BACKGROUND TO THE INVENTION

THIS invention relates to a pneumatic rock drill and in particular to a pneumatic rock drill of hand-held type. The invention also relates to the muffler of such a pneumatic rock drill.

Conventional hand-held pneumatic rock drills of rotary percussive type, such as are normally used to drill holes into a rock mass in mining operations, have a cylinder in which a piston is caused to reciprocate by compressed air. During each forward or working stroke, the piston strikes the end of a drill steel engaged by a chuck at the front end of the drill. The drill steel is also indexed rotationally either through the action of a rifle bar or the action of a separate rotary mechanism driven by the compressed air supply.

In either case, compressed air is exhausted from the rock drill on each cycle. In known rock drill designs air exhausts through a duct structure on top of the cylinder into a generally rectangular muffler above the cylinder. The relatively large volume in the muffler serves a sound damping function. From the muffler the air exhausts sideways to atmosphere through a series of longitudinally spaced apart exhaust ports.

Modern regulations specify that the noise levels generated by rock drills must be with certain limits at specified distances measured to the sides and rear of the rock drill. The noise levels generated by rock drills of the kind described above do not conform to the modern regulations. Also, the known rock drills are considered to be too heavy. SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a pneumatic rock drill having a cylinder, an exhaust opening from the cylinder for exhausting air therefrom and a muffler which extends around at least a part of the cylinder and over the exhaust opening, the muffler comprising:

- a muffler casing which surrounds at last a part of the length of the cylinder and includes casing parts assembled together around the cylinder, the muffler casing defining a muffler space extending around the cylinder with the exhaust opening venting into an upper part of the muffler space towards a rear end thereof;

- spaced apart exhaust ports venting downwardly from a front end of the muffler space through the muffler casing; and

- a baffle structure located in the upper part of the muffler space between the exhaust opening and the exhaust ports, the baffle structure including a series of spaced apart, perforated baffle plates through which air venting into the muffler space through the exhaust opening must pass en route to the exhaust ports.

According to another aspect of the invention there is provided a rock drill muffler for a pneumatic rock drill having a cylinder and an exhaust opening from the cylinder, the muffler comprising:

- a muffler casing including casing parts which can be assembled and held together around the cylinder such that the muffler casing surrounds at least a part of the length of the cylinder over the exhaust opening and defines a muffler space extending around the cylinder with the exhaust opening venting into an upper part of the muffler space towards a rear end thereof;

- spaced apart exhaust ports through the casing parts which are arranged, when the casing parts are assembled around the cylinder, to vent downwardly from the muffler space at a position towards a front end of the muffler space; and

- a baffle structure located in an operatively upper part of the muffler space so as to be between the exhaust opening and the exhaust ports, the baffle structure including a series of spaced apart, perforated baffle plates through which, in use, air exhausting into the muffler space through the exhaust opening must pass en route to the exhaust ports.

The muffler casing may comprise two semi-cylindrical casing parts which are assembled and held together around the cylinder, the muffler including straps which encircle the casing parts to hold them together around the cylinder. The casing parts are preferably of moulded rubber construction, the rubber typically having a Shore hardness of about 80.

The baffle structure may comprise a support plate and a series of spaced apart, perforated baffle plates which stand up from the support plate. The muffler casing may include support formations that support the support plate of the baffle structure. These formations may be opposed, horizontal slots formed internally in the muffler casing to receive opposite edges of the support plate. The muffler casing may also include spaced apart pairs of internal, downwardly extending ribs with vertical slots defined between the ribs of each pair, and upper edges of the baffle plates being received in the vertical slots. Each baffle plate can be perforated by a similar array of holes with the holes of the baffle plates aligned with one another. Alternatively, the baffle plates can be perforated by non-aligned holes.

Preferably the exhaust ports only vent downwardly from the muffler casing and the exhaust ports have an area equal to or greater than the area of the exhaust opening.

