GB2221634A

GB2221634A - Quick release screw drive mechanism

Info

Publication number: GB2221634A
Application number: GB8917443A
Authority: GB
Inventors: Chaolai Fan
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-08-08
Filing date: 1989-07-31
Publication date: 1990-02-14
Also published as: DE68917047D1; DE8907067U1; JPH0288179A; RU2052691C1; BR8903962A; MY104146A; EP0354644A3; NZ229177A; EP0354644B1; RU2052692C1; JPH0543464B2; EP0354644A2; MX172487B; AU3710189A; AU614390B2; DE68917047T2; GB8917443D0

Description

2-22 1634 SCREW DRIVE MECHANISM The present invention relates to a screw

drive mechanism which permits the screw to engage with or disengage from a screw nut by a relative movement between the screw and the nut. Such a mechanism can be widely used, for example in a manual bench vice, or a machine tool vice or a sliding table of a machine tool, where there are requirements for moving quickly through an idle stroke, for speedily adjusting a relative distance or for fast clamping.

There are many known types of fast-clamping vice. US Patent No. 2102602 (1937) discloses a mechanism which the locus of centre of a clamping screw is an arc when the screw disengages or engages its nut. Thus, the nut has to is make a sliding connection with a stationary body, which results in a lack of strength. US Patent No. 2430458 (1947) also discloses a mechanism in which the screw nut will still remain on the screw after the later has disengaged from the former and where the screw nut bears against an outer circumference of the screw by a spring.and a pin, which gives too much resistance. Moreover, the present application has filed European Patent Application No. 88301613 (see Fig. 1) disclosing a fast- clamping mechanism, in which a driving nut is provided outside the vice body, having however a rather comp lex construction.

An object of the present invention is to overcome 2 the aforesaid drawbacks by employing a one-way pawl and cam mechanism to effect transverse relative movement between the screw and the nut for engagement and disengagement of the screw with the screw nut.

A screw drive mechanism according to one aspect of the invention comprises a stationary body, a movable body, a screw, a screw nut seat having two arms between which an eccentric cam is positioned, said screw passing through holes of said nut seat and said cam, said nut seat having at least one thread for engagement with the screw thread to displace said movable body with rotation of the screw, said cam being slidable on cam support surfaces of said nut seat, said screw being arranged to drive said eccentric cam through a pawl device to cause a transverse is relative movement between said screw and said screw nut seat, whereby the thread of said screw is engaged with or disengaged from said thread of the screw nut seat.

In one embodiment of the invention, the two ends of the screw are supported in respective elongated circular holes in two end walls of the movable body, the pawl device comprises a ratchet sleeve, which is positioned between the eccentric cam and an arm of the screw nut seat and is connected to the screw through a guide key. There is provided one (or more) one-way pawls on an end face of the ratchet sleeve, the or each said pawl having a radial face and an oblique face which may engage correspondingly with a radial face and an oblique face on a pawl groove of the eccentric cam. The sleeve is urged against the cam by a 3 spring for said engagement. Under the action of the cam, the screw can be moved transversely relative to the fixed nut seat.

In a second embodiment of the invention, the pawl device comprises a pawl pin, which is located within a radial hole on the cam and into which it is urged by a ring extension spring. The screw nut seat is provided with two elongated circular holes. each said hole having the threads on a lower portion.

In a third embodiment of the invention, the nut seat is provided with a guide pillar which can slide in a transverse holes in the base of the stationary body. The screw is fixed in the radial direction, and under the action of the cam and guided by the pillar the nut seat can is move relative to the radially located screw.

By way of example, the present invention will be described in relation to a bench vice in a number of embodiments shown in the accompanying drawings, in which:

Fig. 1 is a view showing a prior art screw drive mechanism.

Fig. 2 is a sectional view of a bench vice employing a drive mechanism according to the invention. showing the vice in a released position.

