US3740960A

US3740960A - Elastic pressure fluid driven motor

Info

Publication number: US3740960A
Application number: US00086580A
Authority: US
Inventors: E Erma
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-11-07
Filing date: 1970-11-03
Publication date: 1973-06-26
Anticipated expiration: 1990-06-26
Also published as: CA922994A; BE758504A; DE2053336B2; ZA707293B; FR2069110A5; DE2053336A1; GB1325498A; AT302936B; SE339816B

Abstract

The invention concerns an elastic fluid driven motor consisting of a cylinder housing wherein a piston is reciprocably guided. The piston is hollow enclosing a surge chamber and acts as its own distributing valve. For reducing the stresses occuring in the piston it is recoiled at both its end positions by means of elastic fluid cushions. At the forward end position the piston is recoiled by a powertransmitting elastic fluid cushion and at its rear end position the piston is rebounded by an elastic cushion enclosed in a chamber formed in part by the piston during its return stroke.

Description

United States Patent 1 Erma [ ELASTIC PRESSURE FLUID DRIVEN MOTOR [76] Inventor: EeroAntero Erma, 23,

Ottfjallsvagen, Klinten, Sweden [22] Filed: Nov. 3, 1970 [2]] Appl. No.: 86,580

[30] Foreign Application Priority Data Nov. 7, 1969 Sweden 15298/69 [52] US. Cl ..60/542, 91/234, 74/583, 173/116 [51] Int. Cl. F01l 21/02 [58] Field of Search 91/234; 173/116;

[56] References Cited UNITED STATES PATENTS 3,045,433 7/1962 Webber et a1. 60/545 H June 26, 1973 3,601,010 8/1971 Burgess, Jr ..9l/234 3,611,517 10/1971 Giersberg 60/l0.5

Primary ExaminerPaul E. Maslousky Att0rney--Munson & Fiddler 5 7] ABSTRACT 6 Claims, 3 Drawing Figures "noilot ss 20 w ii 4 V 5 i! is; 2/ a g I g f/ 12b 5% 5i; 8 IO mmmmzs ms 3. 740.960

EERO ANTERO ERMA 'g' INVENTOR.

MUNSON & FIDDLER,

Attorneys.

Pmmcnmzsm 3.740.960

SIIII 2 I 3 Fig. 2

EERO ANTERO ERMA INVENTOR.

BY MUNSON & FIDDLER,

Attorneys.

HERO ANTERO ERMA INVENTOR.

MUNSON & FIDDLEI Q,

Attorneys.

ELASTIC PRESSURE FLUID DRIVEN MOTOR This invention relates to an elastic pressure fluid driven motor consisting of a cylinder housing and a piston reciprocably guided in said housing. The piston is hollow and encloses a chamber which is fed with elastic pressure fluid from an elastic pressure fluid source. The piston and the housing are provided with openings and the housing is provided with distributing channels for distributing pressure fluid from the chamber into the cylinder during the return stroke and power stroke of the piston and vent it to the atmosphere, while the piston is reciprocated in the housing.

The invention intends to provide a reciprocating motor, having a great driving force and high frequency. These objects of the invention are obtained by the characteristics stated in the claims.

The invention is further described with reference to the accompanying drawings wherein:

FIG. 1 shows a longitudinal section of a reciprocating motor according to the invention, which motor is built together with an impacting device or percussion device, for instance for a rock drill.

FIG. 2 shows a longitudinal section of a modified motor built together with an impacting device.

FIG. 3 shows a longitudinal section of still another modified motor built together with an impacting device.

The machine shown in FIG. 1 is constructed in accordance with the invention and is intended for rock drilling and the like. It consists of two main parts, namely a pressure fluid driven motor 1 and an impacting device 2 both of which are located within a common housing 3.

The motor 1 consists of a cylinder 4 in which a piston 5 is reciprocably guided. The piston is thin walled and encloses a chamber 6. This chamber 6 is supplied with the driving fluid of the motor from a pressure fluid source. The driving fluid is gaseous and consists preferably of air. (To simplify the terminology the machine will be described in the following with air as driving medium.)