Preferably also, the muffler casing carries an internal, upstanding barrier wall, located rearwardly of the exhaust ports in a lower part of the muffler space, to prevent oil which is carried in use into the muffler space by exhaust air venting into that space through the exhaust opening and which precipitates from the exhaust air in the muffler space from reaching the exhaust ports. -A-

BRIEF DESECRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows an exploded perspective view of a rock drill according to this invention;

Figure 2 shows a perspective view of the assembled rock drill;

Figure 3 shows a side view of the assembled rock drill;

Figure 4 shows a cross-section at the line 4-4 in Figure 3;

Figure 5 shows a rear perspective view of a muffler, according to the invention, which forms part of the rock drill seen in the earlier Figures;

Figure 6 shows a front perspective view of the muffler; and

Figure 7 shows an underplan view of the muffler.

DESCRIPTION OF A PREFERRED EMBODIMENT

Figure 1 shows a rock drill 10 according to this invention. The rock drill has a rock drill body 12 having a back end 14, a front end or cover 16, a cylinder 18 between the back and front ends, a compressed air inlet conduit 20, a handle 22 at the back or rear end, a chuck 24 at the front end for receiving the rear end of a hexagonal section drill steel (not shown), a control lever 26 by means of which an operator can control the flow of compressed air into the cylinder, a bracket 28 to which the upper end of an air leg (not shown) can be attached and a pivoted drill steel latch 29 for engaging and holding the drill steel in the chuck. The numeral 30 indicates exhaust openings, in an upstanding exhaust duct 31 , through which air exhausts from the cylinder during cycling of the rock drill.

Referring to Figure 4, the cylinder 18 accommodates a central water tube 32 around which a rifle bar 34 is located. The numeral 36 indicates a reciprocating piston surrounded by a bronze liner 40.

Persons skilled in the art will recognise that the rock drill body 12 is essentially conventional and of the type sold by the applicant under the trade mark Compair Drill™. Such persons will recognise that in use compressed air fed to the cylinder through the conduit 20 under the control of the lever 26 is suitably valved to cause the piston to reciprocate longitudinally and, on each forward stroke, to impact on the rear end of the drill steel. The interaction of the piston with the rifle bar 34 causes indexed rotation of the piston and chuck, and hence of the drill steel, during reciprocation of the piston. Each impact on the drill steel causes a shock wave to be transmitted through the drill steel to a drill bit (not shown) at the leading end thereof and this, in combination with the indexed rotation of the drill steel, allows the drill bit to perform a rotary percussive drilling action into a rock mass when the drill bit is urged towards the rock mass, typically by the action of an air leg.

In accordance with the present invention, the rock drill 10 includes a muffler 42 having a muffler casing 44 composed of casing parts 46 and 48. Each of these parts has a curved side wall 50; 52, a rear end wall 54; 56 and a front end wall 58; 60. The casing part 46, referred to as a male part, includes a rib 62 extending along its longitudinal and end edges and the casing part 48, referred to as a female part, includes a groove 64 extending along the corresponding edges.

In use, the parts 46 and 48 are assembled around the drill body 12 by moving them sideways towards one another such that the rib 62 enters and seats in the groove 64. The assembled parts are then connected to one another by means of encircling straps 66, shown in Figures 2 and 3 only, which locate in strap- receiving recesses 68 in the external surfaces of the parts.

The casing parts 46 and 48 are of moulded rubber construction. The rubber is typically a synthetic rubber having a Shore A hardness of about 80. The straps 66 are of flexible steel and typically include diagrammatically illustrated fastening buckles 69 of over-centre or crimped type.

The end walls 54, 56, 58 and 60 of the casing parts are shaped to mate closely with the relevant parts of the exterior of the drill body 12 so that, with the casing parts 46 and 48 bound tightly together around the drill body as described above, the resulting muffler 42 forms an enclosure around the body which is, apart from the exhaust ports mentioned below, airtight. As shown in Figure 2, the muffler extends over a part of the drill body. The rear end of the muffler locates behind the duct 31 and the front end locates forwardly of the front end 16. The muffler extends over a part of the cylinder 18 including the exhaust duct 31 , so that the exhaust openings 30 vent into an upper part of the muffler towards the rear end thereof.

Referring to Figure 4 it will be noted that the interior dimensions of the muffler are somewhat greater than the exterior dimensions of the drill body 12, with the result that the muffler defines a muffler space 70 which extends completely around the drill body.

