GB2210840A - A conveyor belt maintenance system - Google Patents

Abstract

A conveyor belt maintenance system for conveyor belts (1) comprising belt joints (3) comprises a belt joint replacement station, belt joint detector means (19) located a preset distance D1 upstream from the belt joint replacement station and adapted, in use, to detect each belt joint (3) in a conveyor belt (1) as it passes the detector means (19), a distance counter (20) for determining the distance travelled by the conveyor belt (1), memory means for storing both the distance of the detector means (19) from the belt joint replacement station and the stopping distance D2 of the conveyor belt (3), processor means (21) for calculating the difference D3 between the distances stored by the memory means (21), and conveyor drive control means for interrupting the conveyor drive. The drive may be interrupted when the distance counter shows the belt has travelled the distance D3, or the drive may be interrupted on detection of the joint to be replaced and when the belt stops, the motor is inched until the distance counter shows the belt has travelled the distance D3. In both cases the joint will then be at the joint replacement station. <IMAGE>

Description

DESCRIPTION A CONVEYOR BELT MAINTENANCE SYSTEM The present invention relates to a conveyor belt maintenance system and, more particularly, to a conveyor belt maintenance system which both aids and semi-automates inspection and replacement of mechanical and/or spliced joints in a conveyor belt. The present invention also provides a conveyor belt maintenance station for use in the conveyor belt maintenance system.

Conveyor belts are widely used in the mining industry to transport coal or ore, for example, to the surface of a mine and thence to storage silos or wagons.

These conveyor belts generally comprise a plurality of belt sections which are joined end to end to form a continuous loop and are supported between a pair of pulley wheels, at least one of which is connected to drive means to drive the conveyor belt round the pulley wheels. Two methods of forming joints between adjacent belt sections are known. One is to splice the ends of the belt sections together, but is time consuming and costly. The other is to use mechanical fasteners and many different types of fastener have been developed for this purpose. In either case the joints themselves are not as strong as the belt sections and must be renewed and replaced at regular intervals to avoid the possibility of failure in use.

In some instances, continuous loop conveyor belts may be several hundred metres in overall length, and at any given instant in time may be carrying hundreds of tons of material. Should one of the joints in such a conveyor belt, loaded to its full capacity, fail, the results could be disastrous. Apart from the inevitable loss of production whilst the conveyor system is cleared and the conveyor belt repaired, there is always the possibility of serious injury, or even death, to any personnel in the vicinity of the conveyor belt. In addition, permanent damage may be caused to the conveyor system as a whole, necessitating its replacement at considerable cost.

In view of the dangers associated with joint failure it is an important part of any conveyor belt maintenance routine to anticipate and prevent joint failure by regularly inspecting and replacing belt joints. Indeed, this applies to conveyor systems used above and below ground.

Conventionally, conveyor belt maintenance is carried out manually and requires an operator to isolate the conveyor drive means so that the conveyor belt is brought to a halt with a joint to be inspected in a predetermined location, where the belt on each side of the joint to be replaced can be clamped (to prevent it from dropping off the supporting pulley wheels under its own weight) and belt joint replacement equipment can be brought into position against the joint, Stopping the conveyor belt so that a joint arrives exactly in the correct position is extremely difficult and involves a considerable amount of skill and judgement on the part of the operator. Being so heavy, even when unloaded, the conveyor belt has substantial momentum and continues to travel forward after drive to it has been stopped for many metres.

In some conveyor systems it may be practicable to provide inching means which allow the conveyor belt to be'inched" forward to bring the joint to the required position, but this is not a favoured solution to the problem because inching means add to the expense of the conveyor system and are prone to cause the drive means to burn out under the load of the belt. Portable belt clamps and portable belt joint replacement equipment may be used so as to make it unnecessary to bring each joint to be inspected to a halt at a precise location, but this is cumbersome to transport to each joint site and even then access to a particular joint requiring inspedtion may not be possible. In any event, it is impossible to bring a preselected joint to a halt at a predetermined location using conventional techniques and equipment.

It is an object of the present invention to provide a conveyor belt maintenance system that obviates and substantially mitigates the problems and disadvantages referred to hereinabove. In particular, the present invention seeks to provide a conveyor belt maintenance system which allows each belt joint to be brought to a halt at a belt joint replacement station.

It is a further object of the present invention to provide a conveyor belt maintenance system which may be used with conveyor drives with and without belt "inching" facilities.

