncture, let it merely be mentioned that the wire reel is provided with a definitely controllable adjusting drive 10 and that the rotary position of the wire reel can be established by a rotary position transmitter 11 capable of indicating the rotary position of the wire reel individually over a plurality of revolutions.

Wound onto the body of the wire reel is a thin wire 6 which carries at its front end a displacement ball 7, the diameter of which is somewhat smaller than the inside diameter of the test piece 1, a thick-gauge injection line for diesel engines. This is a spatially curved relatively long tubular bod which in some places is very sharply curved, comprising a relatively narrow passage which in the most frequently encountered instances is in the region between 1.5 and 3 mm. To accommodate the test piece there is, mechanically upstream of the wire reel 8, a clamping slide 32 mounted for displacement parallel with the wire intake direction. At the place where the wire emerges from the casing, the wire reel 8 comprises a mouthpiece 42 onto which one end of the test piece can be fitted in sealing-tight mannor.For this purpose, the clamping slide 32 comprises, in approximately the same position as the mouthpiece 42, a clamping prism 33 into which the test piece can be clamped by means of a claw. With the test piece mounted securely on it, the clamping slide can be pushed onto the mouthpiece in a manner which is not shown in the drawings pressed thereon in sealing-tight and durable fashion (direction of displacement 34). The entire wire reel 8 including its casing is adapted to rotate about the axis of rotation of the wire reel (push-in shaft 22) b a small angle.

When the wire reel is in the untensioned condition, the lift-off spring 35 can pivot it in an anti-clockwise direction, so that a pressure-tight connection between the wire reel or its casing on the one hand and the pressure feed line on the pressure intake 43 remote from the mouthpiece 42 on the other is broken.

Furthermore, by reason of the lift-off spring 35, after a test piece has been removed from the measuring arrangement, a limit switch 36 is actuated, its actuating member bearing at least indirectly on the casing of the wire reel. This limit switch serves to monitor the clamping status of a test piece; until such time as the test piece is properly and tightly clamped into the measuring arrangement, the limit switch 36 will not have yet reached its response threshold.

In consequence, various functions of the measuring arrangement are still inoperative. For example, the supply of air from the constant pressure source is blocked by a shut-off valve, not shown; furthermore, the adjusting drive 10 for the loire reel and the entire evaluating unit 41 are immobilised.

By means of the wire reel 8 and the wire 6, the displacement ball 7 can be let to any desired distance into the passage in the properly clamped test piece. Jointly with the passage in the test piece, the displacement ball 7 constitutes a measuring throttle. The throughflow resistance of this measuring throttle is indirectly a measurement of the inside diameter of the passage in the test piece. This throughflow resistance is, in the measuring arrangement according to the invention and in the case of the example of embodiment shown in Fig. 1, measured in that the pressure between the fixed throttle 12 and the measuring throttle constituted b the displacement ball 7 and the test piece passage, is measured with the pressure gauge 13.

Regardless of the quantity of air flowing out, the constant pressure source 2 delivers a constant preliminary pressure. By means of a series circuit of throttles, namely a fixed throttle 12 and the aforesaid measuring throttle, the pressure is divided into a total of three stages, namely the constant pressure at the constant pressure source 2 in the region upstream of the fixed throttle 12, a measuring pressure in the region between the fixed throttle 12 and the aforesaid measuring throttle and a relatively low pressure in the region downstream of the measuring throttle. The aforesaid measuring throttle is - as stated - measured by the pressure gauge 13 and converted to an electrical signal which is fed to an evaluating unit 41.This consists essentially of a computer 38 having various peripheral instruments, namely a measuring instrument interface, a programme memory 39 and a plotter 30. With a very slow and steady passage of the displacement ball through the test piece passage and under almost stationary pressure conditions, it is possible at the same time to prepare a measurement record which with corresponding conversional calibration can be made to show directly in mm diameter of test piece passage, plotted in relation to the length of the test piece. Instead of a continuous pass, a "stepped" passage of the displacement ball through the test piece passage is also possible. in small steps; at the individual and separate measuring locations, it is necessary to observe a short waiting time for equalisation of pressures or to await stationary pressure conditions.The choice of the step length must be ascertained empirically from time to time and fed into the computer or its programme.

