Hydraulic impact tool

An impact tool including a cylinder, a piston slidably mounted in the cylinder and a valve for selectively opening and closing oil feed channels to move the piston up and down. A cylindrical member is slidably mounted on the piston around its large-diameter portion or thereunder. Its bottom end defines the top end of a lower chamber formed in the cylinder. The cylindrical member is adapted to descend together with the piston. But when the piston strikes the object and rebounds upwardly, the cylindrical member keeps descending away from the piston owing to the inertia force. Thus the volume of the lower chamber is kept from increasing excessively after the piston has struck the object. This will prevent cavitation in the lower chamber. When thereafter the lower chamber communicates with the oil feed port and pressure oil is fed thereinto, the cylindrical member will be pushed up toward the large-diameter portion under oil pressure in the lower chamber.

The present invention relates to a hydraulic impact tool adapted to be 
mounted on the head of a hydraulic power shovel or the like and used to 
demolish a concrete structure, crush rocks, or excavate a rock base. 
Hydraulic impact tools can be classified roughly into an accumlator type 
and a gas pressure type. With an accumulator type tool, pressurized oil is 
accumulated in an accumulator while a piston is rising and is released 
during its downward stoke to accelerate the piston. 
With a gas pressure type tool, one example of which is disclosed in U.S. 
Pat. No. 4034817, a piston compresses a gas filled in the space above the 
piston to store energy when it rises under oil pressure. During its 
downward stroke, the compressed gas expands to accelerate the piston. The 
prior art impact tool is shown in FIG. 12 in which numeral 1 designates a 
cylinder and a tool 2 such as a chisel is slidably mounted in the lower 
end thereof. 
A piston 4 formed with a large-diameter portion 3 is mounted in the 
cylinder 1 to strike the tool 2. The cylinder 1 has an upper chamber 5 
charged with gas over the piston 4 to exert the gas pressure on the piston 
4 as it reaches its upper limit. 
The piston 4 has small-diameter portions over and under the large-diameter 
portion 3. A middle chamber 6 and a lower chamber 7 are formed between 
these small-diameter portions and the inner periphery of the cylinder 1. 
A valve chest 8 is formed at one side of the cylinder 1. A valve body 10 
formed with a center bore is mounted in the valve chest 8. The valve chest 
communicates with the cylinder 1 through oil channels 14, 15 extending 
from the upper and lower parts of the former to the upper part of the 
middle chamber 6 and to the lower part of the lower chamber 7, 
respectively. Further, the cylinder 1 and the valve chest 8 have their 
respective mid-portions communicating with each other by means of one main 
oil channel 15 and a branch channel. 
The valve chest 8 has its upper and lower parts connected to a discharge 
port 11 and an oil feed port 12, respectively. From the oil feed port 12, 
another oil channel branches and leads to the top end of a plunger 13 for 
pressing down the valve body 10. 
In operation, when the valve body 10 is at its lowermost position, pressure 
oil is supplied through the oil feed port 12 to pressurize the lower 
chamber 7. Since the middle chamber 6 is open to the discharge port 11, 
the piston 4 rises up in the cylinder to compress the gas in the upper 
chamber 5. 
When the piston 4 approaches the uppermost position, the oil feed port 12 
gets into communication with the middle oil channels 15 through which 
pressure oil flows into the valve chest 8 to push up the valve body 10. As 
soon as the valve body 10 clears the bottom of the valve chest 8, the 
lower chamber 7 communicates with the discharge port 11 through the bore 9 
in the valve body 10. Thus, the piston 4 is pushed down by the pressure of 
gas in the upper chamber 5 to strike the tool 2. 
With this prior art impact tool, when the piston 4 rebounds violently 
immediately after striking the tool 2, the pressure in the lower chamber 7 
drops sharply because the chamber 7 is open to the discharge port 11, thus 
allowing air bubbles in the hydraulic oil to grow rapidly. This phenomenon 
is called cavitation. When the valve body 10 descends thereafter and 
pressure oil flows back into the lower chamber 7, the pressure in the 
lower chamber will increase sharply. Thus the air bubbles which have grown 
large will collapse in an instant, producing a very high pressure and a 
shock wave. This happens repeatedly several hundred times a minute. Thus, 
the piston 4 and the cylinder 1 tend to develop erosion on their surface 
after long use. 
