CA1275198C - Rotor of a screw hydraulic downhole motor, method for its production and a device for carrying same into effect - Google Patents

Abstract

ROTOR OF A SCREW HYDRAULIC DOWNHOLE
MOTOR, METHOD FOR ITS PRODUCTION AND
A DEVICE FOR CARRYING SAME INTO EFFECT
Abstract of the Disclosure A rotor of a screw hydraulic downhole motor, made as a hollow multiple-start screw featuring a substan-tially constant wall thickness. The ratio of the length of the rotor cross-sectional outside contour to the length of the circumscribed circle of said contour is substantially within 0.9 and 1.05. When making the rotor a forming element is inserted into a tubular blank, and a fluid pressure is applied to the outside blank surface. A device for making the rotor comprises a hollow housing accommodating a forming element instal-led on centring bushings. The bushings have fitting areas adapted for the ends of the tubular blank to fit thereon.

Description

~L2753L98 ROTOR OF A SCRE~V HYDRAULIC D0~iNHOLE MO~OR, ETHOD FO~ ITS PROnUCTI0~ A DEVICE FOR
CARRYING ~ E INTO EFFECT

l'he present invention relates to drilling equip-ment and more sp~cifically7 to one of tho major units of screw hydraulic downhole motors applicable for drillin~ oil and gas wells, viz., the rotor of a screw hydraulic downhole motor, and to a method for producing said rotor.
Modern progress in well-drilling technigues usin~
screw hy~raulic downhole motors is aimed at the provi-sion of' drilling machines incorporating multi-lobe rotors that feature high torque on the output sha~`t ~
and a reduced rotation freguency thereof. ~rhis enables one to apply up-to-date low-speed high-torgue rolling-cutter drilling bits with oil-filled bearing asse~bly, and drilling bits with polyclystal diamond inserts, which impose stricter reguirements on the ability of a downhole motor to operate und~r heavy axial thrust and at high torq~e.
Known in the art presently is a downhole motor with a multi-lobe rotor made as a solid metallic multiple-thread screw, whereln tho number o~ starts of the helical surface (helical teeth) is in excess of unity (cf. USSR Inventorls Certi~icate No.926,209, Int.C1. E 21B 4/02, published on ;/iay 7, 19~
'l'h~ rotor is accommodated inside a st~tor f'~atur-75~98 ing an inn~r multiple-thread helical surface, wherein the number o~ starts is in excess o~ that o~ the rotor by unity; said helical sur~ace is moulded on the lin-ing made o~ a resilient material, such as rubber pasted to the inner sur~ace o~` the stator ~rame. The rotor axis is o~fset with respect to the skator axis which aligns with the motor axis, by the amount of eccentri-city equal to half the length o~` the rotor and stator teeth, while the ratio of the axial pitch of the rotor helical teeth to the exial pitch of the stator helical teeth equals the ratio between the number o~ teeth on said motor components. When the rotor teeth e~ge the stator teeth, spaces are formed, opening to the rotor top portion and closing over the length of the helix lead. ~'ihen drilling mud is injected into tha screw hydraulic downhole motor from daglight surface along the drill string to the bottom end of ~hich the screw hydraulic downhole motor is connected, tho rotor of the motor perfor~s planetary motion, while the rotor axis rotates about the stator axis in counter-clockwise direction at an angular velocity ~1' and the rotor itsel~ rotates about its own axis in cloock-wise direction at an angular velocity ~ 2 The magnitude o~ the angular velocity ~1 is equal to that o~ the angular velocity ~2 multiplied by the numb~r of L~otor teeth, while the centriI`u~al ~orce acting on the rotor 75~98 i5 proportio~al to its mass and to the square of the angular velocity CJl However, lar~e mass of a solid rotor and high magnitude of the an~ular velocity ~1 at which rotates the rotor re~ult in high centrifugal forces arisin~
during the operation of' the motor. These forces inducs vigorous transverse vibrations which affect adversely the durability o~ the rotor, stator, hinge joints, as well as of the threaded joints of the motor and the drill string.
'~he multi-lobe rotor of the aforediscussed motor is manu~actured by virtue of ~ear hobbin~, i.e., outt-ing with a ~etal-cutting tool called the hob. ~he method is an expensive one, suffers from an inadequate productivity, failq to provide hi~h guality o~ rotor teeth surface finish and involves sophisticated and costly metal-cutting machinery and tools. Further-more, resort should be made'to polishing or grinding of the rotor working surfaces to improve the guality of surface finish, which is a complicated technoligica task on accou~t of i~tricate co~figuration of the rotor and its long overall length.
I~ addition, it is due to a g~r~at length of the multi-lobe rotor that the cutting lips o~ a hob gro~
worn in the course of rotor machining, which affects badly the accuracy o~' the finished product.
~ nother screw hydraulic ~lownhole motor ~nown in ~ 75~8 the present state o1 t~ie art comprises a LlolloY~ multi-lobe rotor. .~or the ~urpose of joining~ with a cardan or ~ flexible sna~t, the rotor is ri~idly connected 9 by virtue of a threa~ed join-t, to -the union coupling (cf.
a textbook "~crew hydraulic downhole motors for -vlell drillin~' by ~ Gusman et al., ;,loscow, ~Tedra i;H, 19~1, pp. 1~5-1~8 (in iius~ian). 'l~he rotor i~ question is hollow-centred by removal of the metal ~`rom ~he central portion thereof either by virtue of a centre hole ~irilled in the rotor or througn the use ol ~ thick-~alled pipe shell.
f~his ma~es it possible to reduce to some extent the centrifu~al forcas applied to the rotor, tnus lo~ier-in~ the dynamics of tran5verse oscillations both of the rotor and of the motor as a whole. However~ a con-sid0rabLe mass OL` ~etàl remains in the bulk of the ro-tor teeth in th~ peripheral portion thereof, with the result that hi~h centrifugal forces arise durin~ the motor operation, which af~ect adversely the motor du rability.
oreover, joining the rotox with a cardan or a flexible shaft through a coupling incorporating threaded joints is unreliablg, sinc~ such joints are likely to disen~a~e under the action of danymic forces resulting f'rom motor op~ration~
L'he helical teeth o~ the rotor of`-the ~otor unaer ~ ~ 75 ~