Fig. 3 is a sectional view of the bench vice of Fig. 2 in a clamping position, Figs. 4 and 5 are respectively, an axial section and a broken-away end view of the screw nut seat of the vice of Figs. 2 and 3, 1 25 4 Figs. 6 and 7 are respective isometric views of the ratchet sleeve and of the eccentric cam of the vice of Figs. 2 and 3, Fig. 8 is a sectional view on the plane AA or DD in Fig. 2, showing the position of the screw neck journal in support holes of the front and back plates of the movable body when the bench vice is in the released position, Fig. 9 is a sectional view on the plane BB in Fig. 2, showing the outer thread of the screw disengaged from the inner thread of an axial hole in the screw nut seat when the bench vice is in the released position, Fig. 10 illustrates the local region E of Fig. 2, showing the pawl engagement that is made when the bench vice is in the released position, Fig. 11 is a sectional view taken on the plane line CC in Fig. 2, showing the relative position of the eccentric cam and the screw nut seat when the bench vice is in the released position, Fig. 12 is a sectional view on the plane AW or D'D' in Fig. 3, showing the position of the screw neck journal in the support holes on the front and back vertical plates of the movab le body when the bench vice is in a position clamping a workpiece, Fig. 13 is a sectional view on the plane B'B' in Fig. 3, showing the outer thread of the screw engaged with the inner thread of the screw nut seat when the bench vice is in the clamping position, Fig. 14 illustrates the local region E' of Fig. 3, showing the disengagement of the pawl when the bench vice is in the clamping position, Fig. 15 is a sectional view on the line C'C' in Fig. 3, showing the relative position of the eccentric cam and the screw nut seat when the bench vice is in the clamping position, Fig. 16 is a view of the second embodiment of the present invention, employed in a machine vice, shown here in a released position, Fig. 17 shows the vice of Fig. 16 in a clamping position, Figs. 18 to 21 are respective isometric views of the pawl pin, the eccentric cam, the ring extension spring and the screw nut seat of the vice of Figs. 16 and 17, Figs. 22 to 24 are respective sectional views on the planes DD, EE and FF in Fig. 16, Figs. 25 to 27 are respective sectional views on the planes D'D', E'E' and F'F' in Fig. 17, Fig. 28 is a view of the third embodiment of the present invention employed in a vice shown here in a released position, Fig. 29 shows the vice of Fig. 28 in a clamping position, Fig. 30 is an isometric view of the screw nut seat of Fig. 28.

Fig. 31 is a sectional view on the plane PP in Fig. 28, and 6 F g. 32 s a sect onal v ew on the plane PIP' n Fig. 29.

Figs. 2-15 show an embodiment of bench vice employing a screw drive mechanism according to the present invention, the vice including among its parts a stationary body 21 having a hollow portion; a movable body 22 positioned within said hollow portion of said stationary body 21, a pair of vice jaws 12,14, a handle 1, a screw 20, an eccentric cam 17, a screw nut seat 19, ratchet sleeve 9, a compression spring 8 and bearing rings 6,42. The movable body 22 can slide along the guide track in the stationary body 21 and the handle 1 mounted in a through hole on the left-hand end of the screw 20 (as seen in Figs. 2 and 3) is used to turn the screw to the left (N-direction) or right (M-direction).

The screw 20 is supported in support holes 7,23 on front and back vertical plates of the movable body 22, the holes 7,23 each having an elongated circular form with two parallel side walls 101, the width of the holes being substantially equal to the diameter of the screw to permit transverse movement of the screw only up and down vertically. The ring 6 is provided between an inner end face 60 on a head of the screw 20 and an outer end face of the front vertical plate of the body 22. Against the right-hand end of thread 25 of the screw 20, there bears one end of a cylindrical compression spring 44, its opposite end bearing through a backing ring 41 against the inner face of the back vertical plate of movable body 22.

7 The right-hand end of the screw 20 has the ring 42 and a stop collar 43 to retain the screw in the hole 23.

The screw 20 passes through an axial hole of the ratchet sleeve 9. The screw is rotatable with but is axially slidable relative to the ratchet sleeve by means of guide key 16 which fits.a keyway 46 in the sleeve and a similar keyway in the screw.