Particularly in the range of curvatures, a narrow step sequence will be expedient because, this was found to be a first result of measurements using the measuring instrument according to the invention, the passage cross-section in the range of curvature narrows by reason of the bending of the tube.

In Fig. 2, the embodiment shown in Fig. 1 is still further simplified and shown elongate, jointly with an associated pressure diagram in which the pressure pattern along the elongate passage to which the air is applied. In the portion of the diagram located underneath the constant pressure source 2 and to the left of the fixed throttle 12 there is a first step-down in pressure caused by the effect of the pressure regulating valve 4 but this does not in any way concern the measuring principle. The pressure obtaining between the pressure regulating valve 4 and the fixed throttle and which is also stored in the meantime in the pressure storage means 5, is the constant air pressure delivered by the constant pressure source 2.The area of the diagram which is on the right of the fixed throttle corresponds to the measuring pressure; this can fluctuate according to the fluctuation in the diameter of the test piece passage, which is shown by the horizontal broken lines above and below the solid pressure line. In the part of the test piece passage between the wire reel 8 and the measuring throttle consisting of the displacement ball 7 and the test piece passage there is, as illustrated by the falling graph shown for this part of the line, a more or less linear pressure drop by virtue of the throttling effect of the relatively narrow test piece passage which is, furthermore, also narrowed by the cross-section of the wire 6 protruding into it. At the actual measuring throttle there is a considerable pressure drop which is illustrated by a lack of steadiness in the pressure line.

In the portion of the diagram to the right of the measuring throttle or displacement ball 7 there is indeed a continuous pressure drop by virtue of the throttling effect of the relatively narrow test piece passage, but by virtue of the larger inside cross-section the incorporated wire is not present - the pressure in this portion of the passage drops more slowly over the length than in the portion to the left of the displacement ball 7. However, this effect only occurs perceptibly when the test piece passages have a clear diameter markedly less than 2 mm, for example when the inside diameter is 1.5 mm.

If such narrow test piece passages are measured by the arrangement according to the invention, then in the diameter trace over the length of the test piece passage there is an apparent conicity which does not actually exist, at least not in the size shown in the diagrams. This is attributable to the fact that with increasing depth of immersion of the displacement ball in the passage in the test piece the throttlingly acting part of the wire increases so that an increasingly greater throttling effect occurs which cannot, however, be exclusively ascribed to the displacement ball 7.

However, as stated, this effect is only of significance from the point of view of measurement if the passage cross-sections are less than 2 mm inside diameter.

This systematic measurement error which resembles an apparent conicity can be compensated in various ways. In the case of the embodiment shown in Fig. 2, this is achieved in that prior to measuring a test piece 1, a comparative measurement is carried out using a master tube 19 which in terms of passage length in the test specimen and also with regard to the inside diameter of the passage, is in accordance with the test specimen but which is, however, rectilinear and has an exactly known and exactly constant inside diameter over its entire length. Since we are concerned here with a master tube produced solely for measuring purposes, it can also be produced in a very thin eaue material which should be readily workable and in which the required criteria can be satisfied quite easily.Even with such a master tube, an apparent conicity is measured which does not in reality exist. A comparison of the measurement curve, namely of the pattern of diameters over the length in the case of the master tube on the one hand and the corresponding measuring curve of the test piece 1 on the other then gives us an actual comparison of diameters for each longitudinal position of the test piece passage.

The inside diameter/passage length curve drawn by measurements using the master tube can be stored in the measuring programme separately for each type of test piece so that a once-only measurement of a corresponding master tube need be carried out; when measuring one specific test piece,m then it is possible to carry out the above-mentioned comparison immediately during measurement and in the computer so that even when measuring narrow passage cross-sections, the deviations from the master tube can be plotted right away. In this way, therefore, the systematic measurement error which resembles an apparent conicity can be compensated for by comparison with a master tube measurement.

The embodiment shown in Fig. 3 illustrates another possibility of compensating for this systematic measurement error. However, before going into this in greater detail, it is intended first of all to describe yet another difference in the embodiment shown in Fig. 3 compared with that shown in Figs. 1 and 2.

In fact, this relates to the bridge circuit 14 of throttles 15, 16, 17 and the measuring throttle constituted by the displacement ball 7 and the passage in the test piece.

