AU620591B2 - A trench digging apparatus - Google Patents

Description

i COMMONWEALTH OF AUSTRALIA PATENT ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE 620591 CLASS INT. CLASS Application Number: Lodged: S Complete Specification Lodged: Accepted: Published: L Priority: 0 0 Related Art-:

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NAME OF APPLICANT: ADDRESS OF APPLICANT: AUSTRALIAN TELECOMMMUNICATIONS eeMM-I-S-S-I-Nu -l-9-9-Wi-l-l--am-St-rzee-t Me--bou-r-ne V-i-e-t er-i-a *Aus.-tr-a-1-i-a- 10 PleckboU/n V (cfo Q fs0 kfo k, CA oc-g S

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NAME(S) OF INVENTOR(S) Ross Allan Vidler ADDRESS FOR SERVICE: DAVIES COLLISON, Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

MOOS1 27/02/89 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: "A TRENCH DIGGING APPARATUS" The following statement is a full description of this invention, including the best method of performing it known to us -1- I 2 4 TRENCH DIGGING APPARATUS" aS The present invention relates to a trench I digging apparatus developed particularly, but not exclusively, for excavating trenches in hard rock, such as limestone and sandstone.

*e a Trench digging apparatuses which comprise an endless digging chain mounted on a boom pivotably affixed to the rear of a tractor are suitable for ,t excavating trenches in hard rock. The boom includes two wheels, one at each end thereof, about which the chain is mounted, one wheel being a toothed wheel used to drive the chain and the other wheel being a plain free running wheel known as an "idler". The boom is mounted on a frame affixed to the rear of the tractor and is able to pivot with respect to the Sframe. The drive wheel shaft is driven by a motor hydraulically coupled to the engine of the tractor.

3 During an excavation operation, the boom is positioned so that the idler is disposed below the drive wheel and furthest from the tractor and the portion of the chain adjacent the idler and between the tractor and the idler is brought to bear against the rock to be excavated. The chain includes a number of flat plates disposed thereon at regular intervals which are used to hold one or more respective cutting teeth. Normally the teeth are positioned so that when the chain is driven around t S..the idler towards the rear of the tractor, the teeth, brought to bear against the rock, cut into the rock ;so as to perform the desired excavation. A trench is completed by moving the apparatus horizontally across B. an area of rock to be excavated.

e B B 9 In known trench digging apparatuses of this o. type the teeth 10 are normally arranged on the chain, I as shown in accompanying Figure 5, so as to be relatively close together with respect to their ;transverse disposition. The plates of the chain are usually adapted to hold either one, two, three or four teeth and Figure 5 illustrates the unique transverse positions of all the teeth 10 on the chain. The teeth 10 are also arranged so as to excavate a trench having a substantially rectangular transverse cross-section 12. The corners 14 of the trench, as shown in Figure 5, are substantially square. A trench digging apparatus of this type excavates a trench by pulverising the rock and breaking it into fine particles. This pulverising action is achieved by transversely arranging the teeth 10, as shown in Figure 5, and driving the chain ij.

i ^ii ~r- S .1 1; ;V S4 at a relatively high maximum speed, normally between 3.8 m/s and 4.6 m/s. Relatively expensive equipment is required to drive the chain at such speeds. This equipment includes, of course, a high speed motor which is normally chosen to have a maximum torque output which is relatively low as a high torque high speed motor would increase the cost of the equipment further. The load which can therefore be placed on the teeth and chain at the portion which the chain pull which can be exerted and the chain t tension which can be withstood is relatively low.

(il A problem associated with such trench digging apparatuses is that they usually have a relatively large "downtime" partly due to the A excessive wear which is placed on the teeth causing them to have a relatively short working life.

Downtime refers to the time spent maintaining the apparatus as opposed to the time which the apparatus spends in operation excavating a trench. The short working life of the teeth substantially increases the cost of operating the apparatus, not only in view of the increased downtime but also due to increased cost of having to replace the teeth. The teeth normally t ^used comprise a flat, conical, or lipped cutting end which is either capped with tungsten carbide or has a tungsten carbide insert at the apex, or tip, of the cutting end. Driving the chain at the relatively high speeds mentioned above causes a substantial degree of friction to be generated at the cutting end of the teeth and it is the heat generated by the friction which causes substantial damage to the cutting end of the teeth. Also as the teeth engage the rock to be excavated they do so under a relatively low load which causes fretting of the teeth.