The screw nut seat 19 is in the form of saddle (see Fig. 4), which is fixed on the stationary body 21 by means of bolts 11. The two arms 40 on the seat 19 have aligned holes 38. The cross-sectional shape of each hole 38 is formed by two circular arcs, ie. the upper arc "a" and the lower arc "b" (see Fig. 5). The centre of circle of the upper arc "a" is 0 1 and the central angle a of the arc "a" is no more than 1800. The radius of upper arc "a" is r 1 which equals to the thread radius of the outer thread 25 on screw 20. The surfaces on the upper arcs "a" of the two holes have the inner threads 15 which can engage with the outer thread 25 of the screw 20. The centre of circle of the lower arc "b" is 0 2 which is beneath the centre 0 1 of the upper arc "a" and there is an eccentric distance "e" between the two centres 0 1 and 0 2 The distance "e" should be greater than the tooth depth of the threads 15,25, and the radius r 2 of the lower arc "b" should be greater than the thread radius of the outer thread on screw 20 in order to ensure that when the screw axis moves from position 0 1 to position 0 2P it does not foul any part of the surfaces of the holes 38, so that it can be freely displaceable in is 8 the axial direction (as shown in Fig. g).

The eccentric cam 17 is positioned between the ratchet sleeve 9 and one arm 40 of the screw nut seat. The curve for the cam comprises a downward stroke curve portion (with the lowest point 32) and an upward stroke curve portion (with the highest point 31) (see Fig. 7). In addition the cam 17 has also a positioning projection 52 and a planar positioning face 51. In left-hand rotation, when the positioning projection 52 touches a horizontal limit plane 56 of the movable body 22, the lowest point 32 on the cam curve will come against the supporting surface 24 of seat 19 so as to cause the cam 17 and the screw 20 to be situated in the fully released position 0 2 (see Fig.

11). Similarly (see Fig. 15) in right-hand rotation, when the positioning face 51 touches the side wall surface 55 on the movable body 22, the highest point 31 on upward stroke curve portion of the cam 17 will come against the cam support surface 24. Under the influence of the upward stroke curve of cam 17 therefore, the axis of screw 20 has then risen from position 0 2 to position Oir so that its outer thread 25 engages with the inner thread 15 on the seat 19 (see Fig. 13).

In assembly, it should be ensured that there is a gap 6 between the end face 60 on the screw head and the opposed end face 61 of the ring 6. The width of 6 is about the pitch of the screw 20. This gap is provided so that if the thread 25 of the screw is axially misaligned when it is brought against its mating thread 15 on the screw nut 9 seat, ie. when displaced from 0 2 to 0 1 the screw 20 has sufficient axial freedom to obtain engagement of the threads.

There is provided on an end surface 45 of cam 17 one (or more) one-way pawl grooves 53 (see Fig. 14) having a vertical surface 50 and an oblique surface 49 (see Fig. 7). The ratchet sleeve 9, which is connected to the screw 20 through the guide key 16, has a flange 57 which is provided with one (or more) one-way pawls 58 having a vertical surface 47 and an oblique surface 48 (see Figs. 6 and 10). Under the action of a compression spring 8 between one arm 40 of the seat and the ratchet sleeve 9, the ratchet sleeve is kept pressed against the cam 17.

The operation of fast clamping and release with the bench vice described will now be explained as follows. There are three steps in clamping a workpiece, of which the second and third steps are performed as a continuous stage and the time for all three steps is about one second; for reopening the vice jaws there are two steps, which are also performed as a continuous stage and they take about 0.5 seconds.

(1) The step for-free adjustment of the opening between the jaws of the bench vice.

With the lowest point 32 on the downward stroke curve of the cam 17 facing the cam supporting surface 24, the cam 17 is in a released position relative to the cam supporting surface 24 of the screw nut seat 19 (see Fig. 11). The ends of the screw 20 are supported on the respective lower supporting surfaces 36 and 37 of the supporting holes 7 and 23 on the front and back vertical plates of the movable body 22 (see Fig. 8). The positions of the supporting holes 7 and 23 ensure the central axis of the screw 20 to be at the centre 0 2 of the lower arc "b", and the outer thread 25 on the screw does not contact any part of the inner surfaces on the holes 38 of the seat 19 (see Fig. 9). Thus, the movable body 22 may be pushed or pulled axially to slide the movable body 22 together with the screw 20 along the guide track in the stationary body 21 to adjust quickly the opening S between the vice jaws according to the size of a workpiece. Depending on the size of workpiece 28, the movable body is pushed to a suitable position to bring the jaws 12,14 into contact with the workpiece 28.

