Apparatus for cleaning air ports of a chemical recovery furnace

Apparatus for cleaning air ports in a chemical recovery furnace includes a plurality of rods, each having a cleaning tip thereon, mounted on a wind box adjacent the furnace. The rods are longitudinally slidable to insert the cleaning tips into the air ports, and are rotatable about an axis perpendicular to the longitudinal axis of the rods so as to index the cleaning tips to a new point of insertion into the air port opening. An air cylinder actuator is employed to insert the cleaning tips into the openings in a ramming motion to dislodge residual buildup in the air port opening, retract the cleaning tips from the air port openings, and index the tips to a new point of insertion. Successive invocation of the actuating means results in removal of residual buildup along the entire length of the air port openings.

BACKGROUND OF THE INVENTION 
The present invention relates to furnaces and particularly to apparatus for 
automatically cleaning ports introducing combustion air into the firebox 
of a chemical recovery furnace. 
Wood pulp for papermaking is usually manufactured according to the sulfate 
process wherein wood chips are treated with a cooking liquor including 
sodium sulfide and sodium hydroxide. The wood chips and the cooking 
liquor, called "white liquor," are cooked in a digester under 
predetermined heat and temperature conditions. After cooking, the used 
liquor, termed "black liquor," containing spent cooking chemicals and 
soluble residue from the cook, is washed out of the pulp and treated in a 
recovery unit where the cooking chemicals are reclaimed. Without 
reclamation and reuse of the cooking chemicals, the cost of the 
papermaking process would be prohibitive. 
In the recovery process, the black liquor is first concentrated by 
evaporation to a water solution containing about 65 percent solids, which 
solution is then sprayed into the firebox of a black liquor recovery 
boiler, a type of chemical reduction furnace. The chemical reduction 
furnace is a reactor wherein the processes of evaporation, gasification, 
pyrolysis, oxidation and reduction all occur interdependently during 
recovery of the cooking chemicals. The organic materials in the black 
liquor, lignin and other wood extracts, maintain combustion in the 
firebox, and the heat produced dries and melts the spent cooking chemicals 
as they fall to the floor of the firebox, where they build a mound of 
material called a char bed. The char bed is further heated to further 
liquify the chemicals into a molten smelt that flows out of the furnace 
through a smelt spout to a collection tank. Concurrently, combustion heat 
is employed to generate steam in a water wall of the boiler for use as 
process steam and for generating electricity. 
The combustion process requires the introduction of large volumes of air 
into the firebox, air comprising about 80 percent of the material entering 
the furnace. The air is forced into the firebox from wind boxes or ducts 
disposed at several levels on surrounding relation to the firebox, through 
a plurality of air ports in the walls of the furnace, viz.:primary, 
secondary and tertiary air ports. The primary air ports, through which 
about 40 to 50 percent of the air enters the furnace, are disposed on the 
side walls of the firebox near the bottom of the furnace close to the char 
bed. The secondary air ports, through which about 35 percent of the air 
enters the furnace, are disposed around the walls of the firebox, higher 
than the primary air ports, and closer to the entry conduits through which 
the black liquor is sprayed into the firebox. While the primary air ports 
provide a relatively large volume of air with considerable turbulence for 
maintaining a fireball in the char bed, the secondary and tertiary air 
ports provide a finer control and distribution of air above the char bed 
and distribute the air evenly in the black liquor spray to support the 
combustion thereof. 
The black liquor sprayed into the firebox, having a consistency like warm 
60 weight oil, swirls, burns and falls toward the bottom of the firebox as 
combustion products comprising char material and smelt. The smelt and char 
material contact the outer walls of the firebox and, cooled by the 
inflowing air, form excrescent deposits around edges of the air ports, 
particularly along the edges of the openings where the excrescent material 
builds up and around the openings under influence of air rushing through 
the air port. Such buildup of char material can block air flow through the 
ports by as much as ten percent, and can even block individual ports 
completely. In accordance with customary practice, the char build-up is 
periodically removed by a worker inserting a rod into the air ports 
successively around the boiler. With manual rodding of the air ports, 
gradual buildup of char material intermittently around the furnace causes 
changes in the volume of combustion air, as well as changes in air 
distribution, velocity and pressure. Therefore, furnace operation tends to 
be inefficient and unpredictable with an attendant decrease in the amount 
of chemicals that can be recovered, a decrease in the amount of steam 
produced per unit of fuel, and increased emission of noxious gases such as 
carbon monoxide and sulfur dioxide. 