In excavating a trench having a substantially rectangular transverse cross-section 12, as illustrated in Figure 5, a significant amount of energy is expended in forming the corners 14 of the trench, as the rock adjacent the corners 14 is generally more difficult to remove. The teeth r. transversely arranged on the chain furthest from the centre-line of the chain and which are used to form o the corners 14 are placed under considerable stress during the cutting operation and this contributes substantially to the wear of those teeth 4* The teeth of the apparatuses of this type are also susceptible to rapid wear because they are 4 invariably mounted on the plates of the chain so as S. to have a negative rake angle with respect to the surface of the rock to be excavated. Teeth having a So negative rake angle are worn each time they strike the rock so as to blunt the cutting end instead of sharpening it. Normally the teeth are mounted on the plates so that the angle between the longitudinal axes of the teeth and the centre-line of the chain is approximately 52°, which ensures that each tooth has a negative rake angle.

SAccordingly, an object of the present invention is to provide a trench digging apparatus which overcomes, at least in part, at least one of the disadvantages or problems associated with trench digging apparatuses of the type described above.

-l 6- In accordance with the present invention there is provided a trench digging apparatus comprising: an endless chain, means for driving said chain and means for supporting said chain, said chain including members disposed thereon at regular intervals which are adapted to receive and hold teeth, said teeth being positioned on said members such that the unique transverse positions of the cutting tips of the teeth are spaced apart so that when said chain is driven and positioned to excavate a trench in relatively hard rock, the teeth perform a chiselling action on said rock so as to dislodge fragments of the rock.

~Preferably said teeth act to induce shear strains behind the exposed surface of said rock.

Preferably said transverse positions of said teeth are such that the teeth are closer together adjacent the edges of said chain than at the centre of said chain.

9 Measuring the transverse tooth spacing as a multiple of the tooth diameter at the cross section lying 25 mm from the apex or tip of the tooth, then preferably in a S• 20 high stress area remote from the centre-line of the chain the transverse spacing is S"between said teeth in the range from 0.5 to 1.5 tooth diameter. Preferably, in a low stress area, adjacent the centre-line of the chain, the spacing is in the range from to 3 tooth diameter.

S 25 Preferably when dislodging said fragments said chain is driven at a relatively low speed under a relatively high load. For example, where the load is measured in tonnes and the chain speed in m/s, the load and speed vary inversely according to a factor in the range of 10 to 20. More particularly, where the load is for inslance 12 tonnes, the chain speed is preferably in the range from 1.1 m/s to 1.4 m/s, and where the load is for instance 22 tonnes, the chain speed is preferably in the range from 0.7 to 0.8 m/s.

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Az _1 I 7 -7- Preferably said teeth are arranged so that the trenches excavated by said apparatus have a curved cross-section. Preferably the transverse cross-section of said trenches has rounded corers. Alternatively the transverse cross-section is substantially semi-circular.

Preferably said teeth are mounted on said members so that the teeth are able to rotate about their respective longitudinal axes with respect to their respective members and have a positive rake angle with respect to the surface of said rock to be excavated when engaging said rock.

Preferably said teeth are tapered at the cutting ends thereof, the angle of the taper being less than 900, and are mounted so that the angle +ri r

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910521,dbwspe.029,trcchIspc,7 8 between the respective longitudinal axes of said teeth and the centre-line of said chain is between 350 and 43°.

Preferably said angle between the axes and the centre-line is approximately Preferably the cutting ends of said teeth are conical and include tungsten carbide.

In accordance with the present invention 9. there is also provided a vehicle having said trench digging apparatus mounted thereon.

e* Preferred embodiments of the present 9* invention will now be described, by way of example o 9 9 9 94 9 only, with reference to the accompanying drawings wherein: .999 99 0 9 0 a 4 I 9 9*9 9.