This bridge circuit of altogether four throttles is provided instead of the single fixed throttle 12 in the embodiment shown in Figs. 1 and 2. In the direction of flow 18, the passage to which measuring air is applied divides into two passages which then extend parallelto each other, in which in each case there are serially disposed two throttles namely on the one hand the two throttles 15 and 17 and on the other the two throttles 16 and the measuring throttle with the displacement ball 7. The pressure gauge is a differential pressure gauge 13'; it lies in that diagonal of the bridge which is crosswise to the direction of flow 18 and measures the difference in pressures between the two flow passages in the region between the two throttles in question.Thanks to this bridge circuit 14, also any influence of viscosity of the air on the inlet side is substantially excluded, so that slight variations in air temperature or environmental air pressure changes will not affect the results of measurement.

The possibility of compensating for the systematic measuring error which is made possible by the measuring arrangement shown in the example illustrated in Fig. 3 lies in the fact that the test piece 1 is measured in two different positions twice, one after the other. This two-fold measurement in a different clamped position is illustrated in Fig. 3 by the two different views of the test piece 1. The graphs obtained thereby must, however, be plotted into a common diagram from different directions so that corresponding longitudinal positions of the test piece passage are located at the same place in the chart.

These two tracers, of which one has a rising while the other has a falling pattern, are shown in Fig. 12 in a broken line and in a dash-dotted line respectively. From the values of these two measuring passes, then, a reel chart in which the systematic measuring error does not appear can be ascertained, to be shown in a solid line in Fig. 12. It should also be mentioned in connection with Fig. 12 that the relevant mean values of the measured values of the two measuring passes are indicated by short horizontal lines disposed centrally between the two corresponding measurement lines.The physical stimuli to arithmetical ascertainment of a graph in which there are no systematic measurement errors are then the following: - With a single measuring pass, the measurement error is diminishingly small when the displacement ball is still right at the start of the passage and the length of wire which has entered the passage is likewise diminishingly small.

- The error within a measurement pass is at the greatest when the dasplacemert ball is entirely at the end of the passage in the test piece, the passage having the supporting wire 6 pulled through its entire length.

- All in all between these two measurement points in the length of passage in the test piece, the actual values which are to be obtained arithmetically are above the diameter values measured by the two passes.

- The actual diameter value which is to be ascertained arithmetically is at every position along the length of the test piece passage a constant amount above the arithmetical mean of the two measured diameters; in other words, the actual diameter diagram graph which has to be ascertained arithmetically is equidistantly above the middle line between the two "inclined" measurement traces.

- This amount of rise corresponding to the equidistant interval, when compared with the relevant arithmetical mean, can likewise be ascertained from the values measured during the two passes, for which purpose various possibilities are available, according to the computer outlay. The amount of rise is half the apparent difference in diameters at the start or end of the passage in the test piece; the amount of rise can be calculated as an arithmetical mean of these two half apparent diameter differences. Even more exactly statistically, it is possible to ascertain the amount of rise by forming the arithmetical mean from the ordinate differences at all measurement points and of all measurement points, and then halving it; this corresponds to the statistical mean distance between the two measured traces.

These physical stimuli have been confirmed as realistic the first time measurements were conducted using the measuring instrument according to the invention. The advantage of the method of compensation which has just been described, with two measuring passes and involving a different clamped position, lies in the fact that by virtue of measuring twice, any measurement errors are mutually reduced in their end effect by the fact that averages are taken. It is also advantageous that no special master tubes have to be made available which, in the case of many types of test piece, could result in the need for a considerable number of master tubes. However, with the method described, a disadvantage is the need for double measurement, which at least in the case of a large number of test pieces can become a burden in terms of time.

In the case of the embodiment shown in Fig. 4, a further possibility of compensating for the systematic measurement error is illustrated for this reason. In fact, there is also in the area to the right of the displacement ball 7 a wire, a compensating wire 20, which has the same diameter as the wire 6 which supports the displacement ball.

Thus, the same cross-sectional conditions are created in front of and behind the displacement ball, so that the resistance to flow through the passage in the test piece is unaffected by the position of the displacement ball 7. In the pressure graph shown in Fig. 4, this is illustrated in that the pressure drop before and after the displacement ball 7 has the same angle of inclination. It is true that in Fig.

