CA1066691A - Helicopter threadable stringing block assembly for power conductors - Google Patents

Abstract

ABSTRACT
A converter yoke and stringing block assembly adapted to be threaded with a helicopter-dispensed conductor hauling line dropped thereonto from either side of a power line. The yoke sub-assembly can be used in lieu of a conventional stringing block yoke thereby to convert a manually threaded block for threading by helicopter, or the features of the invention can be embodied in a stringing block as initially manufactured. The separate yoke brackets are spaced apart to provide an open-ended threading passage and the exit end of the latter is initially extended by dislodgeable guide members shiftable to their normal retracted positions automatically as the advance end of the com-ductor running board enters the stringing block, the guide members being thereafter restrained in their normal retracted position until manually reset. A load transfer latch normally interconnects the two brackets of the yoke and includes means for holding the latch open until dislodged by the conductor hauling line as it is threaded through the stringing block.

Description

106669~ :
1 This invention relates to equipment for ~tringing power line eonductors, ana more particularly to a bundle-type power line stringing block and a eonverter yoke aecessory therefor dosigned to be threaded with a hauling line by helieopter and including means disloaged by the entry of the hauling line to elose a load transfer latch and further ineluding hauling line guides whieh are dislodged to their rstracted po-ition by the entry of a conventional running board interconnecting the trail-ing end of the hauling line and the advance end of the power eonduetors the~Jelves.
Thi~ application mbodies certain improv _ nts over tho related stringing block ass~mhly di~elosed in my coponding applieation for Canadian Letter~ Patent, Serial ~o. 194,484, filed March 8, 1974. The load transfer latch member thero dis-elosed ~ay fail to open by the tension acting in the hauling line a~ it is dropped from the helicopter Thi~ cun be re~edied by o~ploying very light spring means urging th- latch to~ard it~ normal latehed position However, lf this expodient i~
~ploy d th- spring strength may be lnadequato to aJ~ur- full and prop~r clo~ing of the latch after tho pa~sage of the hauling ~-lin-, partlcularly if foreign matter is pre~ent between the parts.
In this event, s-rious damage to the stringing bloek i~ almo~t inevitablo as ~tringing operations proceed with po~ible disas-trou~ re~ult~.
Anoth-r unde~irable charaeteristic of th- earlier eon~truc-tion di~clo~ea in application Serial No. 194,484 is that the hauling lino guide members ~1,62 of that application una-sirably obstruct the conduetor passageway between sheavos 26,27 and tho underside of the stringing blo~k yoke. Furthermore and if made as flexible as is dosirable for certain purposes, the hauling line guide members have inadequate ~trength to serve their inton-dod purpoJo, partieularly along portions of the power line passing