mme 9 9 9 9 99 9 9 9 9 09 99* 99 9 ft *9 9 9 9 4 *S 4 0 j Figure 1 is a schematic view of a tractor having a trench digging apparatus according to the present invention mounted thereon; Figure 2 is a schematic view of the tractor or Figure 1 showing the modifications required to drive the trench digging apparatus; Figure 3 is a schematic side view of the trench digging apparatus; Figure 4 is a partial plan view of the chain of the trench digging apparatus showing a first preferred arrangement of the teeth on the chain; Figure 4a is a partial plan view of the chain showing a second preferred arrangement of teeth on the chain; Figure 5 is a transverse cross-sectional view of an excavated trench showing the relationship between the teeth of a known trench digging apparatus used to excavate the trench; Figure 6 is a transverse cross-sectional view of an excavated trench showing the relationship between the teeth of a trench digging apparatus according to the present invention used to excavate the trench; Figure 7 is a side cross-sectional view taken through the centre-line of the chain showing the angular relationship between the teeth on the chain and the surface of rock to be excavated; Figure 8 is a more detailed view showing the angular relationship between a particular tooth of the chain and the surface of the rock; and Figure 9 is a more detailed view showing the angular relationship between another tooth of the r chain and the surface of the rock.

A trench digging apparatus 2, as shown in Figures 1 and 3, is pivotably mounted on the rear of a tractor 4. The apparatus 2 includes an endless digging chain 6 mounted on a boom 8 which has two wheels 20 and 22 disposed at either end thereof. The chain 6 is fitted around the wheels 20 and 22 with one toothed wheel 20 being used to drive the chain and the other wheel 22 acting as an idler. The drive wheel 20 is in turn driven by a shaft coupled to a Shydraulic motor 24. The motor 24, used to drive the rchain 6, is hydraulically coupled to the engine of I i. the tractor 4. When excavating a trench the .apparatus 2 is positioned, as shown in Figure 1, with i t the idler 22 disposed below the wheel 20 and motor 24 and furthest from the tractor 4 so that a portion 26 of the chain adajacent the idler 22 is used to engage I the rock 28 to be excavated. During excavation the chain 6 is driven in the direction indicated by the arrow 30 and the tractor 4 is moved slowly forward so t as to ensure the portion 26 continually engages the rock 28 under a substantially constant load.

j 1 The tractor 4 is a standard tractor, preferably a Caterpillar* D7 or D8, which includes a number of modifications, as shown in Figure 2, that enable it to drive the trench digging apparatus 2.

AW W The tractor 4 retains a diesel engine 32, a transmission unit 34, a steering clutch 36 and final drive 38, however, a torque converter and drive shaft coupled thereto which normally connect the engine 32 to the transmission 34 have been removed and replaced with a Registered Trade Mark a

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(t t ti 4 4 C *o 4 4' C 4I 4 4 II )t first hydraulic drive pump 40 and a second hydraulic drive pump 42, which is smaller than that the first drive pump 40. Both drive pumps 40 and 42 are driven by the engine 32. The first pump 40 is used to drive the chain motor 24 when excavation is being performed and when the trench digging apparatus is not in use the first pump 40 is used to drive the final drive 38 of the tractor 4. Also added to the tractor 4 is a relatively small drive motor 44 and a reduction gear box 46. The motor 44 and the gear box 46 are mechanically coupled to one another and mounted at the rear of the tractor 4. The reduction gear box 46 is hydraulically coupled to the transmission 34. The motor 44 is hydraulically coupled and driven by the second pump 42 and is used to drive the tractor 4 when the trench digging apparatus 2 is driven by the first pump The trench digging apparatus 2, as shown in Figure 3, includes a H-frame 50 on which the boom 8 is pivotably mounted. The position of the boom 8 with respect to the frame 50 is set by a hydraulic roll ram 52 which is mounted on the frame 50, the piston of the ram 52 being connected to the top of the boom 8. The H-frame 50 itself is pivotably connected to strong backs 54 which are affixed to the rear of the tractor 4 and enable the apparatus 2 to be mounted on the tractor 4. A lift ram 56 is attached to one of the strong backs 54 and has its piston connected to the H-frame 50. The lift ram 56 is used to pivot the boom 8 and the H-frame 50 with respect to the strong backs 54 and the rear of the tractor 4. The apparatus 2 also includes a spoil auger 58 which is disposed adjacent the upper surface 12 of the rock 28 to be excavated and between the rear of the tractor 4 and the chain 6. The auger 58 rotates about its transverse axis and is used to clear debris, such as dirt and fragments of rock, brought up from the trench being excavated by the chain 6. Alternatively, the debris may be removed by a conveyor.

