GB2050977A

GB2050977A - Tow bar

Info

Publication number: GB2050977A
Application number: GB8018086A
Authority: GB
Original assignee: Ateliers Mecaniques du Douaisis SA
Current assignee: Ateliers Mecaniques du Douaisis SA
Priority date: 1979-06-07
Filing date: 1980-06-03
Publication date: 1981-01-14
Also published as: FR2458411A1; DE3020823A1; GB2050977B; FR2458411B3

Abstract

A tow bar formed by parts 1a, 3 and 1b to interconnect a vehicle on tow, for example an aircraft on the ground, with a towing vehicle 5. The part 3 is tubular and carries strain gauges included in bridge circuits to give indication of the stresses of traction, compression and flexion the bar experiences. The resultant electrical signals are used to give visual and other indication to the drivers in both vehicles of the stresses detected so that the drivers can coordinate their actions to reduce the stresses if they are excessive. The tow bar can also include a frangible device which breaks and allows separation of the vehicles if the stresses exceed an upper maximum limit. <IMAGE>

Description

SPECIFICATION Tow bar This invention relates to a tow bar.

The invention relates particularly, but not exclu sively, to tow bars used for manoeuvring aircraft on the ground by means of a towing vehicle.

Generally a tow bar is attached to an aircraft at the level of the landing gear, and in order to protect the landing gear from excessive stress-due for example to a too sudden acceleration or deceleration of the towing vehicle, taking into account the inertia of the aircraft, orto a too wide a turn by the towing vehi clothe known tow bars are constructed in at least two sections joined by bolts, which sections move like shears duringtraction, compression or flexion of the bar. The bolts are made so as to break when they are strained beyond a certain level, corresponding to a predetermined maximum level of stress on the landing gear.

Thus, if the stresses on the landing gear become excessive, the security bolts break and the two sec tions of the tow bar come apart, and in the majority of cases the excessive strain on the landing gear of the aircraft is relieved.

Whilstthis system can protect the landing gear of an aircraft against incorrect manoeuvres, it does have a certain number of inconveniences.

One inconvenience lies in the fact that the shear ing bolts or the safety pins that replace them, give no possibility of controlling the amount of stress being experienced, until the moment of the break. Gener ally, the pilot of the aircraft and the driver of the towing vehicle, who must manoeuvre in accordance with one another without seeing each other and without seeing the tow bar, only become aware of an incorrect manoeuvre when this has already led to the snapping of the safety bolts.

Now, if the breaking of the bolts has the advantage of protecting the landing gear against most incorrect manoeuvres, it does in itself constitute an incon venience in two ways. Firstly, it frees the aircraft which may continue moving out of control under its own momentum, secondly, the changing of the shearing bolts takes a certain amount of time which is prejudicial to the full use of the aircraft and there fore costly because of the very high costs involved when a plane is immobilised on the ground.

In addition, it appears that this safety system at present in use, despite its breaking, does not always prevent stresses being placed upon the landing gear of the aircraft which are greater than what is admiss ible. This is the case, for example, when the tow bar is being used to push the aircraft. Here the breaking of the bolts does not prevent the thrust continuing.

According to the invention there is provided a tow - bar, comprising a bar provided at opposite ends with means for attachment to a vehicle on tow and to a towing vehicle respectively, means for constantly measuring stresses in at least one area of the bar, and means to render these measurements visible in driving cabs of both the towing vehicle and the vehicle being towed.

If desired, a safety system using bolts or security pins can be retained but the constant visualisation for the respective drivers of the two vehicles of stresses, for examples stresses of traction and/or compression and/or flexion, to which the tow bar is subject allows the drivers to act in synchronisation in order to maintain the stresses within acceptable limits, and consequently to avoid breaking the shearing bolts or other equivalent device, and to avoid the inconveniences that such a break may cause.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figs. 1 and 2 are respectively side and plan views of a tow bar formed according to the invention; Fig. 3 is a sectional view, from above, on an enlarged scale, of a test body of the tow bar in Figs. 1 and2; Fig. 4 is a diagram illustrating use of the tow bar in Figs. 1 and 2 joining a rear of a towing vehicle to a front support of landing gear of an aircraft, and Fig. 5 is a diagrammatic representation of a circuit for taking measurements and for representing those measurements visibly, with which the tow bar in Figs. 1 and 2 is provided.

Illustrated in Figs. 1 and 2 is a tow bar comprising two sections 1 a and 1 b, these being for example tubular and cylindrical with a common axis 2. The sections 1 a and 1 b are joined end to end in one piece by a test body 3 itself coaxial with the axis 2.