considerattion are also pro~uced by the ~ear-hobbing technique which suffers from the disadvanta~es men-tioned above.
Furthermore, provision o~ a solid rotor or a rotor made from a thick-~valled pipe lea~s to high consumo-tion OI` stainless steel. i,!otors incorporatin~ the ai'oredescribed rotor ~eature relatively low ef~iciency and power output, since ~,reat mechanical losses occur ~urin~ operation Yor the stator rubber to self-heat.
'here is ~nown more productive an~ ef~`ici~nt ;Dethod for m~Kin~ the sin~le-lobe rotor of the i.luano scr~w oump (cf. US ~atent i~o. ~4~4,011 l~ational Patent ClassiYication 103-117, published on March ~, 1949).
~ he method consists in de~orming a tube blank on a formative helical surface by virtue of a fluid pres-sure applied to said tube blankO
i~he method is carried into ef~`ect through a device comprising a housin~ ~hich accom~odates a ~ormin~
element with the formative surface, the tube blank being situated inside said ~orming element.
'~ne formative helical sur~ace is situated on the inner sur~'ace o~ the formin~ e1elnent which serves at the same time as the aousin~ and has a number of axial joints. A ~luid pressure is built up in the bore (or hollow space) oL the tube blan~ located inslde the ~orminO element ~rovi~e~l ~!Jith seals. i~le proeess o~ ~`or~in~ the rotor of a sin~Jle-scL~ 3U(,ip ~75~