4 the test piece 1 is shown as a straight tube, but nevertheless it could also be a spatially curved test piece; the elongate illustration was chosen solely with regard to the local pressure graph shown underneath in Fig.

4.

The advantage of the method of compensation illustrated in Fig. 4 resides in the fact that only a single measurement pass and thus only a relatively small amount of time is needed. A disadvantage of the method shown in Fig. 4, however, is the additional fact of needing to work with the compensating wire 20 which renders handling difficult.

Particularly in a case where the compensating wire is connected to the supporting wire 6 rigidly in the region of the displacement ball 7, in the event of a collision with the compensating wire 20 which hangs out, uncontrolled forces can easil be exerted on the supporting wire 6 which damage it, for example cause it to be kinked. Thus, smooth running of the displacement ball and the supporting wire 6 in the passage inside the test piece is made difficult.

Finally, Fig. 5 shows a further possibility of compensatinç for the aforesaid systematic measurement error.

In fact. in the bridge circuit 14' shown therein, a wire reel 8, 8' is incorporated into each of the two branches of the bridge, with a supporting wire 6, 6' and a displacement ball 7,7' . However, instead of a test piece, there is associated with the other branch of the bridge an identical elongate master tube 19 while in the lower branch of the bridge the actual test piece 1 is clamped. Compensation, then, is brought about in that a comparative measurement with a master tube is performed simultaneously. This type of compensation is similar to that shown in Fig. 2; there, the comparison of measurement with that of the master tube is, however, carried out after a time lag whereas in the example shown in Fig. 5, the comparison occurs simultaneously.The advantage of a simultaneous comparative measurement lies in the fact that comparison takes place at identical environmental conditions, which allows a greater accuracy of measurement to be anticipated. A disadvantage in the embodiment shown in Fig.

5 is the higher cost of equipment; two wire reels 8, 8' which corresponding adjustment drives and rotary position transmitters are needed; under certain circumstances, both wire reels may be driven by one uniform and common adjusting drive and their rotary position accordingly ascertained with just one single rotary position transmitter.

In conjunction with Figs. 6 to 11, it is intended hereinafter to deal in greater detail with the structural design of the wire reel 8 or its casing 9. The actual winding part of the wire reel consists of a hollow shaft 21 and a disc-shaped winding drum 8 shrunk onto it and into the outer periphery of which there are screwthread-like winding grooves 44. In the case of the example of embodiment illustrated, a special shrunk-on ring is provided for the purpose. In these winding grooves, a wire 6 is laid. What is important is that the diameter of the winding should on the one hand be of equal magnitude for all the different wire reels so that the correlation of wire lengths according to the angular position of the wire reel should be the same for all the different wire reels.Furthermore, it is important that the winding groove 44 be wide enough in the bottom of the groove, so that the wire can drop effortlessly into the bottom of the cylindrical groove. The hollow shaft 21 and the disc-shaped wire reel 8 are enclosed by a pot-shaped casing 9 in one side and by a cover 9' , sealed in air-tight fashion by O-rings in the direction of the shaft.

Tangentially to the periphery of the wire reel 8 there is in the casing 9 a wire pull-off bore which merges into a mouthpiece 42 for the pressure-tight connection of the test piece. For clearly defined supporting of the displacement ball 7 inside the mouth-piece 4 there is a small bore there which is of smaller diameter than the displacement ball so that it constitutes an abutment for the displacement ball when it is in the retracted position.

The transition bore from the mouthpiece 42 to the circumference of the wire reel is wider than the winding width of the wire 6. However, since the diameter of the wire 6 is very small, being for example 0.15 mm, the distance between the winding grooves 44 is very small and the entire winding width is likewise only very small. The diameter of the wire reel or of the groove bottom of the winding grooves is so chosen that a conveniently calculable number of length units of wire, for example 22 cm, can be wound on per revolution. At the outer periphery of the wire reel 8, this is enclosed by the casing 9, just a very small gap being observed; the gap is substantially smaller than the wire diameter so that under all circumstances the wound-on wire is held positively within the winding groove.Also for a different reason, care is taken to see that the wire reel 8 fills the cavity in the casing as completely as possible; in fact, the minimum possible dead space should be left within the casing. In fact, the volume of air in that portion of the line between the fixed throttle 12 (Figs. 1 and 2) or between the throttle 16 (Figs. 3 to 5) on the one hand and the measuring throttle constituted by the displacement ball 7 and the test piece passage on the other, should be as small as possible so that as the displacement ball proceeds from one measurement position to the next, the pressure situation can adjust as quickly as possible to the new measurement location and the gap conditions which obtain at that location.