At one end, provided by the section 1 b, the tow bar carries a ring 4 which fits onto a complementary hook provided on a towing vehicle 5 (see Fig. 4). At its other end, provided by the section 1 a, the tow bar is provided with a coupler head 6 designed to be locked onto a front support 7 of landing gear of an aircraft 8 (see Fig. 4). The coupler head 6 is joined in known fashion to the corresponding end of the tow bar, i.e. to the end of section 1 a by shearing bolts 9.

The end of section 1a receiving the coupler head 6 has two parallel plates 10 and 11 which are horizontal when the axis 2 is itself horizontal. Between these plates fits a plate 12 which is integral with the coupler head 6. The three plates 10, 11, 12 are pierced with holes which coincide to receive the bolts 9.

Of course, the means of coupling the bar to the towing vehicle and to the vehicle being towed may be different without going beyond the compass of this invention as defined in the appended claims.

Likewise the bar could be of a different form, particularlywhen it is a question of towing vehicles other than aircraft on the ground.

The tow bar carries a set of wheels 13 on a horizontal axis perpendicular to axis 2. This set of wheels 13 allows easy movement of the bar on its own or when it is attached to the towing vehicle. These wheels can be retracted once the bar is attached to the towing vehicle and to the vehicle being towed (the retracted position is illuetrated in Fig. 1 and Fig.

4).

At the point of the test body 3, measurements are constantly taken of the stresses of traction or compression on the one hand and of flexion on the other, imposed on the bar during the manoeuvring of the aircraft by means of a towing vehicle. The stresses of traction or compression result from the pull or thrust applied to the bar by the towing vehicle and the aircraft. The stresses of flexion result from the fact that the coupler head 6 on the front support 7 of the landing gear of the aircraft is of such a kind that the tow bar is fast with the support7 which rotates about a vertical axis 15 underthe cockpit of the aircraft.

It should be noted that these measurements do not necessitate the insertion of a test body 3 between the two sections 1 a and 1 b of the bar and that they could equally be taken in a suitably chosen part of the bar having no such a test body, and, for example, in part of a continuous bar not comprised by the two sections 1 a and 1 b.

Nevertheless, the taking of the measurements on a test body 3-the idea being that this localises the stresses to be measuredSives the possibility of a much greater sensitivity in the measuring system and consequently greater precision in the information delivered to the respective drivers of the towing vehicle and of the vehicle being towed.

In the example illustrated, the test body 3, located closer to the coupling gear of the bar to the towing vehicle than to the coupling gear of the bar to the vehicle on tow, has a first section 16 which moves under traction or compression and in which the corresponding stresses can be measured, and a second section 17 which moves under the flexion and in which the corresponding measurements are taken.

If one refers more particularly to Fig. 3 one can see that the two areas 16 and 17 ofthe test body 3 are of a tubular, cylindrical form coaxial with the axis 2 but having different diameters with different thicknesses of wall.

In its section 16, the test body 3 has an external diameter dimension close to that of the sections 1 a and 1 b of the bar and an internal diameterdimen- sion close to the external diameter so that the section has a thin, tubular wall 18 sensitive to the stresses of traction and compression applied along the axis 2.

On the cylindrical external face of the wall 18 and half-way along it with respect to axis 2, are fixed strain gauges 19b and 19c of known kind respectively located in vertical planes on either side of axis 2, and strain gauges 19a and 19d of known kind respectively above and below the testing body 3 in horizontal planes on either side of the axis 2.

As Fig. 5 shows, these strain gauges are joined by a bridge, the gauges 19a and 19b having a common terminal joined to one of the terminals of a constant source V of electrical current. The gauges 19c and 1 9d have a common terminal joined to the other terminal ofthe source V. Aterminal common to gauges 19a and 19c is joined to one of the inputs of an amplifier A, of which another input terminal is joined to aterminal common to gauges 19b and 19d.

The amplifier A amplifies the variations in current to the terminals at the bridge of the gauges, to which these inputs are joined, and transmits these amplified variations to the equipment which renders them visible, for example on dials on a meter20 and an alarm located in the driving cab of the towing vehicle 5. At the same time they are also transmitted to a similar installation 21 located in the driving cab of the vehicle being towed, for example the cockpit of the aircraft.

Preferably, the amplifier A has peak detectors which trigger a visual and/or audible alarm in installations 20 and 21 when the stresses go beyond a predetermined threshold, which can be regulated according to the stresses admissible to the towing vehicle and the vehicle undertow.