_ 6 is carried out in a number of sta~es, each bein~ ~ollow-ed by extracting the tube blan~ from the forming ele-ment for annealing with a view to reducing the hard-ness of` the blani~ and relieving internal stresses ~he-rein .
The aforediscussed method and the device forcarrying it into ef~ect suf~er from too low quality of the rotor outer surface on ~Ihich there ar~ marks left by the joint surface of the forming element, eliminati-on of said marks involving additional machining of the rotor outside surface using special eguipment.
Another disadvantage o~ said method and device resides in a sophisticated process for making the inner surfaces of the split forming element, as well as a complicated procedure OI` bringing the ~ormative helical surfaces in coincidence in the jointing planes. '~he disadvantages mani~est themselves more conspicuously when makin~ rotors featuring hi~h length-to-diameter ratio, thus rendering impossible the production of multi-lobe rotors by the method described above.
One more disad~anta~e inherent in the aforemention-ed known method is the necessity to apply high hydro-static fluid pr~ssure, since -the pipe blank undergoes considerable tensile deformation. '~his, in turn, ac-counts for high sp~cific power consumption o the pro-cess.

75~3 It is a primary and essential object of the inven-tion to pro~ide a rotor of a screw hydraulic downhole ~ motor for drilling wells~ and a method and a davice f`or its production, which would make it possible, due to constructional features of the rotor, to improve out-put power characteristics oX the motor, reduce .~ricti-on loss and increase rotor production eXficiency.
The essence oX the invention resides in that the rotor of a 5crew hydraulic downhole motor made as a multiple-thread sclew having the number o~ teeth on th~ helical surface exceedin~ unity and ri~idly connect-ed to a union couplin~, according to the invention~ is substantially hollow and features substantially cons-tant wall thickness, whils the ratio of the length of the rotor cross-sectional outside contour to the length of a circle circumscribed around said contour is 5ub-stantially within 0.9 and 1.05.
Such a constructional arrangement oX the rotor makes it possible to improve the output power characte-ristics OI the motor, reduce transverse vibrations, add . to the strength oX the rotor with respect to the torgue applied thereto and bending load imposed thereon, dec-rease the rotor mass and its specific metal content, cut down stainless steel consumption, and better the guality oX its manufacture.
'iihe essence of a method for the rotor production 5~

resides in that a tubular blank is subjected to defor-mation on the formative surface by virtu~ of a fluid pressure and in that, according to the invention, the forming element ~lnos~3 outside surface is in ~act the formative sur~ace 9 iS placed inside the tubular blank, while the fluid pressure is applied to the outside sur~`ace of the tubular blank.
This enables one to attain high quality of the rotor helical surface, reduce power and labour consump tion for its manufacture, cut down production ~ime and -thus obtain a rotor featuring improved technical characteristics, higher quality of surface finish and precision, which makes it possible to minimize fric-tion loss and improve oubput power charac-teristics of a motor incorporating the rotor of the present i wen-tion.
On some occasions it is expedient that the form-in~: process of a tubular blank be carried ouk in two stages, at the first of which the tubular blank is given the shape of a helical polyhedron with rounded-off vertices, featuring the diameter of a circumscribed cir-cle drawn therearound somewhat in excess o~ the diame-ter of a circumscribed circle drawn around a finished rotor, and the number of faces is equal to the num-ber of threads (or starts) of the rotor helical sur-face, whereas at the second stage the rotor helical .X75 surface is forrned finall~.
~ his enables one to avoid metal wrinkling during the forming process of a tubular blank and ensure ex-cellent workmanship, hi~h ~imensional accuracy and trueness of ~eometrical shape.
It is expedient that before exerting pressure on the tubular blank a union coupling recessed on its outside sur~ace be inserted into said blank, and the latter be forced a~ainst the surface o~ the union coupling concurrentlg with the formation of the rotor helical surface, thus making the ~lank fast in the rotor.
: This makes it possible to cut down the time spent for producbion of` a rotor with a union coupling due to simultaneous (combined) formin~ of the rotor helical orking~ surface and securing o~ the union coupling in the rotor. Besides, there ~ provided higher reliabi-lity and pressure-tightness of the joint of the rotor with said coupling~
'rhe essence of a device for makin~ said rotor by ~he r~ethod set ~orth hereiabefore consists in that it comprises a housi~g which accorrlmodate~ a fo~ing element having a formative sur~ace, wherein, according to the invention, the forming elernent is in~talled inside the housing on centring bushings, while the ~; ;ormative Sur~aGe is ~rovided on the formin~ element .~ .