If the volume of the line is too great in the portion mentioned, then it is necessary to wait for a long time until stationary conditions have become established for the new measuring condition.

If a per se desirable large number of steps were involved, then this would lead to a very long measuring time.

The ire reel 8 including its casing 9 is constructed as an interchangeable part, rather like a cassette. In fact, for measuring different inside diameters of test pieces, a plurality of completely identical wire reels are kept available together with their casing, differing only by the diameter of the displacement ball at the front end of the wire 6. One and the same displacement ball is capable only of handling a relatively small range of diameters so that to cover a wide measurement range, it is necessary to have ready a relatively large number of wire reels with displacement balls graded in narrow diameter steps. All the various types of connections or links of wire reels to the measuring instrument are therefore constructed as quick release connections.

Firstly, there is the rotary connection of the shaft of the wire reel to the adjusting drive 10 or rotary position transmitter 11. As stated, this is in the form of a hollow shaft 21 which can be fitted onto the fit-on shaft 22 of the adjusting drive or rotary position transmitter. As only relatively small forces or torques have to be transmitted, it can be a force-locking coupling which is closed without clearance by clamping and then rapidly disengaged. In order not to have to exert unintended peripheral forces on the wire reel by the closure of the clamping arrangement, as this might lead to tearing off the retracted displacement ball 7 or to a build-up and kinking of the rolled up wire, actuation of the clamping connection is so contrived that - related to the fit-on shaft 22 - it can be operated solely by axially and/or radially directed hand forces.For this purpose, there is in the fit-on shaft 22, a longitudinal groove into which a thrust strip is moveably fitted. The thrust strip is secured against falling out in a radial or axial direction by resilient wire rings which fit in small peripheral slots. At the bottom end, the thrust strip has on the side towards the bottom of the groove a small bead which serves as a tilting edge 24. At the upper free end of the fit-in shaft there is a thrust screw 25 disposed at a right-angle to the axis and which furthermore extends diametrally. The milled head of the thrust screw projects beyond the outer circumference of the fit-on shaft and its end face so that the head of the thrust screw is accessible manually to quite an extent.By rotating the thrust screw, the free end of the thrust strip can be pushed outwards so clamping the fit-on shaft 22 or thrust strip 23 inside the hollow shaft 21. When the clamping connection is disengaged, the pressure on the thrust strip is relieved so that it can be retracted by the spring rings.

From the fluidic point of view, there is on the one hand a connection between the wire reel and the test piece via the aforesaid mouthpiece 42 and the clamping slide 32.

There is also provided on the outer periphery of the casing 9 a nose in which there is a pressure initiating passage 43 or a corresponding mouthpiece; the mouthpiece lies in an axially parallel plane enclosing the axis of rotation of the wire reel so that upon a pivoting movement of the wire reel about its axis, the mouthpiece can bear on a corresponding mating surface and be free from transverse movement. This point has already been dealt with in conjunction with the description of the embodiment shown in Fig. 1.

Insertion of the wire 6 into the passage in the test piece is carried out while maintaining the pressure in the measuring arrangement; by virtue of the pressure differential past the displacement ball 7, the wire is drawn pneumatically into the passage in the test piece. However, the displacement ball may well become jammed by an extreme narrowing of the cross-section inside the passage, perhaps due to a dent, a particle of dirt or the like. Upon further movement of the adjusting drive in the inserting direction, there is then an accumulation of wire which causes the wire 6 to spring out of the winding grooves 44 of the wire reel. In order to be able to recognise such a dangerous build-up of wire early on, there are on the inside of the periphery of the casing 9 insulated contact strips 26.These are constructed as small let-in bolts which through an interposed sleeve-like insulation, are supported in a corresponding bore in the casing 9. The contact surface of the contact strip ;;hiciì faces the periphery of the wire reel is constructed flush with the rest of the hollow cylindrical surface of the casing, being for example turned out jointly with this surface. The contact strip is connected to a contact thrust piece disposed on the bottom face of the casing and co-operates with a resilient contact piece which is fixed in the measuring device.From there, an electrical conductor leads to the evaluating unit 41 which in the case of an earthing contact with the contact strip, immediately causes a reversal of the direction of rotating at the adjusting drive 10 so that the displacement ball is withdrawn immediately out of the passage in the test piece. The test piece passage must then be cleaned; by rolling small balls of different sizes loosely through the passage, it is necessary at least provisionally to test to see how large is the inside cross-section at the narrowest point of the passage in the test piece.