-2- ~
--~F

. . .
. ... . .
, 1066~91 1 around the bends in the right of way Under these circu~stances, the streqses imposed on the guide means by ~he hauling cable can seriou~ly da~age the guide means Additionally, it io important that any hindrance to the gravitation of the hauling lin- occur a- near as po~sible to a vertical center line of the block to avoid imparting rotary force to the block about its ~u~pens$on axis a~ tension is applied to the hauling line It i- therefore the purpose of the present invention to avoid the foregoing and other shortcomings of prior expodient-for the helicopter threading of stringing blocks According tothis invention, a captive strut or prop cooperates with the latch and the smaller one of the yoke components to hold the lat d po~itivoly in open position dQspite the us- of powerful pring Deans urging the latch clos~d and which ~trut is readily di-lodg-d by contact with the conductor hauling l$ne or knot ~ean- thorein a- the latter is b~ing thr-aded into or pulled through th- tringlng block To thls end, one nd of tho ~trut i~ pivot~d to the lower odge of the latch and it~ oppo~ite end ha~ h-llow seating engagement $n rece-s on the oppoaite ld- of th- th N adlng pa-~age A very slight forco re~e$ved fro~ th- hauling lin~ or an enlargement carr$od th-reby serve-to d$-lodge th- Jt N t from its rest position allowing tho power-ful pring to clo-o the latch 80 that the latch ther-after -rv-s a- a Jtrong load tran~fer connection cro-swiJe of the threading pa-sage.
At l-a-t one and preferably a pair of powerful hauling line guid - aro ~ovably ~ountod below and along either ido of the ~xit end of th- threadlng passage Normally a toggl- spring arrang--~ent bia~-s theJ- guido mQmbers to one or tho other of two rotract-d po~ition- romoto from the string blodk sheave~ ~owever, during hauling line thr-ading operations these guido- are manually et-......
abl- in upright po~tions forming a narrow oxten~ion of the -` ~06669:1 ~
1 threading paseage with the lower ends of the guide members held preesed against a suitable rigid support such as a respective ono of the sheaves to either side of the central sheave This conetruction provide~ a very powerful supplemental guide for the hauling line and restrains the latter and maintains it captive on the periphery of the central groove~ despite strong forces tending to di~place it fro~ the central ~heave of ~tringing block~ locatod along bend~ in the power lino. The running board nor~ally interconnecting the hauling line to the advance end of the conductore readily displace~ the guido me~bere for this ~anually ~et po~ition thereby allowing the pring biased ~ean~ to pivot the guide D~mbers to ono or the other of the genorally horizontal normal rotracted poeitions coqpletely clear of the conauctors being haul d into po-ition along the power line, the diroction of pivoting depending on the diroction of pull past tho block.
accordingly~ it i~ a pri~ary object of thi- inv~ntion to provide an i~provod and novel converter yoko aooeesory and ~tringing block a-se~bly adapted to be threaded with a helicopter-di-pon~od conductor hauling line.
Another ob~-ot of the invention is the provi~ion of a convorter yoke accessory readily attach~ble to the eide fram-s of a bundlo typo Jtringing block iD lieu of a oonvontional yoke and having an improvod load tran~for latch and i~proved hauling line guide ~o~ber~ automatically dislodgeable to an inactivo nor-~ally oooupiod position by ths passago of a running board a~-e~bly therspa~t.
Anothor bb~eot of the invention is the provi~ion of a heli-ooptor throadable stringing block having a latch ~o~ably uppor-ted oro~wiee of it~ threading passage and di~lodgeable to it~
- normal olo~ed po-ition by a force appliod relatively cloee to the su~pen~ion axis of ~aid blodk.

':. -, ~066691 1 Another object of the invention is the provision of a helic~pter threadable stringing block readily threaded in the same flight direction from either side of a power line while a gate for the threading passage is held open by means readily dislodged by the conductor hauling line while being threaded past said gate.
Another object of the invention is the provision of a converter yoke assembly for a stringing block having hauling line guide members mounted thereon each movable between a stable threading position and one or more stable retracted positions remote from the stringing block sheaves.
These and other more specific objects will appear upon reading the following specification and claims and upon con-sidering in connection therewith the attached drawing to which they relate.
Referring now to the drawing in which a preferred embodi-ment of the invention is illustrated:
Figure 1 i8 a fragmentary side elevational view of the upper portion of a stringing block assembly embodying the features of this invention and showing the hauling line guide members set in threading position:
Figure 2 i9 a fragmentary end view of the stringing block assembly shown in Figure 1 with the load transfer latch cocked in threading position;
Figure 3 is a fragmentary view on an enlarged scale partly in section taken generally along line 3-3 on Figure 2 and showing a knot in the hauling line approaching the strut for the latch;
Figure 4 is a view similar to Figure 3 but showing one of the guide members supported firmly in its ~retracted position:
and Figure 5 is a fragmentary view through the upper yoke : . ' :
.~ ' . : . , , . . . . ~ .
. . . .