Apparatus for cleaning openings in a recovery furnace are known--see my 
copending U.S. patent application Ser. No. 829,712, filed Feb. 13, 1986, 
entitled APATUS FOR CLEANING AIR PORTS OF A CHEMICAL RECOVERY FURNACE, 
and my U.S. Pat. No. 4,423,533, entitled FURNACE AIR PORT CLEANER. The 
apparatus disclosed therein includes rods with cleaning tips attached to 
an end of each rod, and the cleaning action is a wiping motion 
accomplished by inserting the cleaning tips into corresponding openings 
and then changing the position of the rods, while the cleaning tips are 
within the openings, so as to cause the cleaning tips to wipe or move 
along the length of the openings to dislodge the buildup therein. It has 
been found that particularly hard deposits of solidified smelt and char 
material resembling a vitreous substance may resist the wiping action of 
the cleaning tip and in extreme instances may cause individual rods to 
stress and even bend. It is desirable therefore to provide a cleaning 
action in which the cleaning tip acts as a ram, wherein force is applied 
longitudinally of the rod, while still cleaning the longitudinal height of 
the air port opening without disrupting or blocking combustion air flow 
through the port either while cleaning or while the cleaning apparatus is 
in an at-rest position. 
It is accordingly an object of the present invention to provide improved 
apparatus for cleaning air ports of a chemical recovery furnace. 
Another object of the present invention is to provide improved air port 
cleaning apparatus for increasing the operational stability of a black 
liquor recovery boiler. 
It is a further object of the present invention to provide improved air 
port cleaning apparatus for enhancing the efficiency of chemical recovery, 
increasing steam production, and reducing sulfur dioxide and carbon 
monoxide emissions from a black liquor recovery boiler. 
Yet another object of the present invention is to provide an air port 
cleaning apparatus in which the cleaning rods are not subject to lateral 
stress. 
SUMMARY OF THE INVENTION 
According to the present invention, in a preferred embodiment thereof, 
furnace air ports are cleaned automatically by apparatus comprising a rod 
having a cleaning tip adapted for insertion through a port by longitudinal 
translation of the rod, the cleaning tip being moved into the air port as 
by ramming to dislodge excrescent material in the air port. The cleaning 
tip is then retracted and indexed, and the ramming operation repeated at a 
different point of insertion in the air port opening. 
In accordance with the invention, an actuator applies force longitudinally 
of the rod to move the cleaning tip into the air port opening and 
subsequently retract the rod, the retraction force being translated to an 
index mechanism that repositions the cleaning tip by a predetermined 
interval of space. With each repeated insertion and retraction of the 
cleaning tip, the tip is moved so that the cleaning operation, i.e., the 
ramming motion of the cleaning tip, is repeated at specific intervals of 
space over the length of the air port opening, thereby cleaning the entire 
opening. 
According to another embodiment, the cleaning tip is provided with serrate 
edges for cutting or chipping hardened excrescent material in the opening. 
The subject matter of the present invention is particularly pointed out and 
distinctly claimed in the concluding portion of this specification. 
However, both the organization and method of operation, together with 
further advantages and objects thereof, may best be understood by 
reference to the following description taken in connection with 
accompanying drawings wherein like reference characters refer to like 
elements.