It is yet another object of the present invention to provide an automated belt joint replacement station for use in the conveyor belt maintenance system.

according to the present invention there is provided a conveyor belt maintenance system comprising detector means located a preset distance upstream from a belt joint replacement station and adapted, in use, to detect each belt joint in a conveyor belt as it passes the detector means, a distance counter for determining the distance travelled by the conveyor belt, memory means for storing both the distance of the detector means from the belt joint replacement station and the stopping distance of the conveyor belt, processor means for calculating the difference between the distances stored by the memory means, and conveyor drive control means for interrupting the conveyor drive.

Initially, the conveyor belt maintenance system is run through an initialisation routine to determine the stopping distance of the belt. In this, the distance between the detector means and the belt joint replacement station (D1) is first entered into the memory means.

Then the conveyor belt is run up to its normal unloaded operating speed, at which speed the conveyor drive control means is set to interrupt the conveyor drive immediately a joint is detected passing the detector means. The distance travelled by the conveyor belt as it comes to rest is determined by the distance counter and corresponds to the stopping distance (D2) of the belt. With the distance of the detector means from the belt joint replacement station and the stopping distance of the belt both stored in the memory means the processor means is able to calculate the difference between the two distances (D1-D2=D3) and the conveyor belt maintenance system is now ready for normal use.

In a first embodiment of the present invention, intended for use with a conveyor system without belt "inching" facilities, the conveyor belt maintenance system operates as follows: When an operator decides to stop the conveyor belt with a joint at the belt joint replacement station, for inspection and possible replacement of the joint, the conveyor belt is run unloaded up to its normal unloaded operating speed.Once this speed is achieved the operator operates the system, by pressingabutton, for example, to stop the conveyor belt when the next joint passes the detector means, Immediately the next joint does pass the detector means the distance travelled by the conveyor belt, as determined by the distance counter, is monitored by the processor means and when this equals the difference (D3) between the two distances (D1 and D2) stored in the memory means the conveyor drive control means interrupts the drive to the conveyor drive. After travelling the stopping distance (D2) the conveyor belt comes to a halt. As will be readily appreciated, the joint has now travelled through a total distance from the detector means of D2 plus D3, which equals the distance D1 of the detector means from the belt joint replacement station.

In a second embodiment of the present invention, intended for use with a conveyor system with belt "inching" means, the conveyor belt maintenance system operates as for the first embodiment, except that immediately the next joint passes the detector means the conveyor drive control means interrupts the drive to the conveyor drive.

After travelling the stopping distance (D2) past the detector means the conveyor belt comes to rest. The distance of the joint from the belt joint replacement station can be simply determined be subtracting the stopping distance (D2) from the distance (D1) of the detector means from the belt maintenance station. Using the belt "inching" facility of the conveyor drive the joint is then inched through this distance (D3) to bring the belt joint up to the belt joint replacement station.

The distance D3 is conveniently measured out by the distance counter.

Whether the conveyor system is provided with belt "inching" facilities or not, no skill is required on the part of the operator to ensure that the joint arrives at the joint belt replacement station, it is simply a matter of pushing a button. Nor is it necessary for the operator to be able to see the conveyor belt to bring a joint to a halt at the belt joint replacement station, as would normally be the case, which means that the controls for the conveyor belt maintenance system can be sited away from the conveyor belt for convenience and safety.

In order to ensure that the conveyor belt always comes to a halt within the predetermined stopping distance it is important that it reaches its normal unloaded operating speed when the joint to be inspected reaches the detector means. Should the conveyor drive be interrupted before this speed is reached an indeterminate shorter stopping distance would result:.

Conveniently, therefore, the conveyor belt maintenance system is provided with an up-to-speed detection circuit and the normal unloaded operating speed of the conveyor belt is stored in the memory means during the initialisation routine. During normal use the up-to-speed detection circuit compares the actual speed of the conveyor belt with the stored value and gives an indication if there is any short fall. This allows the operator to select another joint if the first selected joint passes the detector means before the conveyor belt attains full speed. Alternatively, the up-to-speed detection circuit may automatically inhibit the conveyor drive control means from interrupting the conveyor drive until full speed is reached.

Normally the joints in a conveyor belt comprise metal fasteners in which case the detector means conveniently comprises a metal detector. However, it will be readily appreciated that other types of detector means, such as an ultrasonic sensor or a light reflection sensor, may be used as appropriate to the type of joint and the prevailing operating conditions.

