US 3919513 A
A bolted contact knife switch for high current applications has a two-phase cam-screw clamping and unclamping mechanism. In one arrangement, a nut threaded on the contact bolt has a cammed face that mates with a cammed face on a pressure plate fixed to one of a pair of parallel switch blades. The cammed faces have pocket, ramp, and flat portions in annular sequence. The switch closing motion rotates the nut causing the ramp portions on the cammed nut face to slide over opposed ramp portions on the pressure plate cammed face until the flat portions of each face bear against one another. This camming action quickly displaces the switch blades from an initial wide blade separation into contact with an interposed terminal jamb. Continued rotation of the nut with flat cam faces engaged achieves a high final clamping pressure by screw thread action alone. The final clamping pressure and the initial blade separation are independently adjustable.
Claims available in
Description (OCR text may contain errors)
United States Patent 11 1 Phillips 1 Nov. 11, 1975 1 1 CLAMPING MECHANISM FOR BOLTED CONTACT SWITCH  Inventor: Bernard L. Phillips, Norwood.
 Assignee: Arrow-Hart. Inc.. Hartford. Conn.
122] Filed: Mar. 18. 1974  Appl. No: 451.837
 U.S. Cl. 200/256; 151/34; 85/32: 85/50 R  Int. Cl. HOlH 1/50 [58 Field Of Search 200/254. 255. 256. 162; 151/34. 35. 19 A; 85/32, 50 R  References Cited UNITED STATES PATENTS 2.156.024 4/1939 OBrian 200/169 2.960.590 1 H1960 Kussmaul et al..,. 200/256 3.180.963 4/1965 Hales et a1. 200/256 3.183.335 5/1965 Cobbett 200/254 3.458.680 7/1969 Kussmaul 200/256 3.646.299 2/1972 Stene 200/162 Primary E.wmi11erRobert K. Schaefcr Assistant EXCUHI IICI' JOhlT W. Redman lliurne). Agem. 01' Firmlenway & Jenn)  ABSTRACT A bolted contact knife switch for high current applications has a two-phase cam-screw clamping and unclamping mechanism. ln one arrangement. a nut threaded on the contact bolt has a cammed face that mates with a cammed face on a pressure plate fixed to one of a pair of parallel switch blades. The cammed faces have pocket. ramp. and fiat portions in annular sequence. The switch closing motion rotates the nut causing the ramp portions on the cammed nut face to slide over opposed ramp portions on the pressure plate cammed face until the flat portions of each face bear against one another. This camming action quickly displaces the switch blades from an initial wide blade separation into contact with an interposed terminal jamb. Continued rotation of the nut with flat cam faces engaged achieves a high final clamping pressure by screw thread action alone. The final clamping pressure and the initial blade separation are independently adjustable.
15 Claims, 6 Drawing Figures US. Patent N0v.11, 1975 Sheet1of4 3,919,513
US. Patent Nov. 11, 1975 Sheet 2 of4 3,919,513
US. Patent Nov. 11, 1975 Sheet30f4 3,919,513
l FLQOLJOL US. Patent Nov. 11,1975 Sheet4of4 3,919,513
Allllllll A CLAMPING MECHANISM FOR BOLTED CONTACT SWITCH BACKGROUND OF THE INVENTION This invention relates in general to manual and power operated bolted contact knife switches characterized by a high current carrying capacity and the provisions of bolt means to apply pressure at one or both ends thereof when the switch is in the closed position. This pressure serves to reduce contact resistance and to resist electromagnetic forces generated by high cur rents that may produce an unintended opening of the switch.
This invention relates more specifically to clamping means for applying contact pressure at the ends of such bolted contact switches. U.S. Pat. No. 2,960,590, dated Nov. 15, 1960, to Kussmaul et al. discloses a bolted contact switch with pressure bolting means in which tension is applied to a bolt by relative rotation of two members threaded together, which members are coaxial with the bolt. One of the members is a pressure plate or hub having a set screw to prevent the member from rotating in relation to the blade. The'other member is rotated by a collapsible triangle linkage. The blade and jamb are thus forced together.
U.S. Pat. No. 3,458,680, dated July 29, 1969, to Kimsmaul, discloses means for adjusting the contact pressure in bolted contact switches of the type described. The pressure plate contains a series of holes in a flanged portion that bears on the switch blade. A locking pin simultaneously engages one of these holes and another hole in the switch blade. An adjustment in the lever of contact pressure is made by removing the pin, rotating the pressure plate, and reinserting the pin to secure the adjustment.
These and similar clamping devices have a common problem in that a small rotational movement must be translated into a substantial linear movement along the axis of rotation. The conventional clamping devices meet this problem by employing contact bolts having multiple or coarse threads. This arrangement has numerous disadvantages. First, even a coarsely threaded bolt achieves only a limited linear movement during the clamping rotation. As a result, the switch blades must be spaced close to the terminals at both the pivot and jamb ends of the switch at the start of the clamping rotation in order to develop sufficient clamping pressure at the end of clamping rotation. The closeness of these spacings requires that the blades be machined to a high degree of flatness and that they be aligned with a high degree of accuracy in order to avoid excessive frictioned wear. These requirements are difficult, time consuming, and costly to attain, particularly when the blade dimensions are large to accommodate extremely high currents.
Second, an extremely coarse thread offers a relatively low mechanical advantage and therefore requires a high degree of torque toachieve the desired level of clamping pressure. Third, coarse threads have much higher manufacturing costs than more common threads. Finally, in the switches known in the prior art, there are no means for adjusting the initial blade sepa-' ration between a pair of blades that engage an interposed jamb terminal side without changing the clamping pressure.
