METHOD AND DEVICE FOR INTRODUCING A CABLE INTO A CABLE GUIDE TUBE

Abstract: A method and device for installing a cable, more in particular a glass fibre cable, into a duct of a channelization system, the consideration which has led to this being based on the fact that the dragging working, considered from the cable inlet end, as caused by a flow of compressed gas effected in a relevant duct portion will increase according to a non-constant function and, dependent on the geometry and the length of the duct portion, can be insufficient over the initial part of it to compensate the friction forces exerted on the cable locally. According to the invention it has now been suggested to fulfil such a compensation function by exerting on the cable a pushing force working extend­ ing to the point where the dragging working will be sufficient to compensate the friction forces.

Description: The invention generally relates to a method and a system for introducing (installing) cables, such as e.g. optical glass fibre cables, into a channelization system of guide tubes, in general designated as "ducts", which are disposed, as a rule in the ground, according to a predetermined arrangement. According to a usual technique cables are installed by utilizing tensile force. The amount of the required tensile force, which has to remain smaller than a permissible value in connection with the mechanical properties of the cable, depends on the geometry of the channelization system (in which as a rule curves and/or windings occur), on the friction forces between the cable and the duct, and on the properties of the cable. Without taking into account the stiffness of the cable the following items are of importance for the required tensile force: the friction forces due to the mass of the cable, and the friction forces caused by the tensile stress occurring in the cable (in connection with the curves and/or windings of a relevant duct portion). The former friction forces give rise to a tensile force which increases linearly with the length of cable installed. The latter friction forces give rise to a tensile force which increases exponentially with the number of curves or windings. This imposes an important restriction on the maximum cable length which can be installed in one go. Because of the exponential increase of the required tensile force it can be tried to see to it that the cable tension will be as low as possible. This involves that the friction forces caused by the mass of the cable have to be compensated locally. This means that the installation force has to be exerted divided over the whole length of a cable section to be installed. In a method serving for that purpose a flow of compressed gas (compressed air) is effected from the inlet end of a relevant duct and directed to the outlet end of the same, a relevant cable being introduced into the inlet end. A similar method is known from the European patent 0108590. This patent also discloses a device for employing such a method, which device is provided with a cable injection unit with in it a hollow, substantially rectilinear cable lead-through channel with an entrance end and an exit end for leading in and leading out a cable which has to be introduced into the relevant duct, which cable injection unit is further provided with a gas channel, which debouches into the cable lead-through channel and via which compressed gas can be supplied to the cable lead-through channel, as well as with a set of wheels mounted opposite to each other and partly reaching into the lead-through channel, and serving to move on a cable, disposed between these wheels and touching them, in the direction of the exit end. This known technique is meant for installing lightweight, flexible optical fibre cables. The flow of compressed air led through a duct exerts a drag force working on a cable introduced into the duct, due to which such a cable will be dragged through the duct to its outlet end. As appears from this known technique the velocity of the flow of compressed air is practically linearly dependent (increasing) on the difference in pressure there is between the inlet end and the outlet end of a relevant duct portion. The length of the duct portion over which a cable section can be installed at one processing stroke is in this case limited to about 200 m, even though it is expected in the above-mentioned patent specification that said length can be extended to about 300 m for a cable with a weight of 3 gms/m and when the difference in pressure is about 55 psi (≃ 0,38 MPa). The two wheels forming part of the cable injection unit according to the aforesaid patent specification exclusively serve to compensate the opposing forces exerted on the cable and caused by the difference in pressure there is between the interior and the outer environments of the injection unit. However the cited known technique has the shortcoming that it is not suited for the installation of considerably thicker and consequently stiffer cables into ducts of moreover greater length, than mentioned above. The first object of the present invention is to provide for a method for introducing a cable into a cable duct, which is a scaling up and an improvement of the