.
1 member having an alternate suspension adapter secured thereto.
Referring initially more particularly to Figures 1 and 2, there is shown a bundle-type stringing block, designated generally 10, incorporating the features of this invention.
This block comprises a pair of side frames 11,11 rigidly con-nected to the opposite ends of a shaft 12 by bolts 13. As ;
there shown for use to string a dual pair of bundle conductors are a pair of sheaves 14,15 located to either side of a central ~heave 16.
The upper ends of side frames 11,11 project very substanti-ally above the upper rims of sheaves 14,15,16 and may be connec-ted to a unitary yoke, not shown, of conventional construction by which the stringing block assembly can be suspended from the lower end of a power line insulator or other suitable support.
Such yokes are customarily secured to the upper ends of side frames 11,11 by bolts 19.
This conventional unitary yoke is shown replaced in the present drawings by the invention converter yoke assembly which comprises two yoke members 20,21 extending toward one another from the upper ends of the side frames with tpeir adjacent edges ~paced apart to provide a threading passageway 22 to be described in ~reater detail presently. Yoke members or brackets 20,21 are rigidly secured to the upper ends of the side frames 11,11 by pairs of adapter ~ates 24,24 and pairs of bolts 19,19 and pairs of pins 25,25. It will be understood that adapter plates 24 may be furnished in different designs and dimensions as necessary to accommodate the invention converter yoke assembly for attachment to stringing block as~em-blies of different size and constructions. ~hus~ as is well known to those skilled in this art, such blocks differ consider-ably in size and dimensions and include a detachable yoke secured across the top of the side frames by suitable fasteners. Thi~

, ~
.. ' : : .
: ' ' ' : .

10~;6691 1 yoke is removed and the appropriate design of adapter plates 24 is utilized to connect the universal helicopter-threadable converter yoke assembly of this invention to the side frames of any conventional stringing block.
Threading passage 22 for the hauling line 23 of the type used in the manner described in my copending application referred to in the opening paragraphs of this disclosure, has an inlet end opening laterally through one side of the stringing block and is normally maintained closed by a gate or latch 28 having multiple functions. The lower outlet end of the threading passage 22 opens downwardly directly over center sheave 16.
Latch 28 is hinged to yoke mem~er 21 by pivot pin 29 and is spring-biased to closed position by a torsion spring, not shown but encircling the pivot pin. The free end of the latch includes an adjustable plate 30 clamped to latch member 28 by cap screws 31. The midlength of the latch is provided with an opening 32 for receiving a detent 33 integral with yoke member 20.
As shown in Figure 2, latch 28 is held in open position by a strut or prop 35 having a hooked end 36 seated in a shallow well 37 of yoke member 20. The upper end of strut 35 has an ~ -eyelet 38 pivotally assembled to the edge of latch member 30.
It will be ob9erved that the lower end of strut 35 forms a dowmwardly convsrging passage with the adjacent upper surface of yoke member 20 with the result that line 23 is highly effec-tive in Camming strut 35 out of well 37 to release latch 28.
Hence only a slight force applied from the left of strut 35 by the hauling line as it gravitates downwardly along threading passage 22 suffices to dislodge the hooked end of the atrut from well 37, thereby allowing the aforementioned torsion spring to close latch 28 to the vertical dot and dash line closed posi-tion thereof with its opening engaged over detent 33. In this .. - ...... . , . .................. . : :
. ~ . ~ . . , . , , -. .. .:
.. . .. . ..

