Ram boring implement

A ram boring implement has a pneumatically driven percussion piston (16), which is movable in the axial direction in a reciprocating manner in a housing (12), and a control sleeve (24) which is axially adjustable for reversing the direction of motion of the ram boring implement and is acted upon by the pressure in one (20) of the pressure chambers (18, 20) formed on both sides of the percussion piston (16). The control sleeve (24) can be adjusted by means of a spindle drive (30, 50) by turning a compressed air supply hose (26). According to the invention, the control sleeve is arranged on a core (30, 34), supported in an axially fixed manner on the housing (12), so that the control sleeve itself forms only a relatively small annular effective area (52) acted upon by pressure. This makes it possible for the control sleeve to be moved forward or toward the rear without the compressed air feed having to be interrupted.

BACKGROUND OF THE INVENTION 
The present invention relates to a ram boring implement having a 
pneumatically driven percussion piston, which is movable in the axial 
direction in a reciprocating manner in a housing, a control sleeve, which 
is axially adjustable for reversing the direction of motion of the ram 
boring implement and is acted upon by the pressure in one of the pressure 
chambers formed on both sides of the percussion piston, and a spindle 
drive with which the control sleeve can be adjusted by turning a 
compressed air supply hose. Ram boring implements of this type are used 
for driving earth bores for cables and the like that are laid underground. 
German Pat. No. 1,634,417 discloses a ram boring implement of this generic 
type. The percussion piston has a tubular section displaceable on the 
control sleeve and, in the peripheral wall of this tubular section, is 
provided with radial channels which are periodically opened and closed by 
the control sleeve during the oscillating motion of the percussion piston. 
The compressed air feed to the pressure chamber for driving the percussion 
piston is controlled in this way. If the control sleeve is located in a 
front position, the percussion piston, during its forward stroke, strikes 
against the front end of the housing so that impulse is transmitted to the 
housing and the ram boring implement is driven in the forward direction. 
If the control sleeve is located in the rear position, the motion of the 
percussion piston is pneumatically braked during the forward stroke before 
the percussion piston reaches the front end of the housing. Instead, the 
percussion piston in this case strikes against the rear end of the housing 
during its rearward stroke, so that the ram boring implement moves in the 
rearward direction. 
In the conventional implement, the control sleeve is rigidly connected to 
the compressed air supply hose. The spindle drive for adjusting the 
control sleeve is formed by an external thread which is made on the 
control sleeve and is engaged with a component rigidly connected to the 
housing. 
The pressure chamber located on the rearside of the percussion piston is 
made in the tubular section of the percussion piston and is defined toward 
the rear by the control sleeve made in a pot shape. When this pressure 
chamber is pressurized during the forward stroke of the percussion piston, 
a relatively large force directed toward the rear therefore acts on the 
control sleeve. For this reason, changing over from the rearward motion to 
the forward motion by axially adjusting the control sleeve forward can 
only take place when the compressed air feed is switched off. The thread 
of the spindle drive may only have a small pitch in order that the 
relatively large axial forces can be reliably absorbed. When the direction 
of motion of the ram boring implement is to be reversed and the compressed 
air hose is turned manually for this purpose, the compressed air hose must 
execute a large number of revolutions, as a result of which the reversal 
of the direction of motion is relatively laborious and time-consuming. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
ram boring implement of the generic type mentioned at the outset. 
It is also an object of the present invention to provide a ram boring 
implement wherein the direction of motion can be reversed quickly and 
reliably. 
A further object of the invention is to provide a ram boring implement in 
which the reversal of the direction of motion can be executed when the ram 
boring implement is running, i.e., when the compressed air feed is 
switched on. 
In accomplishing these objects, there has been provided in accordance with 
the present invention a ram boring implement, comprising a generally 
cylindrical housing; a pneumatically driven percussion piston axially 
movable in a reciprocating manner in the housing and defining a first 
pressure chamber in front of it and a second pressure chamber behind it; a 
core member fixed axially within the housing; a compressed air supply, 
comprising a compressed air supply hose connected to the housing; a 
control sleeve which is axially adjustable for reversing the direction of 
motion of the ram boring implement, this control sleeve being arranged on 
the core member and including an annular effective area that is acted upon 
by the pressure in the second pressure chamber; and means, including a 
spindle drive for axially adjusting the control sleeve in response to 
turning of the compressed air supply hose. 
Further objects, features and advantages of the invention will become 
apparent from the detailed description of preferred embodiments that 
follows, when considered in light of the accompanying figures of drawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
According to the invention, the control sleeve is arranged on a core, which 
is supported in an axially fixed manner on the housing, and forms an 
annular effective area acted upon by pressure. 
By means of the core supported on the housing, the effective area of the 
control sleeve to be acted upon by compressed air during the forward 
stroke of the percussion piston is considerably reduced, so that the 
control sleeve only needs to absorb correspondingly low axial forces. 
These low axial forces can be effortlessly overcome during the adjustment 
of the control sleeve so that the control sleeve can be adjusted without 
problem even when the ram boring implement is running. Since the axial 
forces are only small, the spindle drive can have a relatively large 
thread pitch without the control sleeve moving automatically when the 
implement is running. During the reversal of direction, the compressed air 
hose therefore only needs to be turned by a few revolutions or not even by 
one complete revolution. 
A ram boring implement has already been disclosed in German Pat. No. 