The chain 6, as shown in Figures 4 and 4a, includes a plurality of plates 11 disposed thereon at regular intervals. The plates 11 each include selectively positioned tooth holders 60 which are :1 adapted to receive and hold teeth on the plates 11 in a predetermined manner. The plates 11 each have a fplurality of teeth 10 mounted thereon. The teeth are selectively mounted on the plates 11 so that some of the teeth 10 lie on or adjacent to the centre-line 70 of the chain 6 and some are disposed .transversely remote from the centre-line 70 so as to be adjacent the outer sides 72 of the chain 6. In this specification the centre-line 70 is considered to lie substantially in the respective planes of the plates 11. Specific details with respect to preferred arrangements of the teeth are described i hereinafter.

S If all of the teeth 10 disposed on the chain 6 are placed in transverse alignment, a number of the teeth 10 would occupy the same transverse position.

i! Considering only the unique transverse positions of the teeth 10, as shown in Figures 4 and 4a and generally illustrated in Figure 6, the teeth 10 are transversely disposed relatively far apart as compared to the transverse disposition of the teeth

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13 of previously known trench digging apparatuses, as illustrated in Figure 5. This enables the teeth to be used to perform a chiselling action on the rock 28 be excavated instead of being used to pulverise the rLock. The chiselling action involves cutting the rock 28 at spaced cutting points 62 using the cutting ends of the teeth 10 so as to form grooves in the rock 28 at the points 62. The teeth 10 are periodically driven into the grooves in such a manner that they act as chisels on the grooves in the rock and at the same time also act as wedges. This action generates stress and shear strains in the rock 28 behind the exposed surface 64 of the rock 28 and SI between the grooves formed at the cutting points 62.

tall When the stress generated reaches a sufficient level t due to the persistent chiselling action of the teeth 10, portions of the rock 28 are broken off, or dislodged, from between the grooves at the cutting points 62. The portions dislodged are relatively large fragments of the rock 28 and this is distinct S 1 from the fine particles, or powder, which is produced by the action of previously known trench digging 1 i apparatuses.

The chiselling action can be performed by Sdriving the chain 6 at a relatively low speed and under a high load. The high chain load and low speed are desirable to overcome the disadvantages of known trench digging apparatuses, mentioned previously, and assist in driving the teeth 10 into the grooves at the cutting points 62 so as to generate the level of stress required to dislodge fragments of the rock 28. For a trench digging apparatus mounted on a D7 tractor the chain speed is preferably between 1.4 m/s i i 14 and 1.1 m/s and the chain load is preferably approximately 12 tonnes. For the D8 tractor jI preferably the chain speed is between 0.7 m/s and 0.8 m/s and the chain load is approximately 22 tonnes.

The chain load is the tension in the portion 26 of the chain 6, as shown in Figures 1 and 3, when an excavation operation is being performed and is dependent on the speed of the chain and the speed at which the tractor 4 moves forward across the rock 28. The torque which the drive motor 24 for the chain 6 is required to deliver is determined by the 14 desired chain load and the radius of the wheels II :and 22 on which the chain 6 is mounted.

Cft The teeth 10 are also arranged on the plates S11 so that they are adapted to excavate a trench having a curved transverse cross-section 69, as shown in Figure 6, instead of a rectangular cross-section I ,12, as shown in Figure 5. The trench is excavated so as to have either a semi-circular cross-section 69 or a cross-section having rounded corners and this avoids the problems associated with having to form the corners 14 of the rectangular cross-section 12.

The teeth 10a, however, furthest from the centre-line of the chain 6 are placed under the greatest :stress when cutting into the rock 28 and therefore the transverse arrangement of the teeth 10 is such that they are disposed closer together the further they are placed from the centre-line 70 of the chain 6. The transverse tooth spacing is measured as a fraction of the tooth diameter 25mm from the apex or tip of the teeth 10. In a high stress area remote from the centre-line 70 the spacing is preferably between 0.5 to 1.5 tooth diameter and in a low stress

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area adjacent the centre-line 70 the spacing is preferably between 1.5 to 3 tooth diameter.

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C C~ 16 To further assist in extending the life of the teeth 10, the teeth 10 are arranged on the plates 11 so that when they act on the rock 28 their respective cutting ends are sharpened. This is achieved by mounting the teeth in their respective holders 60 so that they are able to rotate with respect to the holders 60 about their respective longitudinal axes and are disposed so as to engage a 98 9 9 9 9999 9 9 9, r 9* 94 9 tne surrace 64 or tne rock 28 witn a positive rake angle, as illustrated in Figures 7 to 9. The type of tooth 10 used is preferably the Kennametal* C-IHDSR or C-3SR. Both have conical cutting ends with the former having a tungsten carbide capped tip 80 and the latter having a tungsten carbide insert in the tip 80. The sharpening of the tungsten carbide insert tooth significantly extends the life of this type of tooth.