- 106669~
1 position the latch is highly effective in transfering high load stresses between the free end of yoke member 21 and the underlying portion of yoke members 20 and thereby to the left hand side of the stringing block assembly. In this connection it will be understood that the stringing block is normally sus-pended by a clevis or other suitable connection to opening 40 in the upper mid-length portion of yoke member 21. If by chance strut 35 is not dislodged in the manner described above a knot or the like 23' in the hauling line assures release of the latch as knot 23' is pulled past and beneath strut 35.
As is best shown m Figure 2, the free end of prop 35 is preferably located reasonably close to a vertical line through the center of the block in order that tension forces acting in line 23 will have a minimum tendency to rotate the block about its vertical axis and a maximum of efféctiveness in dis-lodging prop 35.
A receiving or guide horn 42 i9 secured to yoke member 20 by cap screws 43 and provides an upwardly inclined prolongation of the lower edge of threading passage 22. ~ormagy horn 42 extends outwardly several feet from the left hand edge of the stringing block and catches the hauling line as it i9 dispensed from the reel carried by a helicopter flying along the power line.
Referring now more particularly to Figures 2, 3 and 4 there are shown details of the toggle linkage guide units for the conductor hauling line to assure that it drops through the exit end of the threading passage 22 directly into the groove of center sheave 16. These guide means or units include duplicate linkages, designated generally 50, having a lost motion pivotal connection to brackets 51 extending downwardly from yoke members 20 and 21. The principal moving element of each guide unit comprises a strut 52 having a pair of aligned slots 53 extending r ~L ,~ , .

106~69~
1 lengthwise of its forked upper end which forked ends straddle brackets 51. These brackets have a pin 54 fixed thereto and projecting from the opposite lateral sides into the slotted openings 53 of members 52. The central portion of members 52 are provided with a downwardly converging tubular passage 55 (Figures 3 and 4) in which is seated a headed pin 56. A cross pin 57 extends through the head of pin 56 and its ends are journalled in the notched lower end of bracket 51. A relatively strong compression spring 58 surrounds pin 56 with its lower end pressed against the bottom of the tapering opening 55. The lower ends 60 of guide members 52 are convex and have a radius conforming generally to the radius of the rim of sheaves 14,15 and the opposite ends of this arcuate surface are provided ~:
with reinforcing ribs 61 which project downwardly into the groove of sheaves 14,15.
As shown in Figure 3, the axes of shaft 12 and of pins 54,57 lie in a common vertical plane. Accordingly, when the :
guide members 52 are positioned as shown in Figures 2 and 3, spring 58 holds the guide members firmly seated against the juxtaposed rims of sheaves 14,15. The relatively wide arcuate ends 60 of members 52 cooperate with brackets 51 and the oppo~
site ends of pins 54 to provide a strong, stable toggle support for the guide members and this is particularly true as respects ~:
forces acting transversely of the stringing block, that is axially of shaft 12. However, a much smaller force acting against either the forward or rear edges of members 52 is effective to compress spring 58 sufficiently to permit members 52 to be swung either clockwise or counterclockwise about the axes of pivot pins 54. If either the forward or the rear edge of members 52 pass a vertical line through the axes of pins 54,57 and shaft 12, then the spring 58 is effective to continue the pivotal swinging movement of either guide member to its _g~

- - -, , ~ :
, ... ..

. .
,, . . . ~.

~066691 1 alternate stable position indicated in Figure 4 wherein this member is supported in a substantially horizontal position thereby leaving the passageway overlying the tops of sheaves 1,4~5,16 unobstructed and clear for the passage of the conven-tional running board and the bundle conductors through the stringing block and on toward the next stringing block. Accor-dingly, it will be clear that the guide members 52 remain in their extended active position illustrated in Figures 2 and 3 until the running board conventionally interconnecting the trailing end of the hauling line and the advance end of the bundle conductors contacts one transverse edge of the members 52. When this occurs, the guide members are upset from their extended vertical position whereupon springs 58 continue the pivotal movement of the toggle-type guide assemblies to their re-tracted horizontal positions shown in Figure 4.
Referring to Figures 1 and 2 it will be noted that the upper ends of side frames 11 preferably include guard flanges 65 cast integral with their upper inner transver~e edges which guard flanges embrace the upper exterior rim portions of sheaves 14,15. Guard flanges 65 aid in guiding the running board and holding it centered between the stringing block side frames while passing through the stringing block and they also serve similar purposes as respects the conductors should the latter climb up the sides of the flanges of sheaves 14,15 particularly along curves in the power line and abrupt changes in elevation.
Referring now to Figure 5, there is shown an alternate arrangement for supporting the stringing block from an insula-tor. To this end there is shown a suspension adapter 70 having a length corresponding generally to the length of yoke member 21 and having a bore transversely of its midportion to receive an assembly bolt 71 holding adapter 70 assembled to opening 40.
Additionally, adapter 70 includes U-shaped clips 72 projecting laterally from the opposite sides of the left-hand end of _ . .