DETAILED DESCRIPTION 
Referring to the drawings and particularly to FIG. 1, the present invention 
is adapted for use in connection with a chemical recovery furnace 10 which 
comprises the firebox 12 for a steam boiler. "Black liquor", as 
hereinabove described, is sprayed into the firebox 12 via conduit 14, with 
the chemicals being deposited on the floor of the firebox 12 as a char bed 
16 from which a molten chemical-containing smelt 18 is recovered for 
further treatment. A wind box 20 substantially surrounds the firebox 12 
and delivers combustion air under pressure into the firebox 12 through a 
multiplicity of air port openings 22 in a wall 24 of the firebox 12. 
In accordance with the present invention, apparatus 26 for cleaning the air 
port openings is attached to the outer wall 28 of the wind box 20, and is 
adapted to clean the air port openings 22 in the wall 24 of the firebox 
12. 
Referring now to FIGS. 2-5, illustrating cleaning apparatus 26 in greater 
detail, the apparatus comprises a plurality of rods 30 each provided with 
a cleaning tip 32 adapted for insertion into one of the air port openings 
22 in the firebox wall 24. The apparatus described herein comprises four 
such rods with the cleaning tip on the forward end of each rod, but it is 
understood a greater or lesser number of rods could have been included in 
each unit. The unit as disclosed is conveniently manageable, and can be 
easily removed and transported from the furnace 10 in the event access is 
required to the interior of the wind box 20. The rods 30 are suitably 
formed from stainless steel, while the cleaning tips 32 are suitably 
formed as by casting from a high chromium ferritic alloy such as No. 446. 
Alternatively, a high nickel alloy steel may be employed, but in any case 
the material should be such that the forward end of the tip can withstand 
the high temperatures of the firebox (typically 1,700.degree. F.) for 
short periods. The cleaning tips may extend crossways of the corresponding 
air port opening 22 to fill a substantial portion of the lateral dimension 
of the opening, or alternatively be configured as a ram having a square 
cross-section, in the latter configuration, being approximately one and 
one-fourth inches square with a rounded end. The presently described 
embodiment of the invention preferably utilizes such a ram configuration 
of the tip so as to contact and dislodge relatively hard deposits of 
solidified smelt and char material that build up in and around the air 
port opening 22. The cleaning tips 32, are substantially smaller in 
vertical cross-section than the long distance or height of the air port 
opening 22 into which the tip is adapted to extend. Therefore, as 
hereinafter more fully explained, the tip may be indexed to penetrate the 
opening 22 at a plurality of locations traversing the longitudinal 
dimension of the corresponding air port opening for dislodging the char 
buildup therefrom. 
Mounting means are provided for attachment of the cleaning apparatus 26 to 
the outer wall 28 of the wind box 20 for pivotally and slidably mounting 
the rods 30. The mounting means comprises a faceplate 34 received in and 
attached to a frame 36 covering an opening 38 in the wall 28 of the wind 
box. The frame 36 is disposed at an angle such that the rods 30 suitably 
extend, in relation to the wall 28, angularly downward toward the air port 
openings 22. The mounting means further comprises a plurality of mounting 
assemblies 40, secured by bolts 42 to the faceplate 34, and pivotally 
receiving tubes 44, which are journaled in bearings 46. The rods 30 are 
each slidably disposed within a corresponding tube 44 such that each may 
be longitudinally translated within the tube 44 and rotated about a pivot 
48 having an axis of rotation parallel to walls 24, 28 and perpendicular 
to the longitudinal axis of the rods 30. 