method as known from the European patent application EP 0108590. The scaling up means that the method allows also introducing of ordinary cables, i.e. generally with larger diameters and greater stiffness, in a duct using the drag of a gas flow; the improvement means that moreover larger lengths of cable can be installed at one processing stroke. The consideration which subsequently has led to the present invention is based on the fact that the pressure gradient caused in the duct, and consequently the dragging working, is not constant along the duct, and, dependent on the length of a relevant duct, may be too small to compensate the friction forces exerted on the cable in a first part of the duct. In fact for given cable, duct, and pressure drop there will always be a limiting value for the length of the duct, above which this cable can not be installed in this duct making use of only the drag forces of the flow of a compressed gas. If the length of the duct is larger than this limiting value, then in a first part of the duct the pressure gradient, and accordingly the drag forces on a cable being installed will be too low to overcome the friction forces, which would be exerted on the cable when being installed. In the second part of the duct up to the end of the duct the drag forces will be amply sufficient to compensate said friction forces. Starting from this consideration the invention implies a method for introducing a longitudinal element along a previously installed guide tube of the sort designated as duct with an inlet end and an exit end, whereby the longitudinal element is led into the inlet end and a flow of gas is effected from the inlet end and directed to the exit end of said duct by inserting a compressed gaseous medium into the duct at the inlet end for exerting on the element a drag force distributed throughout the length of the element then existing in the duct so as to further advance the element there into, wherein: the longitudinal element is a cable with an intrinsic stiffness, the length of the duct is greater than a given limiting value below which the drag force exerted onto the cable is, once said cable has been led into the duct inlet end, by itself sufficient, at any point along the duct length, for overcoming the friction forces, so that there is a deficiency in friction forces compensating effect of said drag force in a first part of the duct starting from its inlet end, and the cable is introduced into the inlet end of the duct by pushing forces, which are exerted near the inlet end, to an extent allowed by the intrinsic stiffness of said cable, in a manner such as to be larger than the forces required for overcoming opposing forces on the cable caused by a pressure difference existing at the inlet end of the duct between the inside and outside of the duct as a consequence of said inserting of the compressed gaseous medium at the inlet end, and to be effective over said first part of the duct in order to completely counter said deficiency in friction forces compensating effect of said drag force in said first part. In other words, because of the fact that a cable with a certain own stiffness can be pushed in its longitudinal direction, such a cable can be installed by means of the drag force working of the flow of a gas in a duct longer than said limiting value by adding the extra method step of the exertion of pushing forces near the duct inlet in order to overcome the friction in the first part such that the cable being installed can reach the second part of the duct where the drag forces are large enough to compensate the friction forces which are exerted on the cable in the duct. Thus it appeared possible to install, at one processing stroke, a cable into a duct (with curves and windings) over a length of about 700 m by utilizing the cable stiffness of cables, more in particular optical fibre cables, used in practice. A further object of the present invention is to provide for a system, in which a cable with a certain stiffness can be introduced into at least one cable duct section, which has a length greater than said limiting value, in one processing stroke. To this end the invention implies a system according to claim 5. In a prefered embodiment of the invention the system comprises further a first coupling unit in accordance with claim 6, which is coupled to the outlet end of the first duct section in order to control the strong flow of compressed gas at the end of said duct section, and to lead the cable into a desired direction. In this prefered embodiment also it is made use of the intrinsic stiffness of the cable being installed. A combination of such a coupling unit with the cable advancing means may be very advantageous when used in a tandem arrangement in order to install a cable into more than one consecutive duct sections, still in one processing stroke. Therefore in a further prefered embodiment the invention implies a system according to claim 7. Still further prefered embodiments of the system according to the invention have been summarized in the remaining subclaims, the special features and advantages of which will be described below. Summarizing the invention results in the following advantages: