Dragline bucket

FIELD OF THE INVENTION

This invention relates to a new type of bucket for a rigged dragline assembly.

Draglines are large excavating machines designed to fill, carry and dump loads of material, typically earth. Draglines are often used in open cut coal mines to remove waste overburden covering a shallow coal seam.

BACKGROUND OF THE INVENTION

FIG. 1 illustrates a typical large electric dragline in accordance with the prior art. A conventional dragline includes a rotatable support 1 mounted on a stationary base 2. An outwardly projecting boom assembly 3 is mounted pivotally to the rotatable support. Winches 6,9 are mounted on the support for retrieving or releasing cables or ropes. Normally there are two main sets of ropes or cables, hereinafter referred to as hoist ropes 4 and drag ropes 5. Hoist ropes 4 extend from the hoist winch 6 mounted on the support, up and outwardly along the boom, over pulleys or sheaves 7 mounted at the most distant point of the boom, down to a bucket and rigging assembly 8. Drag ropes 5 extend from a drag winch 9 mounted on the support 1, outwardly to the bucket and rigging assembly 8. The bucket and rigging assembly consists of the bucket itself, and the “Rigging” which refers to the total collection of chains, ropes, cables and other components used to suspend the bucket.

A conventional dragline is equipped with a mechanism for locomotion, typically being reciprocating support feet or crawler tracks. FIG. 2 shows the typical components of the bucket and rigging assembly in accordance with the prior art. While it is acknowledged that there are variations on the arrangements and names of components, the following definitions, familiar to any person skilled in the art, will be used:

Drag ropes 5 which are used to pull the bucket while filling (normally two).

Drag chains 10 which connect the drag ropes to the bucket.

Hoist ropes 4 which are used to lift and carry the bucket (normally two).

Hoist chains 11 (upper and lower) which connect the bucket to the hoist ropes.

Spreader bar 12 which separates the left and right hoist chains to allow the bucket to sit there between. It is situated at the junction of the upper and lower hoist chains.

Dump rope 13 that allows the bucket to be picked up or dumped by applying or releasing tension to the drag ropes.

Dump block 14 which is a pulley around which the dump rope is free to move.

Dump chains 15 which are intermediate chains connecting the dump rope to the leading end of the drag chains.

Miracle hitch 16 which is a three way link connecting the hoist ropes, chains and dump block.

Drag three way link 17 that joins the drag ropes, drag chains and dump chains.

Equaliser links 18 that equalise the loads between various components and allow interconnection, e.g., from the two hoist ropes to the single miracle hitch.

Rope sockets 19 that are used to terminate ropes and allow their connection to other components.

Teeth and lip assembly 20 which is the leading (cutting) edge of the bucket.

Basket 21 which is the main body of the bucket used to carry payload.

Arch 22 which provides structural integrity to the bucket and supplies a point to attach the dump rope.

Dump hitch 23 which is the point on the arch to which the dump rope is attached.

Drag hitches 24 which are the points on the front of the bucket to which the drag chains are connected.

Hoist trunnions 25 which are the points to which the lower hoist chains are attached to the bucket.

Top rails 26 which are structural thickeners along the top edges of the bucket.

Rear rail 27 which is a structural thickener along the top edge of the rear of the bucket.

Other relevant definitions are:

“Carry Angle” which is the acute angle between the floor of the bucket and the horizontal.

“Rated Suspended Load” (RSL) which is the maximum recommended load that can be suspended from the hoist ropes.

“Boom Point”which is the most distant extreme of the boom 3 from the support 1. This point corresponds to the location of the Boom Point Sheaves 7.

“Boom Point Radius” which is the horizontal radius measured outwardly from the centre of rotation of the support 1 to a point directly under the boom point sheaves 7.

The drag and hoist ropes may be retrieved or released from their respective winches to move the bucket freely in space. The rotatable support can “Swing” the upper dragline assembly and thus bucket and rigging through a horizontal arc. The normal operation of a dragline begins with the bucket freely suspended in space above the ground. The bucket is then lowered to the ground and positioned by releasing rope from the hoist winch and/or drag winch. The bucket is then filled with material by retrieving drag ropes onto the drag winch. At some point, the bucket may be lifted or “Disengaged” from the ground by retrieving the hoist ropes. In this operation, tension is developed in the dump rope 13 which causes the front of the bucket to lift via the arch 22. A certain volume of excavated material known as “Payload” is retained in the bucket after disengaging. The bucket may then be moved to its dump point by retrieving and releasing the hoist and drag ropes and/or swinging the support 1. The payload is dumped by releasing drag rope until the dump rope loses tension and allows the bucket to tip forward. This operation can only occur under, or nearly under the boom point sheaves. For a typical large electric dragline (e.g. BE 1370W or Marion 8050), the bucket capacity is approximately 47 cubic metres. The bucket weight is typically 40 tonnes. The combined rigging weight is typically 20 tonnes. The RSL for these machines is approximately 150 tonnes. Therefore, the manufacturers recommend payloads of approximately 90 tonnes.