The piston 5 is at both ends provided with end parts 7 and 8, which are coaxial with and of less diameter than the main part of the piston. The end parts 7 and 8 are guided in extensions 9 and 10 of the cylinder 4. One of the end parts is open at its outer end constituting a tubelike extension of the piston. The corresponding extension 9 of the cylinder 4 is directly connected to a pressurized air conduit by means of a nipple 11. The other end part 8 of the piston 5 is closed at its outer end constituting a drive piston for the impacting device. The cylinder 4 and the piston 5 enclose

annular chambers

12a, 12b at the ends of the main part of the piston. These chambers are intended to be supplied with pressurized air for driving the piston during its work and return strokes. They are also intended to enclose air cushions by means of which the piston is rebounded in its end positions.

To distribute pressurized air to the two cylinder chambers the piston and cylinder walls are provided with openings 13 and 14-17 respectively and distributing channels 18-19. The cylinder 4 is also provided with exhaust openings 20-21. These openings are covered and uncovered by the piston 5 during its movement in the cylinder, so that pressurized air, via the dis tributing channels, always is distributed to the proper side of the piston to drive it forwards and backwards in the cylinder. In the drawing there is shown only one of each of the openings and the distributing channels, but there is in fact a number of them distributed over the piston and the cylinder circumferences.

As the piston 5 moves in the cylinder 4 towards one of its end positions, the upper in FIG. 1, the order of operation is as follows:

At first the front edge of the piston covers the exhaust opening 20. Then the

openings

13 and 15 uncover each other so that pressurized air can flow from the chamber 6 within the piston, through the distributing channel 18 and the opening 14 into the chamber 120. The chamber is pressurized. In the next moment the front edge of the piston covers the opening 14 enclosing the chamber 12a. In this chamber is now enclosed an air cushion by means of which the piston is being slowed down. The air cushion is then compressed. Owing to the elasticity of air the air cushion has now an inherent power which accelerates the piston in the opposite direction. Thus the piston 5 has been rebounded at its end position by means of an air cushion and received a velocity in the opposite direction. At first, moving in this new direction, the piston will uncover the opening 14. At this moment the air cushion has recovered its original volume and its pressure is reduced to the original level or to the same pressure as in the distributing channel 18 and the chamber 6. The pressurized air conducted from the chamber 6 can now drive the piston 5 forwards. Finally, the connection between the chamber 6 and the distributing channel 18 is broken and after some expansion of the air within the chamber 12a the exhaust opening 20 is uncovered so that the pressurized air in the chamber 120 may escape into the atmosphere. The above described order of operation is repeated every time the piston 5 is changing direction.

As the piston 5 is changing direction by means of air cushions the stresses in the piston are considerably lowered. That means that the piston can be made with thin walls and be comparatively light. Consequently the piston can be made with a large cross section area without increasing the piston weight. The piston may preferably be made of a light metal alloy. Owing to the low piston weight and to the fact that the piston is rebounded by means of air cushions at the end positions, the motor can operate with a very high frequency. Another fact which has a determining influence on the frequency, is the chamber within the piston. This chamber will serve as a surge chamber, because one problem in increasing the frequency or impacting rate of a motor of this type is to feed the cylinder chambers with driving fluid at a sufficiently fast rate. This fluid must have a very high velocity during a very short period of time. As the conduit, supplying the motor with pressure fluid, cannot, for practical reasons, have such a cross-sectional area the is sufficiently large to permit such a high speed, a surge chamber must be provided to equalize the intermittent fluid flow.

Moreover, owing to the large cross-sectional area of the piston and the high frequency the power output of the motor the device is especially suitable for driving the impacting device hereinafter described.

As being apparent from FIG. 1, the impacting device 2 consists of a hammer piston 22, a drive piston 23 intended for driving the hammer piston and an auxiliary piston 24. In this case the drive piston 23 is being constituted by the end part 8 of the motor piston 5. The

drive piston 23 drives the hammer piston 22 by means of an air cushion, enclosed within a drive chamber 25 between the two pistons. This drive chamber 25 is fed with pressurized air through an inlet opening 26 which is uncovered by the drive piston when the latter is located at its upper end position. In this position the drive piston is spaced at the largest distance from the hammer piston. During its working stroke towards the hammer piston, the drive piston covers the inlet opening 26 to define a closed chamber. As this chamber is fed with pressurized air the pressure therein will be multipled during the power and compression stroke of the drive piston. This high pressure makes it possible to use a hammer piston having a small effective area with no substantial differences in its mean cross-sectional dimensions throughout its length. A hammer piston having such even mean cross-sectional dimensions throughout its length is very advantageous in that the fatigue stresses in the drill steel will be very low. Although the effective area of the hammer piston is small, the high pressure powertransmitting air cushion makes it possible to use a heavy hammer piston. Thus, this type of power transmitting makes possible the use of a hammer piston having desirable shape and weight and causing less stresses in the drill steel than conventional impacting devices.

