US3581808A

US3581808A - Starting device for continuous casting machine

Info

Publication number: US3581808A
Application number: US786883A
Authority: US
Inventors: Tibor Miklos Vertesi; Bruce Allan Phillips
Original assignee: GAMMA ENGINEERING Ltd
Current assignee: GAMMA ENGINEERING Ltd
Priority date: 1968-12-26
Filing date: 1968-12-26
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01

Abstract

For a continuous casting machine, a strand starting device having Y-shaped links of uniform height, each link having a pair of arms joined by a root, and a bar extending from the root, the bar of one link being pivoted between the arms of the next link. Two pawls in the root of each link, one pawl on each side of the longitudinal center plane of the link, press against a cam face on the tip of the bar of the next link, causing the links to be self straightening. The head assembly of the device contains a pivoted joint permitting rotation in one direction to reduce stresses at its union with a cast strand, the pivoted parts of the head assembly also having a pawl for self straightening.

Description

United States Patent Inventors Tibor Mildos Vertesi 3324,934 6/1967 Hess et Whitbey, Ontario; 3,433,287 3/1969 Greenberger Bruce Allan Phillips, Ajax, Ontario, both 3,464,482 9/1969 Greenberger of, Canada 786,883

FOREIGN PATENTS 3/1955 GreatBritain..............i.

[21] Appl. No. [22] Filed Dec. 26,1968 [45] Patented June 1,1971

4/1955 Great Britain.....

Primary Examiner-J. Spencer Overholser Assistant Examiner.1ohn E. Roethel Attorney-Rogers, Bereskin and Parr [73] Assignee Gamma Engineering Limited Whitby, Ontario, Canada [54] STARTING DEVICE FOR CONTINUOUS CASTING 7 ABSTRACT# For a continuous castin g machine, a strand start- MACHINE k ee nhn m m fium oii hn b eke mm mm um ew f w fmm a m. ae.h..d i dem mam lo 0 r. mM mrae mapom m rne it eh no.m c uoet f b n a .mD.m S nSdd h k u g b w ei fi mm d nm .m h0f W d eed ThC Sm.t.a m k b V r fiuO i mo 8 lea... n mmmumc W e a mm mm Pm ma C fl fn fl gu O d.mm fi v mw U a fafa .lhfappc 38 ,1 4 4 7Mm m 2512." 4 M 6 d6 1 2 1 0 1B W 7 oo 2 whi m4 m3 4 5 0 t S N .m d g u .m n W m" 8 n "u m mu 0" s m m n mu t a cd .1 1 b 1U IF .1 .11 2 0 5 55 1. [1.

the device contains a pivoted joint permitting rotation in one direction to reduce stresses at its union with a cast strand ow .m n e t h .w a H S 4 7 2 4 6 S T N. m .I C 8 Et mTw HAT e 6 PT R H 6 N6 U m l 3 4 9 9 6 U 3 the y also having a pawl for self pivoted parts of the head assembl PATENIEU Jim nan SHEET 2 0F 4 18a 29a 17a 5%\ Fl G98 PATIENTED JUN '1 I97! 35 1. 0

- sum u [If 4 IHHH FIG. 10 (Prior Ari) INVISNIORS TIBOR M. VERTESI B BRUCE A. PHILLIPS m, ma, new

STARTING DEVICE FOR CONTINUOUS CASTING MACHINE This invention relates to an improved starting device for use in continuous casting machines.

In continuous casting machines, molten metal is usually poured into a generally vertically aligned flow-through mold. The metal is chilled in the mold and is withdrawn continuously from the bottom of the mold as a strand of desired cross sectron.

The strand then passes through a spray chamber for further cooling, and then it usually passes through a guiding roller apron (during with time it cools further). During its travel in the roller apron, the strand may, depending on the type of casting machine, be bent from a vertical to a horizontal path of travel.

After its passage through the roller apron, the strand passes through a withdrawal machine (which provides the motive force necessary to withdraw and move the strand), and then moves to a cutting location where it is cut into desired lengths.

Before the start of the casting operation, the bottom of the mold must be closed by a plug, to prevent liquid pouring into the mold from pouring out before it has an opportunity to accumulate and be cooled by contact with the mold walls. In addition, this plug must be somehow connected to the withdrawal machine, so that the withdrawal machine can (when the casting begins) withdraw the plug from the mold and then pull the resultant strand through the roller apron to the withdrawal machine. Usually a starting device or dummy bar is used for this purpose.