Advantageously, the stopping distance (D2) stored in the memory means is updated periodically with the actual distance travelled by the conveyor belt as it comes to rest after the conveyor drive is interrupted and this stopping distance is determined by means of the distance counter. Conveniently, this update is carried out each time the conveyor belt is stopped for a joint inspection. It should be understood that during the working life of a conveyor system the stopping distance of the conveyor belt may vary upwards and downwards as bearings or brake pads, for example, become worn and are replaced. Whilst these changes in the stopping distance may only be as little as half a metre from one inspection to the next, over a prolonged period of use the cumulative change may be quite substantial.

Unless the stopping distance is periodically up-dated the conveyor belt maintenance system could not be relied upon to accurately bring a joint to the belt joint replacement station.

Advantageously, the belt maintenance system comprises means for selecting the joint to be stopped at the belt joint replacement station. In this respect, a datum marker is provided on the conveyor belt which can be detected by the detector means and distinguished from the belt joints. In this way, it is possible to identify the relative position of each joint from the datum position by means of a simple joint counter which is incremented each time a joint passes the detector means following detection of the datum marker, until the datum marker is detected again, whereupon the joint counter is reset to zero.Alternatively, the distance of each joint from the datum marker can be stored in memory and, upon selection of a particular joint by the operator, the distance counter can be employed to determine the distance travelled by the conveyor belt past the datum marker until it corresponds with the stored distance of the selected joint therefrom. Where the distance of each joint from the datum marker is stored in memory it is convenient to update the stored distance with the actual distance, as determined by the distance counter, from time to time. This takes account of changes in the overall length of the conveyor belt during its working life.

Conveniently, the datum marker comprises a pair of markers on the surface of the belt which are closely spaced from each other, by about half a metre, for example, so as to give a double signal on the detector means. The datum marker should be spaced from the nearest joints to avoid confusion therewith and a distance in excess of one metre has been found adequate.

Preferably, the conveyor belt maintenace system comprises joint selector means whereby an operator may preselect a joint to be inspected. The joint selector means enables the operator to input the number of a joint, counted from the datum marker, to be inspected and only when the joint counter reaches this number is the conveyor drive control means operated to interrupt the conveyor drive.

The joint selector may comprise one or more push buttons which increment a numerical display showing the number of the joint selected. Alternatively, it may comprise a keyboard and a visual display unit which in addition to allowing each joint to be preselected also allow information relating to each joint to be input. This information is conveniently held in memory means and can be called up by the operator on demand.

This information may include: a) Type of joint/fastener use.

b) Jointing company.

c) Installation date.

d) Anticipated replacement date.

Other information may, of course, also be stored as deemed appropriate to the particular conveyor belt and belt joints used.

Preferably, the distance counter comprises a length transducer device which may take the form of either a special roller in contact with the surface of the conveyor belt or a retrofit device fitted to an existing roller/idler on the conveyor system. This device gives out N digital pulses for every revolution of the roller/idler which are counted by a counter device.

With the conveyor belt maintenance system of the preferred embodiment of the present invention minimal operator intervention is required to bring a preselected joint to the belt joint replacement station.

According to a second aspect of the present invention there is provided a belt joint replacement station comprising any combination of a lacing machine, a belt shaver and a double belt squarer/cutter. The lacer, shaver and squarer/cutter may be manually, hydraulically or pneumatically operated.

Preferably, the belt joint replacement station further comprises a mobile clamp for retrieving slack belt. This may be powered.

Preferably, the belt joint replacement station, or each of the lacer, shaver, squarer/cutter and mobile 'clamp, is mounted on a bogey for transportation between the work area where joint replacement is carried out and a storage bay.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a partial schematic view of a conveyor system employing a conveyor belt maintenance system embodying the present invention prior to carrying out a belt joint replacement operation; Fig. 2 shows a schematic view of the conveyor belt maintenance system of Fig. 1 carrying out a belt joint replacement operation; and, Fig. 3 shows a schematic block diagram of the conveyor belt maintenance system employed in the conveyor system of Figs. 1 and 2.

Referring to Fig. 1 of the accompanying drawings there is shown the right hand end of a continuous loop conveyor belt 1. The conveyor belt 1 comprises a plurality of belt sections 2 which are each joined end to end by mechanical and/or spliced joints 3. As with any conventional conveyor system the conveyor belt 1 is guided around pulley wheels 4 to 8 and a drive wheel (not shown), and is supported over its length between the pulley wheels t to 8 and the drive wheel by idler rollers (not shown).