Furthermore the impacting device 2 is provided with an annular auxiliary piston 24 surrounding the hammer piston 22 as a sleeve. The auxiliary piston 24 is at one end provided with an annular flange 27 cooperative with a flange 28 on the hammer piston. The object of the auxiliary piston 24 is to limit the hammer piston return stroke and to stop the hammer piston at a predetermined distance from the drive piston irrespective of the recoil energy admitted from the drill steel. Hereby, the kinetic energy of the hammer piston is absorbed by the auxiliary piston as the

flanges

27 and 28 hit each other and the hammer piston is stopped in the predetermined position. As this position is critical with respect to the total impact energy output it is important that the hammer piston always remains in this position before the succeeding stroke.

The weight of the auxiliary piston should be about 40 percent more than that of the hammer piston to make sure that the hammer piston will stand still after hitting the auxiliary piston.

The auxiliary piston 24 is kept in its rest position against the lower end wall 29 of the machine by pressurized air acting upon the upper end surface of the auxiliary piston. Pressurized air leaks through a clearence space between the hammer piston 22 and the housing 4 into a chamber 30 located above the auxiliary piston 24 and acts upon the upper end surface of the auxiliary piston. The space between the lower end surface of the auxiliary piston and the end wall 29 of the housing communicates with the atmosphere.

,As the hammer piston 22 is driven downwardly by the drive piston 23 the following is happening. The hammer piston moves downwardly until it hits the drill steel neck (not shown). Then the hammer piston changes direction by means of the drill steel recoil on one hand and the compressed air cushion in the chamber 31on the other hand. A considerable amount of the recoil energy of the hammer piston 22 is transmitted to the drive piston by means of the air cushion in the drive chamber 25. When the hammer piston reaches its upper direction changing position, the flange 28 of the hammer piston hits the flange 27 of the auxiliary piston so that the remaining kinetic energy of the hammer piston is transmitted to the auxiliary piston. The hammer piston is thereby stopped in the predetermined position while the auxiliary piston starts to move upwardly towards the drive piston 23. The auxiliary piston is retarded to its rest position by means of the air cushion in the drive chamber 30.

Owing to the powertransmitting air cushion arrangement, the impacting device according to the invention, works with a short stroke at a high frequency and develops a high impact energy output per stroke, while the fatigue stresses in the drill steel are low. By fixing the starting position of the hammer piston before every working stroke, the auxiliary piston arrangement is also effective in producing a high total impact energy output per time unit.

FIG. 2 shows another embodiment of the invention. The machine shown is constructed with special intent of reducing its length. According to this intention, the hammer piston has been placed within the motor piston. In other words, the annular motor piston surrounds the hammer piston. The order of operation of this machine is mainly corresponding with the above described machine.

The motor consists of a housing 101, an annular cylinder chamber 102 located within the housing and an annular piston 103 reciprocably guided in the cylinder chamber. Within the piston 103 is enclosed an annular chamber 104, which communicates with a pressurized air source by means of an opening 105 in the inner wall of the piston and

channels

106, 107 in the housing. The conduit leading from the pressurized air source is connected to the machine by means of a nipple 108. (The machine is intended to be operated with any suitable elastic pressure fluid but to simplify the terminology it will be described with air as operating medium.) The piston 103 and the housing 101 are provided with openings 109 and 110-1 13 resp. and the housing is also provided with distributing channels 114-115 for distributing pressurized air to chambers at the ends of the piston causing its reciprocating movement. Furthermore, the housing 101 is provided with two

exhaust openings

116 and 116.