quick and simple installation without the need of a pulling rope, even for conventional, not necessary glass fibre cables; small cable strain during the installation process; relatively long length of installation per cable advancing device; and efficient and simple extension of system for the installation of a cable in several consecutive duct sections in one processing stroke by means of tandem working of a number of cable advancing devices in combination with couple units; during such a tandem working the relevant cable advancing devices are capable of working in series with minimal control needed and with intermediate distances which are practically independent of the route of the channelization system. The invention will be further explained hereinafter with reference to the drawing in which Fig. 1 shows a set of diagrammatic representations with the aid of which the essences of the method according to the invention will be explained; Fig. 2 shows a diagrammatic representation of a duct portion with contained in it a cable section on which a dragging working caused by a flow of compressed gas is exerted; Fig. 3 shows a diagrammatic representation to illustrate the pushing force working which, according to the invention, is exerted on the cable; Fig. 4 shows a diagrammatic cross-sectional view of an embodiment which is illustrative of a device for employing the method according to the invention; and Fig. 5 shows a perspective view of a possible embodiment, taken apart, of a cable injection unit according to the invention. The essence of the method according to the invention will be further explained in the first instance with reference to figure 1. An important consideration underlying the present invention implies that the variation of the pressure in a duct portion having a length l and a pressure at the beginning respectively the end of this portion of p(o) respectively p(l), is a non-linear function of the distance (x) from the beginning of said portion and can be represented for an isothermal flow by so that the pressure gradient dp/dx which is a measure for the dragging working exerted on the cable by the flow of compressed gas can be written as dp dx = p(o)² - p(l)² 21p(x) It is noted that what is expressed by this formula (2) in fact applies to an "empty" duct portion. When such a duct portion contains a cable, the whole will become considerably more complicated. However, in practice it has appeared that what is expressed by the formula (2) is a useful valuation. With the aid of the technique described in EP-A-0108500 it will be possible to compensate the friction forces, caused by the mass of the cable, locally. According to this known technique it is assumed, and this also appears from the article "A radically new approach to the installation of optical fibre using the viscous flow of air" by S.A. Cassidy et al. in Proc. IWCS(1983)250, that the variation of the pressure between the beginning and the end of a duct portion is linear. In other words, according to these known techniques the pressure gradient dp dl considered over the whole length l of the duct portion will be constant. It is also derived that the total drag force per unit of length F l (dragging working caused by the compressed air blown into the duct portion) can be represented by F l = π dp dl r k · rd . See in this connection Figure 2, in which a duct portion with a length l and a cable contained in it are diagrammatically shown. The arrow represents the flow of compressed air blown along the cable. The friction force F w l to be compensated per unit of length can be represented by F w l = f · W . in which f stands for the coefficient of friction between the cable and the duct, and W for the cable weight per unit of length. The diagrammatic representations of figure 1 show the variation of the pressure gradient dp dx as a function of the place x along a duct portion, it being assumed that p(o) = 8.5 bar and p(l) = 1 bar (absolute). The diagrams in this figure are drawn for illustrative duct portions of a length of 437 m respectively 782 m and for a1 and a2 it is assumed that the pressure gradient dp/dx will be a constant p(o) - p(l) l whereas for b1 and b2 it is thought that the pressure gradient dp dx will not be constant according to the formula (2). Moreover, on the basis of the formulae (3) and (4) the arrow on the right indicates the pressure gradient fW πr k r d , which is indispensable to compensate the friction force exerted on the cable (f = 0.25; W = 0.76 N/m; r d = 13 mm; r k = 4.85 mm). In the cases a1 and a2, where the pressure gradient is assumed to be constant, it happens that when the duct lenght l = 782 m, the pressure gradient will just be sufficient to compensate the friction forces, whereas this pressure gradient will be larger than necessary for such a compensation, when the duct length l = 437 m. From the variation of the curves b1 and b2, which are illustrative of the formula (2), the following appears: for the duct length l = 437 m the pressure gradient considered over the whole duct portion will always be sufficient and near the inlet unit just sufficient to compensate the friction forces exerted on the cable; for the duct length l = 782 m the