GB2069399A

GB2069399A - Percussive Tool

Info

Publication number: GB2069399A
Application number: GB8004565A
Authority: GB
Original assignee: V NI I P KONSTRUKT I MEK I RUC
Current assignee: V NI I P KONSTRUKT I MEK I RUC
Priority date: 1980-02-12
Filing date: 1980-02-12
Publication date: 1981-08-26
Also published as: GB2069399B

Abstract

A percussive tool has a housing (1), a cylinder (2) mounted in the housing, and hammer and drive pistons (8, 6) reciprocable in the cylinder with a gas cushion 9 therebetween. Ports (12) in the piston allow escape of gas from the gas cushion when the drive piston is in the bottom dead centre position, and a port (14) in the cylinder admits gas to the gas cushion from a space (3) between the cylinder and housing when the drive piston (6) is in the top dead centre position. <IMAGE>

Description

SPECIFICATION Percussive Tool The present invention relates to percussive tools, and in particular, to electromechanical hammers and rotary hammers employed in building practice, mining industry and geological prospecting work.

The present invention provides a percussive tool comprising a housing, a cylinder mounted in the housing with a space defined therebetween, a hammer piston and a drive piston reciprocable in the cylinder and separated from each other by a gas cushion, at least one port in the drive piston to allow escape of gas from the gas cushion when the drive piston is in a bottom dead centre position, and at least one port in the cylinder immediately below the drive piston when in its top dead centre position for admitting gas to the gas cushion from the space between the cylinder and housing.

It is expedient that the area of said port(s) in the cylinder is in the range of from 0.14 to 0.33 of the area of the port(s) in the drive piston.

An advantage of the inventive tool resides in the fact that gas, e.g. air, is admitted to the air cushion within the zone of the drive piston top dead centre through the port(s) in the cylinder, the area of said port(s) being such that beginning with the moment when the piston opens said port(s) pressure in the air cushion starts growing and by the moment the piston shuts off the port(s) the pressure becomes equal to the pressure in the space between the cylinder and housing. As a result the reduced pressure which is effective during the period of opening of the port(s) in the cylinder causes the piston hammer to raise to a greater height, whereby the hammer piston acquires higher pre-impact speed and hence greater impact energy during the work stroke of the drive piston.

A detailed description of an embodiment of the invention will now be given with reference to the accompanying drawings, wherein: Figure 1 is a sectional view of the percussive tool, showing its drive piston at the bottom dead centre; Figure 2 is a sectional view of the percussive tool, showing its drive piston at the top dead centre; Figure 3 isa cross-sectional view of the piston, cylinder and housing of the percussive tool, showing the drive piston at the bottom dead centre; Figure 4 illustrates a comparison graphic chart of a working impact cycle, wherein: Sp - drive piston displacement; S82 - hammer piston displacement in the tool according to the invention; S, - hammer piston displacement in a conventional tool; and Figure 5 illustrates a graphic chart of a working impact cycle at a higher blow rate, wherein: Sp - drive piston displacement; S53 - hammer piston displacement a conventional tool; S53 - combined hammer piston and drive piston displacement in a conventional tool; S54 - hammer piston displacement in the tool according to the invention; S54, - combined hammer piston and drive piston displacement in the tool according to the invention; Ho - length of the air cushion at the moment of transition idling to hammering; R - is the crank shaft radius of the motion converting crank gear.

A percussive tool according to the present invention comprises a housing 1, wherein a stationary cylinder 2 is accommodated; the outside surface of said cylinder is arranged in a spaced relation to the inside surface of the housing 1 so as to define a space 3 therebetween.

The space 3 communicates with a compartment 4 accommodating a motion-converting crank gear 5, the compartment being defined by an inner face 7 of a drive piston 6. The hollow piston 6 is coupled with the crank gear 5 and is mounted in the cylinder 2. The space 3 is filled with the same medium as the air cushion which will hereinafter be described in detail. The cylinder 2 accommodates also a hammer piston 8 separated from the drive piston 6 by an air cushion 9. Design and calculation of the air cushion parameters (i.e., length Ho and cross-sectional area FD) to suit a required blow rate, and impact energy, are now to a reasonable extent made a common knowledge and therefore offer no difficulties. The hammer piston 8 is adapted to act, by its end 10 opposite to the air cushion 9, on a working tool 11.Air overflow when the drive piston is at the bottom dead centre, occurs through ports 12 in the walls of the drive piston 6 and via segmental grooves 13 provided in the inner surface of the cylinder 2 (Figures 1 and 3), when the drive piston is at the top dead centre air overflow occurs through a port 14 made in the wall of the cylinder 2 and situated immediately under a head 15 of the piston 6.

The tool according to the present invention functions as follows. When the drive piston 6 moves from its botton dead centre upwards rarefaction results in the air cushion 9 between the drive piston 6 and the hammer piston 8, to cause the hammer piston 8 to move upwards to follow the drive piston 6. As the drive piston 6 approaches its top dead centre (see curve Sp in Figure 40, the opening of the port 14 in the cylinder 2 begins with the result that the air from the space 3 is admitted to the space of the air cushion 9, whereby the pressure in this space starts growing. The area of the port 14 is so chosen that the pressure growth in the air cushion continues throughout the entire period of time of opening of the port 14 by the drive piston 6, i.e., from a point "a" to a point "b" of curve Sp (Figure 4).The pressure in the air cushion becomes equal to the pressure in the space 3 at the moment the drive piston 6 shuts off the port 14, which corresponds to the point "b" of curve Sp (Figure 4). As the space of the air cushion 9 becomes closed when the port 14 is shut off by the drive piston 6, and the hammer piston 8 moves to meet the drive piston 6, air pressure in the cushion 9 starts rising. This pressure growth brings the return stroke speed of the hammer piston 8 to zero and reverses it (see curve SS2 in Figure 4i, thus imparting some preimpact speed and respective impact energy to the hammer piston 8 when the latter approaches the working tool 11.