As the piston 103 is moving towards one of its end positions (the upper in FIG. 2) the following happens. At first, the piston covers an exhaust opening 116 in the upper cylinder chamber. Then the openings 109 and 111 uncover each other so as to let pressurized air flow from the chamber 104 within the piston, through the channel 114 to the cylinder chamber at the upper end of the piston. The cylinder chamber is then pressurized. In the next moment the piston covers the opening 110 of the channel 114 closing the cylinder chamber. The piston will hereafter compress and be rebounded by the air cushion enclosed in the cylinder chamber. The piston starts to move in the opposite direction. The channel 114 is opened by uncovering of the opening 110 whereby pressurized air may enter the cylinder chamber and drive the piston on in its new direction. In the following moments the opening 109 in the piston wall is covered by the cylinder wall and the exhaust opening 116 is uncovered by the upper edge of the piston so that the pressurized air within the cylinder chamber may escape out into the atmosphere. A corresponding procedure occurs at the other end position of the piston 103.

The impacting device consists of a hammer piston 117, a drive piston 118 and an auxiliary piston 119. The drive piston drives the hammer piston by means of an air cushion. The drive piston 118 is constituted by a flange provided at the inner wall of the motor piston 103. The flange, as well as the motor piston, is annular and surrounds the hammer piston. Thelatter is also provided with an annular flange 120 which cooperates with the drive piston 118. The drive piston 118 and the flange 120 enclose an annular drive chamber 121 in which a high pressure air cushion transmits driving forces from the drive piston 118 to the hammer piston 117. The high pressure is obtained by feeding the drive chamber with pressurized air from the chamber 104 within the motor piston through an opening 122. The drive chamber 121 constitutes a pressure riser as the drive piston compresses the enclosed air volume during its working stroke. The drive piston 118 drives the hammer piston 117, downwardly in FIG. 2, by means of the powertransmitting air cushion. Thus, the hammer piston is driven downwardly towards a drill steel neck to deliver a blow thereon. Owing to the recoil effect arising in the drill steel and the acting pressure of the air cushion enclosed in the chamber 123 in front of the flange 120 the hammer piston gets a return movement. The main part of the recoil energy of the hammer piston is transmitted to the drive piston by means of the air cushion within the chamber 121. To stop the hammer piston 117 at a distance from the drive piston which is critical with respect to the total impact energy output, the machine is provided with an auxiliary piston 119. The auxiliary piston is located at the rear end of the machine, the upper end in FIG. 2, and hit by the rear end of the hammer piston it is intended to absorb the kinetic energy of the hammer piston and stop it at a predetermined distance from the drive piston. The speed of the resultant return movement of the auxiliary piston is retarded by the pressurized air from the pressurized air source. This pressurized air acts continuously on the upper end of the auxiliary piston and returns the auxiliary piston to its rest position.

FIG. 3 shows a third embodiment of the invention. Like the above described machines, the machine according to this embodiment consists of a pressure fluid driven motor 51 and an impacting device 52. This machine is constructed after the same principle as the first described machine, i.e., the motor and the impacting device are located axially after each other. However, it differs from the above described machine in that it is constructed with the special objective .to reduce the outer diameter. The machine is specially intended for down-the-hole drilling, wherein the machine follows the drill bit down the hole. Such a machine, having a small outer diameter, has been obtained by providing the motor with a number of parallel working stages, wherebya high drive force can be obtained in spite of a small piston diameter.

The motor 51 consists of a housing 53 in which an 'elongated piston 54 is reciprocably guided. The piston 54 is formed with two piston means 55 and 56. In the housing 53 there are two

cylinder chambers

57 and 58 corresponding to the piston means. The piston 54 is hollow and encloses two elongated and

parallel chambers

59 and 60. One of these chambers 59 communicates with a pressurized air source and is provided with

openings

61, 62, 63 to distribute pressurized air to the cylinder chambers. The other chamber 60 acts as an exhaust channel and is provided with

openings

64, 65, 66, 67 to collect exhaust air from the

cylinder chambers

59 and 60.

Furthermore, in the housing 53 there are a number of

channels

68, 69, 70, 71 for distributing pressurized air to the cylinder chambers.