Prior art starting devices, when made flexible for use in a curved roller apron, have suffered from the defect that they are not self straightening when they emerge from the roller apron, and they have not provided a bearing surface on which the withdrawal machine can consistently obtain adequate traction.

The present invention provides a flexible starting device having self straightening properties, i.e. when unconstrained the starting device according to the invention will tend under appropriate conditions to straighten itself, thus reducing the guidance needed for the starting device. The starting device in one of its aspects also employs a Y-shaped links of uniform height to provide increased bearing surface on which the withdrawal machine can act. This starting device is an improvement over that shown in patent application Ser. No. 713,904 ofTibor M Vertesi filed Mar. 18, 1968.

Further objects and advantages of the invention will appear from the following description, taken with the accompanying drawings, in which:

FIG. I is a diagrammatic view illustrating the organization of a continuous casting machine, to show how the starting device of the invention is used;

FIG. 2 is a top view of part of a starting device according to the invention;

FIG. 3 is a' side view of the starting device shown in FIG. 2;

FIG. 4 is a sectional view taken along the centerline of the starting device;

FIG. 5 is a view similar to that shown in FIG. 4, illustrating a second embodiment ofa cam face for the starting device;

FIG. 6 is a view similar to that of FIGS. 4 and 5 and illustrates another type of cam face for the starting device;

FIG. 7 is a top view of a head assembly for the starting device;

FIG. 8 is a sectional side view on lines 8-8 of FIG. 7;

FIG. 9 is a perspective view of a portion of a head member for the head assembly of FIGS. 7 and 8; and

FIG. 10 is a diagrammatic view illustrating separation between strand and a prior art head assembly.

Reference is first made to FIG. I, which shows how the starting device of the invention is used. FIG. 1 shows diagrammatically a continuous casting machine of the type shown in the above referred to Vertesi patent application. This machine has a tundish I which pours molten metal into an oscillating chilled mold 2. A strand 3 ofmetal is withdrawn from the bottomof the mold, passes through a spray chamber 4 for further cooling, and then passes through a curved roller apron 5 where it is guided and supported by rollers 5a to 5g. In the roller apron, rollers 50 to 5d establish a curvature of radius r for the strand, with roller 5b acting as a fulcrum roller. In other words, the strand progresses vertically downwardly through the spray chamber 4 and past roller apron 50 until it reaches fulcrum roller 5b at which time bending commences. By the time the strand has reached roller 5d, its curvature is substantially established, and only a relatively few rollers 5e to 5g are needed for support and guidance in the remainder of the roller apron.

The strand passes from the roller apron 5 to a withdrawal machine 6 having drive rollers 7 urged against the strand by hydraulic pistons and cylinders, not shown. The drive rollers are rotated (typically by hydraulic motors) to withdraw the strand. The strand then passes to a cutting area, not shown.

Before the pouring of metal into the mold 2 is begun, a starting device indicated at 8 normally stored in a storage track 9 is propelled out of the track 9 (usually by means of a piston and cylinder, not shown) to a position between drive rollers 7 of the withdrawal machine 6. The withdrawal machine is then run in reverse to propel the starting device 8 through the roller apron and then vertically upwardly into the bottom of the mold 2 A head assembly I0 on the end of the starting device plugs the bottom of the mold. When casting is initiated, the withdrawal machine is run forwardly to withdraw the starting device 8 (which is shunted back onto the storage track 9) and then to withdraw the strand 3, which disengages from the starting device 8 and continues on to the cutting area.

The starting device 8 must be flexible to pass through the curved roller apron 5, but that part of the starting device 8 that is pushed upwardly from the top of the roller apron to the bottom of the mold is unguided. This portion may therefore tend to bend and will then fail to enter the bottom ofthe mold.

According to the invention, therefore, links are provided for the starting device which tend to resist angular rotation relative to each other and which tend to straighten themselves when rotated out of an in-line position. A number of such links are shown at 11 in FIGS. 2 and 3.

Each link 11 is formed in the shape of a Y and has a U- shaped portion consisting of a pair of parallel arms 12 with a root 13 between them, and a bar shaped portion 14 extending from the root. The arms 12, root l3, and bar shaped portion 14 are all of uniform height as shown in FIG. 3, so that the rollers of the withdrawal machine will obtain good traction on the starting device. The bar portion of each link is located between the arms 12 of the adjacent link and is hingedly secured thereto by a hinge pin 15.