Together the pulley wheels 4 to 8 direct the conveyor belt 1 over a belt joint replacement station, generally designated by the reference numeral 9. The combination of pulley wheels 4, 5 and 7 also serve to provide a tension take-up arrangement for the conveyor belt 1. In this respect, the position of pulley wheel 5 can be adjusted in a horizontal plane towards and away from pulley wheels 4 and 7, thus increasing or decreasing the tension in the conveyor belt 1.

Indicators (not shown) are fitted to the tension take-up arrangement formed by pulley wheels 4, 5 and 7 which indicate when tension is applied to or released from the conveyor belt 1. Additionally, pulley wheel 5 is fitted with an end of travel switch (not shown) which indicates when its limit of travel has been reached in either direction.

The belt joint replacement station 9 comprises a lacing machine 10, a belt shaver 11, a double belt squarer/cutter 12 and a mobile clamp 13 which are each independently movable between a parking bay 14 and a work area along a track 15. The lacer 10, shaver 11 and squarer/cutter 12 may be manually, hydraulically or pneumatically operated. On each side of the belt joint replacement station 9 there is provided a permanent belt clamp 16, 17 and a further permanent belt clamp 18 is also provided in the upper run of the conveyor belt 1.

These permanent belt clamps 16, 17 and 18 grip the conveyor belt 1 whenever a belt joint replacement takes place and prevents the conveyor belt 1 from dropping off the supporting pulley wheels and idler rollers under its own weight. Once gripped the tension take-up arrangement formed by pulley wheels 4, 5 and 7 can be relaxed to allow the section of conveyor belt 1 held between permanent belt clamps 16 and 17 to be dropped down to the belt joint replacement station 9.

Operation of the permanent belt clamps 16, 17 and 18, the tensioning arrangement and the belt joint replacement station 9 can be seen in Fig. 2 in which the permanent belt clamps 16, 17 and 18 have been tightened onto the conveyor belt 1, the tensioning arrangement released and the belt joint replacement station 9 brought out into the work area from its parking bay 14.

Referring again to Fig. 1, a belt joint detector 19, which conveniently takes the form of a metal detector, is positioned adjacent the conveyor belt 1 towards the head end of the conveyor belt 1 and detects the presence of the metal fasteners which comprise each belt joint 3 as they pass it. The distance (D1) of the detector 19 from the centre of the belt joint replacement station 9 is greater than the anticipated maximum stopping distance of the conveyor belt.l.

In the event that the belt joint 3 does not comprise metal fasteners, as, for example, may be the case with spliced belt joints, specially provided metal marker clips may be inserted in the belt joints or an alternative form of detector may be used.

Adjacent the belt joint detector 19 there is provided a length transducer 20 which measures the distance travelled by the conveyor belt 1. The length transducer 20 takes the form of either a special roller or a retrofit device fitted to an existing rollerXidler on the conveyor system. The length transducer 20 provides (N) pulses for every revolution of the roller.

Both the belt joint detector 19 and the length transducer 20 are connected to a control circuit 21 which is described in detail with reference to Fig. 3.

Also shown in fig. 1, in the conveyor belt 1 itself, is a datum marker 22. This datum marker 22 comprises two closely spaced metal markers which can be detected and identified by the belt joint detector 19 and its purpose will be described later, hereinbelow.

Referring now to Fig. 3 of the accompanying drawings in combination with Figs. 1 and 2 operation of the conveyor belt maintenance system according to the present invention will now be described. The control circuit 21 to which the belt joint detector 19 and the length transducer 20 are connected comprises memory means in the form of two down counters 23, processor means- 24 and a conveyor belt drive control 25. Counter C1 is preset from coded switches (not shown) with the number of pulses corresponding to the distance D1 each time counter C2 is set to zero. Both counters C1 and C2 are continuously "clocked" by the pulses from the length transducer 20.

Before the conveyor belt maintenance system is ready for use it must first be run through an initialisation routine. This entails running the conveyor belt 1 up to its normal unloaded operating speed. Once this speed is reached a "next joint switch" is pressed by the operator which causes the detector 19 to be set to detect the first belt joint 3 to pass it and as soon as it is counter C1 is set to zero. At this instant, since the contents of counter C2 have been set to zero, the contents of counter C1 are set to the preset value on the coded switches. At the same time the counter C1 is loaded with this value the conveyor belt drive control 25 signals the conveyor belt 1 to be stopped via a voltage free contact (not shown) and thus the conveyor belt 1 comes to rest.Whilst the belt comes to rest the value held in counter C1 is decremented, such that when the conveyor belt 1 stops, the contents of counter C1 represent the difference D3 between distance D1 and the stopping distance D2 of the conveyor belt 1.