Referring-to FIG. 3 and with special attention to the upper piston-

cylinder unit

55, 57 the order of operation will hereinafter be described. When the piston is in the position shown, the upper part of the cylinder chamber 57 is supplied with pressurized air through the chamber 59, the opening 61 and the channel 68. The piston is then forced downwardly covering the opening 61 and cutting off the pressurized air supply. Approaching its lower end position the piston uncovers the exhaust opening 64, which is situated in an axial extension 73 of the cylinder chamber. The pressurized air within the upper part of the cylinder chamber 57 can now escape through the opening 64 into the chamber 60. The exhaust air is then directed down to the drill bit via the chamber 60 within the piston 54 to serve as flushing medium during drilling. In the lower end position of the piston the opening 62 cooperates with the lower end aperture of the channel 69. Thus, pressurized air from the chamber 59 can enter the lower part of the cylinder chamber 57. The pressurized air is now forcing the piston back upwardly. At first, during the upward movement of the piston, the pressurized air supply to the lower end of the piston means is cut off. Then, the exhaust opening 64 is covered. When, during its return stroke, the piston passes the position shown in FIG. 3 the opening 61 and the aperture of the channel 68 uncover each other permitting pressurized air to enter the upper part of the cylinder chamber 57. However, owing to its kinetic energy the piston continues upwardly recovering the aperture of the channel 68. During the continued upward movement of the piston an axial collar 72 at the upper end of the piston means 55 will cooperate with a corresponding extension 73 of the cylinder covering the lower aperture of the channel 68. As both the inlet channel 68 and the exhaust opening 64 are shut there is enclosed an air volume in the upper part of the chamber. This air volume will act as an elastic cushion which will rebound the piston at its upper end position and deliver to it a downwardly movement.

It the motor, as shown in FIG. 3, is built together with an impacting device in such a way that the motor piston drives a hammer piston 74 by means of an air cushion it is not necessary to fit the motor with air cushions at the lower end positions of the piston means. In this case the piston is recoiled by the powertransmitting air cushion between the motor piston and the hammer piston.

The motor, shown in FIG. 3, has two piston-cylinder units but owing to the power requirement can if of course be fitted with any number of such units.

As the previous described impacting devices the impacting device 52 in the machine shown in FIG. 3 consists of a hammer piston 74, a drive piston 75 intended to drive the hammer piston and an auxiliary piston 76. The drive piston 75 is constituted by a part of the motor piston 54. The piston 54 is also provided with a tubular extension which is extended axially through the hammer piston 74 in order to lead flushing air to the drill bit.

During its power stroke the drive piston 75 compresses an air cushion within a drive chamber 77 at the upper end of the hammer piston 74. The hammer piston 74 is driven downwardly by the compressed air cushion and delivers a blow on the drill steel neck. By the recoil from the drill steel the hammer piston gets a return movement. This movement is absorbed by the auxiliary piston so that the hammer piston stops in a predetermined position before the succeeding working stroke. The order of operation of the impacting device corresponds principally with that of the previously described.

The invention is not limited to the described embodiments but can be freely varied within the limits of the claims.

In a manner usual at rock drills, the impacting devices according to the invention can be fitted with a suitable drill steel chuck and drill steel rotation mechanism.

What I claim is:

1. A motor operated by an elastic pressure fluid, comprising:

a. a cylinder having an end wall and a drive piston having a piston head reciprocating in said cylinder between a power stroke and a return stroke;

b. said piston defining a hollow chamber;

c. means for supplying elastic fluid into said hollow chamber to produce a power stroke by the piston;

d. fluid distributing channel means in the wall of said cylinder having spaced open ends opening into the cylinder cavity adapted to be covered and uncovered by said drive piston during its reciprocation in said cylinder;

e. said chamber having an opening adapted to align with one of the open ends of said fluid distributing channel means to admit pressure fluid into said cylinder between the cylinder end wall and the piston head during the return stroke of the piston;

f. vent means in the cylinder wall for evacuating pressure fluid;

g. said opening and said vent means being so located relative to said open ends as to define a closed chamber between the piston head and the cylinder end wall when the piston approaches the completion of its return stroke to cause the piston to recoil by the force of the elastic fluid and to successively align said opening with said open end to admit the elastic fluid into said closed chamber and to evacuate excess fluid from the closed chamber upon a predetermined length of travel by the piston during its power stroke;

h. percussion means including a hammer piston adapted to be actuated by said drive piston;

i. a drive chamber between said percussion means and said drive piston and j. means for admitting pressure fluid into said drive chamber to provide an elastic cushion effective to absorb the kinetic energy produced by the imping-v ing force of the percussion means and to cause the piston to recoil towards its return stroke position.

2. A motor according to claim 1 further character'- ized in that the chamber within the piston is supplied with elastic pressure fluid through the rear end of the piston, while the forward end of the piston constitutes a driving portion for driving said percussion means.

