GB2117845A

GB2117845A - Hydraulic percussive apparatus

Info

Publication number: GB2117845A
Application number: GB08209962A
Authority: GB
Inventors: Lionel Arthur Reynolds
Original assignee: AF Hydraulics Ltd
Current assignee: AF Hydraulics Ltd
Priority date: 1982-04-03
Filing date: 1982-04-03
Publication date: 1983-10-19
Also published as: FR2524362A1; SE8301571D0; GB2117845B; SE8301571L; DE3311842A1

Abstract

Hydraulic percussive apparatus has a body 104 in which a unitary piston/striker 101 with piston portion 102 and striker portion 103 is reciprocable, for impact with a tool T releasably retained in the forward end of the body 104. The body 104 contains a forward pressure chamber 105 and a return pressure chamber 106. The forward pressure chamber 105 is controlled by a valve V which in a cyclic manner alternately connects the chamber 105 to supply pressure P and to exhaust Ex with fluid pressure in this chamber acting on the piston portion 102 to produce forward strokes of the piston/striker 101 so that the striker portion 103 impacts the tool T. The return pressure chamber 106 is connected to the supply pressure P to provide a continuous return force acting on the piston/striker 101. A channel 113 is closed off during normal reciprocation of the piston/striker 101 but in the event of an overstroke of the latter in the forward direction interconnects the two chambers 105 and 106. The resultant hydraulic flow through the channel 113 pressurises the chamber 105 sufficiently to apply a forward fluid pressure force bias to the piston/striker 101.This holds the piston/striker 101 in the forward position so that reciprocation thereof ceases. <IMAGE>

Description

SPECIFICATION Hydraulic percussive apparatus This invention relates to hydraulic percussive apparatus such as roadbreakers and picks having a piston/striker arrangement which operates to apply successive percussive blows to a cutting tool or the like retained at the front end of the body of the apparatus.

The tools normally used with a roadbreaker, for example, have a shank which is inserted into the body of a breaker and retained therein positioned so that the end of the shank is impacted by a reciprocatory striker. Various tool holding arrangements are employed, usually employing a spring-loaded catch member which engages a retaining flange on the tool shank. While working, the tool shank normally reciprocates freely without the flange striking the catch member but if the tool breaks through the work, or is withdrawn therefrom with the breaker running, the flange impacts the catch member and is retained thereby.

Due to the considerable inertial force involved in arresting a free running piston/striker through the tool, with the latter subject to successive impact blows, the tool retaining means, particularly the catch member and the pivot thereof, have to be correspondingly massive. It will be appreciated that if the apparatus is held with the tool above the horizontal, i.e. inclined upwardly, gravity acts to urge the tool towards the striker so that it is subject to successive impact blows when the piston/striker is free running.With the development during recent years of high powered hydraulic percussive apparatus the provision of tool holding arrangements of sufficient strength and adequate life has resulted in them providing an undue proportion of the weight of the apparatus which is particularly disadvantageous with hand-held apparatus such as a roadbreaker or pick which may be used in the above horizontal position. In fact, the provision of tool holding means of acceptable weight and life has become a problem in the development of small hand-held apparatus of relatively high output power such as is obtainable with modern technology as applied to hydraulic percussive apparatus.

The object of the invention is to solve the foregoing probiem by modification of the hydraulic apparatus so that a free running condition of the striker/piston thereof does not result when the tool is not subjected to an external force.

According to the invention a hydraulic percussive apparatus has a piston/striker reciprocable in a body of the apparatus so that the striker normally impacts a tool releasably retained in the forward end of the body which contains two chambers, namely a valved forward pressure chamber alternately connected to supply pressure and exhaust with the pressure therein producing forward strokes of the piston/striker and a return pressure chamber in use continually subject to the supply pressure to provide a continuous return force acting on the piston/striker, and a channel is provided which is closed off during normal reciprocation of the piston/striker but which in the event of overstroking of the piston/striker interconnects said two chambers so that even though the valved chamber is at this time connected to exhaust the resultant hydraulic flow through the channel pressurises this chamber sufficiently to apply a forward fluid pressure force bias to the piston/striker so that the latter is held in the forward position and reciprocation thereof ceases.

Preferably said channel is provided in the piston/striker and terminates at a peripheral port controlled by the wall of a bore in the body in which the piston/striker reciprocates. Thus, with this preferred arrangement, if the tool is not subject to a working load the resultant overstroke of the piston/striker results in said port being uncovered within the return pressure chamber and the piston/striker held in said forward position without further reciprocation. Reciprocation, with impacting of the tool shank by the striker, does not resume until the operator has applied sufficient force (by pushing against the tool) to return the piston/strikerto a position within its normal range of reciprocation in which said port is closed within the said bore.The flow capacity of said channel is accordingly chosen so that said forward fluid pressure force bias is such that it can be overcome by the operator without undue effort.

