Note: Descriptions are shown in the official language in which they were submitted.
WO 95/23051 PCT/SE95/00209
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Pneumatic impact breaker
This invention relates to a pneumatic impact breaker of
the type comprising a housing with a cylinder bore, a
hammer piston reciprocable in the cylinder bore, a rear
cylinder head with an air distributing valve for
directing motive pressure air to alternative ends of the
hammer piston to reciprocate the latter in the cylinder
bore, a front portion attached to the housing and forming
a guide and support means for a working implement, a
forwardly directed cylinder bore extension coaxial with
and of a smaller diameter than the cylinder bore, said
cylinder bore extension is separated from the cylinder
bore by an annular shoulder, an anvil sealingly guided in
the cylinder bore extension and having a forward end
normally abutting the rear end of the working implement
and a rear end normally located within said cylinder bore
extension, the hammer piston is formed with a piston head
for sealing and guiding cooperation with the cylinder
bore, and a forwardly extending stem portion for cyclical
penetration into the cylinder bore extension to deliver
repeated impacts to the anvil at reciprocation of the
hammer piston, whereby the stem portion and the piston
head form together with the cylinder bore and the annular
shoulder an annular energy absorbing air cushion chamber.
Pneumatic impact breakers of the above type provide an
effective breaking by a high impact energy but generate
at the same time external vibrations and internal blows
which have a detrimental influence on the operator as
well as on the mechanical parts. Detrimental blows or so
called bottom blows of the hammer piston occur as the
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application or feeding force on the breaker is very low,
nill or negative. A negative feeding force is
accomplished as the breaker is lifted up, for instance
when the working implement has become jammed.
In previous impact breakers of the above type, these
bottom blows have been difficult to dampen out fully,
and, in order to make the parts withstand the stress
forces and to ensure a safe assemblage of the breaker,
the parts thereof, including the housing itsel~, have to
be oversized. It has also been necessary to use extra
strong tie bolts and/or threaded joints to keep the parts
safely together. An example thereon is shown in US
3,179,185, Fig 4, in which the damping means for
absorbing the hammer piston energy at bottom strokes
comprises an annular elastomer element. In practice, this
known arrangement has no ability to absorb the kinetic
energy of the hammer piston, and moreover, the service
life of this impact damping elastomer element will be
very short.
Another example is shown in US 3,451,492. The impact
breaker illustrated therein comprises a hammer piston
which has a comparatively long impact delivering stem
portion in relation to the length of the piston head.
This means that, due to the inevitable tilting of the
piston occurring when the stem portion is out of guiding
engagement with the forward small diameter cylinder bore
extension, there must be provided a relatively large play
between the stem portion and the bore to avoid too a
violent metallic contact therebetween. This results in a
relatively wide leak gap around the stem portion, and,
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accordingly, a rather ineffective damping volume entrapped
between the hammer piston, the cylinder bore and the forward
shoulder in the cylinder bore. Despite a rather large play
between the piston stem and the cylinder bore extension and
a following poor air volume energy absorption, there is an
undesirably severe wear of the piston stem and the cylinder
bore extension due to metallic contact therebetween.
The main object of the invention is to accomplish
a pneumatic impact breaker having an improved air cushion
energy absorption at noload or bottom strokes of the hammer
piston by an improved more accurate rectilinear movement of
the hammer piston stem portion.
Another object of the invention is to reduce the
detrimental energy absorbing air volume entrapped between
the hammer piston stem portion and the anvil. If the peak
pressure in this air volume is allowed to be too high there
will be a substantial loss in energy transfer between the
hammer piston and the anvil. This problem is emphasized in
impact mechanisms having a narrow leak gap, i.e. a tight fit
between the piston stem portion and the bore. This problem,
however, is solved by the invention.