3. A motor according to claim 1 further characterized in that the drive piston is provided with a plurality piston means along its length, said piston means are reciprocably guided in separate cylinder chambers in the housing.

4. A motor according to claim 3 further characterized in that the drive piston also encloses a second chamber for collecting the fluid exhausted from the motor.

5. A motor according to claim 4 further characterized in that the drive piston is provided with a tubular extension communicating with the second chamber within the drive piston and leading the fluid exhausted from the motor out through the percussion means.

6. A motor operated by an elastic pressure fluid, comprising:

b. said piston defining a hollow chamber;

e. said hollow chamber having an opening adapted to align with one of the open ends of said fluid distributing channel means to admit pressure fluid into said cylinder between the cylinder end wall and the piston head during the return stroke of the piston;

f. a vent means in the cylinder wall for evacuating pressure fluid;

. percussions including a hammer piston adapted to be actuated by said drive piston;

i. a drive chamber between said percussion means and said drive piston containing an elastic fluid to provide a cushion therein effective to transmit to the percussion means the kinetic energy produced by the impinging force of the drive piston by its power stroke and to cause the drive piston to recoil towards its power stroke and to cause the drive piston to recoil towards its return stroke position. l 1 I!

Claims

1. A motor operated by an elastic pressure fluid, comprising: a. a cylinder having an end wall and a drive piston having a piston head reciprocating in said cylinder between a power stroke and a return stroke; b. said piston defining a hollow chamber; c. means for supplying elastic fluid into said hollow chamber to produce a power stroke by the piston; d. fluid distributing channel means in the wall of said cylinder having spaced open ends opening into the cylinder cavity adapted to be covered and uncovered by said drive piston during its reciprocation in said cylinder; e. said chamber having an opening adapted to align with one of the open ends of said fluid distributing channel means to admit pressure fluid into said cylinder between the cylinder end wall and the piston head during the return stroke of the piston; f. vent means in the cylinder wall for evacuating pressure fluid; g. said opening and said vent means being so located relative to said open ends as to define a closed chamber between the piston head and the cylinder end wall when the piston approaches the coMpletion of its return stroke to cause the piston to recoil by the force of the elastic fluid and to successively align said opening with said open end to admit the elastic fluid into said closed chamber and to evacuate excess fluid from the closed chamber upon a predetermined length of travel by the piston during its power stroke; h. percussion means including a hammer piston adapted to be actuated by said drive piston; i. a drive chamber between said percussion means and said drive piston and j. means for admitting pressure fluid into said drive chamber to provide an elastic cushion effective to absorb the kinetic energy produced by the impinging force of the percussion means and to cause the piston to recoil towards its return stroke position.

2. A motor according to claim 1 further characterized in that the chamber within the piston is supplied with elastic pressure fluid through the rear end of the piston, while the forward end of the piston constitutes a driving portion for driving said percussion means.

6. A motor operated by an elastic pressure fluid, comprising: a. a cylinder having an end wall and a drive piston having a piston head reciprocating in said cylinder between a power stroke and a return stroke; b. said piston defining a hollow chamber; c. means for supplying elastic fluid into said hollow chamber to produce a power stroke by the piston; d. fluid distributing channel means in the wall of said cylinder having spaced open ends opening into the cylinder cavity adapted to be covered and uncovered by said drive piston during its reciprocation in said cylinder; e. said hollow chamber having an opening adapted to align with one of the open ends of said fluid distributing channel means to admit pressure fluid into said cylinder between the cylinder end wall and the piston head during the return stroke of the piston; f. a vent means in the cylinder wall for evacuating pressure fluid; g. said opening and said vent means being so located relative to said open ends as to define a closed chamber between the piston head and the cylinder end wall when the piston approaches the completion of its return stroke to cause the piston to recoil by the force of the elastic fluid and to successively align said opening with said open end to admit the elastic fluid into said closed chamber and to evacuate excess fluid from the closed chamber upon a predetermined length of travel by the piston during its power stroke; h. percussions including a hammer piston adapted to be actuated by said drive piston; i. a drive chamber between said percussion means and said drive piston containing an elastic fluid to provide a cushion therein effective to transmit to the percussion means the kinetic energy produced by the impinging force of the drive piston by its power stroke and to cause the drive piston to recoil towards its power stroke and to cause the drive piston to recoil towards its return stroke position.