Figure 7 is a side cross-sectional view of the chain 6 showing how the teeth 10 disposed on the chain 6 when rotated in the direction 30 engage the surface 64 of the rock 28 to be excavated. The teeth shown in Figure 7 can all be considered to have longitudinal axes 82 which are disposed above and are co-planar with the centre-line 70 of the chain 6.

The teeth 10 travel on the chain 6 towards the bottom 84 of the trench until they engage the bottom 84 at a point 86. The teeth 10 then traverse a rounded corner portion 88 of the trench and continue upwards over an inclined face 90 of the trench. Figure 8 illustrates a tooth 10 engaging the inclined face whereas Figure 9 illustrates a tooth contacting the Registered Trade Mark 17 initial engagement point 86. The rake angle the teeth 10 make with the surface 64 of the rock 28 is smallest at the initial engagement point 86 and increases as the teeth travel around towards the inclined face 90 over which the rake angle is substantially constant. This is because the angle between the plane of the respective plates 11 and the surface 64 is greatest at the initial engagement point 86 and decreases until the plane of the respective plates 11 is substantially parallel to the surface 64 when the teeth 10 traverse the inclined face The rake angle 92, as shown in Figures 8 and 9, is the angle between the normal 94 to the surface S 64 adjacent an engaging cutting tip 80 and the plane 96 of the surface of the cutting tip 80 closest to the normal 94, the furthest surface of the cutting tip 80 engaging or being adjacent to the surface 64 I of the rock 28. A tooth 10 will only have a positive rake angle with respect to the surface 64 of the rock 28 if its cutting tip 80 is disposed wholely within the quadrant 98, illustrated in Figures 8 and 9. Two quadrants opposite the surface 64 of the rock 28 are I delineated by the normal 94, one towards the j direction of travel 30 of the teeth 10 and the other away from the direction of travel 30, the quadrant 98 being the latter quadrant. A positive rake angle is also only achievable if the angle of the taper of the cutting tip 80 is less than 90°. A tooth 10 has a negative rake angle when the plane 96 of the cutting tip 80 lies in the quadrant 100 disposed towards the direction of travel i ci 1 i i~ eeel *t f 49 1g a 4 9 9 9 49 49 e 9 9 o 9 9 4€ 4 *t 9 To give the teeth 10 illustrated in Figure 7 a positive rake angle, the teeth 10 are mounted on their respective plates 11 so that the angle between the longitudinal axes 82 of the teeth 10 and the planes of their respective plates 11 is between 350 and 430, and preferably approximately 40°. This ensures that the angle between the longitudinal axes 82 of the teeth adjacent the inclined surface 90 and the surface 64 of the rock 28 is between 35" and 43°, and preferably approximately 400. This angular relationship also allows for the decreased rake angle 92 when the teeth 10 intially engage the bottom 84 of the trench at the point 86, as illustrated in Figure 9. The angular relationship may also be defined by stating that the longitudinal axes 82 of the teeth are arranged to be approximately 40° above the centre-line 70 of the chain 6.

To ensure that all of the teeth 10 disposed on the chain 6, and not just those illustrated in Figure 7, have a positive rake angle it must be noted, on considering Figures 6 and 7, that the rounded portion 88 of the trench is substantially hemispherical as the bottom 84 and the inclined face 90 have a substantially semi-circular transverse cross-section. Therefore teeth, as shown in Figures 4 and 4a, which are not disposed on the centre-line need to be disposed at a certain angle above the planes of their respective plates and at a certair angle in the planes of the plates with respect to the centre-line 70. These two angles give rise to a compound angle which is the angle between the longitudinal axes 82 of the teeth 10 and the centre-line 70. The compound angle for each tooth I i i