A broad aspect of the invention provides a
pneumatic impact breaker comprising: a housing with a
cylinder bore; a hammer piston reciprocable in said cylinder
bore; a rear cylinder head with an air distributing valve
for directing motive pressure air to alternate ends of said
hammer piston to thereby reciprocate said hammer piston in
said cylinder bore; a front portion attached to said housing
and forming a guide and support for a working implement; a
forwardly directed cylinder bore extension which is coaxial
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with and which has a smaller diameter than said cylinder
bore; an annular shoulder separating said cylinder bore
extension from said cylinder bore; and an anvil sealingly
guided in said cylinder bore extension, said anvil having a
rear end normally located within said cylinder bore
extension and a forward end normally abutting a rear end of
the working implement; wherein said hammer piston includes a
piston head for sealing and guiding cooperation with said
cylinder bore, and a forwardly extending stem portion having
a smaller diameter than said piston head and arranged to
cyclically penetrate into said cylinder bore extension to
deliver repeated impacts to said rear end of said anvil at
reciprocation of said hammer piston, whereby said stem
portion and said piston head together with said cylinder
bore and said annular shoulder form an annular energy
absorbing air cushion chamber; wherein said piston head has
an axial length that is at least three times an axial length
of said stem portion; wherein said piston head comprises two
axially spaced end portions for guiding and sealing
cooperation with said cylinder bore; wherein a radial play
between each one of said axially spaced said end portions
and said cylinder bore is smaller than a radial play between
said stem portion and said cylinder bore extension thereby
enhancing the tightness of said annular energy absorbing air
cushion chamber by aligning said stem portion with said
cylinder bore extension; and wherein said cylinder bore
extension comprises a circumferential groove located at a
predetermined axial distance from said annular shoulder to
form an annular expansion volume for air entranced between
said anvil and said stem portion as said stem portion
penetrates into said cylinder bore extension during impact
strokes.
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A preferred embodiment of the invention is
described below with reference to the accompanying drawings.
On the drawings,
Fig 1A and 1B show longitudinal sections, divided
by a transverse line A-B, through a pneumatic breaker
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according to the invention.
Fig 2 shows on a larger scale a lor~~~tudinal section of
the rear part of the breaker accorc2ing to Fig lA.
Fig 3 shows on a somewhat smaller scale a section along
line 3-3 i Fig 2.
Fig 4 shows on a larger scale a fractional view of Fig
1B, but illustrates a different operating position of the
impact generating parts.
Fig 5 shows on a larger scale a detail view of the device
in Fig 3.
The impact breaker 10 shown in Figs lA, 1B comprises an
elongate housing 11 with a cylinder bore 20 and provided
with a cylinder head 12, handles 18, 19, and a front
portion 13. These parts are interconnected and
symmetrically oriented relative to the longitudinal axis
24 of the cylinder bore 20. The cylinder bore 20 is
extended rearwardly from an annular shoulder 21 through
an enlarged bore 23. The cylinder bore 20 is also
extended forwardly from an inner annular shoulder 25
through a forward bore 45. In front of the bore 45 the
housing 11 is formed with a clamping portion 46 including
an axial slot 47. The clamping portion 46 defines a
further enlarged bore 48 which extends coaxially with the
bore 45 and the cylinder bore 20.
In the bore 45 there is received an intermediate member
in the form of a sleeve 17 which has an outer shoulder
WO 95!23051 PCT/SE95/00209
for abutting cooperation with the annular shoulder 25 and
which extends sealingly into the cylinder bore 20. The
sleeve 17 has an annular end surface 49 which faces the
cylinder bore 20. The sleeve 17 is a part of the front
section of the breaker housing 11 and serves as a guide
sleeve for the impact receiving parts of the tool. The
sleeve 17 has a central coaxial first bore 50 and an
enlarged coaxial second bore 51 separated from the first
bore 50 by an annular forwardly facing shoulder 52. The
front portion 13 of the housing is a separate part which
is formed with a tubular neck 55 to be inserted in the
enlarged bore 48 of the clamping portion 46, thereby
being axially located by the sleeve 17 which defines the
axial position of the front portion 13 relative to the
housing 11 via the annular shoulder 25.
A clamping bolt 56 extends transversely through a bore 57
in the clamping portion 46 and engages a tangential
groove 58 in the neck portion 55 to lock positively the
latter axially relative to the housig 11. By means of a
nut (not shown) the clamping bolt 56 locks frictionally
the neck 55 to the clamping portion 46 such that the
front portion 13 and the sleeve 17 are rigidly secured to
the housing 11.
In the bore 50 in the sleeve 17 there is sealingly guided
an impact transferring anvil 14. The anvil 14 is formed
with an impact receiving end surface 62 facing the
cylinder bore 20 and an annular flange 53 which is guided
in the enlarged second bore 51. The anvil 14 is rewardly
displaceable by the neck portion l5 of the working
implement 16, and the interengagement of the flange 53
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and the annular shoulder 52 defines the-rear working
position of the anvil 14 relative~t~t~~ie housing 11. See
Fig 1B. In the working position~.~o~~'the anvil 14, the rear
impact receiving end surfaces 62 is located substantially
in level with or slightly below the rear end shoulder 49
of the sleeve 17. In a conventional way, the front
portion 13 carries a releasable working implement
retainer 60 which is engagable with the collar 61 of the
working implement 16 while allowing a limited axial
movement of the latter with the neck 15 guided in the
neck portion 55 of the front portion 13. In its
forwardmost position, the working implement 16 is blocked
against further movement by the retainer 16 engaging the
collar 61, which means that the anvil 14 remains in its
extended position in which it abuts against the neck
portion 55 of the front portion 13. The anvil 14 and the
neck 15 forms the impact transferring means of the
working implement 16.