(2) The step for engaging the outer thread of screw with the inner threads of nut seat.

Turning handle 1 in the right-hand or closing direction (shown as the arrow M in Fig. 3), the screw 20 is rotated and through the guide key 16 the ratchet sleeve 9 rotates with it. Initially, the pawl 58 of the ratchet sleeve 9 is in the pawl groove 53 of the cam 17, but the rotation of the ratchet sleeve 9 causes the oblique surface 48 of pawl 58 to bear against the oblique surface 49 of pawl groove 53 (see the position designed by the double dash line in Fig. 14), so that, due to the axial force of the compression spring 8, the thrust from the oblique surface48 of ratchet sleeve 9 onto the cam 17 also 11 gives the cam a right-hand rotation. That rotation slides the upward stroke curve of the cam over the cam supporting surface 24 of the screw nut seat 19 until blocked by the positioning face 51 of the cam contacting the limit surface 55 (see Fig. 15), and at the same time, the highest point 31 on upward stroke curve of the cam 17 just touches the supporting surface 24 of the seat 19. The screw 20 has now been lifted vertically along the parallel side walls 101 of holes 3,27 to its highest position, ie. the central axis of screw 20 rises vertically over the eccentric distance "e" from its original position 0 2-0 2 to a position 0 1_ 01. and the outer thread 25 of the screw engages with the threads 15 in the holes 38 of seat 19. The engagement should ensure a driving connection between the threads 15,25 (see Fig. 13).

(3) The step for clamping a workpiece (see Fig.

3).

Continuing to turn the handle 1 in the right-hand direction (M-direction), since the positioning face 51 of the cam 17 has touched the limit surface 55 of the movable body 22 and has stopped rotation of the cam 17 (see Fig,. 15), the moment of the ratchet sleeve 9 will transmit a force between oblique surfaces 48 and 49 having an axial component along the direction "H" greater than the pressure of spring 8, so that the ratchet sleeve 9 is displaced in the direction "H", until the pawl 58 leaves pawl groove 53 and slides over the end surface 45 of cam 17. Still continuing to turn the handle, since the outer thread 25 of the screw 12 screw 20 has engaged with the inner threads 15 of nut seat 19, which is fixed on the stationary body 21, the screw 20 moves forward in the axial direction 'W' while it turns.

Thus, the screw 20, through the end surface 60 on its head and the ring 6, pushes the movable body 22 in the closing direction until the jaws 12,14 clamp the workpiece 28.

(4) The step for releasing a workpiece.

After the workpiece has been processed and ús to be removed, turning the handle 1 in the left-hand direction (N-direction, see Fig. 2) rotates the screw 20 in that direction. Since the outer thread 25 of the screw is initially in engagement with the inner threads 15 of seat 19r the screw is displaced axially in the direction "H" while it turns to left and, through the ring 42 and stop collar 43, it pushes the movable body 22 in the direction 1'H11 so-as to cause the jaws to release the workpiece 28.

(5) The step for disengaging the outer thread of from the inner threads of seat (see Fig. 2).

In releasing the workpiece, ie. when the handle 1 is turned to the left, the ratchet sleeve 9 driven by the screw 20 and the guide key 16 also turns to the left by a certain angle until the pawl 58 comes into alignment with and falls into pawl groove 53 (see Fig. 10) under the pressure of the compression spring 8. The screw 20 continues the rotation of the ratchet sleeve and so begins to turn the cam 17 in the left-hand direction through the contact between the radial surfaces 47 and 50 on the pawl 58 and pawl groove 53 respectively, causing the lowest 13 point 32 on the downward stroke curve of the cam to turn gradually to its lowest position (see Fig. 11) until the positioning projection 52 of cam 17 engages with the horizontal limit plane 56. Thus, the central axis of screw 20 descends vertically from position 0 1 to position 0 21 dropping the distance "e", and so disengages the thread 25 of the screw wholly from the threads 15 of seat 19. The screw 20 is thereby lowered onto the lower supporting surfaces 36,37 of the holes 7,23 on the front and back vertical plates of the movable body 22 (see Fig. 8), whereupon the screw 20 may be moved forwards or backwards freely with the movable body 22 so that the opening of vice jaws 12,14 can be quickly adjusted as described in the step 1 above.