Located within the root 13 of each link is a pair of

pawls

16a, 16b. The pawls consist of

ball pressure members

17a, 17b, mounted in sockets 18a, 1812, one on each side of the longitudinal center plane 19 of each link. Springs 20a, 20b urge the balls outwardly against a cam face 21 formed on the end of the bar shaped portion 14 facing the root 13. When the links are in-line, which is their normal or rest position, the

balls

17a, 17b rest in

detents

22a, 22b in the cam face. The cam face 21 is radial about the axis of the hinge pin 15 over that portion 24 of its surface between the detents. The

shoulder portion

26a, 26b of the cam face on either side of the detents are substantially tangential to the radial portion 24 of the cam face.

FIG..4 illustrates the action of the

pawls

16a, 16b when one link is rotated about its hinge pin 15. The

balls

17a, 17b are forced out of their

detents

22a, 22b and ride on the cam face 21. As shown in FIG. 4, because of the design of the cam face, one ball 17a is pushed into its recess to a greater extent than is the other ball 17b. This results in a moment about the hinge pin 15 tending to restore the links to their in-line position. Rotation of one link relative to the other is limited by abutment of one of the

shoulder portions

26a, 26b of the cam face against the end of the root 13 as shown in FIG. 4. This prevents the

balls

17a, 17b from popping out of their sockets. A typical maximum angle between the two links is 30.

In practice, the differential in the amount of compression of the two springs 20a, 20b is fairly small. For a typical starting device according to the invention for use with a 4 inch square strand, the height h ofeach link 11 is 4 inches (the width is optional), the depth of each

recess

22a, 22b is l /ainches (by about I inch in diameter), the diameter of each ball is 1 inch, and each spring 20a, 20b is about lV4inches long by about 1 inch in diameter. In the situation shown in FIG. 4, the ball 20a is forced inwardly by about five-sixteenths inch, while ball 20b is forced inwardly by about three-thirtyseconds inch (the depth ofa typical detent), resulting in a difference in compression of only about seven-thirtyseconds inch. Therefore, the springs 20a, 20b will be high pressure springs. Suitable springs are those made by the Associated Spring Company, of Corey, Pennsylvania, USA. and sold by Par Industrial Sales Limited of Scarborough, Ontario under their catalogue no. HP-43.

Such springs require a 148 pound load for a Vs inch deflection, so the difference in compression between springs 20a, 20b is about 259 pounds for a seven-thirtysecond inch difference in compression. In the 4 inch strand starting device in question, the distance between the axis of each spring 20a, 20b and the axis of the pivot pin 15 is l inch, so there is a moment of 259 inch pounds tending to straighten each link relative to the other in the position shown in FIG. 4. In addition, of course, when the links are straight, an additional force is required to force the

balls

17a, 17b from their detents. The result is that the links will tend to remain in-line, and if bent, they will tend to return to an in-line condition.

In use, the starting device will have rotatable caps or rollers 27 fastened to the ends of each pin 15. These rollers 27 will typically be roller bearings the inner races of which are fastened to the pins 15. The outer races of rollers 27 run in a track (not shown) in the roller apron 5 (FIG. 1) in the same manner as described in the above referred to Vertesi patent application. The tracks in the roller apron provide firm guidance to the starting device as long as it is in the roller apron, and yet the tracks are spaced laterally apart sufficiently so as not to interfere with the strand. Similar tracks are provided past the withdrawal machine to shunt the starting device into or out of the withdrawal machine while permitting the strand to continue in a straight path to the cutting area.

As discussed, the links described are useful for that part of the starting device which must be pushed upwardly from the roller apron without guidance. In addition, if such links are used for the entire starting device, instead ofjust for that part which may be unsupported, this will tend to remove slack in the part ofthe starting device located in the roller apron.

Slack in the starting device can occur for two reasons. The first reason is that the spacing between the tracks which guide the rollers 27 of the starting device is usually at least inch (varying up to A inch) greater than the roller diameter. This reduces the risk that the starting device may jam in its tracks due to scale and corrosion, but it also permits "snaking" on undulation of the starting device in the roller apron. The second reason for slack is that at each link in the starting device, there may be a HI 6 inch to a Vs inch slack (caused by clearance between hinge pins and their sockets). The starting device may typically have about fifty links, of which forty may be positioned between the mold and the withdrawal machine, so that the slack caused by looseness between the links can be between about 2 /2inches and 5 inches.