There are a number of limitations that conventional rigging designs place on operating a dragline:

a) After filling the bucket, it cannot be disengaged from the ground until the bucket is sufficiently close to the support 1 to allow enough tension to be developed in the dump rope to lift the bucket arch. FIG. 3 shows that if the bucket is lifted too early, the forward section of the payload is lost. This means that the bucket must be “Over dragged” after it is full, to a point where it can be lifted and retain a satisfactory payload. This adds to cycle time, increases wear and reduces hoisting efficiency.

b) A dragline bucket can only dump at the perimeter defined by the boom point radius. This is because the dump rope will only become slack enough to drop the front of the bucket when the drag rope tension is low, i.e., the drag ropes have been sufficiently released. FIG. 4 shows this effect.

There are dynamic methods for dumping just inside and outside of boom point radius, however these methods are not recommended by the manufacturers.

Various proposals have been made to improve the control of the orientation of the dragline bucket in a vertical plane i.e. “carry angle” control by utilising differential control of the two hoist ropes, one of which is operatively connected to the front of the bucket, and the other operatively connected to the rear of the bucket. By adjusting the position of one hoist rope relative to the other, the vertical orientation of the bucket can be adjusted in order to provide a dumping movement without relying on the dump rope becoming slack with all of the disadvantages set out above.

A construction of this type is described in International Patent Application WO01/32994, where the carry angle of the bucket is controlled by differential hoist rope movement with the hoist rope entrained over two boom point sheaves. FIG. 5 shows the multiple hoist rope dragline assembly described in WO01/32994. A bucket 30 is suspended from two hoist ropes 31 and 32. The first hoist rope 31 is connected to a forward part of the bucket. The second hoist rope 32 is connected to a rear part of the bucket. Two sheaves 33 and 34 are spaced one behind the other at the distal end of boom 35. The first hoist rope 31 is entrained over the first sheave 33, while the second hoist rope is entrained over the second sheave 34. The two hoist ropes 31 and 32 in combination with drag ropes 36 control movement of the bucket 30.

In a dragline assembly having front and rear hoist ropes, as shown in FIG. 5, the payload is distributed between the two hoist ropes. However, the payload is inevitably not distributed evenly between the two hoist ropes. One hoist rope will usually bear more load than the other.

The lifetime of a dragline hoist rope is proportional to the peak cyclic stress experienced by it (stress being the load per unit cross-sectional area of the rope, and peak cyclic stress being the span between consecutive maximum and minimum stress states). This relationship is defined by the “fatigue” lifetime as defined in equation 1:

Rope Fatigue Lifetime is proportional to 1/(Maximum Cyclic Stress)^x eqn. 1

where x lies between 5 and 6.

i.e. for a 10% increase in maximum cyclic stress, rope life is reduced by approximately a half. For a 50% increase in maximum cyclic stress, rope life is reduced by approximately 90%.

Hence, in a dragline assembly having separate front and rear hoist ropes, there is typically a large difference in lifetime between the front and rear hoist ropes, since each rope has differing maximum cyclic stress.

Hoist ropes having unequal lifetimes are highly undesirable from a maintenance point of view. Replacement of a hoist rope is a costly exercise, because it involves lengthy downtime of the dragline assembly. Moreover, because of the expense of this downtime, it is desirable to change both hoist ropes together. However, if one hoist rope fatigues quicker than the other, replacement of both ropes at the same time is inefficient since one rope, having useful remaining lifetime, is unnecessarily replaced.

It is, therefore, an object of the present invention to provide a means for equalising, as far as possible, the loads carried by each dragline in a multiple hoist rope dragline assembly. It is a further object of the present invention to provide a means for equalising, as far as possible, the hoist rope lifetimes in a multiple hoist rope dragline assembly.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method of designing a bucket for a dragline assembly of the type wherein the attitude of the bucket is controllable by differential movement of front and rear hoist ropes attached to front and rear portions of the bucket respectively, said method including the steps of:

analysing the loads in both the front and rear hoist ropes during digging, hoisting and dumping phases of the dragline operating cycle; and

optimising bucket parameters such that the respective loads are equalized as far as possible over as much of the cycle as possible.

