Note: Descriptions are shown in the official language in which they were submitted.
WO 94/18427 PCT/US94/01253
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REVERSIBLE CASING FOR A
DOWN-THE-HOLE PERCUSSIVE APPARATUS
Background of the Invention
This invention relates generally to fluid
actuated, percussive, down-the-hole drills, and more
particularly to casings that serve as the outer body
for such drills.
Fluid actuated down-the-hole drills typically
use a system of internal cylinders or feed tubes to
perform the fluid conveying or porting functions
.
of the air cycle. These internal parts must
required
be retained within the central bore of the drill
casing. In order to maximize cylinder bore diameter
and retain these components in the correct lengthwise
position, a retaining snap ring which fits into a
groove undercut in the casing is provided as a
shouldering means. Manufacturing considerations such
as the need for boring, grinding or honing prohibit the
formation of an internal shoulder which is part of the
casing, or wear sleeve. However, a one-piece casing
with an integral shoulder, or a factory installed and
non-removable ring, would be desirable to keep the
number of serviced parts to a minimum and to avoid
damage to this sensitive area during repair servicing.
In addition, it is desirable to provide the
casing in a form that it is reversible lengthwise
because after the front end of the casing becomes worn
and abraded from use at the drilling interface, the
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casing can be reversed lengthwise to position the
unworn casing end at the drilling interface, thereby
prolonging the useful life of the casing. Such a
reversible casing must provide its snap ring groove i
with a length greater than the body length of the snap
ring, in order to permit installation of the snap ring.
Because the snap ring groove i,s positioned at a
location midway between the casing ends, it is a
problem for the person assembling the drill to
precisely align and insert a snap ring, unless the snap
ring groove length is greater than the snap ring body
length. However, if the snap ring groove length is too
long, for a particular bore and snap ring, the snap
ring can become "skewed" or "rotated" and lose its
interference fit in the groove, and freely fall out
when the casing is empty, as when the drill internal
parts are being repaired.
The foregoing illustrates limitations known
to exist in present reversible casings. Thus, it is
apparent that it would be advantageous to provide an
alternative directed to overcoming one or more of the
limitations set forth above. Accordingly, a suitable
alternative is provided including features more fully
disclosed hereinafter.
Summary_of the Invention
In one aspect of the present invention, this
is accomplished by providing a reversible casing with a
snap ring groove length in the range of 0.25-7.80
inches for a bore diameter between 2.0-10.0 inches when
the groove has a snap ring shoulder ratio between 10.0
150.0 and a snap ring aspect ratio between 1.0-6Ø
The foregoing and other aspects will become
apparent from the following detailed description of the
invention when considered in conjunction with the
accompanying drawing figures.
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Brief Description of the Drawing Figures
Fig. 1 is a longitudinal section of a down-
the-hole drill having the casing of the invention, with
' the piston in the drive position;
Fig.,2 is a view similar to Fig. l, with the
piston in the return position;
Fig. 3 is longitudinal cross section of the
casing of the invention;
Fig. 4 is a schematic longitudinal cross
sectional view of a snap ring in a snap ring groove of -
a casing, with the snap ring skewed to the position
that it is about to lose its interference fit and fall
out of the groove, and the snap ring in normal position
in phantom; and
Figs. 5-8 are curves depicting the
relationship between snap ring groove length, bore,
snap ring shoulder ratio and snap ring aspect ratio.
Detailed Description
In order to disclose this invention, it is
necessary to describe its relationship with a workable
down-the-hole drill. Accordingly, one embodiment of
such a drill is described hereinafter. Referring to
Figs. 1 and 2, a fluid actuated impact tool is shown
generally as 1. The device is adapted to be suspended
from a drill string (not shown). The drill is provided
with a backhead coupling 3 having a threaded section 5
for threadable connection to the drill steel. The
backhead coupling 3 has a bore 7 therethrough for flow
of percussive fluid, as is well known. A hollow,
elongated tubular casing 9 has a first end 11
threadably connected to the backhead coupling 3. Body
portion 13 of casing 9 extends lengthwise to a second
end 15, which is threadably connected to a fronthead
portion, shown generally as 17.
Front head portion 17 includes a drill bit 19
slidably retained in a chuck 21 which is threaded to
second end 15 of casing 9. A flexible retaining ring
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23 positioned in casing 9 above chuck 21 and a sleeve
25 with a second split ring 27 in an undercut in casing
9 completes the fronthead 17 combination. The
' fronthead elements are described for clarity only, and
form no part of the invention.