It is an object of the present invention to provide an impact tool which is 
less susceptible to erosion on the surface of its piston and cylinder 
owing to cavitation. 
In accordance with the present invention, a cylindrical member is slidably 
mounted between the piston and the inner wall of the cylinder. It moves 
down with the piston during the downward movement of the piston, but keeps 
moving down under the inertia force when the piston rebounds and rises 
violently after striking the tool. This prevents a rapid increase in the 
volume of the lower chamber and a rapid decrease in the pressure in the 
lower chamber. 
When both the valve body and the piston are at their lowermost position, 
the lower chamber communicates with the oil feed port, whereas the middle 
chamber communicates with the discharge port. Thus the piston is pushed 
up, compressing the gas in the upper chamber. 
During the upward stroke of the piston, the valve body will begin to rise 
under oil pressure. When it rises to such a level that the lower chamber 
communicates with the discharge port through the bore formed through the 
valve body, the piston will begin to descend at a high speed under the gas 
pressure in the upper chamber to strike the tool. 
When the piston strikes the tool and rebounds upwardly, the cylindrical 
member mounted on the piston keeps going down under the influence of 
inertia force. This will prevent any sharp change in the volume of the 
lower chamber and thus any sharp pressure drop. This will in turn prevent 
the growth of air bubbles contained in the hydraulic oil in the lower 
chamber, so that no cavitation will occur. 
According to the present invention, damage to the piston and the cylinder 
is reduced to a minimum and the durability of the impact tool increases. 
After the piston has risen to a given level, the cylindrical member is 
adapted to be pushed up under oil pressure so as to move together with the 
piston. Thus the cylindrical member will scarcely affect the reciprocating 
motion of the piston so that the piston can move smoothly. 
The tool according to the present invention differs from a conventional 
tool only in that the cylindrical member is fitted on the piston around 
its large-diameter portion or thereunder. The other parts or portions such 
as the oil channels and the valve mechanism are identical in construction 
to those of a conventional impact tool. Thus its construction is simple.

FIGS. 1 to 3 show the first embodiment which differs from the prior art 
tool shown in FIG. 12 only in that the large-diameter portion 3 of the 
piston 4 is slightly smaller in diameter than on the prior art tool, and a 
cylindrical member 21 is slidably mounted on the large-diameter portion 3. 
It is integrally formed at its bottom end with an inturned annular flange 
23. 
The tool of this embodiment operates in substantially the same way as the 
prior art tool shown in FIG. 12. But its operation immediately after the 
piston has struck the tool 2 is slightly different from that of the prior 
art tool. 
Namely, when the piston 4 is descending under the gas pressure in the upper 
chamber 5 with the lower chamber 7 in communication with the discharge 
port 11, the cylindrical member 21 mounted on the large-diameter portion 3 
of the piston 4 descends together with the piston until the piston 4 
strikes the tool. (FIG. 2) 
Upon striking the tool, the piston 4 will rebound and rise violently. But 
the cylindrical member 21, which is slidable with respect to the piston 4, 
will keep descending owing to the inertia force. In other words, the 
bottom edge of the cylindrical member 21 will move away from that of the 
large-diameter portion 3 as shown in FIG. 3. 
Since the cylindrical member 21 keeps descending while the piston 4 is 
rebounding, the volume of the lower chamber 7 will be kept from increasing 
sharply immediately after impact. This will keep the lower chamber 7 from 
being put under negative pressure and thus avoid the development of 
cavitation. 
Although immediately after impact the bottom of the cylindrical member 21 
will move away from the bottom of the large-diameter portion 3, when the 
lower chamber 7 communicates with the oil feed port 12 and hydraulic oil 
begins to flow into the lower chamber 7, the cylindrical member 21 will be 
pushed up under the pressure in the lower chamber 7. 
The cylindrical member 21 will rise until its flange 23 come into abutment 
with the bottom end of the large-diameter portion 3. Then the piston 4 and 
the cylindrical member 21 will rise together and descend together as if in 
one piece until the piston 4 again strikes the tool 2. 
In the second embodiment shown in FIG. 4, the large-diameter portion 3 of 
the piston 4 is short in length and flange-like. A cylindrical member 21 
is slidably mounted on the piston 4 so that its top end abuts the bottom 
end of the large-diameter portion 3. Otherwise, this embodiment is 
identical in construction and function to the first embodiment. 