Referring to FIGS. 6 and 7 in conjunction with FIG. 3, the mounting 
assembly 40 for pivotally and slidably mounting the rod 30 to the 
faceplate 34 includes a housing 50 having a transverse bore 52, 54 through 
both side walls 56, 58 thereof for receiving bearings 60, 62 mounted on a 
fulcrum rotating cylinder 64. The tube 44 comprises a T-shaped member 66 
having a cylindrical sleeve 68 into which the cylinder 64 is slidably 
received. The bore 70 of tube 44 is perpendicular to the axis of the 
sleeve 68 and extends through the back wall 72 of the sleeve 68 for 
slidably receiving and supporting the rod 30. The cylinder 64 is adapted 
to receive the bearing 60 through which a bolt 74 extends for threadable 
engagement in an end of the cylinder 64. A lock washer 76 is suitably 
disposed between the head of bolt 74 and the bearing 60. The fulcrum 
rotating cylinder 64 is provided with a transverse bore 78 coaxial with 
the bore 70 of tube 44 when the cylinder 64 is disposed inside the 
cylindrical sleeve 68. The rod 30 extends through the bore 70 of tube 44 
and the transverse bore 78 of the fulcrum rotating cylinder 64 into the 
wind box. The housing 50 is also provided with a vertically elongated 
opening 80 through which the rod 30 extends, and teflon seals 82, 84 in 
the rear corners of the housing 50. This construction provides an 
effective air seal for preventing the escape of any substantial amount of 
air from the wind box while allowing pivoting motion of the rod 30. The 
rod 30 is thus free to move both longitudinally and in rotation about the 
pivot 48 in the direction indicated by the arrows 86, FIG. 7, as the rod 
30 effects a cleaning cycle. 
Referring now to FIGS. 2-5, means for translating the rods 30 within the 
tubes 44 comprises a crossbar 90 suitably made of steel tubing, and an 
actuator or air cylinder 92. The crossbar 90 extends laterally of the rods 
30 and is affixed by bolts 94 to each of rods 30 at an end 96 thereof 
opposite cleaning tips 32, thus unifying the longitudinal translation of 
the plurality of rods 30. The air cylinder 92, having a piston rod 98 
rotatably connected to faceplate 34 by a pivot 100, is affixed to crossbar 
90 by a bolt 102 and is employed to actuate rods 30 in longitudinal 
translation with respect to faceplate 34. Pivot 100 provides an axis of 
rotation substantially colinear with that provided by pivot 48 such that 
air cylinder 92 and rods 30 rotate about a substantially common axis. 
Cleaning tips 32 may be inserted within openings 22 by actuating air 
cylinder 92 to cause crossbar 90 to move toward the faceplate 34, and are 
retracted from openings 22 by reversing the stroke of the air cylinder 92. 
A boot or bellows 104, suitably made from synthetic rubber, protects the 
otherwise exposed portions of rods 30 when the rods are in a retracted 
position. 
The cleaning tips 32 of the rods 30 are indexed, in accordance with one 
embodiment of the invention, by rotating the rods about the axis of pivot 
48 perpendicular to the longitudinal axis of the rods, as the cleaning 
tips are being withdrawn from the openings 22. An indexing mechanism 110 
comprises a clutch assembly 112 coupled between the rod assembly and the 
faceplate 34. A reference plate 114 affixed to the faceplate 34 by bolts 
116 extends outward from the faceplate toward the actuator 92, as seen in 
FIG. 4, and provides a reference bearing 118 for attachment of the clutch 
assembly 112. A second crossbar 120 is attached to each of the tubes 44 by 
bolts 122 through a weldment 124 to unify the rotational translation of 
the rod assembly about the pivot 48. The clutch assembly 112 is attached 
to crossbar 120, and is employed to move crossbar 120 incrementally in 
relation to the reference plate 114 so as to cause an angular displacement 
of the rods 30 about the pivot 48, and thus index the cleaning tips 32 to 
provide a new point of insertion of the cleaning tips 32 into the openings 
22. 
Referring now to FIG. 8, the clutch assembly 112 is shown in greater detail 
comprising a first clutch 126, which is a no-back clutch, coupled 
coaxially with a second clutch 128, the latter being a torque-limiting 
clutch of the type having spring segments wrapped about the shaft. The 
no-back clutch 126 comprises an input shaft 130 journaled in a bearing 
132, which bearing 132 is pressed into a cylindrical housing 134 of the 
clutch assembly 112, and an output shaft 136 journaled coaxially with the 
input shaft 130 in a second bearing 138, the bearing 138 being pressed 
into the cylindrical housing at the end thereof opposite the first bearing 
132. 