A timer circuit in the processor means 24, which is started when the joint 3 is detected, transfers the contents of counter C1, corresponding to difference D3, to a latch at the end of its timing period T. The timing period T is preset to give a time delay which is greater than the time taken for the conveyor belt 1 to come to rest (typically 45 seconds). The contents of counter C2 are then preset with the contents of the latch each time a joint 3 is detected after the "next joint button" is pressed.

The conveyor belt maintenance system is now ready for use in normal operation. When a joint is to be parked at the belt joint replacement station 9, the next joint button" is pressed which causes the next joint detected by the detector 19 to load counter C2 with the difference value D3. This counter 3 then decrements until it reaches zero, whereupon the conveyor belt drive control 25 signals the conveyor drive to be stopped via the voltage free contact. The system will come to a stop in the stopping distance D2 of the conveyor belt which brings it exactly to the middle of the belt joint replacement station 9.

Thus, the conveyor belt maintenance system delays stopping the conveyor drive after the joint has been detected by the time taken for the conveyor belt to travel the shortfall in distance between the actual distance D1 of the detector 17 from the belt joint replacement station 9 and the stopping distance D2 of the conveyor belt 1. Each time the conveyor belt 1 stops, the contents of counter C1 decrements by the stopping distance and updates the latch and in turn the counter C2 with a revised value of the distance D3.

Thus, the conveyor belt maintenance system parks the next joint within the jointing area correcting the system for any changes in belt stopping distance, due to changes in inertia, braking, etc.

It is important that the stopping distance of the conveyor belt 1 be determined whilst it is not loaded as the load will certainly be variable and make it impossible to accurately determine the stopping distance.

Variations in the stopping distance will also arise if the conveyor belt 1 is not run up to its normal unloaded operating speed before a belt joint 3 to be inspected passes the detector 19. In order to prevent this from happening an up-to-speed detection circuit 26 is provided which inhibits the conveyor belt drive control 25 until the required speed is reached.

As will be readily appreciated the conveyor belt maintenance system can also be applied to conveyor systems employing drive means having belt "inching" facilities.

In this respect, the conveyor belt drive control 25 signals the conveyor drive to be stopped immediately a joint 3 is detected by the detector 19. The joint 3 will come to a halt in the stopping distance of the conveyor belt 1 and can then be inched through the distance D3 corresponding to the shortfall.

As indicated hereinabove, the conveyor belt 1 may be provided with a datum marker 22 in its surface. The purpose of this is to allow individual joints 3 to be selected and brought to the belt joint replacement station 9.

As the datum marker 22 passes the detector 19 it gives a double signal which causes a belt joint counter 27 (shown in dotted line in Fig. 3) to be incremented each time a belt joint 3 passes the detector 19.

Obviously, this allows an operator to stop the conveyor belt 1 with a particular belt joint 3 at the belt joint replacement station 9 simply by checking the belt joint counter 27p However, the conveyor belt maintenance system also' allows the operator to preselect a particular belt joint 3. This is achieved by means of a belt joint selector 28 (shown in dotted line in Fig. 3) which enables the operator to input the number of the required joint 3, counted from the datum marker 22, to the processor means 24. When the selected belt joint number corresponds to the number of the belt joint 3 passing the detector 17 the conveyor belt drive control is prompted to interrupt the conveyor drive.Thus, the required belt joint 3 can be brought into the belt joint replacement station 9 with the minimum of effort on the part of the operator and certainly without the difficulties associated with conventional techniques.

A further advantage of the belt joint counter 25 is that it allows details of each belt joint 3 to be stored in memory means (not shown). As each belt joint 3 can always be identified the information relating to it can always be called up. The information stored may relate to: a) Type of joint/fastener use.

b) Jointing company.

c) Installation date.

d) Anticipated replacement date.

Other information may also be stored as deemed appropriate to the particular conveyor belt and belt joints in use.

As an alternative to counting the number of joints 3 from the datum marker 22, the conveyor belt maintenance system may be provided with memory means in which the distance of each joint 3 from the datum marker 22, as determined by the length transducer 20, is stored. To bring a selected joint 3 to the belt joint replacement station 9 the output of the length transducer 20, after the datum marker 22 has passed the detector 19, is compared with the stored distance of the selected joint 3. When the two values are the same the conveyor belt maintenance system operates as described above for conveyor drives with or without belt "inching" facilities.

