GB2221418A

GB2221418A - Reciprocating percussive device

Info

Publication number: GB2221418A
Application number: GB8917175A
Authority: GB
Inventors: John Graham Davies; Bruce Foster
Original assignee: Boart UK Ltd
Current assignee: Boart UK Ltd
Priority date: 1988-07-27
Filing date: 1989-07-27
Publication date: 1990-02-07
Anticipated expiration: 2009-07-27
Also published as: GB8817907D0; GB2221418B; GB8917175D0; ZA895716B; CA1325562C

Abstract

A hydraulic/pneumatic percussive device comprising a double-acting piston (60) which executes reciprocal motion and a tool or tool holder (90) which executes limited reciprocal motion due to the piston (60) striking it. The position of the tool or tool holder (90) relative to the piston stroke is maintained substantially constant by use of a spool valve (210, 211) which acts, through a drive gear (90B) and insert (90D) to apply greater or lesser fluid pressure to the tool or tool holder (90) in response to the axial thrust acting thereupon. The spool valve may be made adjustable to allow the maintained position of the tool or tool holder to be adjusted to cope with variations in axial thrust levels. The sleeve 211 of the spool valve can be made movable (left or right) thereby altering the metering position of the spool 210 and hence the strike position. The tool or tool holder may be rotated by a drive gear. <IMAGE>

Description

RECIPRDCATMG PERCUSSIVE DEVICE The present invention relates to hydraulically or pneumatically actuated reciprocating devices, and in particular to percussive devices such as rock drills.

United Kingdom patents GB 1526048 and GB 2157220 describe such devices having preferred embodiments in the form of rock drills and other percussive tools such as impactors (e.g. paving-breakers).

Such devices comprise a cylinder containing a double-acting piston having recessed portions which co-operate with a control port formed in the cylinder wall. This port leads to a controlling valve arrangement which controls the reciprocation of the piston. This reciprocating motion is then imparted to a drill rod or shank adaptor axially aligned with the piston. The above two patents, GB 1526048 and GB 2157220, relate specifically to such devices in which the frequency of reciprocation may be varied.

In the case of rock drills and similar rotary percussive tools, the drill rod or shank adaptor is caused to rotate by gearing linked to a driving motor. The gearing is arranged to allow some degree of axial movement of the drill rod, the generation of rotary motion is thus independent of the percussive mechanism.

In use, there is an optimum position of the drill rod relative to the stroke of the hammer piston, such that the piston strikes the drill rod just prior to the end of the piston power stroke.

Achieving this optimum positioning will result in neuualm transfer of energy to the drill bit with increased penetration rates and minimised reflected or wasted energy which can cause excessive wear of the drill bit and damage to the rock drill components.

Because of the wear of the strike faces which can occur in use together with the build up of tolerances which affect the strike position, wide variations in strike position from one machine to another can result. We have appreciated that a mechanism is required to maintain the optimum strike position and compensate for its possible variation depending on piston power, rock conditions and thrust levels.

In accordance with the present invention there is provided a hydraulic or pneumatic percussive device acmprising a double-acting piston which executes reciprocal mation and a tool or tool holder executing limited reciprocal notion due to the piston striking the said tool or holder near the end of the piston power stroke wherein a spool valve is provided which in response to the axial thrust acting on the tool or holder applies a greater or lesser fluid pressure to the tool or holder to maintain a substantially constant position thereof relative to the piston stroke.

Thus, when drilling, thrust levels will vary depending on rock oonditions and piston power which will vary the forces s on the tool.

During moments of increased force the tool will tend to move backwards (into the drill). By increasing the fluid pressure on the tool or tool holder, the position of the tool or tool holder relative to the device (and hence the piston stroke) is maintained substantially constant. With a corresponding drop in pressure which occurs when drilling forces are reduced, the device maintains the optimum strike position.

In a further preferred embodiment, the system is adjustable, to cope with drilling different types of rock, varying piston power, and varying thrust levels.

The invention will now be further described by way of example and with reference to the acoampanLng drawings in which: Fig. 1 shows a sectioned part-assembly of a percussive rock drill, Fig. 2 is an enlarged view of the spool valve assembly of Fig.l, Fig. 3 shows a variable sleeve position control used in conjunction with the spool valve of Fig. 2, Fig. 4 shows diagrammatic representations of the hydraulic circuits for the spool valve of Fig.2, with (Fig. 4A) or without (Fig. 4B) the control valve of Fig.3.

The structure and operation of the drill is similar to that of our earlier application (GB 2157220) and, where appropriate, the same reference numerals have been used.

Referring initially to Fig. 1, the drill comprises a hammer piston 60 at its left hand end for inverting blows to a drill rod (not shown) retained by a shank adaptor 90 at its right hand end.

The e shank adaptor 90 and drill rod are caused to rotate by means of the engagement of splined portion 86 with an internally splined sleeve 88B which in turn engages via dogs 92B a gear 90B which inoorporates an insert 90D. The gear is rotated by a pinion 93B which is connected to a motor output shaft 94B via splines 200. The pinion is supported in bearings 201 and the motor output shaft 94B is limited in axial movement by stop 203. The e gear 90B is supported in bearings 204 and 205 which are housed in housings 206 and 207. The rotation is independent of the percussive mechanism.