The piston/striker is preferably a unitory member with a rear piston portion and forward striker portion. However, the piston and striker may be separate members in which case the striker is preferably a freely-sliding anvil positioned between the piston and the tool.

When the apparatus exhausts into a long hydraulic return hose the back pressure therein may be sufficient to provide the required forward force bias to the piston/striker with an acceptable flow cross-section of said channel. However, in some cases it may be necessary to introduce a small exhaust restrictor in order to increase the back pressure although in many cases such a restrictor will already be present in order to control the operating frequency of the apparatus.

The invention may conveniently be applied to apparatus as disclosed in our Patent Specification No. 1 396 307 which employs a further and reduced pressure chamber into which a valve operating pilot exhausts to provide rapid valve changeover at the end of each forward piston stroke, and in which further chamber fluid leakage past the piston collects. In this case it is necessary to exhaust from that chamber such leakage while said forward pressure force bias is applied to avoid a pressure build up therein, and to this end the exhaust restrictor provided may be replaced by a venturi the throat of which still provides the appropriate restriction and is connected to the third chamber to exhaust the latter as a result of the flow to exhaust through the venturi.

The invention will now be further described with reference to the accompanying diagrammatic drawings which illustrate, by way of example, the basic concept of the invention and its application to a practical form of apparatus.

In the drawings: Figure 1 illustrates the basic concept; and Figure 2 illustrates the practical embodiment.

Figure 1 illustrates diagrammatically hydraulic percussive apparatus of basically known form with a unitary piston/striker member 1, having a piston portion 2 and a striker portion 3, reciprocable in a body 4 having a bore 4a which closely fits the piston portion 2. The body 4 contains two chambers, namely a valved forward pressure chamber 5 and a return pressure chamber 6. The chamber 5 is controlled by a valve V which alternately pressurises and exhausts that chamber in dependence on piston position. Pressurisation of the chamber 5 produces forward impacting strokes of the piston/striker 1 and exhausting of this chamber results in return strokes under the effect of the fluid pressure in the chamber 6 which is permanently connected to the supply pressure P. This pressure acts on an annular step 7 between the piston and striker portions 2, 3.

The valve V, which is connected to supply pressure P and exhaust return line EX, is controlled in dependence on piston/striker position by a pilot port 8 in the bore 4a. In Figure 1 the piston/striker 1 is shown at an intermediate stroke position, and at the end of a return stroke the port 8 is uncovered and pressure P in the chamber 6 acts through a valve pilot connection 9 to changeover the valve V whereby to pressurise the chamber 5 and effect the next forward stroke at the end of which the striker portion 3 impacts a tool T. At the end of a forward stroke the pilot port 8 is connected to an adjacent exhaust port 10 via a peripheral groove 11 in the piston portion 2, and this results in changeover of the valve V to exhaust the chamber 5. Thus a return stroke commences with pressure P in the chamber 6 acting on the step 7.

Overstroking of the piston/striker 3, if the tool T impacted by the striker portion 3 is not subjected to an external force, is arrested by means of a damping dashpot 1 2 disposed at the forward end of the chamber 6 and which is entered by the step 7. However, with known apparatus restriction of the piston/striker 1 continues with continuous overstroking for as long as the tool T is unloaded, so that the tool if in the above horizontal position is impacted on each overstroke with considerable shock loading of the tool retaining means. This is prevented, according to the invention, by the provision of a channel 13 through the piston portion 2. At the inner end the channel 13 communicates with the chamber 5, and at the other end it terminates in a peripheral port 1 4 which during a normal working stroke is closed off within the bore 4a.

In the event of an overstroke of the piston/striker 1, as the step 7 enters the dashpot chamber 12 the port 14 is uncovered within the chamber 6. The result is a flow of pressure fluid from the chamber 6 into the chamber 5 which is at this time valved to exhaust. The flow crosssection of the channel 13 is so chosen, in relation to the flow restriction provided by the exhaust line and the differential piston areas, that the resultant pressure in the chamber 5 builds up to a value such as to provide a resultant force bias on the piston/striker 1 in the forward direction. Thus the latter comes to rest in a forward position within the damping chamber 12, and reciprocation does not recommence until the operator pushes against the tool with sufficient force to return the piston/striker 1 to a position in which the port 14 is closed off within the bore 4a.

Thus if during working the tool T breaks through the work, or is lifted off, the apparatus makes only one overstroke and the unloaded tool T can receive only a single blow. The forward fluid pressure force bias is chosen so that it reliably holds the piston/striker 1 in the forward position but the latter can be returned without undue operator effort which it will be appreciated may be assisted by the weight of the tool. For example, a forward pressure bias requiring a return force of about 30 Ibs may be employed.