pressure gradient considered over a relatively large part of the duct portion will not be sufficient to compensate the friction force exerted on the cable locally. From the above it can be concluded that the above-mentioned technique will be insufficient to install per injection unit a cable length longer than a certain limiting value, in the aforesaid case 437 m, into a duct. In other words, on the basis of the assumption made in connection with the case a2 and the possible installation length calculated from it, it will not be possible to install a cable into a duct portion with such a length by making use of only a flow of compressed gas. According to the present invention it has now been suggested to exert, from the inlet end of a relevant duct portion and by making use of the stiffness of a cable to be installed, a pushing force working on such a cable. If somewhere in the duct the dragging working of the flow of compressed gas blown into the same is still insufficient to compensate the friction forces, this function will be fulfilled by such a pushing force working. In this way the length of the duct portion over which a cable can be installed by means of only one injection unit can be considerably extended, which is also established by experiment. A factor of two with regard to a known blowing-in technique has proved to be possible. Illustrative of a cable used in practice is a cable with a stiffness of about 0.9 Nm². Such a stiffness will be sufficient to hinder "buckling" of the cable when pushing it into a duct in such a way that the cable will not press itself fixed against the wall to any extent, and on the other hand the friction forces particularly arising consequent on the stiffness of the cable in curves and/or windings in the relevant duct portion still prove to be compensable. By employing the method according to the invention it has proved to be possible to install a cable into a duct portion with a length of more than 700 m. Briefly summarized the method according to the invention results in the following advantages: quick and simple installation, small cable strain during the installation process; considerable extension of the length of installation, which can be effected by means of only one injection unit; and several injection units can be used in series (tandem connection) in an efficient and simple way. Illustrative of the invention is the following example: The duct portion in this example has a length of 667 m and an inside diameter of 26 mm, and right-angled curves at distances of 150, 250, 400 and 600 m from the cable inlet end, which curves have a radius of curvature of 1 m; moreover, the duct portion has a winding variation with a period of 4 m and an amplitude of 5 cm. The cable to be installed has a diameter of 9.7 mm, a weight of 0.65 N/m and a stiffness of 0.9 Nm². The coefficient of friction between the cable and the inner wall of the duct is 0.25. Compressed air supplied by a compressor with a capacity of 75 l/sec (atmospheric) and a maximum working pressure of 7.5 bar (overpressure) is used for installing the cable. By means of the formula (2) a value of 5.34 * 10⁸/p p d /m can be calculated as an estimation for the pressure gradient. Only after 386 m this pressure gradient will be sufficiently large to generate aidragging force sufficiently high to compensate the effective friction force fW of 0.19 Nm (consequent on the existing curves and windings the effective cable weight per unit of length has become larger due to the stiffness of the cable) exerted on the cable locally. The variation of the necessary pushing force F has been numerically calculated as a function of the distance x (from the inlet end of the duct portion) as illustrated in Figure 3. It has appeared that the cable can be installed into the portion in question by means of the combination of such a pushing force working and a flow of compressed air. For the sake of completeness it is noted that the various negative effects (curves hinder the development of pushing force) and positive effects (moving the position where the pushing force is zero because of the fact that in the part after it the compressed air exerts an effective dragging working) can play a part. Moreover, a change of the geometry can have considerable consequences. For example, if the number of windings and/or curves is decreased, the length of the duct portion into which the cable can still be installed by means of the combination of a flow of compressed air and a pushing force working, will be higher. Figure 4 is illustrative of an embodiment of a device for employing the method according to the invention. More in particular figure 4 shows a diagrammatic cross-sectional view of such a device. A device of that type or a cable injection unit is in its generality designated by 1. This unit comprises a housing 2, in which a substantially rectilinear cable lead-through channel 3 is formed, which channel 3 has an inlet end 4 through which a cable 26 can be led into the injection unit 1, and an outlet end 5, which has been adapted to be coupled to a revelant duct 6, into which a cable has to be installed, while at the same time forming a gas-tight closing. An inlet tube 7 debouching into