Maximum reliability and the stability of the impact energy, as well as high energy performance are attained in the tool with the ratio of the area of the port 14 in the cylinder to the area of the ports 12 in the drive piston 6 ranging within 0.14 to 0.33.

When this ratio exceeds 0.33, the pressure in the air cushion 9 rapidly grows after the drive piston 6 has opened the port 14 at the top dead centre (the point "a" of curve Sp in Figure 4). The reduced pressure does not act upon the hammer piston 8 within major part of the time the port 14 remains exposed. This reduces the length of the return stroke of the hammer piston 8 (the pathway of the hammer 8 being represented by the curve SS, in Figure 4) to reduce the preimpact speed of the hammer piston due to a smaller distance of acceleration of the hammer piston during the work stroke, which is proportional to the tangent of the angle of inclination (tg,B) of the tangent line drawn to the pathway SS, of the hammer piston 8 at the impact point c' (Figure 4).

Lower preimpact speed of the hammer piston 8 results in a reduced impact energy thereof.

With the abovesaid ratio below 0.14 the hammer piston 8 and the drive piston 6 are likely to collide with each other under transient conditions, e.g., when the tool changes idling to hammering. Said disadvantage manifests itself in tools driven by a commutator motor featuring a drooping speed-torque characteristic. When changing over to the hammering the tool operates initially, at the rate of strokes of the drive piston 6 which remains the same as during the idling, i.e., it can be 1.5 to 2.0 times the rate during the hammering. As it is known from the theory of impact mechanisms, the higher the rate of drive piston strokes the smaller the steepness of hammer piston retraction (see curve S83 in Figure 5).When the ratio is below 0.14 (with the area of the port 14 assumed to be zero for the sake of clarity), a vacuum is effective in the space between the hammer piston 8 and the drive piston 6 during major part of the cycle, and the zone of pressure build-up (beginning with the point "d" in Figure 5) is already formed as the drive piston 6 approaches the bottom dead centre (a distance from the point "e" to the point "f" in Figure 5). When the space of the air cushion 9 is opened through the port 12 in the drive piston 6, the pressure effective therein falls to the atmospheric pressure. The time during which the pressure acts upon the hammer piston 8 is too short so that even though the hammer is decelerated it has no time for reverse.After the ports 12 in the drive piston 6 have been shut off (the point "f" in Figure 5) during the return stroke of the drive piston, a vacuum is established in the space between the hammer piston 8 and the drive piston 6 to cause the hammer piston 8 resume its move following the piston. In this case the hammer piston 8 starts moving when the length of the air cushion 9 is equal to H, rather than to Ho (Figure 5), i.e., at a shorter air cushion length.

Thus, the length of the air cushion 9 becomes shorter after every stroke cycle of the drive piston 6 until the hammer piston 8 and the drive piston 6 are at the closest distance from each other at the drive piston bottom dead centre. Inasmuch as said contact is of the impact nature this unfavourably affects the reliability of the machine as a whole.

Collision of the hammer piston 8 with the drive piston 6 under transient conditions of the machine is ruled out when the abovesaid ratio exceeds 0.1 4.

In this case the drive piston return stroke opens the space of the air cushion 9 through the port 14 so that the pressure in the air cushion 9 grows to a value of the pressure in the space 3 at the moment when the piston 6 shuts off the port 14 (the point "b" on the curve Sp of Figure 5).

Forasmuch as the drive piston 6 and the hammer piston 8 move to meet each other after having passed by the point "b" in Figure 5, the pressure in the air cushion 9 starts growing. The time of action of said pressure till the moment the air cushion 9 starts communicating through the port 12 in the drive piston 6 is in this case much longer (from the point "b"' to the point "e" on the curve Sp in Figure 5), and the hammer piston has enough time for its reverse (see the pathway S84 in Figure 5). As a result, the working cycle terminates in an impact of the hammer piston 8 against the working tool 11 at the point "g" (Figure 5). After recoil during the next cycle the hammer piston 8 moves back following the drive piston through a still greater distance, thus providing for a higher preimpact speed, hence the impact energy of the hammer piston 8. The rate of piston strokes becomes lower as the impact energy of the hammer piston 8, hence the load upon the drive motor increase from cycle to cycle.

This occurs until the rate of strokes of the drive piston 6 and the impact energy of the hammer piston 8 reach their rated values specified for steady-state conditions. No collision of the hammer piston 8 with the drive piston 6 occurs when starting the machine.

Claims

1. A percussive tool comprising a housing, a cylinder mounted in the housing with a space defined therebetween, a hammer piston and a drive piston reciprocable in the cylinder and separated from each other by a gas cushion, at least one port in the drive piston to allow escape of gas from the gas cushion when the drive piston is in a bottom dead centre position, and at least one port in the cylinder immediately below the drive piston when in its top dead centre position for admitting gas to the gas cushion from the space between the cylinder and housing.

2. A percussive tool as claimed in claim 1, wherein the area of said port(s) in the cylinder is in the range of from 0.14 to 0.33 times the area of the port(s) in the drive piston.

3. A percussive tool substantially as herein described with reference to the accompanying drawings.