.

.

106669~

1 adapter 70 and sized to slidingly receive the reinforcing ribs 73 extending horizontally along the lateral edges of yoke member 21. The opposite ends of adapter 70 are provided with openings 75 by which the stringing block is suspendable in well known manner from the tower or from the lower end of the insu-lator to which the bundle conductors are to be attached.
The operation and use of the converter yoke components and the stringing block to which they are attached will be quite evident from the foregoing detailed description. It will be understood that the stringing blocks are suspended from insula-tors distributed along the power line with the guide horns 42 facing outwardly along one outer lateral side of the power line and in position to receive a conductor hauling line as it is dispensed from a reel on board a helicopter flying along the power line. The trailing end of this hauling line is attached to conventional running board means having it~ rear end firmly attached to the bundle conductors to be strung.
After or at the time the stringing blocks are secured in place on the lower end of the insulator, the workman takes care to cock latch 28 to the full line position shown in Figure 2 with the lower end 36 of strut 35 firmly seated in well 37.
At the same time the workman makes certain that each of the guide members 50,50 is pivoted to its vertical extended posi-tion with the lower end 60 pressed against the upper edges of a respective one of sheaves 14,15. After all of the stringing blocks have been prepared in this manner, the helicopter flies along the power line and drops the hauling line onto first one and then another of the guide horns 42. The hauling line gravi-tates downwardly along these horns into the entrance end of threading passage 22 and contacts or engages the lower hooked end 36 of strut 35 thereby easily and positively dislodging this strut from well 37. Immediately, the torsion~pring --11-- . . ..

. .- . . . ~ . -::.. ' -......... :. .

106669~
1 encircling pivot pin 29 of latch 28 snaps the latch closed to embrace and engage latch detent 33. As this occurs the hauling ~he drops downwardly between the two guide units 50,50 into the groove of central sheave 16.
After the hauling line has been thus installed and threaded onto the central sheave of each of the s~inging blocks, the advance end i8 disc~nnected from the helicopter and connected to ground-supported winch means to advance the bundle conduc-tors through each of the stringing blocks 10. Throughout the threading operation of the hauling line as well as during winching of the hauling line through the stringing b~ocks, guide members 52 strongly resist any tendency of the hauling line to :
escape from the groove of sheave 16 by riding up out of the groove of central sheave 16, a tendency which is likely to arise around curves or changes in elevation of the power line.
As the advance end of the running board contacts the transverse edge of members 52, it easily pivots these members forwardly out of engagement with sheaves 14,15 until toggle springs 58 become effective to snap the snap memberq 52 to their fully retracted position shown in Figure 4 or to the alternate stable position thereof on the left side of member 51. The members are thereafter firmly held in this position until forcibly and manually reset in their extended positions in readiness for the rethreading of another conductor hauling line.
It should also be pointed out that during advance winching of the hauling line and of the conductors latch 28 remains firmly and positively engaged with detent 33 with the result that it interconnects yoke members 20,21 so that the very high loads carried by the stringing block are arched by both yoke members and particularly by the opposite ends of the longer yoke member 21.
After the conductors have been installed along one side 106669~
1 of the power line they are rotated 180 degrees so that the entrance to threading passage 22 faces outwardly and then -installed and used as previously described to string bundle conductors along the other side of the lane. All parts function ~ -in the same manner described previously except that the guide linkages 50,50 are contacted from their other edges and pivot ~-in the opposite direction to their alternate stable positions upon being contacted by the advance end of the running board. -~
Although the conductor hauling line has been described ~ ~
herein as dispensed from a reel mounted on suitable aircraft, ~ -such as a helicopter, it will be understood that the supply ;
reel need not be mounted on the air-borne craft but can be - -mounted at ground level and pulled into position along the power line while beingd~livered or threaded to the stringing blocks. Accordingly, these terms as used in the claims are to be understood as embracing either de for mounting the hauling line reel. It will also be understood that the central sheave employed to receive the conductor hauling line may be similar to the other sheaves or it may be a special sheave such as one having annular flanges projecting laterally from the rims of its groove structure, such as a grooved drum. ;~
While the particular helicopter threadable stringing block assembly for power conductors herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it i8 merely illustrative of the presently preferred embo-diment of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims.