Claims (1)

1. A conveyor belt maintenance system for conveyor belts comprising belt Joints, said system comprising a belt joint replacement station belt joint detector means located a preset distance upstream from the belt joint replacement station and adapted, in use, to detect each belt joint in a conveyor belt as it passes the detector means, a distance counter for deberrlining the distance travelled by the conveyor belt, memory means for storing both the distance of the detector earns fro the belt joint replacement station and the stopping distance of the conveyor belt, processor means for calculating the difference between the distances stored by the mer.-:orv means, and conveyor drive control means for interrupting the conveyor drive.

2. A system according to claim 1, wherein the difference between the distance of the detector means fro the belt joint replacement sta+ on and the stopping distance of the belt is determined by:: a) entering the distance (D1) between the detector means and the belt joint replacement station into the memory means; b) running the conveyor bel up to its normal unloaded operating speed; c) interrupting the conveyor drive immediately a joint is detected passing the detector means; d) determining by means of the distance counter the distance (D2) travelled by the conveyor belt as it comes to rest which is the stopping distance of the belt; e) calculating by means of the processor means the difference (D3) between distances Dl and D2; and f) storing said difference D3 in the memory means.

3. A system according to clain 2, wherein a belt joint is stopped at the belt Joint replacement station by: a) running the unloaded conveyor belt up to its normal unloaded operating speed; b) operating the distance counter to detenmlne the distance travelled by a joint to be inspected past the detector means; c) comparing this distance with the difference D3 stored in the memory means; and d) shen the distance travelled by the conveyor belt equals distance DS interrupting the conveyor drive to bring the conveyor belt to a ilalt within the conveyor belt stopping distance D2.

4. A system according to claim 2, wherein a belt joint is stopped at the belt joint replacement station by: a) running the unloaded conveyor belt up to its normal unloaded operating speed; b) intewrupving the conveyor drive meciate1y a joint to be inspected passes the detector means to bring the conveyor belt to a halt within the conveyor stopping distance 92; and c) inching the conveyor belt forward through difference D3 to bring the belt joint up to the belt joint replacement station.

5. A system according to claim 4, wherein difference D3 is measured by the distance counter

6. h system according to any preceding claim comprising an up-to-speed detection circuit for comparing the actual speed of the conveyor belt with the normal unloaded operating speed of the conveyor belt and indicating any difference therebetween.

7. A system according to claim 6, wherein the up-to-speed detection circuit automatically inhibits the conveyor drive control means from interrupting the conveyor drive until the conveyor belt reaches its normal unloaded operating speed.

8. A system according to any preceding claim, wherein the detector means comprises a metal detector.

9. A system according to any preceding claim, wherein the detector means comprises an ultrasonic sensor.

10. A system according to any preceding claim, wherein the stopping distance (D2) stored in the memory means is periodically updated with the actual distance travelled by the conveyor belt as it comes to rest after the conveyor drive is interrupted, the actual stopping distance being determined by te distance counter.

11. A system according to any preceding claim, wherein a datum marker is provided on the conveyor belt ':;1iCh can be detected by the detector .eans and distinguishec from the belt joints.

12. A system according to claim 11, wherein the datum marker comprises a pair of markers which are closely spaced from each other.

13. A system according to claims 11 or 12, comprising a Joint counter which is zeroed each time the datum marker passes the detector means and incremented by each joint that passes the detector means.

14. A system according to claims 11 or 12, wherein the distance of each joint from the datum marker is predetermined and stored in the memory means, or comparison with the distance travelled by the datum marker past the detector means as indicated by the distance counter.

15. A system according to claims 13 or 14, comprising joint selector means whereby an operator may preselect a joint to be inspected, which joint selector means enables the operator to input the number of a Joint, counted from the datum marker, to be inspected and only when the joint counter reaches this number is the system operated to halt the preselected joint at the belt joint replacement station.

15. A system according to any preceding claim, wherein the distance counter comprises a length transducer device in the form o a rolled ifl contact with the surface of the conveyor belt which gives out IX digital pulses for every revolution of the roller and a counter for counting these pulses.

17. A system according to any preceding clam, wherein the belt joint replacement station comprises any combination of a lacing machine, a belt shaver. and a double belt squarer/cutter.

18. A system according to any preced-nc claim, wherein the belt joint replacement station is mounted on a bogey for transportation between 2 work area where joint replacement is carried out and a storage bay.

19. A conveyor belt maintenance system or conveyor belts comprising belt Joints, substantially as hereinbefore described with reference to the accompanying drawings.