The third embodiment shown in FIG. 5 slightly differs from the 
above-described embodiments in the hydraulic circuit. This arrangement 
requires a longer large-diameter portion 3 and the provision of an annular 
groove 24 around the cylindrical member 21. 
In this embodiment too, the cylindrical member 21 is adapted to move away 
from the large-diameter portion 3 immediately after the piston 4 has 
struck the tool 2, to prevent a sharp drop in pressure in the lower 
chamber 7. 
While the above-described embodiments are related to gas pressure type 
impact tools, the fourth embodiment shown in FIG. 6 is an impact tool of 
an accumulator type. 
In this figure, numeral 29 designates an accumulator and 30 designates an 
oil pressure changeover valve. The piston 4 is adapted to move up and down 
by switching hydraulic circuits leading to the upper chamber 5, the middle 
chamber 6 and the lower chamber 7, respectively. In this figure, the 
cylindrical member 21 is slidably mounted on the large-diameter portion 3 
of the piston 4 and the inturned flange 23 is formed at the bottom edge of 
the member 21 as in the first embodiment. But the cylindrical member 21 
may be formed as in the second and third embodiments. In this embodiment, 
a gas is sealed in a chamber formed over the upper chamber to auxiliarily 
apply pressure to the top of the piston. 
FIG. 7 shows a modification of the first embodiment in which the 
large-diameter portion 3 is integrally provided at its top end with a 
flange 32 so that when the cylindrical member 21 rises with respect to the 
piston 4, the cylindrical member 21 will abut the flange 32 at its top 
end, thus keeping the inturned flange 23 out of contact with the bottom 
end of the large-diameter portion 3. 
This arrangement will be effective in preventing cracks from being formed 
at the top end of the inturned flange 23 and at the inner periphery of the 
cylindrical member 21 near its bottom end. 
FIGS. 8A and 8B show a further modification of the example shown in FIG. 7 
in which the piston 4 is formed in its outer periphery below its 
large-diameter portion 3 with an annular groove 34. 
With this arrangement when the cylindrical member 21 lowers with respect to 
the piston 4 (FIG. 8A), a gap 35 will be formed between the annular groove 
34 and the inturned flange 23 so that a sufficient amount of oil can be 
supplied into a cushioning chamber 36. 
FIGS. 9A and 9B show another example, which has the same function as the 
example shown in FIG. 8. In this example, the cylindrical member 21 is 
formed with a channel 37 therein. When the cylindrical member 21 lowers 
with respect to the piston 4 (FIG. 9B), oil will flow into the cushioning 
chamber 36 through the channel 37. 
In still another example shown in FIGS. 10A and 10B, the flange 32 is 
stepped so that its lower half portion 38 has a smaller diameter and the 
cylindrical member 21 has its top end recessed circumferentially along its 
inner edge as shown at 39. When the cylindrical member 21 lowers with 
respect to the piston 4 (FIG. 10B), oil will flow into the recess 39 to 
serve as a cushion when the cylindrical member 21 rises again. 
The larger the size of the piston 4 and hence the larger the mass of the 
cylindrical member 21, the larger the inertia force acting on the 
cylindrical member 21 will be. This will cause the cylindrical member 21 
to descend too far downward and thus badly influence the operation of the 
impact tool. 
To solve this problem, another inturned flange 40 may be provided at the 
top end of the cylindrical member 21 as shown in FIG. 11 to restrict the 
downward stroke of the cylindrical member 21. 
In the preferred embodiments, there has been shown only impact tools having 
the piston mounted in direct contact with the cylinder for simplification. 
But an impact tool having its piston mounted in a cylinder through a 
bushing is also within the scope of the present invention. 
In the preferred embodiments, there has been shown only impact tools in 
which the lower chamber communicates with the oil discharge port during 
the downward stroke of the piston. But, the present invention is also 
applicable to impact tools in which the lower chamber does not communicate 
with the oil discharge port during the downward stroke of the piston, but 
is always in communication with the oil supply port. 
Although the above-described embodiments are related to impact tools, the 
concept of the present invention will be applicable to not only an impact 
tool but also other tools and devices such as a pile driver in which an 
impact is given to an object during its downward stroke.