2,157,259 in which the control sleeve is arranged on an axially fixed core 
and only has one annular area acted upon by pressure. However, the 
direction is not reversed here by means of a spindle drive but directly by 
axial displacement of the control sleeve by means of the compressed air 
hose. During the forward motion of the ram boring implement, the control 
sleeve is arrested in the front position by means of a locking device. To 
change over to the rearward motion, the locking device is released by 
means of a cable line. In this implement, the effective area of the 
control sleeve is of such a large size that the control sleeve is 
displaced pneumatically toward the rear after the locking device is 
released. If a changeover to forward motion is again to be made, the 
compressed air feed has to be interrupted so that the control sleeve can 
again be pushed forward and locked. In this known implement, the principle 
according to the invention of reducing the effective area of the control 
sleeve in such a way that it can be moved forward against the working 
pressure is therefore not realized. 
Advantageous developments of the ram boring implement according to the 
invention as well as a preferred exemplary embodiment of the invention are 
described in greater detail below with reference to the drawings. 
A ram boring implement has a housing 12 which is provided with a percussion 
tip 10 and is closed at the rear end by an end piece 14. Inside the 
housing 12, a front pressure chamber 18 and a rear pressure chamber 20 are 
formed by a percussion piston 16 guided in an axially movable manner in 
the housing. The rear pressure chamber 20 is located in a tubular section 
22 of the percussion piston 16 which is displaceable in a sliding manner 
on a control sleeve 24. A compressed air hose 26 for supplying the ram 
boring implement with compressed air is connected non-rotationally via a 
connection piece 28 to a control tube 30 which is rotatably mounted in the 
end piece 14 and is supported in the axial direction on a shoulder 32 of 
the end piece. At the front end, the control tube 30 is surrounded by an 
elastomeric cushioning element 34 which is in tight contact with the inner 
wall of a thin-walled cylindrical section of the control sleeve 24. The 
rear pressure chamber 20 is therefore defined toward the rear by the 
control tube 30, the cushioning element 34 and the thin-walled cylindrical 
section of the control sleeve 24. A rear section 36 of the control sleeve 
is in screw-thread engagement with the control tube 30. The control sleeve 
24 is preloaded toward the front by a right-hand helical compression 
spring 38 supported on the end piece 14 and has ventilation channels 40 
for ventilating the hollow space 42, defined by the cushioning element 34, 
inside the control sleeve. 
The tubular section 22 of the percussion piston 16 is provided with radial 
channels 44, and the end piece 14 has axial air channels 46 which are 
fluidly connected to the inside of the tubular section 22 of the 
percussion piston. 
In the position of the percussion piston shown in FIG. 1, the front 
pressure chamber 18 is relieved of pressure via axial channels 48 of the 
control piston, via the radial channels 44 and the axial channels 46 of 
the end piece 14. When compressed air is introduced into the rear pressure 
chamber 20 via the compressed air hose 26 and the control tube 30, the 
percussion piston 16 moves toward the front. The radial channels 44 are 
temporarily closed by the control sleeve 24 and then come into fluid 
connection with the rear pressure chamber 20, so that the compressed air 
expands into the front pressure chamber 18 via the radial channels 44 and 
the axial channels 48. Since the effective area of the percussion piston 
16 relative to the front pressure chamber 18 is greater than relative to 
the rear pressure chamber 20, the piston is braked and moved back again so 
that it does not strike against the front end of the housing 12. At the 
end of its rearward stroke, the percussion piston 16 strikes against the 
rear end piece 14 of the housing and imparts a rearward impulse to the 
housing. The actions described above are repeated periodically so that the 
ram boring implement is on the whole driven in the rearward direction. 
On the other hand, if the control sleeve is located in the front position 
shown in FIG. 2, the percussion piston, during its forward stroke, strikes 
against the front end of the housing 12 and, during the rearward stroke, 
the percussion piston is pneumatically braked before it reaches the end 
piece 14. In this case, therefore, the ram boring implement is driven in 
the forward direction. 
During the reversal of the direction of motion, the control sleeve 24 is 
adjusted by the compressed air hose 26 being turned, for example, 
manually. The thread 50 of the rear section 36 of the control sleeve and 
of the control tube 30 is, for example, a four-start thread with an axial 
thread spacing of 10 mm. The pitch of an individual thread turn is 
therefore 40 mm so that the control sleeve, can be moved the entire 
displacement distance "x" (FIG. 3) by means of only one to three 
revolutions. Since the effective area of the control sleeve 24 facing the 
pressure chamber 20 is only very small, when the ram boring implement is 
running, the control sleeve can be moved forward against the pressure 
prevailing in the pressure chamber 20. The helical compression spring 38 
assists the forward motion of the control sleeve and at the same time 
forms an anti-rotation means which prevents the control sleeve from 
turning when the control tube 30 turns. Alternatively, however, another 
anti-rotation means can be additionally provided. 
If a high pressure prevails in the pressure chamber 20, a relatively low 
axial force acts on the control sleeve 24, which force, on account of the 
action of the thread, tends to turn the control tube 30. At the same time, 
however, a substantially larger axial force acts directly on the control 
tube 30 so that rotation of the control tube is prevented on account of 
the friction between the control tube and the shoulder 32. In this way, 
despite the relatively large thread pitch, the control sleeve 24 is held 
in a self-locking manner in the respective position.