Figs. 16-27 show the second embodiment according to the present invention, in which a vice for the table of a machine tool comprises a stationary body 63, a movable body 64, a screw 20, an eccentric cam 65, a screw nut seat 68, a one-way pawl pin 73, a ring extension spring 76, a positioning pin 74. In this embodiment, the one-way pawl device employs the pawl pin 73 instead of the ratchet sleeve 9 in first embodiment. The movable body 64 can slide along the guide track in the stationary body 63, the screw 20 passing through the holes in the eccentric cam 65 and the screw nut seat 68.

The screw nut seat 68 is in the form of saddle with an upper top portion 88 (see Fig. 21), and is fixed on the stationary body 63 by means of bolts 75. The holes of 14 screw nut,seat 68 each have an elongated circular form with two parallel side walls 87, an upper arc portion 86, and a lower arc portion provided with the inner thread 15 engagable with the outer thread 25 of the screw 20. The width of the elongate-circular holes is substantially equal to the diameter of the screw 20.

The eccentric cam 65 is located within the screw nut seat 68, curve portion 92 of the cam being circular and able to slide between upper cam support surface 94 and lower cam support surface 95 of nut seat 68. The eccentric cam 65 is provided on its plane face 89 with a radial hole 67 and a threaded hole 90. It is also provided with a groove 77 for receiving the ring extension spring 76, said groove surrounding the outer circumference of cam 65 and is passing through the central axis of the radial hole 67 (see Fig. 19). The positioning pin 74 is secured in the hole 90 of cam 65 by a threaded engagement therewith.

The pawl pin 73 is radially slidable in the hole 67 of the cam 65 and is provided with a one-way pawl formed by a radial surface 79 and an oblique surface 78. The pawl pin is further provided with a hole 80 through which the ring extension spring 76 passes. The screw 20 is provided with an axially extending one-way pawl groove 81 formed by a radial surface 83 and an oblique surface 82 (see Figs. 24 and 27). The front end plate of movable body 64 is provided with two holes in which springs 71 are located to ensure that there is a gap between the front end wall-and bearing ring 70, and allow the screw 20 to have a little axial movement to ensure that a suitable threaded engagement can be achieved with the screw nut seat. A further bearing ring 93 is located against the inner face of the front end wall of the movable body and prevents the screw 20 from sliding out.

When releasing a workpiece (see Fig. 16), as the screw 20 is turned in the left-hand direction (N-direction), the pawl pin 73 falls into the pawl groove 81 of screw 20 under the action of ring extension spring 76. The screw 20 then drives the eccentric cam 65 through the two contacting radial surfaces 83 and 79 on the pawl groove 81 and the pawl 73 respectively (see Fig. 24). Thus, the eccentric cam 65 between the limits of two cam support surfaces 94 and 95 rotates to cause the screw 20 to rise vertically along two parallel side walls 87 of the holes of the nut seat 68, until the head of the positioning pin 74 contacts limit surface 84 of nut seat 68 (see Fig. 23). The axis of screw 20 has now risen from its lowest position 0 2_ 0 2 to its highest position 0 l_01. and the thread 25 of the screw is disengaged from the threads 15 of nut seat 68 (see Fig. 22). The movable body 64 may thus be pushed or pulled axially to adjust the opening "S" of the vice jaws quickly (see Fig. 16).

When clamping a workpiece, the screw 20 is turned in the right-hand direction (M-direction) to bring the oblique surface 82 of the pawl groove 81 of screw against the oblique surface 78 of pawl pin 73 (see Fig. 27), where they are held by the action of the ring extension spring 16 76, and thus to rotate the eccentric cam 65 until the head of the positioning pin 74 contacts with the upper limit surface 85 of the nut seat 68 (see Fig. 26). At the same time, the rotation of eccentric cam 65 causes the screw 20 to descend along two parallel walls 87 of the elongated circular holes of the nut seat 68, with the axis of the screw falling from its highest position 0 1-0 1 to its lowest position 0 2-0 2 Thus the thread 25 of screw 20 engages with the threads 15 of nut seat 68 (see Fig. 25).