the lead-through channel and meant to be connected to a source of compressed gas (a compressor which is not shown in the figure) is designated by 7. In the case of a usual flow resistance of the relevant duct portion and a conventional compressor with a capacity of 75 ltr/ sec and a maximum working pressure of 7.5 bar (overpressure), a flow of compressed gas (flow of air) in the order of magnitude of 75 ltr/sec will stream via said inlet tube. A set of wheels 8, 9, 10 and 11 is pivotally mounted in a housing and partly reaches into the lead-through channel 3. The set of wheels 8 and 9 is supported by a frame 12, which is pivotally coupled to a piston rod of a piston 13, which is movably mounted in a pneumatic cylinder 14. As diagrammatically shown in figure 4, the two wheels 8 and 9 can be set turning via a transmission mechanism by means of a pneumatic motor 15, which is mounted on the housing. Opposite to the embouchure of the gas inlet tube the cable lead-through channel is bounded by a streamlined small pipe 16, which ensures that the course of a cable disposed in the lead-through channel will remain substantially rectilinear, in spite of the strong flow of compressed gas. In other words, the cable will be prevented from being blown to a "buckling" in that portion, which would seriously hinder the introduction of the cable into the duct. A cable led into the lead-through channel via the inlet opening 4 forms via a diagrammatically drawn washer 17 a gastight closing. In consequence of the compressed air supplied there will be a difference in pressure between the interior and the exterior of the housing. Owing to this a force working will be exerted on the cable disposed in the lead-through channel 3, which takes the opposite direction with regard to the desired direction of movement of this cable. When compressed air is supplied to the cylinder 14 and to the motor 15, this "oppositely directed" force working will be compensated. A pneumatic motor has the advantage that the driving couple provided by it is proportional to the pressure caused in the housing; moreover, when compressed air is supplied, a pneumatic motor can be slowed down to a standstill without harmful consequences, and also be kept in such a state (if desired for a long time). The latter is a particular advantage when several cable injection units are used in series (or in tandem connection). Within the scope of the present invention, however, the pneumatic motor is considerably more powerful than necessary for compensating the above-mentioned "oppositely directed" force working. Illustrative is e.g. a motor capable of providing a force working which is thrice as large as the force working necessary for the above-meant compensation. By means of a motor thus dimensioned it will be achieved that over a length zone extending over a certain distance from the beginning of the relevant duct portion, a pushing force working is exerted on the cable disposed in the duct. As explained with reference to figure 1, such a pushing force working adds to the dragging working to neutralize the local friction force working, caused by the friction between the cable and the inner wall of the duct, and by the weight of the cable, at those places where the pressure gradient respectively the dragging working caused by the flow of compressed air is still too small to compensate the friction force working referred to. In order to ensure that the pushing force working will be effective it is essential that the relevant cable has a certain stiffness. The cables used in practice meet this requirement. In practical situations it has appeared that the length of the duct portion over which a cable can be installed by means of only one injection unit, can be extended by a factor of about two, when use is made of such a pushing force working. The speed at which the cable is introduced into a duct can be regulated by means of a pressure regulator (not shown in figure 4). To promote the engagement between the wheels and the cable to be moved on by them, each of these wheels has a hollow tread, which is provided with transversal knurls. The advantage of such a construction is that said wheels cannot get filled up with material from the cable sheath and pollutants, if any, taken along by the cable, and that slipping, even when the cable sheath is covered with a lubricant, will be effectively hindered. In the embodiment represented in figure 4 two driven wheels and two "counter pressure" wheels are used. However, if need be, also an embodiment with one respectively more than two driven wheels and one respectively more than two counter pressure wheels can be made use of. In the latter case it is advisable to use at least two pneumatic cylinders, each of the pneumatic cylinders operating two wheels at most, which are in this case mounted in a frame, which is pivotally coupled to the relevant piston rod. The pneumatic cylinder/piston combination serving to cause the pressing working has the advantage that in the case of a certain supply of compressed air the pressure force working exerted on the cable will be substantially constant. That means that notwithstanding variations in the thickness of the cable, this pressure force