AU612522B2 - Pneumatic shoe for high speed filamentary capstan - Google Patents

Description

I- OPI DATE 28/05/90 AOJP DATE 05/07/90 APPLN. ID 43444 89 PCT NUMBER PCT/US89/04187 PCr INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 5 (11) Internatio l Publication Number: WO 90/05105 51/16 B65H 23/24 Al (43) Internati Pu1catio ate 7 990 (17.05.90) (21) International Application Number: PCT/US89/04187 (81) Designated States: AU, CH (European patent), DE (European patent), FR (European patent), GB (European pa- (22) International Filing Date: 28 September 1989 (28.09.89) tent), IT (European patent), JP, KR, NL (European patent), NO, SE (European patent).

Priority data: 268,421 8 November 1988 (08.11.88) US Published With international search report.

(71) Applicant: HUGHES AIRCRAFT COMPANY [US/US]; 7200 Hughes Terrace, Los Angeles, CA 90045-0066 (US).

(72) Inventors: LeCOMPTE, George 8620 North Pomona Road, Tucson, AZ 85704 EISENTRAUT, Rudolph, A. 5855 South Old Spanish Trail, Tucson, AZ 85747 COXON, Moran 6822 East Nasumpta Drive, Tucson, AZ 85715 (US).

(74) Agents: SALES, Michael, W. et al.; Hughes Aircraft Company, P.O. Box 45066, Bldg. Cl, M/S A126, Los Angeles, CA 90045-0066 (US).

(54) Title: PNEUMATIC SHOE FOR HIGH SPEED FILAMENTARY CAPSTAN 16 An apparatus and methodology for uniformly applying a compressive force against a filament to pneumatically drive the filament or fiber optic cable against a capstan without any concentrated amount of stress at any point on the filament (12) is effectuated by pneumatically forcing the filament (12) into an equatorial V-groove (46) defined in the capstan Pneumatic pressure is applied to a predefined segment of the capstan (16) by a pneumatic shoe (10) having an internal shoe pressure chamber The pressurized gas or air is applied to the segment from the chamber (20) into the proximity of the equatorial V-groove (46) on the capstan The V-groove (46) is vented to atmosphere so that the filament (12) is forced or blown into the V-groove (46).

Side and end clearances between the rotating capstan and the shoe are sized to allow the viscosity of the pressurized gas to operate to retard the escape of the pressurized gas from the predefined segment of the capstan Utilizing such an arrangement, approximately 10 kilometers or more of filament can be 20 paid out in approximately 32 seconds or less.

i .i i i PCT/US89/04187 WO 90/05105 PNEUMATIC SHOE FOR HIGH SPEED FILAMENTARY CAPSTAN Backqround of the Invention 1. Field of the Invention The invention relates to the field of machines for handling filamentary or fiber optic cables and in particular to a pneumatic shoe and capstan design for paying out filament at high speed.

2. Description of the Prior Art In the course of optically testing filamentary or fiber optic cable for various types of applications it is necessary to pay out the cable at a very high speed in a laboratory or test enviroment. The problem is to apply a sufficient force to a fiber optic cable to pull it from a supply reel or other supply source and to move it at a high rate of speed to or through a test station. The force must be applied to the filament in such a manner that damage or distortion to the cable is avoided and without applying any I 1 i, i -2- 1 permanent strains to the cable, abrading or otherwise blemishing the optical surface of the fiber optic cable,or otherwise affecting its optical properties.

Therefore, what is needed is a methodology and apparatus whereby a force may be gently and uniformly applied to filament with a sufficiently high magnitude that enough tensile force can be applied to the filament to pull it at a high rate of speed without damaging, nicking, kinking, abrading or otherwise applying a distorting or 10 degrading force to the filament.

o* o* *4 p go -i ~I '0 q 3 According to one aspect of the present invention there is provided an apparatus for pulling a filament at a high rate of speed comprising: a source of pressurized gas; a capstan for frictionally engaging said filament along a predetermined segment of said capstan; a shoe disposed over at least said segment of said capstan which segment is in frictional engagement with said filament, said shoe having defined therein a shoe pressure chamber, said shoe pressure chamber being :"pneumatically communicated with said source of pressurized

OS*S

15 gas so that said pressurized gas is supplied through said goo: shoe into the proximity of said predetermined segment of said capstan in frictional engagement with said filament; and o°°o means defined in at least said capstan for goo* forcing said filament into frictional contact with the predetermined segment of said capstan by flow of said pressurized gas through said shoe into the proximity of said segment of said capstan, 0S e whereby a uniform stress is applied to said filament thereby allowing frictional engagement between said filament and said capstan and permitting rapid payout of said filament by rotation of said capstan.

see* 00. 0.