Within the torque connection between the fit-on shaft 22 on the one hand and the adjusting drive 10 on the other there is a torque limiting friction clutch which slips in the event of the ball becoming jammed during retraction or in the event of the displacement ball striking the retracted extreme position and the adjusting drive continuing to run; consequently, upon retraction of the displacement ball or when the wire 6 is rolled up, the sensitive wire and its connection to the displacement ball 7 are protected. When the wire is unwound, a build-up of wire can result and this as stated - can be recognised in good time.However, so that the wire cannot be unwound too far - the wire 6 could be kinked radially outwardly at the clamping point at the reel end, so that it no longer lies flat in the winding groove 44 - the wire reel 8 is provided with a mechanical rotary travel limiter and this responds at least half a turn before the wire 6 is completely unwound. This rotary travel limiter is, in the example of embodiment shown in Figs. 6 and 7, provided in that machined into the end face of the wire reel 8 is a flat screwthread groove 28. A radial groove 29 is machined into the closely opposite axially perpendicular inside the face of the casing 9. At a point of intersection of the two grooves there is a locking ball 30. As the wire reel rotates in the casing 9, the locking ball 9 "screws" itself into the flat screwthread groove moving thereby radially inside the radial groove 29.

When the wire is completely rolled up, the ball is completely on the inside radially; when the wire is almost unrolled it is close to the outer periphery. At the outer periphery, the flat screwthread groove 28 is limited by an abutment pin 31 against which the locking ball 30 strikes, preventing further rotation of the wire reel 8. Thus the point at which the wire 6 is clamped at the reel end is effectively safeguarded against uncontrolled force influences and deformation.

In conclusion and with reference to the diagram in Fig.

13, it is intended to discuss a result of measurement which it was possible to obtain using the measuring arrangement according to the invention. In the upper part of the diagram in Fig. 13 there are two measurement traces which show the pattern of diameters of two injection lines of identical construction. The predetermined range of tolerances for the inside diameter of the injection lines is indicated by two horizontal lines adjacent to each of which there is an area of shading. The measurement trace shown in solid lines keeps substantially within the range of tolerances; when used in an engine, this injection line worked perfectly. In this connection, reference should also be made to the lower part of the diagram in Fig. 13 in which there is shown the pattern of curvature over the length of the test piece, indicated as a reciprocal of the radius of curvature.The injection line is shown as a composition of variously long portions of arc having different curvatures and straight portions. It is evident that in the region of the curvatures, the inside cross-section is markedly smaller than in the region of the straight portions; it is also evident that the narrowing of the cross-section is the more pronounced the more intensely curved the passage in the test piece is. In the past, it was assumed that in the case of thick-walled injection lines, the inside cross-section was negligibly small in the radius of curvature. At least, there was not hitherto any non-destructive method of measurement of justifiable cost which made it possible to measure this. The measurement trace shown in solid lines also reveals a conicity which does, however, keep substantially within the field of tolerances.The measurement trace shown in broken lines - as far as the straight portions of the injection line are concerned - does indeed indicate a substantially constant inside diameter, but in the straight portions this inside diameter is far beyond the range of tolerances. When used in an engine, this injection line proved to be disadvantageous; it was evident that the injection processes produced secondary splashing when this injection line was used.

In order clearly to show the accuracy of measurement which can be achieved by using the measuring apparatus of this invention, it should be mentioned that the width of the tolerance field in Fig. 13 amounts to two-tenths of a millimetre. Since the individual points of measurement are reproducible within a substantially narrow range, the premise can be adopted that the accuracy of measurement is at least within the range of one-tenth of the width of the field of tolerance, in other words in the range of + one hundredth of a millimetre. Such an accuracy of measurement was formerly impossible when using relatively simple and destruction-free means applied to spatially curved test pieces in which there were passages of such a length.