Preferably the parameters include, but are not limited to:

(a) the positions of the points at which the front and rear hoist ropes ultimately join the bucket;
(b) the position of the centre of gravity of the payload;
(c) the position of the centre of gravity of the bucket itself; and
(d) the carry angle of the bucket with respect to the horizontal.

Preferably, the method of designing a bucket is performed on a dragline assembly having two spaced apart boom point sheaves located at or adjacent the distal end of a boom assembly, one sheave being positioned closer to the distal end of the boom assembly than the other

Preferably, the bucket parameters are optimised such that the respective front and rear hoist rope loads are within 30% of their mean value during a major part of the hoisting phase of the operating cycle, being the time between the point of disengagement of the bucket from the ground, and dumping of the payload.

Preferably, the bucket parameters are optimised such that the respective front and rear hoist rope loads are within 20% of their mean value during a major part of the hoisting phase of the operating cycle.

Preferably, the bucket parameters are optimised such that the respective front and rear hoist rope loads are within 10% of their mean value during a major part of the hoisting phase of the operating cycle.

In a further aspect the invention provides a bucket for a dragline assembly when designed in accordance with the method above.

Preferably, the bucket includes a base and side walls extending upwardly therefrom, wherein the bucket has a height dimension shorter than a length dimension, and wherein the height dimension at a front part of the bucket is less than or equal to the height dimension at a rear part of the bucket.

Preferably, the bucket has a substantially flat base with two tapering side walls along its length dimension.

Preferably, the overall length dimension of the bucket is at least twice the overall height dimension.

Preferably, the length dimension is at least three times the height dimension.

Preferably, the bucket does not include an arch between the upper front portions of the bucket side walls.

The bucket of the present invention has generally shallow sides which may be tapered from rear to front. This means that when the bucket carries a payload, the centre of gravity of the combined payload and bucket is shifted further towards the rear of the bucket than in a conventional bucket design. This advantageously tends to equalise the load carried by the front and rear hoist ropes. This effect is illustrated in more detail below in the Example and Comparative Example.

Preferably, the bucket has a substantially flat base with two tapering side walls along its length dimension.

A further advantage of the bucket of the present invention is that it has a short height dimension relative to its length dimension and, preferably, its width dimension. Preferably, the length dimension is at least twice the height dimension, more preferably at least, or about, three times the height dimension. This flat bucket geometry minimises the amount of bucket steel required to constrain the payload and thus increases the efficiency of the carrying operation, i.e. maximum payload for minimum steel.

Preferably, the bucket includes attachment means at the front and rear parts of the bucket for attachment to front and rear hoist ropes respectively.

Preferably, the bucket includes attachment means at the front part of the bucket for attachment to a drag rope/s.

The attachment means may be, for example, hooks or openings in the bucket. Preferably, the attachment means for the rear hoist rope comprise a single rope and/or chain attached to a central point at the rear of the bucket, or a dual attachment to multiple rear points via an intermediate “bridal” or “yoke” chain or rope. The dual arrangement minimises the chance of the bucket twisting or uncontrollably rotating in situations where the bucket is hanging almost exclusively by the rear hoist rope, e.g. during dumping or “chopping”. Preferably, the attachment means for the front hoist rope comprises a pair of flanges having a plurality of openings therein, each flange extending upwards from respective side walls forming the length of the bucket. Preferably, the attachment means for the drag rope comprises a pair of flanges having a plurality of openings therein, each flange extending in a forward direction from respective side walls forming the length of the bucket.

Preferably, the bucket does not include an arch. As shown in FIG. 2, traditional dragline buckets include an arch 22. The incorporation of the arch 22 has the effect of shifting the centre of gravity of the bucket towards the front part. The absence of an arch, in this preferred embodiment of the present invention, contributes to shifting the centre of gravity more towards the rear of the bucket. In addition, the elimination of the arch reduces the total bucket weight and thus increases the total payload that may be carried without exceeding the machine's RSL.

Preferably, the bucket of the present invention is substantially cuboid- or trapezoid-shaped, having an open top and an open front section. Preferably, the side walls forming the length of the bucket are flared outwards from the base, which increases the volume of the bucket and helps to retain material in the bucket.