8304S/JM 4 According to a further aspect of the present invention there is provided a method for applying tension to a filament to pay out the filament at a high rate of speed comprising the steps of: frictionally engaging said filament with a segment of a rotating capstan by pneumatically forcing said filament against an equatorial vent line defined in said capstan; retarding escape of a pressurized gas around the proximity of said segment of said capstan frictionally engaging said filament in order to maintain said pressurized gas pressure against said filament during a precetermined 15 payout period; and Soo: maintaining a predetermined exit pressure magnitude of said pressurized gas in the proximity of said segment of said capstan frictionally engaging said filament during said payout period, whereby a tensile force is applied to said filament in order to pull said filament from a source of supply without applying concentrated stresses to said filament at any point.

S*S

V

8304S/JM -Lii -1 i i 5 Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings.

Brief Description of the Drawings esee 0*S*

S

*SCS

S

S Sr S@ S. 5

S

S.

9 Fig. 1 is a diagrammatic side view of a pneumatic pinch 10 shoe according to the invention for pulling a filament and a mechanism for operating the pinch shoe.

Fig. 2 is an end elevational view of the pinch shoe of Fig. 1.

Fig. 3 is a diagrammatic fragmentary cross-sectional view in enlarged scale of the portion of the pinch shoe of Figs. 1 and 2 as seen through section lines 3--3 of Fig. 1.

Fig. 4 is a diagrammatic fragmentary cross-sectional.

view of an alternative embodiment of the pinch shoe as would be seen through section lines 3--3 of Fig. 1 of a second 20 embodiment of the invention.

The invention and its various embodiments may be better understood by now turning to the following detailed description.

"e WO 90/05105 PCT/US89/04187 1 Detailed Description of the Preferred Embodiments An apparatus and methodology for uniformly applying a compressive force against a filament or fiber optic cable to pneumatically drive the filament against a capstan without any concentrated amount of stress at any point on the filament is effectuated by pneumatically forcing the filament into an equatorial V-groove defined in the capstan.

Pneumatic pressure is applied to a predefined segment of the capstan by a pneumatic shoe having an internal shoe pressure chamber. The pressurized gas or air is applied to the segment from the chamber within the shoe into the proximity of the equatorial V-groove on the equator of a disc shaped capstan. The V-groove is vented to atmosphere so that the cable is forced or blown into the V-groove. Side and end clearances between the rotating capstan and the shoe are sized to allow the viscosity of the pressurized gas to, operate to retard the escape of the pressurized gas from the predefined segment of the capstan. Annular interleaved baffles may also be provided on the equatorial surface of the capstan and the opposing surface of the shoe to provide additional frictional engagement between the pressurized gas and the elements in the proximity of the predetermined segment of the capstan. Utilizing such an arrangement, approximately 10 kilometers or more of filament can be paid out in approximately 32 seconds or less.

IAt 1 31 -l WO 90/05105 PCT/US89/04187 -7 1 Fig. 1 is a highly diagrammatic side view of an apparatus in which a pneumatic pinch shoe, generally denoted by reference numeral 10, is depicted for driving or paying out a filament 12. The various elements of the apparatus in Fig. 1 are not to scale and where the detail is irrelevant to the primary thrust of the invention, various details of certain elements have been omitted. For example, it must be understood that capstan 16 is driven by means of an axle 14 which in turn is driven by a conventional motor, typically an air turbine motor, which is shown symbolically as motor 13. Electric motor 13 in turn is powered and controlled by various types of conventional electronic circuitry which are of no consequence to the invention. Pinch shoe 10 includes a thin disc V-groove capstan 16 which is driven by axle 14.