The muffler 42 also includes a baffle structure 72 comprising a base plate 74 and a series, in the illustrated case five, spaced apart, perforated baffle plates 76 mounted on edge on the plate. The base plate 74 and baffle plates 76 are typically made of steel, although other suitably rigid materials could also be used. In the illustrated case the baffle plates are perforated by similar arrays of round holes 78 which are aligned with one another. In other embodiments, there could be more or less baffle plates and different , possibly non-aligned hole arrays in different baffle plates. The muffler casing parts 46 and 48 are formed with horizontal slots 73 to receive end edges of the base plate 74 when the casing parts are assembled together around the drill body 12. They are also formed with spaced apart pairs of ribs 77, defining vertical slots 79 between them, to receive free upper edges of the baffle plates when they are so assembled. As shown in Figure 4, the baffle plates of the baffle structure 72 locate in front of the exhaust openings 31 in the assembled muffler.

A lower part of each of the casing parts 46 and 48 is formed with a series, in the illustrated case three, exhaust ports 80 located towards the front end of the casing part. As will be apparent from Figures 6 and 7, the exhaust ports 80 are downwardly oriented in a lower quadrant of the assembled muffler.

Referring to Figure 1 , a lower part of each of the casing parts 46 and 48 includes an internal, upstanding wall formation 82 situated just behind the exhaust ports 80. In each case the wall formation extends from the lower longitudinal edge of the casing part to an upper extremity 84 located roughly half way up the curved side wall of the casing part. When the casing parts are assembled together the wall formations cooperate to form a continuous barrier wall which extends across the lower part of the muffler.

In use, with the muffler 42 assembled about the rock drill body 12 and the drill running, air is exhausted from the cylinder 18 through the duct 31 and exhaust openings 30 into the upper part of the muffler space 70 towards the rear end of the muffler. The exhausted air impinges on the baffle plates 76 and must pass through the holes 78 into the downstream part of the muffler space. It will be understood that entry of the exhausted air into the relatively large muffler space, together with the impingement of the air on the baffle plates, decelerates it substantially. The deceleration of the exhausted air results in damping or muffling of the sound which is generated. The air eventually flows through the muffler space 70 to the exhaust ports 80 through which it exhausts to the surrounding atmosphere. In practice, the cross-sectional area of the ports 80 will be equal to or greater than the cross-sectional area of the openings 30 in order to prevent a build-up of exhaust pressure in the muffler space 70.

In cases where the baffle plates are formed with non-aligned and/or different hole arrays, streams of exhaust air can be forced to follow nonlinear or tortuous paths through the baffle structure and it can be expected that this will further enhance the sound damping which is achieved.

In initial tests with a muffler of the kind described above it has been shown that exhaust noise can be kept to a value of 103dB or less. Persons skilled in the art will recognise that such noise levels are acceptable in terms of modern regulations.

It is common for the compressed air which is supplied to a rock drill to be passed through an oil pick-up where it entrains oil droplets to lubricate the working parts of the drill. In a conventional rock drill, the entrained oil droplets are exhausted into the ambient atmosphere along with the exhaust air. With a muffler 42 as described above, oil droplets carried by the flow of exhaust air which enters the muffler space are caused to precipitate by the deceleration of the air flow. These oil droplets migrate into the lower part of the muffler space, but are prevented from leaving the muffler space through the ports 80 by the barrier wall formed by the wall formations 82. Thus the oil is captured in the lower part of the muffler space during use of the rock drill. The accumulated oil may periodically be removed to waste by appropriately manipulating the rock drill or disassembly of the muffler.

The use of a hard rubber to form the muffler casing is advantageous both in that it can reduce the overall weight of the rock drill and in that the muffler casing is particularly robust and provides substantial protection in the event that the rock drill is dropped on a hard surface or is otherwise impacted.

A further advantage of the rock drill and muffler described above is the fact the air exhausted from the muffler space is only directed downwardly rather than to the side. This can be expected to achieve a further reduction in noise levels measured above and to the side of the rock drill in use.

In this specification directional terms such as upper, lower, front, rear and so on refer to a normal orientation of the rock drill as shown in Figure 3, i.e. held generally horizontally with the chuck facing towards the face of a rock mass which is to be drilled.