Continuing to turn the screw 20, the oblique surface 78 of pawl pin 73 will slide along the oblique surface 82 of pawl groove 81 of the screw when the force component acting on two oblique surfaces is greater than the holding force of the ring spring 76, until the pawl pin has risen out of the pawl groove (see Fig. 27). Since the outer thread 25 of screw 20 has engaged with the inner threads 15 of nut seat 68 with the seat 68 is fixed on the stationary body 66, continuing right-hand rotation of the screw 20 will move the screw forward in the axial direction 'W' and will apply pressure to the movable body 64 through the bearing ring 70 to clamp the workpiece.

Figs. 28-32 show a third embodiment according to the present invention wherein the structures of the pawl pin, the eccentric cam and the nut seat are basically identical-to those of the second embodiment, but the nut seat-68 is provided on its bottom surface with a guide pillar 96 which can slide vertically in a guide hole 97 on the base 100 of stationary body 63. The screw 20, 17 supported in holes in opposite end walls of the movable body 64 is not displaceable up and down and can only turn in those holes, while the nut seat 60 can move vertically, under the action of eccentric cam 65, in order to engage or disengage the threads 25,15 of the screw 20 and the nut seat 68.

When clamping a workpiece, the screw is rotated in the right-hand direction (M-direction) and drives the eccentric cam 65 through the pawl groove and pawl pin 73.

The eccentric cam 65 is thereby rotated while it is held between the two surfaces 94 and 95 of the nut seat. and causes the nut seat 68 to rise vertically over the distance "h", because the screw 20 is fixed in the radial direction, (see Figs. 29 and 32). Thus the threads 15 of nut seat 68 are engaged with the thread 25 of screw 20. Continuing to turn the screw until the pawl pin 73 slides from the pawl groove of the screw 20, allows the screw 20 to move in the axial direction "K" as it rotates and so apply pressure to the movable body 64 through the bearing ring 70 to clamp the workpiece (see Fig. 29).

When releasing a workpiece, left-hand rotation of the screw 20 (Ndirection) drives the eccentric cam 65 through contact between the radial surfaces 83 and 79 respectively on the pawl groove 81 and the pawl pin 73.

The cam 65 thus rotates to cause the nut seat 68 to descend relative to the radially fixed screw 20, so that the thread 25 of the screw disengages from the threads of nut seat, and the movable body can then move freely in the axial 18 direction (see Fig. 28).

The mechanism may have a variety of modifications, which should be considered with the scope of the present invention.

1.

19

Claims

1. A screw drive mechanism comprising a stationary body, a movable body, a screw, a screw nut seat having two arms between which an eccentric cam is.positioned, said screw passing through holes of said nut seat and said cam, said nut seat having at least one thread for engagement with the screw thread to displace said movable body with rotation of the screw, said cam being slidable on cam support surfaces of said nut seat, said screw being arranged to drive said eccentric cam through a pawl device to cause a transverse relative movement between said screw and said screw nut seat, whereby the thread of said screw is engaged with or disengaged from said thread of the screw nut seat.

2. A screw drive mechanism according to claim 1 wherein said nut seat thread is provided in at least one hole of elongated circular shape including a pair of vertical side walls between which the width of the hole is at least equal to the diameter of said screw, a first end arc portion of said elongated circular shape being provided by a wall having said threads for engagement with the thread of said screw with the co-operation of an upper cam support surface of the nut seat with the eccentric cam.

3. A screw drive mechanism according to claim 2 wherein said at least one hole has a further end arc portion opposite the first end arc portion said further portion having a centre spaced below the centre of the first portion by a distance greater than the depth of the screw thread, and the radius of said further portion being 5 greater than the outer radius of the screw thread.

4. A screw drive mechanism according to any one of claims 1 to 3 wherein the eccentric cam is arranged to displace the screw transversely to its axis to engage it with and to disengage from said nut seat thread.