Preferably, the bucket includes a scooping means for scooping material into the bucket. Preferably, the scooping means comprises a downwardly angled flange extending from a front part of the base. These types of scooping flanges are known in dragline bucket design. Preferably, the flange includes a plurality of tapered fingers which define the flange's front leading edge. The tapered fingers facilitate scooping of material (e.g. earth) into the bucket.

Preferably, the bucket is used with a dragline assembly including differential control means for adjusting the length of one hoist rope relative to the other, thereby allowing control of the angle of inclination of the bucket in a vertical plane.

Preferably, the first hoist rope is wound onto a first drum located on the base, and the second hoist rope is wound onto a second drum located on the base, the first and second drums being independently rotatable to achieve differential control.

Preferably, the first and second boom sheaves are spaced apart from each other by a fixed distance such that the first sheave is located closer to the support than the second sheave. This type of rigged dragline assembly (excluding the bucket of the present invention) is described in more detail in WO01/32994.

In a further aspect, the present invention provides a method of moving a payload, including the steps of

providing a rigged dragline assembly as described above;

depositing a payload into the bucket; and

adjusting the front and rear hoist ropes and/or the drag rope(s) to move the payload in a desired manner.

Preferably, the front and rear hoist ropes are adjusted independently to control the carry angle of the bucket, thereby controlling the combined centre of gravity of the payload and bucket. Thus, in addition to the bucket design being used to distribute evenly the load between the front and rear hoist ropes, the carry angle may also be used to facilitate even weight distribution. A large carry angle will tend to shift the centre of gravity towards the front hoist rope due to the extra payload being carried on the front payload face, whereas a small carry angle will tend to shift the centre of gravity towards the rear hoist rope due to the loss of payload from the front payload face.

Hence, the present invention recognises the problem of unequal loads between front and rear hoist ropes in the known dragline assembly described in WO01/32994. Moreover, the present invention recognises that this problem may be addressed by optimising bucket design. In addition, the present invention allows the payload to be varied to ensure that the total hoist rope load is matched to the Rated Suspended Load (RSL) of the Dragline by virtue of altering the bucket's carry angle to control the bucket payload, yet still maintaining an even load distribution between the two hoist ropes during the parts of the dragline cycle in which maximum hoist rope loads are experienced.

Preferably, the analysis is performed on a dragline assembly having two spaced apart boom point sheaves located at or adjacent the distal end of a boom assembly, one sheave being positioned closer to the distal end of the boom assembly than the other. This type of boom arrangement is described in WO01/32994.

Preferably, the bucket parameters are optimised such that the respective front and rear hoist rope loads are within 30% of their mean value during a major part of the hoisting phase of the cycle, more preferably within 20%, and more preferably within 10%. The hoisting phase is defined as the time between the point of disengagement and dumping of the payload in a typical dragline assembly cycle.

Preferably, the major part of the hoisting phase comprises at least 50% of the hoisting phase, more preferably at least 60%, more preferably at least 70%, and more preferably at least 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail with reference to the following Figures in which:—

FIG. 1 illustrates a conventional dragline;

FIG. 2 illustrates conventional components of a bucket and rigging assembly;

FIG. 3 illustrates a disadvantage with a conventional dragline;

FIG. 4 illustrates a conventional dragline dumping at the boom point radius;

FIG. 5 illustrates a prior art dragline having front and a rear hoist rope;

FIG. 6A is a front view of a bucket according to the present invention;

FIG. 6B is a side elevation of FIG. 6A;

FIG. 6C is a perspective view of FIG. 6A;

FIG. 7 is a graph of hoist rope load over time for both conventional and separate front and rear end hoist ropes; and

FIG. 8 is a similar graph to FIG. 7 showing the loads from a bucket designed in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In a dragline assembly having front and rear hoist ropes as shown in FIG. 5, the payload is distributed between the two hoist ropes. The payload is inevitably not distributed evenly between the two hoist ropes and the inventor of the present invention has realised that this uneven distribution can be minimized by careful bucket design.

This uneven distribution of rope load during hoisting is a direct result of:

- (a) the positions of the points at which the various ropes and chains ultimately join the bucket;
- (b) the position of the centre of gravity of the payload;
- (c) the position of the centre of gravity of the bucket itself; and
- (d) the carry angle of the bucket with respect to the horizontal.

In addition the rope load distribution may be uneven during other parts of the dragline cycle such as dumping or when the bucket is disengaged out of the ground.