According to an alternative design of the impact
mechanism, the anvil 14 is omitted and the shank portion
15 of the working implement 16 is extended to reach the
impact receiving position defined by the surface 62 of
the anvil 14. The rearmost position of the shank portion
15 and, accordingly, the impact receiving surface 62 is
determined by interengagement between the collar 61 of
the working implement 16 and the front portion 13.
At its rear end, the housing 11 is formed with two side
walls 29, 30, Fig 2, which extend rearwardly beyond the
cylinder head 12 and the central portions of the handles
18, 19. In opposite coaxial bores 67, 68 in the side
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walls 29, 30 there is inserted a wedge bolt 32 which
comprises a cylindrical steel tube having an axially
extending zigzag shaped slot 33 for obtaining radial
compressability. Thanks to the zigzag shaped slot 33, the
wedge bolt 32 gets a smoother outer surface without any
straight cutting edges which could damage the bores 67,
68 at mounting. The wedge bolt 32 forms a mounting pivot
for the central parts of the handles 18, 19, Fig 3,
thereby connecting the handles 18, 19 to the housing 11.
Vibration damping pretensioned springs 35 are located
between the housing 11 and each of the handles 18, 19 to
bias the handles toward a rear end cover 31. This end
cover 31 is formed of a plastic material and is secured
in opposite grooves 74 in the side walls 29, 30.
Inside the cover 31, the handle 19 supports a pivot lever
36 which by means of a push rod 40 is arranged to control
an air inlet valve 38. The latter is biassed by a spring
39 toward closed position. By manipulating the lever 36,
thereby activating the inlet valve 38, a connection
between a pressure air inlet 80 and an inlet passage 81
in the housing 11 and the rear bore 23 of the cylinder
bore 20 is controlled.
Resting on the axial shoulder 21 in the enlarged bore 23,
there is inserted a valve housing 27 of a distributing
valve, Fig 2, 4,. The cylinder head 12 comprises a plug
of metal or a plastic material which is introduced into
' the enlarged bore 23 and abuts and~locks axially the
valve housing 27 via a seal ring 82. At its rear end the
plug 12 is formed with two rearwardly extending heals 83
which are formed with indentations 79 and which are
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located on both sides of the handles 18, 19. The heals 83
rest against the wedge bolt 83 such that the plug 12 is
axially locked in the bore 23. The plug 12 has a radially
extending air distributing passage 84 which via a
longitudinally extending feed passage 86 in the housing
11 communicates with a front end of the cylinder bore 20.
The passage 89 is open toward the valve housing 27 via a
central axially extending opening.
The valve housing 27 is formed in a plastic material,
preferably acetal plastic (delriri), and comprises a
rotationally symmetric and substantially cup-shaped mair.
part having an outer circumferential groove 87
communicating with the a'ir inlet passage 81 in the
housing 11. In the valve housing 2?, there is shiftably
disposed a valve plate 26, also of a plastic material,
fo.r alternative cooperation with a forward valve seat 41
which is open to the cylinder bore 20 and a rear valve
seat 42 which is open to the radial air passage 84 in the
plug 12. The bottom 88 of the circumferential groove 87
is provided with radial openings 89 which are disposed in
axially separated rows between which the valve plate 26
is shiftable. The rear valve seat 42, also formed in a
plastic material such as acetal plastic, comprises a lid
which is inserted in the valve housing 27 and locked by a
lock ring 93. See Fig 2.
In the cylinder bore 20, between the valve housing 27 and
the end surface 49 of the sleeve 17,~ there is
reciprocably guided a hammer piston 28. The latter is
formed with a piston head 63 having a rear end portion 65
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and a forward end portion 66 which are sealingly guided
in the cylinder bore 20, and a piston neck 64 which is
intended to deliver hammer blows onto the impact
receiving surface 62 of the anvil 14.
The anvil 14 is sealingly guided in the cylinder bore
extension 50 formed in the sleeve 17 and may occupy any
axial operating position therein depending on the actual
magnitude of the feeding or application force applied on
the breaker handles 18, 19. If an extremely high feeding
force were applied, the anvil 14 would occupy its
rearmost position as illustrated in Fig 1B. In such a
case, the rear impact receiving end surface 62 of the
anvil 14 would be flush with the shoulder 49 in the
cylinder bore 20 and the stem portion 64 of the hammer
piston 28 would not at all penetrate into the cylinder
bore extension 50.