Backhead assembly portion, shown generally as "
30, includes an air distributor 32, disposed in casing
9. A check valve 34 in bore 7 is also part of backhead
assembly 30, to prevent reverse flow of air in the
drill, as is well known. Air distributor 32 includes a
valve member 35 that opens and closes during operation
of the device to permit fluid flow into various pas-
sageways, as is well known. The air distributor 32
includes a body portion 36 that has an extended exhaust
rod 38 thereon, also as is conventional. The air
distributor 32 also includes a cylinder sleeve 40 that
extends lengthwise along casing 9. The combination of
elements described, including the check valve 32, the
air distributor 32, the exhaust rod 38, the cylinder
sleeve 40, are retained in bore 42 of casing 9 by
coupling 3 threaded into casing end 11 and pressing
against first collar 44, which rides on spring 46
seated on second collar 48. The backhead portions are
described herein for clarity, and form no part of the
invention.
The backhead assembly 30 is supported at a
forward end in casing 9 by a split snap ring 50. Snap
ring 50 is centered between first and second casing
ends 11 and 15, respectively, as described hereinafter.
Piston 52 is slidable between backhead
assembly 30, as shown in Fig. 1 and fronthead 17, as
shown in Fig. 2, as is well known. Piston 52 forms ,
part of the backhead assembly 30 when piston 52 is
located in the drive position, as shown in Fig. 1, and ,
it forms part of the fronthead 17, when piston 52 is
located in the return position, as shown in Fig. 2.
Piston 52 has a plurality of grooves, undercuts and
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land contact surfaces, as is well known. During drill
operation, drive chamber 54 and return chamber 56
alternately expand and contract in volume, as well as
exhaust and compress the percussive fluid, as is well
known.
As shown more clearly on Fig.3, the casing of
this invention has an internal surface 60 that forms a
bore 62 that has an axis 64 that extends lengthwise in
the same direction as body 13 of casing 9, and which
coincides with the axis of bore 7 of backhead coupling
3. The internal surface 60 of casing 9 has a profile
that is provided by a plurality of undercut portions
66, alternating with land portions 68, which profile is
manufactured by a boring or machining procedure, as is
well known. The purpose of the profile is to combine
with surfaces or undercuts in the piston 52, the
backhead assembly 30 and the fronthead 17 to form
various fluid passageways for flow of percussive fluid,
to operate the drill, as is well known. The exact
combination of lands 68 and undercuts 66 in the casing
9 and the surfaces or grooves in the other elements may
vary from drill to drill, so long as the percussive
reversal of the piston is achieved along with the
exhaust of percussive fluid, as is well known.
The casing 9 of this invention is reversible
lengthwise, as measured about centerline plane 70 (Fig.
3) transverse to the lengthwise direction of casing
body 9, which plane is centered between first and
second casing ends 11 and 15. In order for casing 9 to
be reversible, first groove means (undercuts 66 and
lands 68) between first casing end 11 and centerline 70
must be substantially the same as the second groove
means (undercuts 66 and lands 68) between second casing
15 end and centerline 70. By substantially the same,
we mean that the grooves at either end of casing 9 must
provide the same functional fluid passageways, in
combination with the backhead assembly 30 and fronthead
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17, regardless of which casing end is connected to the
fronthead or backhead. For best performance, and ease
of manufacture of casing, we prefer that first groove
means and second groove means are mirror images about
centerline plane 70. However, slight variations away
from mirror image can work, so long as the fluid
passageways are formed substantially the same,
regardless of lengthwise orientation of casing. Snap
ring groove 72 is centered over plane 70, as seen in
Fig. 3. Snap ring groove'72 is a flat-bottomed groove
that extends radially from axis 64, as viewed in plane
70. Groove 72 consists of a pair of spaced apart, ,
parallel radially extending sidewalls 74 with a base
surface 76 therebetween. Groove 72 extends lengthwise
along casing 9 and is centered about plane 70, to
extend an equal distance on either side of plane 70.
As seen in Figs. 1 and 2, the length of
backhead assembly 30, indicated by numeral 80, is such
that it does not fall on the exact centerline plane 70
of casing 9. Therefore, there will be a slight
lengthwise movement of snap ring 50 back and forth in
snap ring groove 72, depending upon which end of casing
9 the backhead assembly 30 is connected to.