The slack in the starting device can cause difficulty to the operator of the casting machine. The operator attempts to keep the liquid level in the mold constant during casting, but with reciprocation of the mold causing alternate tightening and loosening of the starting device, the slack in the starting device can cause the liquid level in the mold to rise or drop suddenly by as much as 5 inches. The suddenly dropping liquid level can fall below the bottom of the mold, causing voids in the strand, while the suddenly rising liquid level can overflow the top ofthe mold which is also highly undesirable.

Because the links of the invention tend to straighten themselves, undulations of the starting device in the roller apron are reduced, and also, the springs 20, which are precompressed, tend to reduce the slack between individual links. The result is to reduce fluctuations in the liquid level in the mold while the starting device is being used, thus facilitating control of the casting process.

Various configurations are possible for the cam face 21 while remaining within the concept of the invention. A second embodiment of the cam face is shown at 21 in FIG. 5, in which primed reference numerals indicate parts corresponding to those of FIGS. 1 to 4. Cam face 21' is the same as cam face 21 of FIGS. 1 to 4 except that the radial portion 24 between the detents in FIG,3 is replaced by a flat area 28. The ball 17b in this embodiment does not move into its recess 18'b quite so far as it does in the embodiment shown in FIG. 4. At the same time the ball 17'a moves into its recess 18a by the same amount as that shown in FIG/1. The resulting moment about the pin 15' in this second embodiment is therefore larger than the resulting movement in FIG. 4 for the same angular movement of one link relative to another. Consequently this link will tend to return to the in-line position more readily than will the link shown in FIG. 4.

A third embodiment of the cam face is illustrated at 21" in FIG. 6, in which double primed reference numerals indicate parts corresponding to those of FIGS. 1 to 4. The cam face 21" is defined by a portion of a cylinder having a radius of curvature R which is larger than the distance from the center of the pin 15" to the cam face 21". The radius R is chosen so that the ball 17"b is moved into its recess 18"b by a smaller amount than the ball 17"a moves into its recess 18"a. The choice of the radius R depends upon the required angular movement between links since the larger the radius the less angular movement there is possible.

In all cases the cam face 21 is preferably hardened to give prolonged life since the

balls

17a, 17b will tend to form tracks in the cam face 14.

The head assembly 10 for the starting device will next be described, with reference to FIGS. 7 to 10. As shown, the head assembly 10 includes a head member 30 having an axial socket 32 for receiving a starting bolt 33 (FIG. 9). The bolt is adapted to connect the strand to the starting device and is held in the head member by a taper pin 34 (FIG. 9) which fits into a tapered hole 35. The tip of the head member 30 fits into the bottom of the mold, with about a Va inch clearance (which is sealed by asbestos cord) to seal the bottom ofthe mold before metal pouring commences.

The head member 30 is pivotally connected through a link pin 36 (of the same type as pins 15) to a disengaging link 38. The disengaging link 38 is in turn pivotally connected to a locking link 40 by a link pin 42 mounted in a slot 43 in the disengaging link. The locking link 40 is in turn pivotally connected through a link pin 44 to an adapter link 46 similar to the links 11, and link 46 is connected to a series of links 11 as shown in FIG. 2.

The head member 30 includes a first shoulder 48 which engages a second shoulder 50 in the disengaging link 38 and prevents rotation of the head member in one direction relative to the disenegaging link. This is because relative bending of these parts is only required in one direction, i.e. to accommodate the curve of the roller apron. A pawl 52 of the same type as those shown for links 11 includes a ball 54 pressing on a cam face 56 of the head member to ensure that the head member 30 and disengaging link 38 will remain in line except when forced to bend in the roller apron,

The locking link 40 comprises a pair of side members 58 carrying rollers 27. The side members 58 tend to remain in the in-line position under the influence of pawls 59 mounted in the adapter link 46 and pressing on cam faces 60 of the locking link 40.

The locking link 40 and the adapter link 46 interengage between the side links 58. This interengagement is effected by a first projection on the adapter link 46 meeting a second projection on the second member. The first projection consists of a first longitudinal face 61 extending between a first transverse face 62 and a second transverse face 64. The second projection formed on the second member comprises a second longitudinal face 66 extending between a third transverse face 68 and a fourth transverse face 70. The transverse faces 61 and 66 are in contact when the adapter link 46 and the locking link 40 are in the in-lin'e position. A small angle a between the first and third transverse faces 62, 68 allows a slight angular displacement ofthe locking link 40 relative to the adapter link 46. A relatively large angular gap between the second and fourth transverse faces 64, 70 allows the starting device to follow the track leading to the storage track under the influence of the rollers 27 which are guided by the track. As the rollers 27 enter the storage track (as indicated by the chain dotted outline in FIG. 8) the head member 30 and disengaging link 38 continue in a straight line and disengage from the locking link 40 as the side members 58 pull the hinge pin 42 down the slot 43 and into the storage track.