Claims (13)

1. An arrangement for pneumatically ascertaining the inside diameter of a cross-sectionally circular passage in a test piece, the passage being at least indirectly connectable to a source of constant either negative or preferably at above-atmospheric pressure, comprising a variable-length pressure-tightly encased thread, wire or the like having at its free end which projects into the passage in the test piece a cross-sectionally likewise circular displacement body which almost completely fills the cross-section of the passage, it being possible to hold the displacement body s+.ationars- at an desired position along the length of the passage, and also comprising a device for ascertainin the throttling effect of the throttle formed by the displacement body and the passage, as a measurement of the inside diameter of the passage at that location, wherein a) the displacement body is constructed as a small displacement ball, b) a row of wire pressure-tightly encases the smallest possible amount of dead space and with a definitely cuntrollable adjusting drive and a rotary position transmitter is provided, the circumference of the reel being tangential to the beginning of the passage in the test piece, and c) between the constant pressure source and the connection of the passage in the test piece there is a fixed throttle, a pressure gauge connected to the portion between the fixed throttle and the passage connection to measure the pressure in this zone to provide a measurement for the inside diameter of the passage.

2. An arrangement according to Claim 1, wherein fluidically downstream of the constant pressure source there is instead of the fixed throttle, a bridge circuit of throttles, one of which is a measurement location constituted by a displacement ball and the passage, the pressure gauge being a differential pressure gauge disposed in the bridge diagonal located transversely of the direction of flow.

3. An arrangement according to Claim 2, wherein in the bridge circuit there are fluidically parallel two reels of wire adapted to be driven simultaneously and at identical speed, and two test pieces, of which one is constructed to be of the same diameter and length as the other test piece, but being a rectilinear master tube of exactly known and constant inside diameter.

4. An arrangement according to Claim 1 or Claim 2, wherein - viewed from the wire inlet side of the test piece there is in that portion of the test piece passage which is beyond the displacement ball, a compensation ire of the same diameter as the ball supporting wire, the compensating wire corresponding at least to the length of the passage in the test piece and being immediately adjacent the displacement ball at all the measurement positions.

5. An arrangement according to Claim 4, wherein the compensating wire and the wire supporting the displacement ball are connected in one piece and merge continuously into each other at the displacement ball, the hollow bored displacement ball being threaded onto the wire and being glued or soldered securely to the wire at the transition point.

6. An arrangement according to Claim 4, wherein the compensating wire is, in comparison with the wire supporting the displacement ball, constructed as a separate piece of wire adapted to be pushed through the passage in the test piece by the displacement ball as the measurement position progresses.

7. An arrangement according to any one of Claims 1 to 6, wherein the wire reel including its casing is constructed in the form of an interchangeable cassette.

8. An arrangement according to Claim 7, wherein between the wire reel provided with a hollow shaft and a push-in shaft of the adjusting drive there is a circumferential backlash eliminating means operatively acting manually closeable or separable rotary entraining means which - in relation to the push-in shaft - can be actuated by exclusively axially and/or radially directed manual force.

9. An arrangement according to any one of Claims 1 to 8, wherein there is in the casing of the wire reel, on the inside of the periphery, at least one electrically insulated contact strip which contacts the wire which sprints up out of the roll on the wire reel in the event of the ball encountering a blockage.

10. An arrangement according to any one of Claims 1 to 9, wherein the wire reel is provided with a mechanical rotary travel limiter which responds at least half a revolution before the wire is completely unwound.

11. An arrangement according to Claim 10, wherein the rotary travel limiter is constituted by a flat screwthread groove on the one hand, provided with end stops and a radial groove on the other, in an oppositely disposed pair of axially perpendicular faces of the wire reel and casing and by a locking ball engaging a point of intersection of the two grooves.

12. An arrangement according to any one of Claims 1 to 11, wherein the adjusting drive and/or the constant pressure source can be engaged only when the test piece is properly fluidically and mechanically coupled to the wire reel.

13. An arrangement for pneumatically ascertainin the inside diameter of a cross-sectionally circular passage substantially as hereinbefore described and with reference to the accompanying drar;ings.