will remain constant. Moreover, an upper limit corresponding to the maximum working pressure of the supply compressor of 7.5 bar, has been set to the pressure force working exerted. This means that the permissible squeezing pressure of a cable used in practice will not be exceeded when using the pneumatic cylinder/piston combination. By means of the above-described construction the motor can be brought to a standstill by slowing down and bringing to a standstill the cable moved on by that motor. Figure 4 also shows a coupling or closing unit 18. Such a unit comprises a housing 19, which leaves space to a room 20, which is connected to the outer environments via an inlet opening 21, an outlet opening 22 and a compressed gas outlet pipe 23. The inlet end has been adapted to form a gastight coupling with the final part of a duct portion 24, at the beginning of which use is made of a cable injection unit similar to the one shown in figure 4. The velocity at which the compressed air flows into the room 20 via the inlet end 21 is substantially determined by the magnitude of the flow of compressed air. In the case of values of about 7.5 bar and 75 ltr/sec this velocity will be in the order of magnitude of 150 m/sec. For reasons of security and for the protection of an injection unit located near the outlet opening 22 the compressed air outlet pipe 23 has been designed in such a way that the compressed air supplied will be slowed down, via a widening initial part 25, without causing whirls, and led away, via this outlet pipe 23, to a place which is safe for the operating staff. As an alternative said outlet pipe 23 can have been dimensioned in such a way that the velocity at which the compressed air streams out has been reduced to a safe value. The cable 26 installed in the duct portion 24 can be led to the next cable injection unit, in this case the injection unit 1, via the outlet opening 22, which is provided with a lining 27′ capable of slowing down quick particles which may come along with the cable and/or the flow of compressed gas. As a rule the opening 22 is closed by means of a valve 27 pivotally mounted on the housing 19, which valve will attach itself firmly to the outside of the housing due to the acceleration of the flow of air there, when there is no cable at that place, in such a way that the valve will offer protection against the particles moving on at a high speed. By means of the cable supplied this valve can be opened, in which case the lining 27′ will then fulfil the function of the valve 27. After that this cable can be installed via the injection unit 1 into the duct portion 6, which follows after the duct portion 24. For that purpose the two injection units have to work in series or in tandem. In connection with such a tandem working each cable injection unit 1 and coupling or closing unit 18 which has to be used in that process, has been designed as two parts which are detachably fixed to each other. This is illustrated in figure 5, in which these parts 38 and 39 are detached from each other. In this figure the relevant parts are indicated as follows: 2; 2′ component parts of the housing; 3 cable lead-through channel; 4; 4′ component parts of the inlet end; 5 outlet end (one half of it); 6 duct; 7 compressed air inlet tube; 8 wheel with hollow tread provided with transversal knurls; 13 cable pressure mechanism; 15 pneumatic motor; 16 streamlined pipe (one half of it); 17 recess for one half of a washer; 26 cable; 28 on/off switch compressed air; 29 reducing valve for controlling the speed of the pneumatic motor; 30 pressure gauge for measuring the pressure supplied to the pneumatic motor (indicative of the motor torque); 31 reversing switch for reversing the direction in which the motor runs; 32 pressure gauge for measuring the pressure at the entrance of the duct portion 6; 33, 33′ fixing clamps for detachably fixing the parts 38 and 39 to each other; 34 centring bosses. For the sake of completeness it is noted that in the embodiment according to figure 5 the motor 15 drives only one wheel 8, which wheel can co-operate with a wheel (not shown in figure 5) mounted in the part 39 opposite to the former wheel. Moreover, the drive mechanism 13 in this embodiment is provided with a pressure spring (not shown in the figure), due to which the latter wheel is pressed against the cable. It is obvious that this pressure mechanism can be replaced by the construction comprising a pneumatic cylinder/piston combination and described in connection with figure 4. The special construction of the wheels described in what precedes makes it possible that a cable disposed between them will be moved on effectively without causing damage to the cable sheath, and no matter whether such a cable sheath is covered with a lubricant or not. The latter is an advantage when the tandem method is employed. For in that case a cable provided with a lubricant is supplied to an injection unit. For a further improvement of the working of the wheels it may be advisable to provide the outer sheath of a cable to be installed with corresponding transversal knurls. These knurls may also result in an enhancement of the dragging working caused by the flow of compressed gas.