The head member 30 and disengaging link 38 continue with the strand through the cutting area and into strand storage racks. They are then removed from the strand by disengaging the starting bolt 33 from the head member 30. This is accomplished by knocking out the taper pin 34 from the tapered slot 35. The head member 30 and disengaging link 38 can then be reused when starting a new strand;

The flexibility in the head assembly provided by the joint at pin 36 between the head member. 30 and the disengaging link 38 decreases the likelihood of the strand breaking away from the starting bolt 33 during their travel through the curved roller apron.

It may be noted that the head member 30 and disengaging link 38 do notcontain any rollers 27. This is because the starting bolt 33 of the head member must be thrust up through a roller assembly (not shown) on the bottom of the mold (used to guide the emerging strand) and then up about 6 inches into the mold itself. Any rollers 27 on the part of the starting device that passes through the mold guide rollers would interfere with the mold guide rollers, and therefore, the leading l2 inches or so of the head assembly should be without rollers 27.

It used to be the practice to make such leading portion rigid, instead of in two linked parts such as head member 30 and dis engaging link 38. Such a long rigid head member often caused separation of the strand from the starting bolt as the strand entered the roller apron. The reason for such separation is best seen from FIG. 10, where a prior art rigid head member 80 for the starting device is shown as connected to the strand 3. As the head member 80 enters the roller apron, it is forced to assume a path of travel angled to that of the strand. Since the head is rigid and cannot bend, the joint 82 between the strand 3 and starting member 80 undergoes considerable stress and tends to separate (the interior of the strand is still soft at this stage).

The invention shortens the rigid head member attached to the strand and connects it to another member (also without rollers), namely, the disengaging member 38. Bending can thus occur in the head assembly, reducing the stresses at the point between it and the strand.

It may be noted that the angle between

surfaces

55, 56 is allowing head member 30 to rotate by an angle of no more than 10 relative to disengaging member 38. The reason for the limitation of the angle at which the head. member can rotate relative to the disengaging member is because if too i great an amount of rotation were allowed, then the leading edge of the head member 30 would, in the situation shown in FIG. 10, fail tube-pulled sufficiently to the right by the disengaging member 38, and the head member might tend to strike roller 5d, causing jamming of the machine.v The maximum angle of rotation allowed between the head and disengaging

members

30, 38 will in practice depend on the length ofthese members, the spacing of the rollers in the roller apron, and the sharpness of the curve in the roller apron.

Reference is next made to the chain dotted line 84 in FIG. 8. This line is present to indicate that the head member 30 and disengaging member 38 could have a sloping top surface defined by the line 84. Such head and disengaging members will be used when it is desired to cast a larger strand using essentially the same starting device. For example, the device illustrated, with the measurements given previously, is for use with a 4 inch square strand. By using the enlarged head and disengaging members indicated by chain dotted line 84, the starting device can be used to start a 5 or 6 inch strand. The slope provided on the upper surface of the head assembly ensures that the pressure drive rolls 7 (FIG. I) of the withdrawal machine 6 will be forced apart gradually as the head assembly passes through them, to accommodate the change in size between the 4 inch height of the starting device and the 5 or 6 inch height of the strand. The increased pressure in the hydraulic system (not shown) which forces the drive rolls 7 of the withdrawal machine against the starting device or strand will be exhausted through a relief valve (not shown).

Although in the links illustrated, the

pawls

16a, 16b are mounted between the arms 12 of the U-shaped portion, the pawls could instead be mounted in the end of the bar portion 14. In that case the root 13 between the legs of the U-shaped portion will form the cam face. This would normally require extending the end of each bar portion 14 further past its link pin 15 and recessingmore deeply than the U-shaped portion of each link.

In addition, if desired the pawls can be placed in the end of the root 13 facing away from the free ends of the arms 12. Such an arrangement is used for the adapter link 46 of FIGS. 7 and 8, and in that case the cam faces are formed on the free ends of the arms 12 of the adjacent link. This facilitates the use of four pawls instead of two, to increase the straightening forces acting on the links.