However, as shown in Fig. 4, the lengthwise
movement of snap ring 50 also permits it to "rotate" or
"skew" out of the centerline plane 70. This "skewing"
under certain conditions, permits the snap ring to lose
its interference 90 with the snap ring groove sidewalls
74, and to thereby freely come out of the snap ring
groove 72, when the operative elements of the drill
backhead assembly 30 are not inside the casing 9, as
when the drill is being repaired.
Referring to Fig. 4, various dimensional
parameters will be described that must be balanced for ,
proper design of down-the-hole drills, including the
following:
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a. casing bore 42 (termed D), having a
circular cross section as measured in a radial
direction from axis 64, in a plane 70, perpendicular to
axis 64 and perpendicular to lengthwise direction of
casing body 9;
' b. a snap ring groove 72, having a length
100 (termed L), as measured in lengthwise direction of
casing body 9;
c. a snap ring groove depth 102 (termed t),
as measured in a radial direction from axis 64 in a
plane 70, perpendicular to axis 64 and also
perpendicular to lengthwise direction of casing body 9;
d. a snap ring groove shoulder 104, as
measured radially from axis 64, and equal to snap ring
groove depth t;
e. a snap ring shoulder ratio (termed r),
calculated as the ratio of D/t;
f. a snap ring body length 106 (termed 1),
as measured in lengthwise direction of casing body 9;
and
g. a snap ring aspect ratio (termed k),
calculated as 1/t.
The snap ring shoulder ratio (r) is a measure
of the relative snap ring should size (load carrying
area) within the casing 9. Design considerations such
as load carrying capacity required to support the
backhead assembly 30 and minimum wall thickness of the
casing 9 must be taken into consideration when
selecting the appropriate snap ring shoulder ratio.
For example, a shoulder ratio of 10.0 provides a large
and strong shoulder size but also minimizes the wall
thickness of the casing 9. A shoulder ratio of 150.0
provides a small shoulder and a maximized casing wall
thickness.
The snap ring aspect ratio (k) is a measure
of the relative bending strength of the snap ring 50.
Design considerations such as strength and ease of
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installation must be considered when selecting this
parameter. For example, a snap ring aspect ratio of
1.0 would provide a thin ring which would install
easily but provide minimal bending strength. A snap
ring with an aspect ratio of 6.,0 would provide a very
strong ring but may be difficult to install. a
We have discovered that there is a
relationship among the above-listed design elements
whereby the maximum length (L) of a snap ring groove 72
can be predicted, which length (L) will permit
lengthwise movement of the snap ring 50, while still
retaining the snap ring 50 in the groove 72 by
interference fit. The outer limits of such groove
length (L) are listed in Table I, and shown graphically
in Figs. 5-7.
TABLE I
Range of
Range of Max. Range of Range of Snap Ring
Groove Length,Bore Diameter,Shoulder Aspect Ratio,
L, (inches) D, (inches) Ratio, r=D/tk=1/t
2 0.25-7.80 2.0-10.0 10.0-150.0 1.0-6.0
0
0.25-1.22 2.0-10.0 150.0 1.0
0.75-3.9 2.0-10.0 30.0 4.0
1.75-7.80 2.0-10.0 10.0 6.0
In Fig. 5 the preferred combination of
shoulder ratio (r) and snap ring aspect ratio (k) are
shown, for predicting the maximum acceptable groove
length (L), for various diameter bores (D). Any groove
length falling below the curve 110 will retain the snap
ring 50 in the groove 72. Above the curve 110, the
snap ring 50 will freely "skew" out of the groove 72.
Fig. 6 shows the outer extreme~of acceptable groove
lengths (L) for a shallow groove depth (t) [r = 150.0]
and narrow ring body length (1) [k = 1.0]. Any groove
length (L) below the curve 120 is acceptable. Fig. 7
2~.~~~~~
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shows the outer extreme of acceptable groove lengths
(L) for a deep groove depth (t) [r = 10.0] and long
ring body length (1) [k = 6.0]. Any groove length (L)
below the curve 130 is acceptable. Fig. 8 shows a
combination of the curves of Figs. 5-7.
" Finally, it is understood that as used herein
the term "snap ring" refers to the conventional,
substantially circular, elastically deformable ring-
type element that can be elastically deformed into
small diameter, for insertion into a bore 42, during
assembly of the drill. The ring will expand or "snap"
into the groove 72 to press its outer diameter surface
lightly against the bottom surface 76 of the groove,
with its inner~diameter surface extending a slight
distance into bore 42, resulting in the support of
backhead assembly 30, as described hereinabove. In
such position, ring 50 is substantially permanently
positioned in groove 72, being removable only with
special tools and or special effort.