5. A screw drive mechanism according to claim 4 wherein said pawl device comprises a ratchet sleeve, which is positioned between one arm of said screw nut seat and an end surface of said eccentric cam and which is keyed to the screw for axial movement relative thereto, opposed end faces of said sleeve and said cam carrying cooperating elements of said pawl device that are releasably engagable to transmit a rotation of the screw, spring means urging said pawl device elements axially together.

6. A screw drive mechanism according to claim 5 wherein at least one of the pawl device elements comprises respective engagement surfaces substantially radial to the axis of rotation and oblique to said axis.

7. The screw drive mechanism according to claim 5 or claim 6 wherein said eccentric cam has a positioning 1 21 projection engageable with limit means on the stationary body to determine an end position of said cam at which it has caused said relative movement of said screw to bring the threads of said screw and nut seat out of engagement, said cam also having a positioning face that is engageable with a limiting side face of said movable body with the engagement of the screw thread with the thread or threads of said nut seat, when a portion said cam is brought into contact with a cam support surface on said screw nut seat.

8. A screw drive mechanism according to any one of claims 4 to 7 wherein said screw nut seat is in the form of a saddle axially fixed on the stationary body, the saddle having two arms with respective concentric axial holes.

9. A screw drive mechanism according to any one of the preceding claims wherein opposite end portions of the screw are supported in respective holes of elongated circular shape in said movable body, each said hole having a pjair of parallel side walls between which the width of the hole is at least equal to the diameter of said screw, to permit movement of the screw move in said holes for said movement relative to the screw nut seat.

10. A screw drive mechanism according to any one of claims 1 to 3 wherein said pawl device comprises a radial pawl pin positioned within a radial hole in said eccentric cam, the pawl pin having respective radial and oblique 22 engagement surfaces, and resilient means urging said surfaces towards a further member of the pawl device for releasable engagement therewith.

11. A screw drive mechanism according to claim 10 wherein said resilient means is in the form of a ring spring extending around the cam and through a hole in said pawl pin.

12. A screw drive mechanism according to claim 10 or claim 11 wherein the screw is provided a groove extending axially along it forming said member of the pawl device engageable with the pawl pin, said groove having surfaces complementary to said surfaces of the pawl pin.

13. A screw drive mechanism according to any one of claims 10 to 12 wherein the cam has a circular curved portion eccentric to a bore for receiving the screw, and a positioning pin is fixed on said eccentric cam to project therefrom to set at least one end limit position of the cam.

14. A screw drive mechanism according to any one of 20 claims 1 to 3 or any one of claims 9 to 13 where dependent on any one of claims 1 to 3, wherein said screw is fixed in the radial direction and said eccentric cam causes said nut seat to move transversely relative to the screw axis for said engagement and disengagement between the threads of 23 said screw and nut seat.

15. A screw drive mechanism according to claim 14 wherein the screw nut seat is provided with a guide pillar slidable in a hole in said stationary body for guiding movement of the nut seat for said engagement and disengagement of the screw thread.

16. A screw drive mechanism comprising stationary and movable bodies, a drive screw and a threaded seating engageable with the thread of the drive screw for displacement of said movable body by rotation of the screw, an eccentric held captive by said seating axially of the screw for relative movement between the screw and the seating transversely of the screw axis, and pawl means cooperating with said eccentric for moving the screw and seating threads into engagement with one direction of rotation of screw relative to the seating and for moving said threads out of engagement with the other direction of said rotation.

17. A screw drive mechanism according to any one of the preceding claims wherein said transverse movement takes place in a substantially vertical direction.

18. A screw drive mechanism constructed and arranged for use substantially as described herein with reference to the accompanying drawings. - 24

19. A vice having a screw drive mechanism according to any one of the preceding claims.

Published 1990 at The Patent Office, State House, 6671 High Holborn, LondonWC1R4TP.Further copies maybe Obtainedfrom The Patent Office. Sales Branch. St Mary Cray. Orpingtor- Nei.t BRS 3FX. Pr,rEed by Mlilliplex techniques ltc. St Mary Cray. Kent. Con- 1'87