This uneven distribution of rope load can be overcome by providing a means for equalizing, as far as possible, the hoist rope lifetimes in a multiple hoist rope dragline assembly.

The inventor of the present invention has realised that this can be achieved by providing a bucket design that allows the total hoist rope load to be matched to the Rated Suspended Load (RSL) of the Dragline by virtue of altering the bucket's carry angle to control the bucket payload, yet still approximate an even load distribution between the hoist ropes during the parts of the dragline cycle in which maximum hoist rope loads are experienced.

The present invention recognises that in a typical multiple hoist rope dragline assembly, having a front and a rear hoist rope, the maximum load carried by the front hoist rope is often greater than the maximum load carried by the rear hoist rope. Furthermore, the present invention recognises the importance of bucket design in equalising the loads carried by front and rear hoist ropes during the parts of the dragline cycle in which maximum hoist rope loads are experienced.

Typically, this can be achieved by carefully considering the bucket parameters including those listed in items (a) to (d) above when designing the bucket to be used with the dragline.

Referring to FIGS. 6A-6C, there is shown a bucket 100, having a base 102 and side walls 103, 104 and 105 extending upwardly therefrom. Two side walls 103 and 104 define a length of the bucket, and one side wall 105 defines a width of the bucket. From FIG. 6B, it can be seen that the bucket 1 has a length dimension which is approximately three times the height dimension.

The bucket 100 has a rear part 106 and a front part 107. From FIG. 6B, it can be seen that the height of the bucket at the rear part 106 is greater than the height of the bucket at the front part 107. The effect of this tapering is that more payload is carried in the rear part than the front part, meaning that when the bucket is filled with a payload, the centre of gravity is located towards the rear part of the bucket.

To facilitate construction and reduce weight in the bucket, the rear corners 108 and 109 may be left open, braced by welded rods across the top of the opening. These rods may also be used to mount a bridle (not shown) for attachment to the rear hoist rope, although the rear hoist rope may be attached to a single central point on the rear wall 105 of the bucket.

At the front part of the bucket 1, a pair of flanges 110 and 111 extend upwards from respective side walls 103 and 104. Each flange 110 and 111 has respective openings 112 and 113 for attachment of a front hoist rope via an intermediate bridal or yoke.

Also at the front part of the bucket 1, a pair of flanges 114 and 115 extend frontwards from respective side walls 103 and 104. Each flange 114 and 115 has respective openings 116 and 117 for attachment of a drag rope.

From FIG. 6A, it can been seen that side walls 103 and 104 are flared outwards from the base, facilitating retention of material in the bucket. Hence, the bucket 100 is substantially trapezoid-shaped, having an open top and an open front end.

The bucket 100 includes a downwardly angled flange 118 extending from a front part of the base 102. The flange 118 is comprised of a plurality of fingers 119. The fingers 119 are tapered to define a front leading edge 120 of the flange 118. As with conventional dragline buckets, material may be scooped into the bucket 100 using the flange 118 by dragging it along the ground.

The efficacy of the bucket shown in FIGS. 6A-6B was compared with a conventional bucket, as illustrated in FIGS. 1-5.

COMPARATIVE EXAMPLE

The hoist rope loads were measured for a typical prior art bucket design (as shown in FIGS. 1-5) for two cases. The first where the bucket was rigged with conventional hoist ropes, i.e. side by side, and the second in the front/rear configuration using a conventional bucket as described in WO01/32994. FIG. 7 shows the hoist rope loads over time during the various stages of digging, hoisting, dumping and return for the two cases. The trace corresponding to the conventional hoist rope load represents exactly half the total hoist load because there are actually two identical ropes working together, joined by an “equalising bar” at or near the bucket hitching point. This trace is used as a reference for comparison for the individual front and rear hoist ropes in the new invention.

It can be seen that during the hoist part of the dragline cycle, the hoist rope loads for the front/rear configuration system diverge significantly from the conventional trace. Since the peak load for the front hoist rope during hoisting is greater than that of the rear hoist rope, its lifetime is significantly shorter.

EXAMPLE

The loads on a front hoist rope and a rear hoist rope were measured on a rigged dragline bucket assembly of the type shown in FIGS. 6A, B and C. The total suspended load was matched to the example of the conventionally designed bucket of prior art. FIG. 8 shows the hoist rope loads over time during the various stages of digging, hoisting, dumping and return.

It can be seen that the peak front and rear hoist rope loads are nearly equal during the hoisting part of the cycle, meaning that they will have similar lifetimes.

Dragline bucket

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