Claims: 1. A method for introducing a longitudinal element (26) along a previously installed guide tube (6) of the sort designated as duct with an inlet end and an exit end, whereby the longitudinal element (26) is led into the inlet end and a flow of gas is effected from the inlet end and directed to the exit end of said duct (6) by inserting a compressed gaseous medium into the duct (6) at the inlet end for exerting on the element (26) a drag force distributed throughout the length of the element (26) then existing in the duct (6) so as to further advance the element (26) there into, wherein: the longitudinal element (26) is a cable with an intrinsic stiffness, the length of the duct (6) is greater than a given limiting value below which the drag force exerted onto the cable (26) is, once said cable (26) has been led into the duct (6) inlet end, by itself sufficient, at any point along the duct length, for overcoming the friction forces, so that there is a deficiency in friction forces compensating effect of said drag force in a first part of the duct (6) starting from its inlet end, and the cable (26) is introduced into the inlet end of the duct (6) by pushing forces, which are exerted near the inlet end, to an extent allowed by the intrinsic stiffness of said cable (26), in a manner such as to be larger than the forces required for overcoming opposing forces on the cable (26) caused by a pressure difference existing at the inlet end of the duct (6) between the inside and outside of the duct (6) as a consequence of said inserting of the compressed gaseous medium at the inlet end, and to be effective over said first part of the duct (6) in order to completely counter said deficiency in friction forces compensating effect of said drag force in said first part. 2. Method in accordance with claim 1, wherein the cable (26) is introduced into an auxiliary guide channel (3) disposed upstream said duct inlet end and connected thereto, in which auxiliary guide channel (3) said compressed gaseous medium is inserted and wherein said pushing forces, the direction of which coincides with the desired direction of movement of the cable (26), are exerted on the cable section installed in said auxiliary guide channel (3) in an amount of a few times larger than the forces required for overcoming said opposing forces. 3. Method in accordance with claim 2, wherein forces transversely exerted on the cable (26) are made use of to cause said pushing forces, and wherein for these transverse forces a value has been chosen which is practically equal to, but smaller than a transverse force value corresponding with the maximum squeezing pressure permissible for the relevant cable (26). 4. Method in accordance with claim 3, wherein a lubricant is applied to the outside of the cable (26), which lubricant is maintained on the outside of the cable (26) upstream of the location where said transvers forces are exerted on the cable (26). 5. System for introducing a cable into a cable duct, which system comprises a first duct section (6) with an inlet end and an outlet end, in which a flow of gas can be effected from the inlet end and directed to the exit end and a relevant cable (26) can be led into the inlet end, a first cable advancing device (1), coupled to the inlet end of said first duct section (6), comprising: cable advancing means (3, 4, 8--15) for advancing a cable (26) downstream into the first duct section (6), gas inserting means (7) for inserting compressed gas in order to cause said flow of gas, wherein: the cable (26) to be introduced has an intrinsic stiffness; the length of the duct section (6) is greater than a given limiting value below which the drag force exerted onto the cable (26) is, once said cable (26) has been led into the duct section inlet end, by itself sufficient, at any point along the duct section length, for overcoming the friction forces, so that there is a deficiency in friction forces compensating effect of said drag force in a first part of the duct section (6) starting from its inlet end; and the cable advancing means (3, 4, 8,--,15) of said first cable advancing device (1) comprise means adapted for exerting pushing forces on the cable (26) being introduced, to an extent allowed by the intrinsic stiffness of said cable (26), in a manner such as to be larger than the forces required for overcoming opposing forces on the cable (26) caused by a pressure difference existing at the inlet end of the duct section (6) between the inside and outside of the duct section (6) as a consequence of said inserting of the compressed gaseous medium at the inlet end, and to be effective over said first part of the duct section (6) in order to completely counter said deficiency in friction forces compensating effect of said drag force in said first part. 6. System according to claim 5, which further comprises a first coupling unit (18) coupled to the outlet end of the first duct section (24), which coupling unit (18) comprises means for coupling said unit (18) to the outlet end (21) of the first duct section (24) in a gastight way; means (23) for diverting a flow of gas, leaving said outlet end (21), from the direction of a cable (26) being introduced; means (27′)for slowing down particles, which may be carried along by said gas flow; means for leading a cable (26) being introduced into a predetermined direction. 7. System for introducing a cable into a cable duct, comprising one or more further duct sections (6) similar to and aligned with said first duct section (24), according to claim 5, wherein in said system each of said further duct section (6) is sandwiched between a further cable advancing device similar to the first cable advancing device according to claim 5 and a further coupling unit similar to the first coupling unit according to claim 6, the cable advancing means of a further device belonging to a next further duct section are immediately contiguous to and aligned with the predetermined direction of the cable leading means of the coupling unit of the immediately previous duct section in such a way that said cable advancing means can receive automatically a cable (26) led by said leading means. 