A spindle 140 machined on the internal end of output shaft 136 is received 
in a corresponding aperture 142 machined in the end of the input shaft 
130. The input and output shafts 130, 136 are loosely joined by way of a 
pin 144 such that the input shaft 130 rotates several degrees before the 
output shaft 136 follows. The no-back clutch mechanism comprises an input 
bearing block 146 disposed in substantially surrounding relation of the 
input shaft 130 and keyed to a flat 148 of the input shaft 130, the input 
bearing block 146 being affixed to the shaft 130 for rotation therewith by 
a set screw 150. A finger or actuating member 152 of the bearing block 146 
extends longitudinally beyond the internal end of the input shaft 130. An 
output bearing block 154, similarly affixed to the output shaft 136 by a 
set screw 156, is disposed in substantially surrounding relation of the 
output shaft 136 and includes a finger or actuating member 158 extending 
longitudinally toward and interdigitated with the actuating member 152 of 
the input bearing block 146. A coil spring 160, compressed and disposed 
interiorly of the housing 134, surrounds the interdigitated bearing blocks 
146, 154, and normally bears against the interior surface of the housing 
134. End tabs 162, 164 of the coil spring 160 protrude such that either of 
the bearing blocks 146, 154 makes contact with the tabs to coil or uncoil 
the spring 160. 
Referring to FIG. 8A, when torque is applied to the input shaft 130, the 
input bearing block 146 contacts the end tab 162 of the coil spring 160 
and applies force thereto in a direction indicated by the arrow 166, which 
tends to compress the coil spring 160. Simultaneously, the output bearing 
block 154 holds the second end tab 164 and the spring 160 compresses, 
which relieves the grip of the coil spring on the interior of the housing 
and allows the output shaft 136 to rotate with the input shaft 130. When 
torque is applied to the output shaft 136, however, the output bearing 
block 154 contacts the end tab 164 and applies force thereto in the 
direction indicated by the arrow 168 to uncoil or open the coil spring 160 
against the interior of the housing 134 and provide a binding force 
thereagainst that prevents the output shaft 136 from rotating with respect 
to the housing 134. The no-back clutch 126 thus has the following 
operational characteristics:Torque applied to the input shaft 130 is 
transferred to the output shaft 136; however, the output shaft 136 will 
not move in response to torque applied thereto, and such torque applied to 
the output shaft 136 is not transferred to the input shaft 130. It is 
possible that excessive torque could be applied to the output shaft 136 
and thereby damage the internal components of no-back clutch 126; 
therefore, the torque-limiting clutch 128 is connected between the output 
shaft 136 of the no-back clutch 126 and an output shaft 170 of the clutch 
assembly 112. When torque applied to the output shaft 170 of the clutch 
assembly 112 exceeds a predetermined limit, 300 inch-pounds in the 
presently described embodiment of the invention, the torque-limiting 
clutch 128 allows the output shaft 170 to turn independently of the shaft 
136 output of the no-back clutch 126. The construction and operation of 
spring segment torque-limiting clutches is well known in the art. 
Referring again to FIGS. 2-5, a lever arm 200 is connected by an indexing 
or backstopping clutch 202 to the input shaft 130 of the clutch assembly 
112. The backstopping clutch 202 is suitably a drawn cup roller clutch, 
which transmits torque to the shaft in one direction and allows free 
overrun in the opposite direction. The lever arm 200 extends radially from 
the clutch assembly 112 generally in the direction of a cam follower 204, 
which is connected to the actuator 92, and includes a dogleg portion 206 
oriented with respect to the line of travel of the cam follower 204 such 
that the impulse imparted to lever arm 200 upon initial contact with cam 
follower 204 is reduced. The orientation of the dogleg portion 206 also 
serves to reduce the total angular rotation of lever arm 200. As the 
actuator 92 completes a retraction stroke, cam follower 204 contacts the 
lever arm 200 on its dogleg portion 206 and rotates the input shaft 130 
counterclockwise with reference to FIGS. 4 and 5 to produce a torque on 
the input shaft 130. A return spring (not shown) urges the roller clutch 
202 clockwise, in its direction of overrun, such that as the actuator 92 
performs an insertion stroke, lever arm 200 is free to move against an 
adjustable stop 208 in preparation for the next retraction stroke. Thus, 
upon each retraction stroke of the actuator 92, lever arm 200 is in a 
position to receive the cam follower 204 and translate force from the 
actuator to torque on the input shaft 130, the amount of angular 
displacement of input shaft 130 being controllable by adjustment of the 
stop 208. 
