GB2035866A - A device for applying an impact - Google Patents

Abstract

A device for applying an impact such as to load bearing piles for driving the latter comprises an anvil (12) abuttable with an object or substance to which an impact is to be applied such as a load bearing pile (20), a piston (14) movable towards and away from the anvil for applying an impact thereto, guide means (10) for guiding the piston during said movement and a liquid cushion (32) interposed between the anvil and the piston so that an impact applied by the piston is transmitted to the anvil by the liquid cushion. <IMAGE>

Description

SPECIFICATION A device for applying an impact This invention reiates to a device for applying an impact and particularly but not exclusively to such a device for applying repeated impacts to an elongated element, such as a load bearing pile, so as to drive the element into the ground.

Known pile driving devices comprise an anvil which is supported in abutting relationship with a pile to be driven, a piston guided in relation to the anvil and a cushion of solid yieldable material, specifically of wood, between the anvil and piston for extending the duration and reducing the amplitude of a resultant stress wave set up in the pile being driven. Such a device possesses two major drawbacks, namely that the cushion needs to be replaced frequently, e.g. after circa every 500 impacts, and that the noise level on impact is high.

It is thus an object of the present invention to provide a device for applying an impact in an improved form.

According to the present invention, a device for applying an impact comprising an anvil abuttable with an object or substance to which an impact is to be applied, a piston movable towards and away from the anvil for applying an impact thereto, guide means for guiding the piston during said movement and a liquid cushion interposed between the anvil and the piston so that an impact applied by the piston is transmitted to the anvil by the liquid cushion.

Advantageously, the liquid has a gas (such as air) entrained therein and/or the piston includes means for trapping a pocket of gas (such as air) between the piston and the cushion upon impact therebetween. The trapping means may comprise a depression in the impact transmitting surface of the piston.

Preferably, the liquid cushion is disposed in a cylindrical sleeve sealingly engaging the anvil. The guide means may comprise an extension of said cylindrical sleeve and in this event the sleeve has one or more gas escape ports adjacent to the surface of the liquid cushion for the egress of gas as the piston moves towards the anvil. Conveniently, the ports communicate with a liquid reservoir which collects liquid, which may escape through the ports during movement of the piston towards the anvil, for return to the interior of the sleeve after an impact has occurred.

The anvil may be mounted for movement relative to the sleeve in which case means for limiting movement between the anvil and the sleeve or the piston are provided.

Conveniently, means are provided for causing gas to bubble through the liquid cushion.

The invention will now be more particularly described with reference to the accompanying drawing which is a sectional view of one embodiment of a device according to the invention.

Referring to the drawing, the pile driving hammer shown therein comprises a cylindrical sleeve 10 to the lower end of which is connected an anvil 1 2 and the upper end of which serves as a guide for a piston 14. The anvil 1 2 has a stepped circumference which defines a shoulder 1 6 between its ends.The upper, smaller diameter end extends into the lower end of the sleeve 10 and carries one or more sealing rings which engage the inner wall of the sleeve 1 0. The lower, larger diameter end has a cavity 1 8 in its lowermost surface, the cavity 18 having a cross-sectional shape similar to that of a pile 20 to be driven so that when the inner surface of the cavity 1 8 abuts the end of the pile 20, the collar 22 surrounding the cavity serves to locate the anvil 1 2 with respect to the pile 20. Secured to the shoulder 1 6 and upstanding therefrom are a plurality of equidistantly spaced bolts 24 which extend through correspondingly spaced apertures formed in a radially outwardly directed flange 26 which is integral with the lower end of the sleeve 10.The free end of each bolt 24 is engaged by a nut 28 having an associated washer and a compression spring 30 surrounds each bolt 24, one end of each spring engaging the flange 26 whilst the other end engages the aforesaid washer. Between the flange 26 and the shoulder 1 6 and surrounding each bolt 24 is a short spring or element of shock absorbing material 46 to prevent the face of the flange 26 from striking the anvil shoulder 16. Such a connection between the sleeve 10 and anvil 1 2 permits limited movement of the anvil 1 2 with respect to the sleeve 10 and this is particularly desirable when the anvil is subjected to repeated impact as hereinafter described since, if the anvil and sleeve are rigidly connected together, their combined inertia may well be such as to result in severe damage thereto.

The piston 14 in the form of a drop hammer effecting impact under the influence of gravitational force alone is sealingly slidable in and thereby guidable by the upper end of the sleeve 1 0. Retraction or upward movement of the piston 14 is effected by any suitable lifting device and the cavity defined between the anvil 12, sleeve 10 and piston 1 4 when the latter is in the retracted position is partially filled with a liquid 32, such s hydraulic oil, serving as a cushion between the anvil 1 2 and piston 14 during impact therebetween.

The lower end of the piston 1 4 has a central, concave depression 34.

The sleeve 10 has a plurality of circumferentially spaced ports formed at a level immediately above the normal level of the upper surface of the cushion 32 and an annular trough 42 is secured to and surrounds the sleeve 10, the lower part of the trough being in direct communication with the ports 40. An annular plate 44 is also secured to the sleeve 10 and is spaced upwardly of but masks the trough 42 for a reason which will become apparent hereinafter.