8. System according to claim 5, 6 or 7, in which A. said cable advancing means comprise a hollow, substantially rectilinear cable lead-through channel (3) with an entrance end (4) and an exit end (5) for leading in and leading out a cable (26) which has to be introduced into the relevant duct section (6), at least one pair of wheels (8, 10; 9, 11) mounted opposite to each other for moving on a cable (26) disposed between these wheels into the direction of the exit end (21) of a relevant duct section, a motor (15) coupled to at least one of said wheels for providing a driving couple to it, B. said gas inserting means comprises a gas channel (7), which debouches into the cable lead-through channel (3) and which is adapted for inserting compressed gas into said lead-through channel (3) between said wheels and the exit end (5) of said lead-through channel (3), wherein a piston (13) has been provided, which is on the one hand movably mounted in a pneumatic cylinder (14) and on the other hand coupled to at least one of said wheels (8; 9), in such a way that when compressed gas is supplied to the pneumatic cylinder (14), transverse forces will be exerted on the cable (26) disposed between the wheels (8, 10; 9, 11), the motor (15) is a pneumatic motor capable of providing said driving couple, which is larger than the driving couple, which has to act on the cable (26) to compensate the difference between the pressure inside and the pressure outside the cable lead-through channel (3), whereby said pushing forces are exerted by the wheels on the cable (26) as a consequence of the cooperative effect of the driving couple and the transverse forces. 9. System according to claim 8, wherein the pneumatic motor (15) and the pneumatic cylinder (14) are adapted to be coupled to a common source of compressed gas. 10. System according to claim 9, wherein said gas channel (7) is adapted to be coupled together with the pneumatic motor (15) and the pneumatic cylinder (14) to a common source of compressed gas, and between the connection for this source of compressed gas and the pneumatic motor a reducing valve has been placed for controlling the speed of the motor. 11. System according to one of the claims 8-10, wherein two wheels (8, 9) are mounted in a frame (12), which is pivotally mounted to said piston (13), which wheels can turn and are adapted to cooperate with two wheels (10, 11) disposed opposite to the former wheels and partly reaching into the cable lead-through channel, and the pneumatic motor is coupled in a driving connection to one of the pairs of wheels. 12. System according to one of the claims 8-11, wherein means (16) are disposed between the wheels and the exit end (5), and also at the place where the gas channel (7) debouches into the cable lead- through channel (3), which means are adapted to prevent the flow of gas supplied via the gas channel (7) from having a negative effect on the working of said pushing forces. 13. System according to one of the claims 8-12, wherein the wheels have a hollow tread, such a tread being provided with knurls substantially running parallel to the axis of rotation of the relevant wheel. 14. System according to one of the claims 8-13, wherein the cable advancing device (1) is composed of two parts which are detachable fixed to each other, in such a way that said device can be removed from a cable extending through the same. 15. System according to claim 13 in combination with a cable (26) engaged therein for being introduced, wherein the outer sheath of the cable (26) is provided with knurls, the knurls extending in a transverse direction with regard to the longitudinal direction of the cable axis and wherein said knurls of the hollow tread of said wheels of said device mesh with said knurls of said cable (26). 16.System according to claim 6 or 7, wherein said coupling unit (18) comprises a mainly hollow housing (19), a duct inlet opening in said housing for receiving and coupling the outlet end of the cable duct section (24); an outlet pipe (23), extending from the interior of said housing (19) through said housing, which pipe is suitable for leading away from said housing a high velocity flow of compressed gas, which flows from the outlet end of the cable duct section (24) into the housing as a consequence of the insertion of compressed gas at its inlet end, in a controlled manner; a cable outlet opening (22) in said housing mainly directly across from the duct inlet opening, which cable outlet opening is provided with a cover valve (27) pivotally mounted on the outside of the housing (19), which cover (27) is capable of closing firmly said outlet opening (22), when there exists under-pressure inside the housing near said cover valve as a consequence of said leading away from said housing said high velocity flow, and which cable outlet opening is further provided with a lining (27′) capable of slowing down quick particles dragged along with the high velocity flow of compressed gas and/or a cable (26) being introduced in said duct section (24), the interior of said housing having at least partly a profile as to lead the front end of a cable (26) being introduced into the direction of said cable outlet opening (22). 17. System according to claim 16, wherein the coupling unit (18) is composed of two parts which are detachable fixed to each other, in such a way that said unit can be removed from a cable (26) extending through the same.

Inventors: DE JONG, CORNELIS LEENDERT, GRIFFIOEN, WILLEM
Applicants: KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1988
Main classification: G02B6
IPC codes: G02B6, H02G1