In the other extreme, the feeding force applied on the
breaker housing 11 is negative, i.e. the housing 11 is
lifted up in relation to the working implement 16. In
such a case, which occurs rather frequently during
operation of the breaker, the anvil 14 is displaced to
its forwardmost position, thereby allowing the piston
stem position 64 to penetrate by its full length into the
cylinder bore extension 50.
In normal operating positions of the anvil 14, however,
the rear end surface 62 is situated somewhere in front of
the annular shoulder 49, which means that the piston stem
64 always penetrates to some extent into the cylinder
bore extension 50. Since, accordingly, the piston stem 64
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normally enters the cylinder bore ~~ension 50 air
volumes are entrapped both in th~\ring chamber 59 formed
between the cylinder bore 20; the shoulder 49 and the
piston stem 64, and in the cylindrical chamber formed
between the piston stem 64 and the rear end surface 62 of
the anvil 14 in the cylinder bore extension 50. The air
volume in the ring chamber 59 has a very important
purpose, namely to form a piston damping and energy
absorbing air cushion to prevent the piston 28 to hit
mechanically against the shoulder 49 in cases of no load
strokes i.e. when the feeding force is very low or
negative.
In contrast thereto, the air volume entrapped between the
piston stem 64 and the anvil 14 has a negative influence
on the operation of the breaker. This is because this air
volume acts as an energy absorbing cushion which prevents
an efficient energy transfer from the piston 28 to the
anvil 14.
The invention aims to solve these two problems, namely
how to improve the efficiency of the impact damping
annular air cushion entrapped in the ring chamber 59 and
how to avoid influence of the energy absorbing air volume
entrapped in the cylinder bore extension 50.
The latter problem is dealt with by providing the sleeve
17 with an inner circumferential groove 54 located at a
certain distance from the shoulder. 49 which forms the
rear end of the cylinder bore extension 50. This groove
54 forms an annular expansion volume by which the
magnitude of the pressure peaks in the entrapped air
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volume are substantially reduced as is the damping
effect.
The other problem together with the previously mentioned
problem of how to avoid metallic contact between the
piston stem 64 and the cylinder bore extension 50 are
solved both by designing the hammer piston 28 so as to
accomplish an accurate guidance of the hammer piston 28
in the cylinder bore 20 and minimize radial misalignment
of the stem portion 64 in relation to the cyinder bore
extension 50, and by arranging the radial plays between
the different piston portions so as to enable an improved
tightness of the annular air cushion chamber 59.
This is obtained by giving the piston head 26 a
considerably larger axial extent than the piston stem 64,
by forming the piston head 26 with two axially spaced end
portions 65, 66 for sealing and guiding cooperation with
the cylinder bore 20, and by providing a radial play
between each one of these end portions 65, 66 and the
cylinder bore 20 that is smaller than the radial play o
between the piston stem 64 and the cylinder bore
extension 50. The piston head 26 should be at least three
times longer than the stem 64.
By these measures, the guidance of the hammer piston 28
is very accurate and the play between the piston stem 64
and the cylinder bore extension 50 could be comparatively
small, less than 0,05 mm, without risking metallic
contact, which means that the annular air cushion chamber
59 is very tight and provides a very good energy
absorption. Thereby, the breaker housing 11, hammer
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piston 28, bushing 17 and other partsn,are protected from
damage at bottom strokes of the ~mmer piston 18 at no-
load operation.
As the operator applies the impact breaker 10 against the
working surface the working implement 16 as well as the
anvil 14 are displaced rearwardly to their normal
operating positions. (See Fig 1B.) As the lever 36 is
pressed down, pressure air will be supplied to the valve
housing 27 from the air inlet 80, through the inlet valve
38 and the passage 81. By cooperating alternatively with
the valve seats 41, 42, the valve plate 26 will
distribute pressure air to the respective ends of
cylinder bore 20, to thereby make the hammer piston 28
reciprocate in the cylinder bore 20 and deliver
repetetive hammer blows on the anvil 14. During the
reciprocation of the hammer piston 28, the respective
parts of the cylinder chamber 20 are vented to the
atmosphere through outlet openings 70, 71 which are
located at different axial levels in the housing 11. The
outlet openings 70 vent the rear part of the cylinder
chamber 20 behind the hammer piston 28, while the
openings 71 vent the forward part of the cylinder chamber
20 in front of the hammer piston 28.