:

75~

outside surface 9 and tAe centring bushes have fitting areas adapted for the tubular blank ends to fit tightly thereon.
'~his provides ~or reliable location o~` the forming element with respect to the housing and tubular blank and production of a rotor having high-quality outside workin~ surface, as well as simplifies the manufacture of the forming element.
It is expedient that each centring bushing be pro vide~ ~vith a projection adjacent to its ~ittin~ area and adapted for tne tubular blank set on said ~itting area, to rest against, and that ~aid projection have an a~-nular groove whose width is substantially equal to the thicknes~ of the tubular blank, said groove being adapted for a seal to accommodate.
This provides for reliable ori6inal hermetic seal-ing of the high-pressure chamber of the device before be~innin~ the process of deformation of a tubular blank on the fritting ar~9 of the centrin~ bushings, as well as makes it possible to attain more reliable operation of the rotor manufacturi~ device.
It becom~s necessary, on some occasions, that the formin~ element should be replaceable in the housing and that a preforming element be provided for preli-~inary formatio~, made as a helical polyhedron with rounded-off vertices, featurin~ the diameter of its , ,~

,:, ~7S~

: circumscribed circl~ so~ewhat in excess of the diameter of a circumscribed circle of the forming element for fi-nishing formation, the number of the ~aces of said poly-hedron being egual to the numoer of threads on the rotor helical surface.
~his makes it possible to prevent wrinkling on the rotor working surfaces and provide high quality oX
said surfaces, high dimensional accurac~ and trueness of geometric shape.
.- In what follows the invention is illustrated b~ a detailed description of a specific embodiment thereof with reference to the accompanying drawin~s, wherein:
FIG. 1 is a schematic, partly longitudinal section-al view of a screw hydraulic downhole motor for drill-ing oil and gas wells, incorporating the rotor, accord-ing to the invontion;
FIG. 2 is a cross-sectional view of the motor, taken along the line II-II; !
`~ FIG. 3 is a longitudinal-section view o~ the rotor, according to the invention;
FIG. 4 is a cross-sectional view of the rotor, : tak~n aIong the lin~ IV-IV;
FIG. 5 is a cross-sectional vi~w of the rotor, ta~en ~long the line V-V;
FIG. ~ is a longitudinal-sectional view of a devics for making the rotor, according to the invention;

. . .

.;275 FIG. 7 is a cross-sectional ~iew of a devico for makin~ the rotor, taken along the line VII-VII;
FIG. 8 is a cross-sectional view of the forming cores for preli~inary and finishing Yorming process;
and ~ IG. 9 is a fra~mentary longitudinal-scctional view of a de~ice for making the rotor with simultaneous forcinK of a union coupling.
A rotor 1 is in effect one of the major compo-nents of a downhole motor (FIG. 1); it i5 made as a multiple-thread screw havin~r external helical teeth 2, the number OL threacLs (teeth) on the helical surface bein~ in excess of unity. r~he rotor 1 is accommodated ins~de a stator 3 which is provided with a lining 4 made of such a resilient material as rubber. '~he lin-ing 4 has an inside helical surface which Yorms he-lical teeth 5 the number of which exceeds the number of teeth on the rotlor 1 by unity. An axl~ l (F~g. 2) of th~ rotor 1 is offset with respect to an axis 2 of the stator 3 by an amount 7e ~ of eccentricity. The rotor 1 (EIG. 1) is associated with a shaft 6 of a bearing unit 7 of the motor throu~h a flexible shaft 6 or a cardan shaft (not shown). '~he bearing unit 7 comprises axial and radial bcarings (not shown) adapt-ed to take up bottom-hole loads. Connected to the lower encL of the shaft 6 of the bearing unit 7 is a `:

' ~ ' '' ~ `

~.27~ 8 , rock destruction tool 9. '~he stator 3 of the-motor is connected, through an adaptor lO, to thc lower ~nd of a drill string 11.
The rotor 1 (~IGS 3, 4) , according to the inven-tion, is a hollow structure 9 comprising a tubular shell 12 (housing) and a union couplin~ 13 (FIG. 3) rigidly h~ld to said.shell and adaptod for assoclation with the flexible shaft 8 (FIG. 1). ~he union coupli~g13 (FIG. 3) is provided with el~m~nts 14 ~or connecting the flexible sha~t8~ e.g.~threadS~ though some alter-natives may be resorted to, such as welding, joinin~ by m~ans of cones, etc.
It is a preferable method of holding the union couplin~ 13 to th~ tubular sh~ll 12 by forcing the latter against the shaped outside Sur~aGe o~ ths union coupling 13, wherein r~cessss 15 are provid~d by the method described below. ~h~ recesses 15 may be 9haped as radial blind holes, longitudinal or cross slots or ~lats, annular or helical ~rooves, or a~ combinations ther~o~. It is important that projections 16 that aro established on the inner ~urface of the tubular shell 12 as a result of forcing the terminal portion of th~ tubu-lar shell 12 against the shaped outsido sur~aco o~ th~
union couplin~ 13, should interact with th~ recesses 15 o~ the union coupling 13 so as to transmit the torquo and axial :load.

75~

Shown as an exa~ple in FIGS 3 and 5 i9 -the re-cess 15 shaped as an annular groove having a diameter dl and bein~ eccentrical with respect to an outside cylindrical surface 17 of the union coupling 13.
'~he ratio of the lengtn of an outside contour 18 of the cross-section of the rotor 1 to the length of a circle 19 circumscribed around said contour, is sub-stantially within 0.9 and 1.05~ ~hen said ratio is below 0.9, other things being egual, this-results in adve~sely affected output power characteristics of the screw motor as to the torque developed and ths output power (due to a reduced number of rotor threads)7 in reduced torsional and bending stren~th of the hollow rotor9 as well as in d~teriorated quality of rotor manufacture by the method and device proposed herein and described in detail beloW, due to wrinkl-ing on the rotor surface and d~parture ~rom true geo-metric shape of the rotor.
ihen said ratio exceeds 1 05 this results in re-duced efficiency of the motor (du~ to an increased num-ber of the rotor threads), in affected torsional and bendin~ strength of the rotor, and in some difficul-ties encountered in the manufacture of the rotor accord-in~ to the m~thod and devico proposed in this inv~n~
tion and described in detail her~inbelow, due to con-siderably incr~eased values o~ working pressure, as ,. . . .
.-- .

~, , .

" ~IL,275~L~8 well as on account o~ hig~ pow0r consumption of the rotor production procoss.
Th~ rotor disclosed in this invention operat~s as follows. Wh~ drilling mud is ~ed from dayli~ht sur-facc along-the drill string 11 (FIG. 1), the rotor 1 is urged to rotat~, under the actlon of an unbala~ced fluid pressure applied to its lateral helical surface, thus rolling ov~r the teeth of the stator 3. The tor-gue and axial (thrust) load developed on the rotor as a result, are transmitted to the sha~t 6 o~ the bearing unit 7 through the flexible shaft 8 connected to th~
rotor 1 through the union coupling 13. Further on ro-tation ~rom the shaft 6 of the beari~g unit 7 is trans-lated to the rock destruction tool 9.
~ he rotor o~ a SCL ew hydraulic downhole motor des-cribed above is manufactured as follows. ~ ~`orming element having an outer ~`ormative surface shaped as a multiple-tnread helisal sur~ace, is placed in a tubular blank that has preliminarily b~0n machined on its outside sur~ac~ to a reguired guality of surface finish (e.g. , by grinding~ polishin~, etc.). There-upon the ends o~` the tubular blank are herm0tically s~aled with respect to the forming element, at the same time mutually centre-ali~ning the tubular bla~k and the forming olement, and a pressur~ o~ such a fluid as, e.g., mineral oil is applied to the outside . .