-13- #

Claims (34)

WE CLAIM:

1. A converter yoke accessory attachable across the upper ends of the side frames of a stringing block assembly equipped with a plurality of sheaves for use to thread said assembly with a conductor hauling line while the line is being delivered by an airborne craft, said yoke accessory comprising a pair of brackets each rigidly securable to the upper end of a respective one of the stringing block side frames and having adjacent edges of said brackets cooperating to provide an open-ended threading passage having an exit end overlying a middle sheave of the stringing block assembly, conductor hauling line guide means movably mounted below and along the opposite sides of the exit end of said threading passage thereby to provide an exten-sion of said threading passage for the hauling line while being threaded to a stringing block assembly, said hauling line guide means being bodily shiftable to a retracted position remote from said plurality of sheaves as the hauling line pulls a conductor running board therepast, and means for retaining said guide means in said retracted position.

2. A converter yoke accessory as defined in claim 1 characterized in that each of said guide means includes an elongated member having one end pivoted to a respective one of said brackets for swinging movement in parallel planes trans-versely of the exit end of said passage, and said retaining means for said guide means including spring means effective to hold each of said guide means pivoted to the retracted position thereof in response to passage of a conductor running board past said guide means.

3. A converter yoke accessory as defined in claim 2 characterized in that the outer free end of each of said guide means is adapted to be pressed against the rim edge of one of said plurality of sheaves by said spring means when said con-verter yoke assembly is installed on a stringing block assembly and cooperating therewith to hold said guide means in a stable position to guide a conductor hauling line onto one of said plurality of sheaves.

4. A converter yoke accessory as defined in claim 1 char-acterized in that one of said brackets is sufficiently long to span the distance between the side frames of a stringing block assembly to which the upper ends of said brackets are securable, and the other of said brackets being relatively short and adap-ted to underlie and be spaced beneath said long bracket suffici-ently to form said hauling line threading passage.

5. A converter yoke accessory as defined in claim 4 charac-terized in the provision of load transfer latch means pivoted to the underside of said long bracket near the free end thereof and spring biased into latching engagement with detent means carried by the shorter of said brackets, and means for holding said latch means open and adapted to be dislodged from latch holding position by a conductor hauling line passing along said threading passage.

6. A converter yoke accessory as defined in claim 5 char-acterized in that said holding means comprises a pivoting strut having one end engageable with said latch means to hold the same open until said strut is dislodged by the passage of a conduc-tor hauling line along said threading passage.

7. A converter yoke accessory as defined in claim 6 char-acterized in that the longer one of said brackets includes means in the mid-length thereof for suspension of a stringing block assembly to which said yoke accessory is secured from adjacent the lower end of a power line insulator.

8. In combination, a bundle-type power line stringing block assembly having at least three sheaves rotatably supported between a pair of rigid side frames cooperating to support said assembly by suspension adjacent the lower end of a power line insulator, said side frames cooperating to provide an open-ended threading passage through which an aircraft-delivered con-ductor hauling line can be dropped for gravitation onto a cen-tral one of said sheaves, and a pair of guide means movably sup-ported along the opposite sides of the exit end of said thread-ing passage and movable between active and inactive positions, said guide means being in the active position thereof to guide said hauling line onto said central sheave and including means for holding each of said guide means in said inactive position remote from said sheaves after a hauling line has been threaded onto said central sheave and an object attached to the end of said hauling line has been hauled into contact with said guide means.