The sleeve .0 has an opening therein below the trough 42 and in direct communication with the liquid cushion 32. A non-return valve 48 is located in the opening to permit air to flow into the liquid cushion when the latter is subjected to sub-atmospheric pressure as described hereinafter. but to prevent liquid exiting through the opening.

When driving a pile, the device is mounted on the upper end of a pile and supported by any convenient structure which may be attached to the lifting device used to retract the piston 14. Starting with the piston 14 in a retracted position, the piston is released and falls under gravitational force. As it falls air in the aforesaid cavity and above the surface of the liquid cushion 32 is mainly ejected under the influence of the failing piston through the ports 40, although some of the air is compressed in the cushion 32. The air ejected through the ports 40 vents to atmosphere whilst most of the liquid droplets entrained in this ejected air fall back into the trough 42 upon impact with the mask 44.Upon impact of the piston 36 with the liquid cushion 32, the cushion yields to a degree due to the entrained air in the liquid and the effect of this is enhanced by the provision of the depression 34 in which an air pocket is trapped upon impact. It is also believed that the provision of the depression 34 will effect noise reduction on impact. The impact thus transmitted to the anvil 1 2 is partially absorbed by the cushion 32 and this cushion in effect reduces the amplitude of and extends the duration of the resultant stress waves set up in the pile 20.

After impact, the piston 14 is retracted during which time air re-enters the aforesaid cavity from the atmosphere through the ports 40 and the non-return valve 48. It will be appreciated that this will be fresh air and will aid cooling of the device. During retraction of the piston 14, liquid which has collected in the trough 42 will return to the aforesaid cavity through the ports 40 to thus replenish the liquid cushion.

The liquid cushion will require only very infrequent replacement, although it may be necessary to top it up occasionally due to liquid loss over the trough 42 and beneath the mask 44. The noise on impact between the piston 14 and liquid cushion 32 is very much less than the noise of an equivalent impact on a wooden cushion and it is thought that the degree of noise reduction is improved by the provision of the depression 34. The frequency at which the device can be oper ated is governed to a great extent by the rate at which the constituent parts of the device can be cooled, and thus, if desired, the sleeve 10 may be water or air cooled.

The liquid cushion 32 may be additionally cooled by bubbling air therethrough during the power stroke. This could be achieved by providing a further set of ports within the sleeve 10 but above the mask 44. This further set of ports (not shown) could communicate through ducts and by way of non-return valves with the lower end of the aforesaid cavity, so that during initial downward movement of the piston 14 in the course of an impact stroke, air within the cavity will be forced through this further set of ports and will be introduced into the liquid cushion at the lower end thereof. Such an arrangement has the additional advantage that some of the air bubbling through the liquid cushion will be entrained therein, this being especially desirable under controlled conditions since it will increase the degree to which the cushion 32 will yield when it is struck by the piston 14.

In an alternative embodiment, the further set of ports in the sleeve 10 could be replaced by one or more passages arranged in the piston 14 and forming part of one or more interconnecting ducts between the aforesaid cavity and the lower end of the liquid cushion 32.

In a modification of the above-described embodiment, the piston is reciprocated by power operated means.

In addition to pile driving applications, the above-described device may be used to drive any other elongated element into the ground or, alternatively, could be used, inter alia, for compacting earth.

Claims (11)

1. A device for applying an impact comprising an anvil abuttable with an object or Substance to which an impact is to be applied, a piston movable towards and away from the anvil for applying an impact thereto, guide means for guiding the piston during said movement and a liquid cushion interposed between the anvil and the piston so that an impact applied by the piston is transmitted to the anvil by the liquid cushion.

2. A device as claimed in claim 1 wherein the liquid cushion has a gas entrained therein.

3. A device as claimed in claim 1 or claim 2, wherein the piston includes means for trapping a pocket of gas between the piston and the cushion upon impact therebetween.

4. A device as claimed in claim 2 or claim 3 wherein the gas is air.

5. A device as claimed in any one of the preceding claims wherein the liquid cushion is disposed in a cylindrical sleeve sealingly engaging the anvil.

6. A device as claimed in claim 5 wherein the guide means comprises an extension of said cylindrical sleeve and the sleeve has one or more gas escape ports adjacent to the surface of the liquid cushion for the egress of gas as the piston moves towards the anvil.

7. A device as claimed in claim 6 wherein the ports communicate with a liquid reservoir which, in use, collects liquid, which may escape through the ports during movement of the piston towards the anvil, for return to the interior of the sleeve after an impact has occurred.

8. A device as claimed in any one of claims 5 to 7, wherein the anvil is mounted for movement relative to the sleeve.

9. A device as claimed in claim 8, wherein means for limiting movement between the anvil and the sleeve or the piston are provided.

10. A device as claimed in any one of the preceding claims wherein means are provided for causing gas to bubble through the liquid cushion, in use.

11. A device for applying an impact substantially as herein described with reference to and as shown in the accompanying drawing.