. ~ . ,.

-`

~75 surface of the tubular blanX. Under the effect of said fluid pressure the tubular blank loses stability and ~ets defor~ed cross-sectionally, with the re~lt that the blank becomes snug against the formative sur-face of the forming element, thus acquiring the requir-ed geometric shape of a multi-lobe rotor of a screw hydraulic downhole motor~ In some cases, particularly with a great length of the rotor teeth and their low number, the rproce~s of forming the rotor teeth by the aforedescribed method is expedient to carry out in two stages. At the first stage the tubular blank is subjected to partial deformation for an incomplete tooth lengt~l, thus imparting to it the shape o~ a he-lical polyhedron with rounded-off vertices, while at the second stage the rotor helical surface is finish-formed. In this case a guality helical surface free from wrinkles and other departures from true geomet-ric shape is obtained at the first stage due to a re-duced amount of radial deformation. ~he first sta~e of the process may be conducted at a reduced fluid pres-sure, since that stage is aimed at overcoming the stability of the tubular blank cylindrical shape and preforming a helical surface having the same number o~
th~eads and the same helix lead as in the finished rotor. '~he tubular blank obtained at the first stage as a helical polyhedron is subjected to final forming ~;

- ~ ~75:~g~3 to establish the helical surface of the rotor, by the same method, i.e., by applying a ~lu~pressure to the outside surface of the tubular olank inside which the forming element is placed.
On many occasions an optimum method for making the rotor is the one, wherein the process for forming a helical surface on the rotor proceeds simultaneously with the joining of its tubular shell 12 with the union coupling 134 ~0 this end there is inserted in the interior of the tubular blank before its ~orcing b~
the fluid pressure, t~e union coupling 13 whose outside surfaca is made profiled or shaped, that is, is provided -vith recesses having this or that form, e.g., radial blind holes, longitudinal cros~ slots or flats, annular or helical grooves, or any combinations thereof. iuhen forcing the terminal portion o~ the rotor tubular shell, projections are formed on the shell inner sur-face, which are adapted to interact with the recesses in the union coupling, thus making it possible t-o impart the torque and axial forces developed on the rotor tubular shell, to the union cou~ling and further on to the ~lexible shaft.
The aforedescribed method for producing a rotor of a screw hydraulic downhole motor can be carried into ~; effect with the aid of ~ device shown in FIG. 6 in a longitudinal section, and in FIG. 7, in a c~oss-section.

:

~ 275~

'l'he device comprises a thick-walled tubular housing 20 which accommodates a formin~ element 21 cenkre-aligned with the housing 20 by means OI` centring bushings 22, 22' (FIG. 6). ~he outside ~ormative surface of the forming element 21 is shaped as helical teeth 23 having the same hand o~ helix and helix lead as the rotor being manufactured, whereas the cross-sectional dimension o~
the forming element 21 is equidistant with respect to the rotor cross-sectional outside contour. 'The amount of equidistance equals the thickness ~ (FIG~ 4) of the wall of a tubular biank 24. Fitting areas 25 are provided on the outside sur~ace of the centring bus-shings 22 (FIG. 6), on which the end portions of the tubular blank 24 are fitted. I
The centring bushings 22, 22' are provided with seals 26, 26' located at the places of contact o~ said bushing~ with the hous~ng 20. ~he af`oresaid seals are in the form o~ e.g. 9 rubber 0-rings.
'~he centring bushing 22 has a projection adjacent to the fitting area 25 and having an end annular groove 27, which receives a seal 28 ~ade of rubber or any other elastic material. ~he width of the groove is substantially equal to the thic~ness ~ of the tubular blank 24. The tubular blan~ 24 is located on the fitting areas 25 (only one of these being shown in the FIGUR~) of the centring bushings 22, 22' in such a 75~L3~3 manner that the ends of the blank 24 rest against the ~aces of the seals with some axial tension applied to the rubber. Axial holding of the tubular blank 24, the centring bushings 22, 22~ with the ~eals 28 (only one of these being shown in FIGUR~), and the forming ele-ment 21 is by means of the inside faces 29 of circular nuts 30 (only one of' these being shown) turned onto the end threads of the housing 20.
A chamber 31 is established between -the outside surface oY the tubular blank 24 and the inside surface of the housing 20 for a fluid under pressure to feed~
~orts 32 and 33 are provided in the housing 20-for the purpose.
According to the herein-proposed metaod, when the rotor is manufactured in two stages, the forming ele-ment 21 (FIG. 8) is made replaceable. A formin~
element 21l for preliminary forming is made as a helic-al polyhedron having the cross-sectional shape o~ a polygon with rounded-off vertices and features a reduced length hl of helical teeth and an increased outside dia~neter d2 as ompared with respective di mensions h3 and d3 of the forming element 21 for ~inish-for~ing. FIG. 8 represents the superposed cross-sectional contours of ~he forming elements 21' and 21 for preliminary and finish forming respecti-vely.

~7S3L9~3 ~ 20 The device is assembled and operates as follows.
~he forming element 21 is :inserted in the tubular blank 24 that has preliminarily been machined on its outside surface to a quality of sur~ace ~inish requir-ed t`or the rotor (e.g., by grinding, polishing, etc.).
~he centring bushing 22' is set on one end of the forming element 21, simultaneously engaging the end portion of the tubular blank 24 with the fitting area of the centring bushing 22'. Then the ~orming ele-ment 21 with the tubular blank 24 and one of the centring bushings 22, 22' is placed in the housing 20.
Next the other centring bushing 22 is set on the free end of the forming element 21, simultaneously bring-ing its fitting area into the tubular blank 24, and the outside surface o~ the centrin~ bushings 22, into the hou.~ ng 20. '~hereupon the thus-assembled compo-nents are held in place in the housing 20 by means of nuts 30 until the ends of the tubular blank 24 are some-what forced into the bulk o~ the rubber seals 28. 'llhen a fluid, e.g., a mineral oil is ~ed to the chamber 31 of the device through the port 32 of the housing 20 to expell air from the chamber 31 through the port 33.
~s soon as oil appears ~rom the port 33~ the latter is shut with a cock (omitted i~ the ~rawlng). As the feed ~f the ~luid continues the cylindrical tubular bla~k is liable to lose its stability under the effect of externally applied pressure, thus becoming forced ~.~75~98 against the formative helical sur~`aces of' the forming element 21, whereby the rotor helical teeth are formed on the outside surface of the tubular blank 24. The seals 26 establish pressure-tightness in the joint clearances between the housing 20 and the centring bushings 22 (and egually the bushing 22~), while the clearances between the centring bushings 22, 22' and the tubular blank 24 are pressure-tightened at the initial instant due to the fact that the ends of the tubular blank 24 are somewhat forced into the rubber seals 28. As the fluid pressure in the chamber 31 rises and deformation of the tubular blank 24 progresses, the clearances between the t~bular blank 24 and the fitting areas ~5 of the centring bushings 22, 22' a~e pressure-tightened by virtue of hydraulic forcing of the tubular blank 24 against said fitting areas.
On completion OL' the aeformation process applied to the tubular blank 24, which is judged by a rapid fluid pressure rise, the pressure is relieved, the device is disassembled and the forming element 21 is removed from the tubular shell of the rotor.
~ IG. 9 illustrates an embodiment of the method for making the rotor of a screw hydraulid downhole motor with a simultaneouS pressing-in of the union coupling 13.
According to said embodiment, one end of the forming element 21 is located in the housing ~0 by means of ., , ~"~d75~L98 a centring bushing 3~ which accommodates the union coupling 13 whose outside surface serves as the ~itting area ~or the tubular blank ~4 and is provided with the recess 15 shaped as an eccentric groove. 'l'he process of forming the rotor helical sur~ace proceeds con-currently with the Eorci~g o~ the union coupling, with the result that a projection is formed on the tubular shell inner sur~ace, The projection engages the recess 15 of the union coupling 13 and is adapted to interact therewith when transmitting the torgue and axial load.
It is due to the forclng o~ the tubular blank 24 against the outside surface o~ the union coupling 13 under the e~fect of high fluid pressure that hermetic sealing of the joint is attained.
The aforedescribed invention is e~ficiently appli-cable for the proYision o~ high-torque screw h~draulic downhole motors for drilling oil and gas wells, such motors featuring improved output power and performance characteristics.