Figure 2 shows the invention applied to apparatus as disclosed in our Patent Specification No. 1 396 307 to which reference may be made for a detailed description of the basic operation of this apparatus. It is described herein merely sufficiently to enable the application of the present invention thereto to be understood. Corresponding parts in Figure 2 utilise the reference numerals of Figure 1 increased by 100, and the basic operation is as has been described except that centre of the pilot port 108 is reversed -- it is now connected to the chamber 106 through the piston groove 111 and uncovered for connection to exhaust.

However, in this case the forward pressure chamber 105 is provided by a blind axial bore in the piston portion 102 which slides on a concentric fixed tube 11 5 which provides the hydraulic connection to the chamber 1 05. This construction enables a further third chamber 11 6 to be introduced within the body 104, this chamber being connected to exhaust through a one-way valve 11 7 such that a forward stroke of the piston/striker 1 results in a reduced pressure within the chamber 11 6. At the end of a forward stroke the valve pilot 108 is uncovered to exhaust into this chamber, the reduced pressure in which provides rapid and reliable changeover of the valve V. In accordance with the invention a channel 113 is drilled through the piston portion 2 so that, in the event of an overstroke and as has been described, its outer end port 114 is uncovered within the chamber 106 to provide a pressure build up in the chamber 105 to provide a forward fluid pressure force bias which holds the piston/striker 1 in a forward position within the dashpot 112 until it is returned by pushing against the tool.

In its original form the basic apparatus employs an exhaust restrictor chosen to control the operating frequency, and in the present embodiment this restrictor is replaced by a venturi 11 8 the throat of which is connected through a conduit 11 9 to the downstream (exhaust) side of the valve 11 7. Thus the flow which as before now passes through the channel 113 and the chamber 105 to exhaust passes through the venturi 11 7 which operates to exhaust the chamber 116. This prevents a pressure build up in the chamber 11 6 due to fluid leakage from the chamber 105 past the tube 11 5, and from the chamber 106 past the piston/striker 1, while the piston/striker 1 remains stationary and held in the inoperative forward position.

As in the basic construction of Figure 1 , the channel 11 3 becomes operative when the outer end port 114 enters the chamber 106 at which time the inner end of the channel has cleared the inner end of the tube 11 5. However, it will be appreciated that the operative port 114 could instead be at this inner end and controlled by the tube 115.

Claims

1. Hydraulic percussive apparatus having a body in which a piston/striker is reciprocable, for impact with a tool releasably retained in a forward end of the body, and which contains a forward pressure chamber and a return pressure chamber, said forward pressure chamber being valved and alternately connected to supply pressure and to exhaust with pressure therein acting to produce forward strokes of the piston/striker and said return pressure chamber being in use connected to the supply pressure so as to provide a continuous return force acting on the piston/striker, wherein a channel is provided which is closed off during normal reciprocation of the piston/striker but which in the event of an overstroke of the latter in the forward direction interconnects said two chambers, so that even though the valved forward pressure chamber is at this time connected to exhaust the resultant hydraulic flow through said channel pressurises this valved chamber sufficiently to apply a forward fluid pressure force bias to the piston/striker which is thus held in the forward position so that reciprocation thereof ceases.

2. Hydraulic apparatus according to claim 1, wherein said channel is provided in the piston/striker and at one end terminates at a peripheral port which is controlled by the wall of a bore in which the piston/striker reciprocates.

3. Hydraulic apparatus according to claim 2, wherein the other end of said channel terminates at a surface on the piston/striker which is normally exposed in said forward pressure chamber, and in that said one end of the channel is exposed in said return pressure chamber only in the overstroke condition.

4. Hydraulic apparatus according to claim 1, wherein said forward pressure chamber is formed at the inner end of an axial bore in the piston/striker, and wherein said channel is provided in the piston/striker and at one end terminates at a port in said axial bore, which port is exposed only in the overstroke condition, with the other end of the channel normally exposed in said return pressure chamber.

5. Hydraulic apparatus according to any one of the preceding claims, wherein said piston/striker is a unitary member with a rear piston portion and a forward striker portion.

6. Hydraulic apparatus according to any one of claims 1 to 4, wherein said piston/striker comprises separate striker and piston members and the striker is a freely-sliding anvil positioned between the piston and the tool.

7. Hydraulic apparatus according to any one of the preceding claims, wherein an exhaust restriction is introduced to provide a back pressure to said hydraulic flow which is sufficient to provide said forward force bias for the piston/striker.

8. Hydraulic apparatus according to claim 7, wherein said restriction is provided by a plain restrictor.

9. Hydraulic apparatus according to claim 7, wherein the body of the apparatus contains a third chamber under reduced pressure into which a valve-operating pilot exhausts and in which fluid leakage past the piston collects, and said exhaust restriction is provided by a venturi the throat of which is connected to said further chamber.

1 0. Hydraulic percussive apparatus constructed and arranged substantially as herein particularly described with reference to Figure 1, or Figure 2, of the accompanying drawings.