An eccentric 210, see FIG. 3, is connected to the output shaft 170 of the 
clutch assembly 112 and is rotationally disposed within a ring bearing 212 
held against motion relative to the reference plate 114 by a link 214. 
Upon rotation of the output shaft 170, eccentric 210 is rotated within 
ring bearing 212, and the second crossbar 120, being rigidly affixed to 
the clutch assembly 112, is moved relative to the reference plate 114. 
Thus, torque transmitted to the input shaft 130 by the retracting air 
cylinder 92 results in an angular displacement of rods 30 about the pivot 
48. 
The link 214, which is proximally affixed to the ring bearing 212 and 
rotatably connected at the distal end thereof to the reference bearing 118 
of reference plate 114, is adjustable in length to control the vertical 
offset of the range of displacement of the cleaning tips 32. Referring to 
FIGS. 9 and 10, which show the rotatable connection of link 214 to 
reference plate 114 in greater detail, a threaded rod 216 is rigidly 
connected at one end thereof to ring bearing 212 (see FIG. 3), and at the 
other end 218, is threaded within a connecting member 220 and held by a 
lock nut 221. A wear block 222 is disposed within a U-shaped portion of 
connecting member 220 and held therein by a bolt 224 engaging both 
connecting member 220 and wear block 222. Wear block 222 includes a bore 
226 therethrough dimensioned to receive a hardened pivot pin 228 rigidly 
connected to and extending perpendicularly from the reference plate 114, 
the wear block 222 being held on the pivot pin 228 by a C-clip 230. Pivot 
pin 228 and the wear block 222 thus form the rotatable connection between 
link 214 and reference plate 114. The length of the link 214 is determined 
by the extent to which threaded rod 216 is threaded within connecting 
member 220. When wear block 222 is worn, it may be easily replaced without 
affecting the length adjustment of link 214, as would be the case if the 
entire link 214 were to be replaced. 
Referring to FIGS. 2-5, in operation, a cleaning cycle consists of a series 
of insertions and retractions of the cleaning tips 32, into the openings 
22, each insertion and retraction occurring at a different point along the 
longitudinal dimension of the openings 22. The cleaning cycle begins from 
a rest position with cleaning tips 32 retracted from openings 22, and as 
the air cylinder 92 is activated to perform an insertion stroke, the 
cleaning tips 32 enter openings 22 at a first point of insertion. When the 
air cylinder 92 reaches the end of its insertion stroke, it reverses 
direction to retract cleaning tips 32 from openings 22, and over the last 
few inches of the retraction stroke, cam follower 204 contacts the lever 
arm 200 to actuate the clutch assembly 112 to rotate the rods 30 about the 
pivots 48. Rotation of the rods 30 allows cleaning tips 32, upon the next 
insertion stroke, to enter openings 22 at a second point of insertion 
spaced apart from the first point of insertion. During one cleaning cycle, 
the cleaning tips 32 are indexed successively a sufficient number of 
times, eight in the presently described embodiment of the invention, to 
clean all portions of the openings 22. 
The cleaning tips 32 dislodge residual buildup from the edges surrounding 
openings 22 during their insertion and not during the change in angular 
position of rods 30. In the prior apparatus, the cleaning tips dislodge 
residual buildup by a wiping action effectuated while the cleaning tips 
are within the openings, and if a cleaning tip encounters an exceptionally 
resistant residual buildup, the corresponding rod may be bent as the 
apparatus nevertheless continues to attempt to change the position of the 
tip. The present invention avoids this problem by not substantially 
changing the position of the tips at a time when they could be bound 
within the openings 22. 