During drilling the drill steel with its bit is thrust in to the rock and the action of percussion and rotation causes it to drill a hole. The reaction from the thrust and drilling forces is resisted by the gear 90B due to its contact, by way of the insert 90D, with the shank adaptor 90. The reaction is then normally absorbed in a thrust bearing situated between the gear 90B and housing 206 (as in OB 2157220A).

To optimise the strike position and hence the effectiveness of the percussive action the gear 90B is in the form of a piston 90C at the left hand side which moves in a cavity 208 counterbored into housing 206. The allowable axial movement of the gear 90B is limited in its forward direction (right hand) by a thrust bearing 209 and in its rearward direction (left hand) by the rear wall of the cavity 208.

To sense and control the axial movement of the gear 90B (and hence the shank adaptor 90) a spool 210 housed in a sleeve 211 is provided in the rear wall of the cavity 208 (see Fig.2).

Pressurised fluid either from a high pressure cavity within the machine or from an external source enters passage 212 at the rear (left hand) end of the spool 210. A central hole 210A through the spool 210 emerges at the right hand end via a smaller choke hole 210B in a hydrostatically balanced pad 210C. The balance is such that contact is maintained between the pad 210C and the piston 90C irrespective of the position of the gear 90B. Connected to the central hole 210A are holes 213 which connect to a first recess 214 on the outside of spool 210. A second recess ss 215 on the outside of the spool 210 is separated fran the first by a land 216.

Holes 218 in sleeve 211 connect to recess 21so. This in turn connects to a passage way which links up with the tank/return line of the rock drill (not shown). Holes 219 in sleeve 211 connect to recess ss 219A and to axial holes 219B which emerge into cavity 208.

In operation pressurised fluid entering passage 212 forces s the spool 210 with its hydrostatic pad 210C into contact with piston 90C. In the position shown the gear 90B and hence the shank 90 are in the optimum strike position. The position is maintained by virtue of the positional relationship between land 216 and hole 219 metering sufficient fluid into or out of cavity 208 to maintain the required pressure acting on area al, to resist the forward thrust of the machine.

The condition of over thrusting or encountering greater resistance to penetration will cause the shank 90 and gear 90B to move into the drill (to the left of Fig. 2) taking spool 210 with it. Now land 216 opens hole 219 and hence connects recess 214 and hole 213 (which are pressurised) to cavity 208 via recess 219A and axial holes 219B. This raises the pressure in cavity 208 to counter the additional forces, and causes the gear 90B and shank 90 to move forward towards the designed strike position.

Conversely during under-thrusting conditions or sudden increased penetration into a fissure or softer rock the gear 90B and shank 90 will move forward (to the right of Fig. 2). Pressure on the end of the spool at 212 will cause the spool to follow the gear hence connecting hole 219 to the tank/return line via recess 215, hole 218 and recess 218A. Pressure in cavity 208 is now reduced and the gear 90B and shank 90 return to their correct strike position by virtue of the normal feed force fram the thrusting mechanism.

The combination of the available pressure and area al of the piston section 90C of gear 90B ensures more than ample reaction force to counter any thrust levels likely to be encountered during drilling. The nett effect is optimum performance with reduced wear and hence increased reliability.

The sleeve 211 can be made movable (left or right) thereby altering the metering position of the spool 210 and hence the strike position. moving the sleeve forward (right) will cause the effective strike position of shank 90 to move forward. MOving the sleeve to the left will cause the strike position to move to the left. The strike position can be retarded or advanced to suit the dynamics of the hammer piston and the type of rock being drilled.

The e sleeve 211 can be moved hydraulically, pneumatically, mechanically or by any oa'bination.

One suitable method of axially moving the sleeve 211 is shown in Fig. 3. Pressure in cavity 220 acts on an area a2 of the sleeve 211 and is resisted by the force fran spring 221. Increasing the pressure moves the sleeve 211 to the right until a balanced situation is maintained and a new strike position achieved.

Fig. 4A illustrates the hydraulic (or pneumatic) system of such a drill having a movable sleeve: Fig. 4B illustrates the corresponding system for a fixed sleeve.

Although the invention has been described with particular reference to percussive rock drills, it will be understood that possible applications are not so limited.

Claims

1. A hydraulic or pneumatic percussive device wtprising a dodble- acting piston which executes reciprocal motion and a tool or tool holder executing limited reciprocal motion due to the piston striking the said tool or holder near the end of the piston power stroke wherein a spool valve is provided which in response to the axial thrust acting on the tool or holder applies a greater or lesser fluid pressure to the tool or holder to maintain a substantially constant position thereof relative to the piston stroke.

2. A device according to claim 1, in which the spool valve comprises a spool movably housed in a sleeve and in which the spool surface has recessed portions cooperating with a pressurised fluid port in the inner surface of the sleeve and so shaped that movement of the spool in response to the axial thrust acting on the tool or holder directs the pressurised fluid to increase or decrease the fluid pressure applied to the tool or holder.

3. A device according to claim 1 or 2, in which the tool or holder is rotated about its axis by a drive gear connected to a motor and in which fluid pressure applied by the spool valve is directed against the drive gear which in turn transmits the resultant force to the tool or holder.

4. A device according to any preceding claim, in which the position of the spool valve may be adjusted, thereby adjusting the maintained position of the tool or holder relative to the piston stroke.

5. A hydraulic or pneumatic percussive device substantially as hereinbefore described with reference to the acwtpanying drawings.