A shoe 18 is disposed over an azimuthal segment of capstan 16 as shown in the side view of Fig. 1 and encloses capstan 16 to the extent of their mutual overlap as best depicted in the end elevational view of Fig. 2. Shoe and capstan 16 are spaced apart and do not touch at any point. Shoe 18 ludes an internal shoe pressure chamber 20 shown in dotted outline in Fig. 1. Pressure chamber 20 is communicated via a duct 22 through piping 24 to a pressure reduction valve 26. Pressure reduction valve 26 in turn is coupled via piping 28 to a pressurized air tank 30. Air tank 30 is coupled via a supply pipe 32 to a dryer 34 which removes water vapor and other contaminants from the air j WO 90/05105 PCT/US89/04187 1 which is pressurized within tank 30. The pressurized air is delivered to dryer 34" through pipe 36 from a conventional air compressor 38. Removal of water vapor is required since the exit temperature of gas from pressure chamber 20 is low enough to cause freezing or condensation.

Turn now to Fig. 3 wherein the operation of pneumatic pinch wheel 10 in relationship to filament 12 can be better depicted and understood. Fig. 3 is a fragmentary crosssectional view taken through lines 3--3 of Fig. 1 as seen in enlarged scale. Fig. 3 clearly illustrates that capstan 16 is fabricated from two symmetric halves split along an equatorial plane of circular disc-shaped capstan 16. Halves 16a and 16b of capstan 16 are bound together through an axially disposed disc-shaped spacer 40. Spacer 40, however, only extends radially to a predetermined point 42 between capstan halves 16a and 16b, thereby leaving an annular gap between halves 16a and 16b which extends from spacer 40 to the radial outermost perimeter of capstan 16. This gap serves as a vent 44 and clearly those portions of vent 44 outside of shoe 18 freely communicate with the atmosphere.

The interior edge of each capstan half 16a and 16b is chamfered so as to form a dihedral V-groove 46 along the equator of capstan 16 when halves 16a and 16b are assembled together. V-groove 46 is the radial most extension or orifice of vent 44 and circumferentially extends about capstan 16.

i,_i WO 90/05105 PCTr/US89/04187

/Y

1 The function of pinch sho- 10 is to force filament 12 into V-groove 46 so that there will be sufficient frictional force developed between capstan 16 and cable 10 to exert the required pull on filament 12.

Filament 12 is pressed into V-groove 46 by means of pneumatic pressure. Pressurized dried air is applied through the compressor, dryer and tank system of Fig. 1 through pressure reducing valve 26 and ultimately to shoe pressure chamber 20. A thin, uniform-width azimuthal slot 48 marks the boundary between capstan 16 and shoe 18. Slot 48 forms a minimum or throat area into which is injected high pressure air from the interior of pneumatic shoe High pressure air is communicated between shoe pressure chamber 20 and space 50 of shoe 10 by means of a plurality of ducts 52, one of which is shown diagrammatically in cross-sectional view in Fig. 3.

It is believed, as a first approximation, that the airflow within space 50 is isentropic and that a sonic condition exists at slot 48 and in side slots 54. Given the conditions of stagnation pressure and temperature, the mass flow of air required for delivery to shoe 10 can then be estimated.

Air is supplied to pneumatic shoe 10 from high pressure tank 30 through reduction valve 26 so that the stagnation pressure within pneumatic shoe 10 is maintained at a desired constant level. In the illustrated embodiment the _ii WO 90/05105 PCT/US89/04187 1 stagnation pressure is 150 psia. The required run timp is estimated from the length of filament 12 which must be paid out and the payout speed. The size of tank 30 is then derived from considering the mass of air which needs to be supplied from tank 30 at the estimated rate.

However, these estimations merely set an upper bound because in practice the effects of the viscosity of the flow of air will be significant. In order to obtain a more realistic assessment, the flow of air in slots 48 and 54 must be considered. In the present analysis, slot flow has been treated as a Fanno flow, i.e. characterized by constant cross-sectional area in adiabatic flow with friction. The effect of rotation of capstan 16 due to viscous loading is very small and is generally neglected. The reduction of mass flow through slot 46 due to viscosity turns out to be significant in terms of reducing the necessary tank volume for a given run time.