What We claim as our invention is:

l. A starting device adapted to be positioned between a mold and a withdrawal mechanism of a continuous casting machine and to be pulled along a path of travel to withdraw the leading edge of a newly formed metal strand from said mold and to draw said leading edge to said withdrawal mechanism, said starting device including a. first and second links,

b. a link pin rotatably connecting said links so that said links can be rotated relative to each other in a first plane at right angles to the axisof said pin, said links having an inline position in which they have a common longitudinal center plane orientedat right angles to said first plane and passing through said axis ofsaid pin,

c. said first link having a first end surface and said second link having a second end surface facing said first end surface, said end surfaces extending on either side of said center plane in a direction substantially across said path of travel,

d. a pair of pawls located in one of said end surfaces, one on each side of said center plane, each pawl having a pressure member and means urging said pressure member towards the other of said end surfaces,

. said other of said end surfaces having a cam face formed thereon, said cam face including a pair of detents in which said pressure members ride when said links are in in-line position to oppose rotation of said links from their in-line position, said cam face being shaped for angular rotation of one link relative to the other to cause one of said pawls to exert a larger turning moment about said hinge pin that the other to tend to restore said links to their in-line position.

2. A starting device according to claim 1 wherein said end surfaces are shaped to engage in abutting relationship upon rotation of one link relative to the other to a predetermined extent, said predetermined extent being such that said other end surface at all times limits egress of said pawls from said one end surface.

3. A starting device according to claim 2 wherein each said link is Y-shaped, having a U-shaped portion comprising a pair of arms and a root joining said arms, and a bar portion projecting from said root in a direction opposite to that of said arms,

said bar portion of one of said links extending between said arms of the other of said links and said hinge pin passing through said arms and said bar portion, said, arms, roots and bar portions all being of uniform height to improve the traction on said device by said withdrawal mechanism.

4. A starting device according to claim 3 wherein one end surface is formed on said root of said other link facing the tip of said bar portion of said one link, said cam face being formed on the tip of said bar portion.

5. A starting device according to claim 3 wherein said one end surface is formed on the tips of said arms of said other link, and said other end surface is formed on the end of said root of said other link facing said tips of said arms of said one link.

6. A starting device according to claim 4 wherein said cam face of each link forms part of a cylindrical surface having a radius of curvature larger than the distance between said axis of said hinge pin of such link and that part of said cam face which crosses said center plane on such link.

7. A starting device according to claim 4 wherein said cam face of each link forms part of a cylindrical surface between said detents; said cam face further comprising a pair of flat areas extending outwardly from said detents and being tangential to said cylindrical surface.

8. A starting device according to claim 7 wherein said cylindrical surface has a radius of curvature centered at the axis of said hinge pin.

9. A starting device according to claim 4 wherein said cam face of each link comprises a flat area between said detents lying in a plane at right angles to said center plane of such link.

10. A starting device according to claim 9 wherein said cam face further comprises two planar areas extending outwardly at an angle to said flat area and and wherein said cam face is symmetrical about said center plane.

11. A starting device according to claim 3 wherein a guide roller is mounted on an outer face of each of said arms.

Claims

1. A starting device adapted to be positioned between a mold and a withdrawal mechanism of a continuous casting machine and to be pulled along a path of travel to withdraw the leading edge of a newly formed metal strand from said mold and to draw said leading edge to said withdrawal mechanism, said starting device including a. first and second links, b. a link pin rotatably connecting said links so that said links can be rotated relative to each other in a first plane at right angles to the axis of said pin, said links having an in-line position in which they have a common longitudinal center plane oriented at right angles to said first plane and passing through said axis of said pin, c. said first link having a first end surface and said second link having a second end surface facing said first end surface, said end surfaces extending on either side of said center plane in a direction substantially across said path of travel, d. a pair of pawls located in one of said end surfaces, one on each side of said center plane, each pawl having a pressure member and means urging said pressure member towards the other of said end surfaces, e. said other of said end surfaces having a cam face formed thereon, said cam face including a pair of detents in which said pressure members ride when said links are in in-line position to oppose rotation of said links from their in-line position, said cam face being shaped for angular rotation of one link relative to the other to cause one of said pawls to exert a larger turning moment about said hinge pin that the other to tend to restore said links to their in-line position.

3. A starting device according to claim 2 wherein each said link is Y-shaped, having a U-shaped portion comprising a pair of arms and a root joining said arms, and a bar portion projecting from said root in a direction opposite to that of said arms, said bar portion of one of said links extending between said arms of the other of said links and said hinge pin passing through said arms and said bar portion, said, arms, roots and bar portions all being of uniform height to improve the traction on said device by said withdrawal mechanism.