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9949 oft 4 9~11 4 9r 9 9. 9 99 4 lies in the plane defined by the longitudinal axes 8; of the tooth 10 and the centre-line 70. To ensure all of the teeth 10 are sharpened on engaging the surface 64 of the rock 28 the compound angle of each tooth 10 is between 350 and 43°, and preferably approximately The teeth 10, as shown in Figures 4 and 4a, are arranged in a series of distinct groups and the series is repeated over the length of the chain 6, each group being disposed on a respective plate, to give a pattern repeat length of 526.5 mm. The series 110 for the rench digging apparatus 2 mounted on the D7 tractor is illustrated in Figure 4. The first group of the series 110 comprises 3 teeth 10c and mounted on a first plate lla. One tooth 10c has its tip 80 transversely aligned with the centre-line and the longitudinal axis 82 of the tooth 10c is disposed 37° above the plane of the plate lla and forms an angle of 20 in the plane of the plate lla with respect to the centre-line 70. This gives the tooth 1Oc a compound angle of substantially 37° with respect to the centre-line 70. The other two teeth are disposed symmetrically on opposite sides of the centre line 70 and have their respective cutting tips 80 220 mm apart. The tips 80 of the teeth log are also disposed 65 mm from the tip 80 of the tooth in the direction of travel 30. The longitudinal axes 82 of the teeth 10g are disposed 30° above the plane of the plate lla and the angle in the plane of the plate lla between the respective longitudinal axes and the centre-line 70 is 20°. This gives the teeth lOg a compound angle of 36° with respect to the centre-line 22 A second plate lib, above the first plate lla and going back in the opposite direction to the direction of travel 30, includes two teeth 10d. The teeth 10d are symmetrically disposed on opposite sides of the centre-line 70 with .heir tips 80 170mm apart. The tips 80 of the teeth 10d are also 116.5 mm from the tips of the teeth lOg on the lower plate lla, in the direction of travel 30. The angular disposition of the teeth 10d with respect to the centre-line 70 is the same as that for the tooth and, accordingly, they exhibit a compound angle of 370 with respect to the centre-line A third plate 11c, above the second plate lb and going back in the opposite direction to the direction of travel 30, includes four teeth 10e and A pair of teeth 10e are symmetrically disposed on opposite sides of the centre-line 70 and a further pair of teeth 10f are also symmetrically disposed on opposite sides of the centre-line 70, the tips 80 of the teeth 10e being 90mm apart and the tips 80 of the teeth 10f being 245mm apart. The tips of the teeth are disposed 60mm from the tips 80 of the teeth 10e, which are disposed 175mm from the tips of the teeth 10d, in the direction of travel 30. the angular relationship between the longitudinal axes of the teeth 10e and the centre-line 70 is the same as I that for the tooth 10c. The longitudinal axes 82 of the teeth 10f are disposed 200 above the plane of the plate lic and form an angle of 35° in the plane of the plate llc with respect to the centre-line The compound angle of teeth 10f with respect to the centre-line 70 is therefore

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21 All of the teeth 10 in the series 110 are angled or directed outwards from the centre-line paR o a Ca C

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9@ Re CC a *C 0 C OR f4 22 towards the sides 72 of the chain 6, except for the tooth 10c which is angled towards the centre line from one side of the centre-line 70. To preserve the symmetry of the arrangement of the teeth 10 on the chain 6, the tooth 10c is angled from the opposite side of the centre line 70 in every second series 110 disposed on the chain 6. The tip 80 of the tooth is also disposed 110mm from the tips 80 of the teeth of another series disposed below the first plate lla, in the direction of travel The series 111 for the trench digging l apparatus 2 mounted on the D8 tractor includes the same teeth 10c, 10d, lOe, 10f, and l0g as that of the ,t series 110 for the D7 tractor, however, the teeth are Ui:' arranged in different groups on respective plates 11 and are disposed differently with respect to one another. The angular relationship between the respective longitudinal axes 82 of the teeth 10 and tthe centre line 70 is the same as that for the series 110, described previously. The series 111 comprises tonly two distinct groups of teeth mounted on two respective plates Ila and llb, as shown in Figure 4a. Like reference numerals are used to indicate teeth of the series 111 which are similar to the i teeth of the series 110.