Consider now the specific embodiment shown in Figs. 1- 3. In the illustrated embodiment capstan 16 has a radius of 6 inches and a full width of 0.5 inch. Side slot gap width 54, between the sides of capstan 16 and the adjacent sides of shoe 18, is 0.002 inch. The distance of shoe and capstan overlap 56 is 1 inch. Finally, the sectorial angle 58 of shoe 18 adjacent to capstan 16 is 2/3 pi radians. Given a desired cable speed of 1,000 feet per second and a cable length of 32,808 feet (10 kilometers) with a capstan I i WO90/05105 PCT/US89/04187 1 rotational speed of 19,099 rpm, an initial tank pressure of 200 psia and a shoe system stagnation pressure of 150 psia at a temperature of 288 decrees Kelvin, it is determined, that under Fanno flow conditions, the mass flow rate is 0.003819 slug per second with an exit plane pressure of 49.83 psia requiring a tank volume of 18.1 cubic feet for a 32.8 second run time. This is substantially less than the volume of 29.23 cubic feet for the tank volume which is the computed estimate when the viscosity of the air is ignored.

The air stored in the tank expands in a polytropic process as mass flow occurs from tank 30 to pinch shoe pressure chamber 20. If the stagnation pressure in shoe is maintained at a fixed value by using a suitable pressure reduction valve, then it can be shown that the reduction in stagnation temperature during the run is fairly small from the range of tank pressure normally considered. A choke system with constant stagnation conditions will provide a fixed mass flow rate. This allows some simplification in the estimations leading to the prediction of necessary tank size and pressures. Ultimately the approximation formula given below will provide an estimate of the required tank volume.

filament and said capstan and permitting rapid payout of said filament by rotation of said capstan.

WO 90/05105 PC/US89/04187 1 V T m'cl n (p'l/n po1/n where V equals tank volume required for tank T is the time required for the pressure drop between an initial pressure of P1 in the tank to a final pressure of P 0 m' is the mass flow rate; n is 1.2; and C is 1.767 x 10 6 The effect of viscosity on the flow is estimated by assuming a Fanno flow, that is, a constant area, adiabatic flow with friction in a pipe equivalent to the actual slot having a length L and a hydraulic diameter D which is equal to four times the total area of the exit slot divided by the wetted perimeter of the exit slot.

The use of a high pressure chamber 20 assures of a sonic flow condition in the slot exit plane. A friction factor, f, is arbitrarily assumed. The sl6t inlet mach number is determined using flow tables. The remaining flow quantities at the inlet are calculated and a mass flow is determined. Inlet conditions are then used to find the Reynolds value and to obtain a new value for friction factor f. If the computed magnitude for the friction factor, f, does not agree with the originally assumed value, the 1~-e c WO 90/05105 PCT/US89/04187 1 procedure is repeated in an iterative process until the assumed and calculatedvalues match within the desired error percentage. The tank requirement and exit pressure, as stated above, are then derived from the final iteration.

Using the physical dimensions of pneumatic shoe stated above, a total area of exit slot of 0.05589 square inch and a wetted perimeter area of 51.89 inches can be calculated. The hydraulic diameter is then 0.0043 inch leading to an isentropic mass flow rate of 0.006171 slug per second. The run time is given as 32.8 seconds corresponding to a payout of approximately 10 kilometers of filament at 1,000 feet per second. Without accounting for the effects of viscosity of the fluid, the exit plane pressure would be expected to be 79.2 psia and the required tank volume, for a 200 psia initial pressure, is expected to be 29.23 cubic feet. This corresponds to a cylindrical tank of 2.5 feet in diameter and 6 feet in length.

However, when viscosity is considered, assuming a friction factor for Fanno flow given by f 103.2/Re or f 0.01, a mass flow rate of 0.03819 slug per second and an exit plane pressure of 49.83 psia results. Under these 4 conditions a tank 2.5 feet in diameter by 6 feet in length would allow a run time of 52.8 seconds, far in excess of that required. Instead, for the run time under consideration, a tank volume of only 18.1 cubic feet is required, a tank which allows a 38% reduction in tank volume c crate to eturu e e ape o tme pressurized gas from the predefined segment of the capstan Utilizing such an arrangement, approximately 10 kilometers or more of filament can be 20 paid out in approximately 32 seconds or less.

r4 f WO 90/05105 PCT/US89/04187 1 as compared with an isentropic flow which does not consider viscosity of the air.

Turn now to Fig. 3 which shows an alternative embodiment of the invention, wherein shoe 118 is similar to shoe 18 and capstan 116 similar to capstan 16 with the exception the inner facing surfaces of each have been provided with annular, alternately positioned baffles to form a complex labyrinthian duct. More specifically, turning to Fig. 4, shoe 118 is provided with a plurality of annular radially extending baffles 120. Each baffle 120 extends into a corresponding indentation 122 defined in the radially outermost opposing surface of capstan 116.

