This invention relates to a trench cutting assembly.

Machines which use a rotating wheel fitted with cutters to cut trenches are well known. One kind of such trenching machine employs a wheel which is mounted on and driven by an axial shaft. This allows the wheel to be relatively thin and so to cut a narrow trench, required say for installing cables or pipes under roads or for some types of land drainage. A drawback of this type of wheel is that the drive shaft and its associated drive mechanism have to be above the trench top, which restricts the depth to which the wheel can cut to under half the wheel diameter, and frequently to only one third of that diameter. There is another known type of trenching machine which has a wheel provided with an eccentric drive, where a drive pinion engages external drive teeth near the upper periphery of the wheel. The wheel is rotatably supported on a frame that is narrower than the trench to be excavated so that both the wheel and the frame can be lowered into the trench as it is cut. With this type of wheel trenches can be excavated to a depth of well over half the diameter of the wheel and typically to two thirds thereof, often allowing a wheel of half the diameter to do the same work as a centrally-driven wheel.

The advantage of using a smaller diameter wheel is most important, as for any given depth of trench a much smaller machine may be used. It is also possible to dig deeper trenches than otherwise when the wheel is used as an attachment on plant such as a skid steer loader or a backhoe excavator.

Eccentrically driven wheels as described above are necessarily more complex than those centrally driven

and the frame with the bearings needed to support the wheel and the drive teeth needed to drive it are usually relatively wide, so needing a wider trench to accommodate them. The disadvantage of the extra trench width is not only that more power is needed to drive the wheel but that handling the extra excavated material, as well as providing, carting, infilling and compacting a different backfilling material and reinstating the greater surface area all substantially increase the costs of the overall trenching operation. A principal aim of this invention is to provide a robust arrangement for rotatably supporting and driving a cutting wheel which can be made relatively narrow, whilst maintaining the advantage of depth capacity given by an eccentric drive.

According to the present invention there is provided a trench cutting assembly comprising a rotatable element adapted to carry a plurality of external cutters and having a plurality of rollers mounted internally of any carried cutters which rollers are spaced around a common pitch circle, a support provided with an arcuate support surface on which said rollers of the rotatable element engage so as radially to support the rotatable element for rotation about the axis of said pitch circle, said trench cutting assembly further comprising at least one drive pinion rotatably mounted on said support so that said drive pinion drivingly meshes with said rollers to effect rotation of the rotatable element, and drive means to rotate the drive pinion.

Most preferably, the support surface is of a discontinuous circular form, defined for example by a disc with a cut-out portion in the periphery thereof, said drive pinion being mounted within the discontinuity so that the pinion axis lies within the pitch circle of the rollers and internally meshes

therewith. In this way, the assembly may be made most compact and the rotatable element may be particularly narrow, allowing slit-like trenches to be cut.

Two or more separate drive pinions may be provided within the discontinuity, each pinion having its own drive means, for instance in the form of an hydraulic motor coupled to the respective pinion. This arrangement reduces the forces on each pinion and on the rollers engaged thereby, and also allows the use of lower-powered motors for a required total power input to the rotatable element.

Two specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 shows a trenching machine of this invention mounted on the arm of a backhoe excavator, with the arm pulled in;

Figure 2 shows the arrangement of Figure 1 but with the arm extended; Figure 3 is a diagrammatic end view of the wheel and the support frame;

Figure 4 is a vertical section through the centre of the wheel, face on;

Figure 5 is a vertical section through the centre of the wheel, edge on; and

Figure 6 is a view similar to that of Figure 4, but of a second embodiment of trenching machine.

Referring to Figures 1 and 2, the trench cutting wheel 10 with its support frame 11 is shown attached to an hydraulically operated arm 12 of a backhoe excavator. The trench cutting wheel 10 is rotatably driven using the hydraulic supply of the backhoe excavator. The arms 12 and 13 are used to lower the trench cutting wheel 10 and its support frame 11 to cut a trench to the depth set by depth wheels 14 and then to provide forward motion by pulling the cutting wheel

towards the backhoe excavator.