The first plate Ila of the series 111 includes 4 teeth 10e and lOg. The pair of teeth lOg have tips 80 disposed 250mm apart and the pair of teeth 10e have their tips 80 disposed 100mm apart, the distance between the tips 80 of the pairs 10e and lo0g in the direction of travel 30 being I-7i The second plate lib above the first plate lla and going back in the opposite direction to the direction of travel 30, comprises five teeth 10c, and 10f the pair of teeth O1d being disposed with their tips 80 200mm apart and the pair of teeth 1Of having their tips 80 280mm apart. The tooth 10c has its tip 80 aligned with the centre-line 70. The displacement of the teeth 10 of the second plate llb in the direction of travel 30 is as follows: the tips of the teeth 10f are 70mm from the tip 80 of the tooth 10c, which is 80mm from the tips 80 of the teeth 10d, and the tips 80 of the teeth 10d are 100mm from the tips 80 of the teeth 10e. The pattern repeat length of the teeth, in the direction of |r ,travel 30, is 431.8mm, the distance between the tips of the teeth 10g in one pattern length to the tips of the teeth 10f in an adjacent pattern length being 101.8mm in the direction of travel As for the series 110, the tooth 10c of the series 111 is the only tooth angled and directed towards the centre line 70 and the angling of the tooth 10c is from an opposite side of the centre-line t. 70 for every alternate series 111 disposed around the chain 6.

Many modifications of the present invention will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.

Claims (14)

1. A trench digging apparatus comprising: an endless chain, means for driving said chain and means for supporting said chain, said chain including members disposed thereon at regular intervals which are adapted to receive and hold teeth, said teeth being positioned on said members such that the unique transverse positions of the cutting tips of the teeth are spaced apart so that when said chain is driven and positioned to excavate a trench in relatively hard rock, the teeth perform a chiselling action on said rock so as to dislodge fragments of K the rock.

2. A trench digging apparatus according to claim 1 wherein said teeth act to induce shear strains behind an exposed surface of said rock.

3. A trench digging apparatus according to any preceding claim wherein said transverse positions of said teeth are such that the teeth are closer together adjacent the edges of said chain than at the centre of said chain.

4. A trench digging apparatus according to any preceding claim wherein when dislodging said fragments said chain is driven at a relatively low speed under a relatively high load. A trench digging apparatus according to claim 4, wherein where the load is 12 S 25 tonnes, the chain speed is in the range from 1.1 m/s to 1.4 m/s, and where the load is 22 tonnes, the chain speed is in the range from 0.7 to 0.8 m/s.

6. A trench digging apparatus according to any preceding claim wherein said teeth are arranged so that the trenches excavated by said apparatus have a curved cross- section. 911119,dbwspel36,te1ecnon,24 r 25

7. A trench digging apparatus according to any one of claims 1 to 4 wherein the transverse cross-section of said trenches has rounded corners.

8. A trench digging apparatus according to claim 6 wherein the transverse cross section is substantially semi-circular.

9. A trench digging apparatus according to any preceding claim wherein said teeth are mounted on said members so that the teeth are able to rotate about their respective longitudinal axes with respect to their respective members and have a positive rake angle with respect to the surface of said rock to be excavated when engaging said rock. *i "10. A trench digging apparatus according to any preceding claim wherein said teeth are tapered at the cutting ends thereof, the angle of the taper being less than 90°, and are mounted so that the angle between the respective longitudinal axes of said teeth and the centre-line of said chain is between 35" and 43°.

11. A trench digging apparatus according to claim 10 wherein said angle between the axes and the centre-line is approximately

12. A trench digging apparatus according to any preceding claim wherein the cutting ends of said teeth are conical.

13. A trench digging apparatus according to any preceding claim wherein the cutting ends of said teeth comprise tungsten carbide. S14. A trench digging apparatus as claimed in any one of the preceding claims, wherein in a high stress area remote from the centre-line of the chain the transverse spacing is between said teeth in the range from 0.5 to 1.5 tooth diameter, said tooth diameter being the diameter of one of said teeth at the cross section 25 mm from the apex of said tooth. 910521,dbwspcfl29,rnchl .spc, 2 :I -11 -11111 i- i IC1 liilU lllllllr -l- 26 A trench digging apparatus as claimed in claim 14, wherein in a low stress area, adjacent the centre-line of the chain, the spacing is in the range from 1.5 to 3 tooth diameter.

16. A vehicle having a trench digging apparatus according to any preceding claim mounted thereon.

17. A method of trench digging which comprises performing a chiselling action on rock to be excavated. .0

18. A trench digging apparatus substantially as hereinbefore described with reference to any one of Figures 1 to 4, or 6 to 9, of the accompanying drawings. I C *t L r 6- I C, C Ir C CI tIC 4 20 DATED this 21st day of May, 1991. t I) CI'- CC AUSTRALIAN TELECOMMUNICATIONS CORPORATION By its Patent Attorneys DAVIES COLLISON 910521 ,dbwspe.029,trenchl .spc,26 1