9. The combination defined in claim 8 characterized in that said guide means has two stable positions in one of which said guide means lies in said active position extended toward and in proximity to the opposite lateral sides of said central sheave and in the second of which said guide means is held re-tracted in said inactive position remote from each of said sheaves.

10. The combination defined in claim 8 characterized in that said guide means includes toggle spring means operable in one stable position to hold the lower end of said guide means pres-sed against the juxtaposed rim edge of one of said sheaves and operable at other times to hold said guide means retracted and remote from said sheaves.

11. The combination defined in claim 8 characterized in that said object attached to the trailing end of a conductor hauling line comprises a running board operable to dislodge said guide means and automatically shift the same to said in-active position as said running board is hauled therepast there-by leaving the area closely overlying said sheaves free and un-obstructed for hauling bundle-type conductors through said stringing block assembly.

12. The combination defined in claim 8 characterized in that said guide means include means rigidly supporting the same substantially immovably in said active position so long as a conductor hauling line is being pulled through said string-ing block assembly.

13. The combination defined in claim 8 characterized in that the outer free ends of said guide means are broad in a dir-ection extending in the plane of rotation of said sheaves and shaped to bear against the peripheries thereof in areas spaced to either side of a vertical bisector plane through the sheave axis.

14. The combination defined in claim 13 characterized in the provision of stiff spring means holding the outer free end of each of said guide means firmly seated against said sheaves while a conductor hauling line is pulled past said guide means.

15. A power line stringing block assembly adapted to be aircraft-threaded with a conductor hauling line and of the type having a main frame including a shaft interconnecting and ex-tending between upright legs thereof and supporting at least three sheaves in side by side relation and which main frame has an upwardly and outwardly extending threading passage for a con-ductor hauling line between the upper ends of said upright legs, characterized in the provision of guide means movably supported in a first stable position between the inner end of said passage and a generally central one of said at least three sheaves to provide a temporary prolongation of said passage to guide a conductor hauling line only onto said generally central one of said at least three sheaves as it is delivered thereto from an aircraft and gravitates downwardly through said passage, and said guide means including means for supporting the same in a second stable position when deliberately dislodged from said first stable position and for holding said guide means in said second stable position and spaced from conductors when being hauled through said stringing block assembly.

16. A stringing block assembly as defined in claim 15 characterized in the provision of load transfer means movably supported across said threading passage for a conductor hauling line and operable to transfer load stresses across said threading passage and between said upright legs of said main frame, and means for holding said load transfer means retracted along one side of said threading passage to permit a conductor hauling line to pass along said threading passage and onto a generally central one of said sheaves.

17. A stringing block assembly as defined in claim 16 characterized in the provision of means for holding said load transfer means in a first stable position to permit a conductor hauling line to gravitate downwardly therepast and along said threading passage and onto one of said sheaves as said load transfer means moves to a second stable position across said threading passage.

18. A stringing block assembly as defined in claim 15 characterized in that said guide means comprises at least one spring biased toggle linkage assembly movably supported on said main frame and movable between first and second stable positions, and operable in said first stable position to hold said guide means in said first stable position thereof to guide a conductor hauling line onto said generally central one of said at least three sheaves and operable in the second stable retracted posi-tion thereof to hold said guide means retracted and to one side of the path of conductors while being hauled through said stringing block assembly.

19. A stringing block assembly as defined in claim 18 characterized in that said guide means comprises a pair of generally similar toggle linkage assemblies supported for pivotal movement through wide arcs in parallel planes between said upright legs of said main frame.