Referring now to FIG. 11, an improved cleaning tip 250 is shown having a 
plurality of serrations 252 spaced substantially along the length of the 
tip 250 and extending around the periphery thereof. A base 254 of the tip 
includes a tapped hole 256 receiving the rod 30 (not shown) therein, and a 
transverse bore 258 through the base 254 receiving a pin for holding the 
tip 250 attached to the shaft. The tip 250 is suitably formed as by 
casting from a heat resisting material such as No. 446 alloy, which is a 
high chromium ferritic alloy. The cleaning tip 250 is substantially square 
in cross section, the serrations 252 being approximately one and 
three-eighths inches square on serrate edges 260 and one inch square at 
bases 262 of the serrations. An end 264 of the tip 250 is suitably rounded 
to facilitate guiding the tip into the air port opening. It has been found 
that cleaning action wherein the tip 250 is inserted by ramming into the 
opening, i.e., repeated longitudinal translation rather than wiping the 
tip inside the opening, provides superior cleaning because the serrate 
edges 260 of the tip 250 can chip away hardened excrescent deposits that 
otherwise would resist the action of the cleaning tip or even bind the tip 
in the opening. Further, the serrated tip 250 is easily withdrawn from the 
opening and is not subject to binding therein. It is seen that 
approximately two to three serrations 252 are inserted into an opening 
during a cleaning cycle, the tip being adjustable inward as it erodes away 
with extended use, by loosening the bolt 94 and adjusting the rod end 96 
inward (FIGS. 2 and 3). 
In the working environment, workers are often required to work in the 
vicinity of the cleaning apparatus 26, and it has been found that these 
workers sometimes find it necessary to stand upon the crossbar 90, this 
activity potentially applying force to the rods, which if translated to 
the output shaft 170, could produce torque that exceeds the design limits 
of the no-back clutch 126. To relieve such excess force, the 
torque-limiting clutch 128 allows the output shaft 170 to rotate with 
respect to the output shaft 136 of the no-back clutch 126 upon application 
thereto of a predetermined level of torque. As the output shaft 170 moves, 
the rods 30 move to the limit of their range of vertical motion, i.e., to 
top-dead-center or bottom-dead-center of the eccentric 210, where torque 
is no longer applied to the output shaft 170. 
While a preferred embodiment of the present invention has been shown and 
described, it will be apparent to those skilled in the art that many 
changes and modifications may be made without departing from the invention 
in its broader aspects. For example, the indexing clutch could be such 
that an absolute point of insertion could be selected instead of simply 
incrementing or decrementing its current position. The indexing clutch 
could be replaced by a number of mechanisms, such as servo and stepper 
motors, to control the angular position of the rods. An alternative 
embodiment of the indexing mechanism utilizes a constant-torque slip 
clutch which is responsive to torque applied thereto within a range of 
approximately 150 to 300 inch-pounds to transmit the applied torque to the 
output shaft. 
While discontinuous or stepped indexing is the preferred embodiment and the 
best mode of implementing the invention at present, it is seen that the 
tip may be moved incrementally by slowly (with respect to the longitudinal 
translation of the rods) and continuously pivoting the rod so as to move 
the tips to a new point of insertion each time the tip is successively 
inserted into the opening. In this latter described embodiment of the 
invention, virtually no wiping action of the tip occurs while the tip is 
inserted in the opening because a plurality of insertion and withdrawal 
cycles occur during the time when the tip translates through the height of 
the opening, and the cleaning action is essentially a ramming action. 
Further, rods 30 need not be pivotable if, for example, positioning means 
were provided to displace said rods in a direction perpendicular to their 
longitudinal translation so as to control their point of insertion and 
retraction. The appended claims are therefore intended to cover all such 
changes and modifications as fall within the true spirit and scope of the 
invention.