The area 17 of the testing body 3, joined to area 16 by a ring-shaped wall 22 centred on the axis 2, has for its part an external diameter less than that of area 16, for example one half of the external diameter of wall 18, and an internal diametersuchthatthethick- ness of tubular wall 23 of the test body in this area is greaterthan the thickness of the wall 18.

Area 17, concentrating thus the stresses of flexion, has on its external cylindrical circumference strain gauges 24a and 24c located in the same horizontal plane above and parallel to the horizontal plane including the axis 2, and two other similar strain gauges 24b and 24d symmetrical with the gauges 24a and 24c, and disposed in a common horizontal plane below the axis 2.

As Fig. 5 shows these gauges are joined by a bridge, gauges 24a and 24b having a common input terminal joined to one of the terminals of a constant source V of electrical current of which the other ter minal is joined to a common in put terminal of gauges 24c and 24d. A common output terminal of gauges 24a and 24c is joined to an input terminal of amplifier A having another input joined to an output terminal common to gauges 24b and 24d.Amplifier A processes the signals it receives from the bridge formed by gauges 24a, 24b, 24c and 24d in the same way as the information received from the bridge formed by gauges 19a, 19b, 19c and 19d, in orderto render visible on units 20 and 21 the variations in the stresses of flexion and preferably to trigger an alarm on these units when the flexion stresses become excessive. Amplifier A has different processing circuits which can be regulated independently-forthe traction compression measurements and for the flexion measurements.

The invention as defined by the appended claims is capable of many variations in its realisation and the expert can choose in each case the most appropriate method of measuring the stresses of traction-compression and the stresses of flexion or, as the case may be, one or other of these categories of stress, and the method of processing these meas urements. Should the occasion arise, the information made visible on installations 20 and 21 located respectively in the driving cab of the towing vehicle and the driving cab of the vehicle being towed, may be translated, fore:'ample atthe level of amplifierA, into information other than an evaluation of the stresses in the area of the tow bar where the meas urements are taken. For example, in the case of a tow bar designed for towing aircraft it is possible to provide a direct visualisation in the cockpit of the aircraft and on the dashboard of the towing vehicle of the force couples about axis 15 corresponding, for the support 7 of the landing gear, to the flexion stresses measured on the tow bar.

When units 20 and 21 comprise alarm means these latter are regulated so they will operate before the bolts 9 or other equivalent device break. Thus the breaking of these latter is only the ultimate safeguard.

Claims

1. Atow bar, comprising a bar provided at oppo- site ends with means for attachment to a vehicle on tow and to a towing vehicle respectively, means for constantly measuring stresses in at least one area of the bar, and means to renderthese measurements visible in driving cabs of both the towing vehicle and the vehicle being towed.

2. Atow bar as claimed in claim 1,wherein the means for constant measurement of stresses in at least one area of the bar comprises meansforcon- stant measurement of stresses of traction and/or compression in said area.

3. A tow bar as claimed in claim 2, wherein the bar has an area sensitive to traction and/or compression, and the means for constantly measuring stresses of traction and/or compression are located in the area sensitive to traction and/or compression.

4. Atow bar as claimed in claim 3, wherein the area sensitive to traction and/or compression comprises a hollow section of the bar.

5. Atow bar as claimed in any one of claims 1 to 4, wherein the means for constant measurement of stresses in at least one area of the bar comprise means for constantly measuring stresses offlexion in the area.

6. Atow bar as claimed in claim 5 wherein the bar has an area sensitive to flexion and the means for constant measurement of the stresses of flexion are located in said area sensitive to flexion.

7. Atow bar as claimed in claim 6, wherein the area sensitive to flexion comprises a hollow section of the bar, which hollow section has external transverse dimensions less than those of the rest of the bar.

8. A tow bar as claimed in claims 4 and 7, comprising a portion formed by a tubular test body having an axis coincident with the general direction of the bar, the test body having first and second sections, the first section being of greater diameter than the second section and having a wall forming an area sensitive to traction and/or compression, the smaller diameter second section having a thicker wall than the first section and forming an area sensitive to flexion, the means for constant measurement of stresses of traction and/or compression are located on the first section, and the means for constant measurement of stresses of flexion are located on the second section.

9. A tow bar as claimed in any one preceding claim, further comprising means fortriggering an alarm in the respective driving cabs of the towing vehicle and of the vehicle being towed when measured stresses exceed a predetermined threshold which can be varied.

10. Atow bar, as claimed in claim 9, having a safety device which will cause the bar to break when the stresses exceed a second predetermined threshold, the first mentioned stress threshold which triggers the alarm being less than the second threshold.

11. Atow bar substantially as hereinbefore described with reference to the accompanying drawings.