Indentations 120 can alternatively as defined by annular, radially extending baffles 124. The non-contacting intermeshing disposition of baffles 120 and 124 thereby dei"2Te a serpentine air duct designed to enhance the effect of the viscosity of the air and to further reduce the amount of high pressure air required to complete a predetermined run of filament payout. By making the assumption that the "pipe length" of the entire duct is equal to the full path length of the labyrinth as seen in the view of Fig. 4 and assuming a Fanno flow of a system of two pipes in parallel which are fed from a common reservoir, namely shoe pressure chamber 20, a similar calculation, taking into account viscosity, as described above, can be made in connection with the embodiment of Fig. 4. The total mass flow for the I &r WO 90/05105 PCr/US89/04187 -if 1 system thus turns out to be 0.003292 slug per second and the resulting tank value for a run time of 32.8 seconds is 15.59 cubic feet, or a further reduction of 13.8% as compared to the embodiment of Figs. 1-3.

Alternatively, the labyrinthian baffles may be deleted and elastomeric seals may be substituted.

Therefore, it can be seen that the use of a pneumatic shoe in conjunction with a V-groove capstan wheel as a means of deploying filament at very high speed under laboratory conditions can be practically achieved according to the invention. In all cases which have been considered, the pressures in the system remained well above the minimum required to prevent separation of the filament from the rim due to the action of centrifugal force on the cable.

Furthermore, over a major portion of the cable length the full stagnation pressure is operative. The pressure applied by the pneumatic shoe capstan is very evenly applied without' any pinch points or points of high concentration of stress so that a tensile force required to pull the cable quickly from a supply reel is achieved without any of the debilitating effects or damage encountered in the prior art.

Many modifications and alterations may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it is understood that the illustrated embodiment has been set :r forth only for the purposes of example and illustration and 'NT r. I i I. i-- L i- 1 ss~-~l---LIIU~~ WO 90/05105 PCT/US89/04187

A"

1 should not be taken as limiting the invention which is defined in the following claims.

Claims (6)

1. An apparatus for pulling a filament at a high rate of speed conprising: a source of pressurized gas; a capstan for frictionally engaging said filament along a predetermined segment of said capstan; a shoe disposed over at least said segment of said capstan which segment is in frictional engagement with said filament, said shoe having defined therein a shoe pressure chamber, said shoe pressure chamber being pneumatically communicated with said source of pressurized gas so that said pressurized gas is supplied through said shoe into the proximity of said predetermined segment of 15 said capstan in frictional engagement with said filament; Sm.. D 0 and means OSre S:L defined in at least said capstan for forcing said filament into frictional contact with the predetermined t segment of said capstan by flow of said pressurized gas *04* t 20 through said shoe into the proximity of said segment of said capstan, whereby a uniform stress is applied to said filament thereby allowing frictional engagement between said filament and said capstan and permitting rapid payout of 25 said filament by rotation of said capstan. so so •9•Qm *ee 8304S/JM -i 1 i~ JilI.lIl Ill I I Il III WO 90/05105 PCT/US89/04187 .d 1 2. The apparatus of Claim 1 where said means for forcing said filament'into frictional engagement with said capstan comprises a vent defined into said capstan, said filament being forced by said pressurized gas into said vent. 1 3. The apparatus of Claim 2 wherein said capstan is disc shaped and has an equatorial surface radial most on its perimeter and wherein said vent comprises a V-shaped groove defined into said equatorial surface of said disc shaped capstan. 1 4. The apparatus of Claim 3 wherein said capstan is comprised of a&kfirst disc portion, a second disc portion and a spjara -t serving to space said first and second disc portions apart, said spacer radially extending to a predetermined distance radially inset from said equatorial plane of said disc, said V-groove being defined by said first and second portions of said capstan, said vent comprising said separation between said first and second portions of said capstan as produced by said spacer between said first and second portions of said capstan. 1 5. The apparatus of Claim 2 wherein said means for forcing said filament into frictional engagement with 0o -L I -rl i" L 0 i I I II WO 90/05105 PCT/US89/04187 IC( said capstan comprises means for' increasing friction of said pressurized gas with said capstan. 1 6. The apparatus of Claim 5 wherein said means for increasing friction further comprises means for increasing friction of said prescirized gas with said shoe. 1 7. The apparatus of Claim 5 wherein said means for increasing friction of said pressurized gas with said capstan comprises a plurality of annular radially extending baffles defined in said equatorial surface of said capstan. 1 8. The apparatus of Claim 6 wherein said means for increasing friction with said shoe comprises a plurality of radially extending annular circular baffles projecting from said shoe toward said equatorial surface of said capstan. 1 9. The apparatus of Claim 1 wherein said means for increasing friction with said shoe comprises a plurality of radially extending annular circular baffles projecting from said shoe toward said equatorial surface of said capstan. vI rr rl~r i ;i r~l l- T I 20 The apparatus of Claim 9 wherein said plurality of radially extending baffles from said shoe and from said capstan are interleaved.