Referring now to Figures 3, 4 and 5, the cutting wheel comprises a rotatable element 15 including a side disc 16 having a series of equi-spaced holes on a common pitch circle, adjacent its outer periphery. Located in each hole is the end portion of a respective pin 17 mounted on an annular plate 18 of the same outer diameter as disc 16, a respective roller 19 being rotatably mounted on each pin 17 between the disc 16 and the plate 18.

Four cutter carriers 20 (only one of which is shown in Figure 4) are arranged end-to-end around the rotatable element, the carriers being located between the peripheral margins of the disc 16 and plate 18, and being clamped in position by bolts 21, which bolts also serve to hold together the assembly of disc 16 and plate 18. Each carrier 20 supports five cutters 22 each having a replaceable cutting tip, the cutters being arranged to cut different tracks across the trench width, as shown diagrammatically in Figure 5, for three cutters. The bolts 21 also serve to clamp a replaceable annular wear-plate 23 to the disc 16 and another replaceable wear-plate 24 to the annular plate 18. Wear plate 24 is in two semi-circular sections 24A and 24B, and is formed with a part of a labyrinth seal

25 around its inner periphery. A circular seal strip

26 is fixed to disc 16, to extend between the disc 16 and plate 18, to surround the rollers 19 and prevent the ingress of foreign matter thereto. A stationary support plate 27 has a smooth outer periphery of part-circular form, the rollers 19 running on that periphery so as rotatably to mount the element 15 on the plate 27. The support plate 27 is demountably and concentrically attached to an inner disc 28 and a seal plate 29 by countersunk bolts 30. The inner edge of the annular plate 18 is a close

running fit around the inner disc 28, and the outer periphery of the seal plate 29 co-operates with the inner periphery of wear-plate 24 to complete therewith the labyrinth seal 25. Other seal designs may instead be employed.

Bolts 30 serve to attach the inner disc 28 and seal plate 29 to the support frame 11 by an arm 31 so positioned that the arm is always above the level of the ground at the trench top 32 when the trench cutting wheel is in use, and lowered until the depth control wheels 14 engage the ground.

The support plate 27 has a cut-out portion 33 in which is positioned a pinion 34 mounted on, radially supported by and driven by a shaft 35 of an hydraulic motor 36 which is mounted on the arm 31 in such a manner that the pinion 34 is in the same plane as the rollers 19 and the support plate 27, and meshes with the rollers 19 to effect rotation of the rotatable element 15. The rollers 19 thus perform the double duty of providing the radial bearing for the element 15, and also of transmitting the driving force to the element. The rotatable element 15 is held against axial movement in one direction by the side disc 16 engaging one side of the support plate 27, and in the other direction by the inner peripheral region of the annular plate 18 engaging the other side of the support plate.

Figure 6 shows a second embodiment of cutting wheel assembly, in which two drive pinions 34A and 34B are provided, mounted on respective shafts 35A and 35B. Each shaft is driven by an independent hydraulic motor (not shown) . The cut-out 33 in plate 27 must correspondingly be enlarged to accommodate the two pinions, but in other respects the assembly is similar to that described with reference to Figures 3 to 5. Like reference characters have been used to indicate

like parts and these will not be described again, here.

The assembly of Figure 6 allows the load, for a given trenching operation, on each pinion and imparted to the rollers to be halved. Also, hydraulic motors of a lower capacity may be employed.

In an alternative drive arrangement, an hydraulic motor is mounted remote from the pinion shaft 35 (or 35A and 35B), and is coupled thereto for example by a reduction chain-drive. In this case, a flywheel may be mounted on the motor shaft, to assist the cutting action of the wheel.

The rotatable cutting wheel assemblies described above are provided with robust drive arrangements, and are also robustly supported both radially and axially, all within the confines of the axial width of the wheels.