20. A stringing block assembly as defined in claim 19 characterized in that said toggle linkage assemblies include spring means for selectively holding each assembly extended and effective to guide a conductor hauling line while being threaded through a stringing block assembly or retracted and displaced out of the path of conductors being pulled through said stringing block assembly.

21. A stringing block assembly as defined in claim 18 characterized in that said toggle linkage assembly includes means for holding the same selectively in one of two retracted positions depending upon the direction in which said hauling line is passing through said stringing block assembly.

22. A stringing block assembly as defined in claim 15 characterized in the provision of gate means extending cross-wise of and pivoted to one side of said threading passage and normally blocking travel of a hauling line therethrough.

23. A stringing block assembly as defined in claim 22 characterized in the provision of means for holding said gate means open during threading of said threading passage which holding means is adapted to be dislodged by a conductor hauling line while gravitating downwardly along said threading passage in an area thereof adjacent the vertical center line of said stringing block assembly.

24. A stringing block assembly as defined in claim 23 characterized in the provision of means operatively associated with said gate means for moving and holding the same closed after dislodgment of said holding means.

25. An aircraft threadable stringing block assembly for use in stringing bundle-type power line conductors, said stringing block assembly comprising a pair of side frames rigidly interconnected to the opposite ends of a shaft rotatably supporting in side by side relationship at least three sheaves, means for suspending said side frames from a power line insulator and including an open ended passage for threading an aircraft delivered conductor hauling line therethrough for guid-ance onto a generally central one of said at least three sheaves, guide means disposed on either side of the outlet end of said passage for guiding a conductor hauling line as it gravitates from the lower end of said passage toward the central one of said at least three sheaves, said guide means including means for holding the same releasably in a first stable position while guiding a conductor hauling line onto said central sheave and including means responsive to contact with means being pulled therepast by a conductor hauling line to dislodge said guide means to a restricted second stable position offering no obstruc-tion to the passage of power line conductors through said string-ing block assembly with each conductor resting in the groove of a respective one of said at least three sheaves.

26. A stringing block assembly as defined in claim 25 characterized in that the upper ends of said side frames extend toward one another in spaced-apart overlapping relation thereby to provide said open-ended threading passage therebetween and which threading passage opens outwardly past one of said side frames.

27. A stringing block assembly as defined in claim 26 characterized in the provision of guide horn means extending outwardly and upwardly from and forming a continuation of the lower side of said threading passage to intercept and channel a conductor hauling line into said passage after being dropped thereonto from a helicopter dispensing reel.

28. A stringing block assembly as defined in claim 26 characterized in the provision of high strength latch means movably supported on one side of said passage and normally latched to a portion of the side frame on the opposite side of said passage thereby to interconnect said pair of side frames with a load transmitting barrier extending crosswise of said threading passage.

29. A stringing block assembly as defined in claim 28 characterized in that said latch means includes means normally effective to hold the same in closed latching position.

30. A stringing block assembly as defined in claim 28 characterized in the provision of means for holding said latch means temporarily open to permit movement of a conductor haul-ing line therepast along said passage.

31. A stringing block assembly as defined in claim 28 characterized in the provision of strut means for holding said latch means temporarily open and so positioned as to be dis-placed by contact with a conductor hauling line after the lat-ter passes said latch means while moving along said passage.

32. A stringing block assembly as defined in claim 26 characterized in that said guide means comprises separate haul-ing line guide units movably mounted on said side frames along the opposite sides of said passage adjacent the lower outlet end thereof and effective to guide a conductor hauling line from said passage onto a central one of said at least three sheaves.

33. A stringing block assembly as defined in claim 32 characterized in that said guide units are selectively movable in one or the other of opposite directions to one of two of said second stable positions.

34. A stringing block assembly as defined in claim 33 characterized in that said holding means for said guide units comprises spring-biased toggle linkage means operable in a first position thereof to hold said guide units extended to-ward a respective side of said central one of said at least three sheaves and operable in a second position thereof to hold said guide units retracted in a selected one of said two second stable positions remote from each of said at least three sheaves.