Claims (7)

1. A rotor of a screw hydraulic downhole motor, made as a multiple-thread screw with the number of threads of the helical surface exceeding unity and rigidly connected to a union coupling, said rotor being a hollow structure and featuring substantially constant wall thickness, while the ratio of the length of the rotor cross-sectional contour to the length of a circle circumscribed around said contour is substantially within 0.9 and 1.05.

2. A method for the production of a rotor for a screw hydraulic downhole motor, wherein a tubular blank is forced against a formative surface by virtue of the fluid pressure exerted thereon, while a forming element whose outside surface serves as the formative surface, is placed inside the tubular blank, and the fluid pressure is applied to the outside surface of the tubular blank.

3. A method as claimed in claim 2, wherein the forming process applied to the tubular blank, is carried out in two stages, at the first of which the tubular blank is given the shape of a helical polyhedron with rounded-off vertices, featuring the diameter of a circumscribed circle drawn there-around somewhat exceeding the diameter of a circumscribed circle drawn around a finished rotor, while the number of faces is equal to the number of threads of the rotor helical surface, whereas at the second stage the rotor helical surface is formed finally.

4. A method as claimed in Claim 3, wherein before exerting pressure on the tubular blank, a union coupl-ing having a shaped outside surface is inserted into the tubular blank, and the process of forming the rotor helical surface is carried out simultaneously with the forcing of the tubular blank against the shaped surface of the union coupling to make the blank fast on the rotor.

5. A device for making the rotor as claimed in Claim 1, comprising a housing which accommodates a forming element having a formative surface and a num-ber of seals which establish, together with the hous-ing, a chamber for pressure-feeding of a fluid, said housing being provided with a number of centring.
bushings on which the forming element is installed, while the formative surface is situated on the outside surface of the forming element, and the centring bushings have fiting areas adapted for the ends of the tubular blank to fit tightly on said areas.

6. A device as claimed in Claim 5, wherein each of the centring bushings has a projection adjacent to its fitting area and adapted for the tubular blank set on said fitting area, to rest against, and said projection have an annular groove whose width is sub-stantially equal to the thickness of the tubular blank, said groove being for a seal to accommodate.

7. A device as claimed in Claim 5, wherein the forming element is installed in the housing with a possibility of being replaced, and a forming element is provided for preliminary forming, said element being in fact a helical polyhedron with rounded-off vertices, featuring the diameter of its circumscribed circle somewhat in excess of the diameter of a circumscribed circle of the forming element for finishing formation, while the number of the faces of said polyhedron is equal to the number of threads on the rotor helical surface.