11. The apparatus of Claim 1 further comprising a pressure reducing valve and wherein said pressurized gas applied from said source to said shoe pressure chamber within said shoe is throttled by the pressure reducing valve.

12. The apparatus of Claim 1 wherein said source of pressurized gas provides dry pressurized air.

13. A method for applying tension to a o ee 15 filament to pay out the filament at a high rate speed ~comprising the steps of: frictionally engaging said filament with a segment of a rotating capstan by pneumatically forcing said filament against an equatorial vent line defined in said fe. Goat 20 capstan; retarding escape of a pressurized gas around the proximity of said segment of said capstan frictionally engaging said filament in order to maintain said pressurized ooeoo gas pressure against said filament during a predetermined 25 payout period; and maintaining a predetermined exit pressure magnitude of said pressurized gas in the proximity of said *see segment of said capstan frictionally engaging said filament 30 during said payout period, go whereby a tensile force is applied to said filament in order to pull said filament from a source of supply without applying concentrated stresses to said filament at any point. 8304S/JM 1 i- I -21

14. The method of Claim 13 where in said step of frictionally engaging said filament by pneumatically forcing said filament into the equatorial vent line, said filament is pneumatically forced into a V-groove defining an orifice to said equatorial vent line, said V-groove being annularly defined on an equatorial surface of said capstan. The method of Claim 13 wherein, said pressurized gas is supplied to a proximity of said segment of said capstan frictionally engaging said filament from a compressed gas tank having a volume to provide sufficient mass flow for said predetermined payout period through a throttle to provide a constant stagnation pressure. 15 16. The method of Claim 13 where in said step fee: of retarding escape of pressurized gas, said escape is retarded by providing a labyrinthian duct through which said pressurized gas must flow to reach ambient pressure. *ooo* C C C C C C^ CC* 8304S/JM II -e I I i. I 1 i- WO 90/05105 PCT/US89/04187 3 's 1 17. An apparatus for paying out filament at a high rate of speed without application of any concentrated stress at any point comprising: a shoe for receiving pressurized gas, a shoe pressure chamber defined within said shoe for distributing said pressurized gas through said shoe to an arcuous azimuthal segment; and a capstan disposed at least in part in said arcucus segment provided by said shoe, said capstan defining an equatorial vent line for providing an escape slot through said capstan for said pressurized gas, said filament being forced by said pressurized gas suppliable through said shoe into said equat 1 i nl- olot, whereby a uniform normal force is applied to said filament forcing said filament against said capstan without concentration of stress at any point. 1 18. The apparatus of Claim 17 wherein said shoe radially overlaps said capstan in said segment for a predefined distance, said radial overlap defining a side slot, azimuthal overlap of said capstan and shoe defining end slots, said end sl and side slots being sized to permit viscosity of said pressurized gas flowing through said end slots and side slots to retard escape of said pressurized gas from said arcuous segment. 4 t 4 "A.-'17r 1 ~r I- 1 WO 90/05105 PCT/US89/04187

19. The apparatus of Claim 18 further comprising means for enhancing frictional engagement between said pressurized gas and said shoe and capstan to retard escape pr-s orisec\ of sa~Ai gas pressur. from said arcuous segment. The apparatus of claim 17 wherein said esccp.e equatori t slot further comprises a V-shaped orifice, said filament being forced into said V-shaped orifice by said pressurized gas. 0 K p^ f