Accordingly, a principal object of this invention is to provide a clamping mechanism yielding a high final clamping force with an initial wide blade separation.
It is another object of this invention to provide a clamping mechanism which has a rapid lateral displacement, low-torque operation.
Still another object is to provide a clamping mechanism having convenient, independent adjustment of both the final clamping force andthe initial blade openmg.
A further object is to provide a clamping mechanism that permits a high degree of blade misalignment due to warping or other causes without sacrificing ahigh final clamping pressure and low contact resistance.
A still further object is to provide a clamping mechanism that has excellent wear characteristics and isxadjustable to compensate for wear.
Yet a further object is to provide a clamping mechanism requiring a small angular travel to achieve a high clamping force.
SUMMARY OF THE INVENTION The features of this invention, which can be adapted to a number of different structural environments, but are described with reference to a knife switch, include a clamping nut threaded on a clamping bolt, one face of the nut or the head of the bolt having formed thereon an annular cammed surface that mates with a corresponding annular cammed surface on a face of a flanged pressure plate bearing on a switch blade. The cammed surfaces each consist of at least one, and preferably more, segments, each segment having pocket, ramp and flat portions in annular sequence. Once the switch is pivoted to a closed position, further closing action collapses a triangle linkage, which causes the relative rotation of the bolt and nut and of the cammed surfaces.
In a cam phase of the rotating clamping action, rampto-ramp contact of the annular cams, in combination with the screw action of the threads, results in a large lateral displacement of the pressure plate relative to the nut or bolt head that quickly brings the switch blades from an initial wide separation into contact with an interposed terminal side such as the blade of a jamb. With the camming action completed, flat-to-flat contact of the annular cams is attained and the continued screw action of the nut rotating on the bolt threads generates a high final clamping pressure through action of the screw threads alone. This constitutes the screw phase of the clamping action. Since the cam action effects a large lateral movement of the blades, the bolt threads can have a relatively fine pitch. In both the cam and screw phases, the clamping action requires a relatively low torque. since low mechanical resistance exists during the cam phase and high mechanical advantage exists during the screw phase when mechanical resistance is high.
Another significant feature of this invention is that the final clamping pressure and the initial blade separation are conveniently and independently adjustable. The final clamping pressure is adjusted by rotating and locking the clamping bolt. The initial blade separation is independently adjusted by rotating and locking the cammed pressure plate.
These and other features of this invention will become more fully evident from the following detailed description of a preferred embodiment to be read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view of a bolted contact switch embodying the invention;
FIG. 2 is a side elevation corresponding to FIG. 1;
FIG. 3 is a side elevation similar to FIG. 2, showing the blades in the open switch position;
FIG. 4 is a detailed plan view of the novel cam-screw clamping mechanism;
FIG. 5 is a developed and enlarged view of the annular cammed surfaces of FIG. 4; and
FIG. 6 is a detailed plan view of an alternative embodiment of the cam-screw clamping mechanism.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrated and preferred embodiment of this in vention has a pair of fixed hinge terminal sides 12a and 12b and pair of fixed jamb terminal sides 14a and 14b. The entire structure as shown carries the current of one conductor in a single or multiphase circuit. These terminals are formed of sheet metal of sufficient thickness to carry a high current load. For example, the terminals may be three-quarters inch thick copper. The terminal sides are mounted in a parallel spaced relationship on a pair of support castings 16a and 16b that can be machined from a suitable material such as manganese bronze. Mounting screws 18 fasten the terminal sides to upright flanged portions 20 of the support castings. The supports 16a and 16b can be mounted on a suitable insulating base 22 by screws or other suitable means not shown. Metal spacers 24 and a metal cam support spacer 26 are held by screws 28 to support the terminal pairs 12a and 12b and 14a and 14b in the alignment shown. The terminal sides contain a plurality of holes 30 that provide means for attachment of other circuit parts such as busses or fuses.
The switch has two pairs of identical pivotal blades, each pair having an outer blade 32 and an inner blade 32a, the blades being of a generally rectangular shape formed from sheet metal of suitable thickness for the desired current carrying capacity. In the illustrated em bodiment the blades 32 and 32a are one-half inch thick copper with a silver plate. All of the edges are chamfered and the corners 34 at the pivot or hinge end are cut away for clearance. A pair of blades 32 and 32a is mounted with a portion of a hinge terminal 120 or 12b sandwiched between their hinge ends and a portion of a jamb terminal 140 or 14b sandwiched between their jaw ends. All of the blades are in a mutually parallel spaced relationship. The blades are pivoted on bolts 33a and 33b in the hinge terminals.
Means for mounting the blades 32 and 32a comprise a triangle linkage operating mechanism and a camscrew clamping mechanism. The legs of the triangle linkage include respectively left and right hand threaded, slotted hinge arms 36b and 36a, respectively left and right hand threaded wrench arms 38a and 38b, and the blades 32 and 32a. This triangular relationship can be seen most clearly in FIG. 3 showing the switch in the open position. The hinge arms 36a and 36b and wrench arms 38a and 38b are linked by means of a wrench arm pin 40 secured between the hinge arms 36a and 36b by snap-on washers 42. The pin 40 passes freely through aligned closed slots 44 in the hinge arms 36a and 36b and through holes in the wrench arms 38a and 38b. Bearings 46 on the pin 40 ride in the slots 44. Washers 48 are provided between the bearings 46 and the wrench arms 38a and 38b. The slots 44 allow the triangle linkage to collapse as shown in FIGS. 1 and 2 when the switch is in the closed position.
A cam 49, acting in cooperation with a cam follower 50, prevents the triangle from collapsing until the switch is nearly in the closed position. The cam 49 is located at the hinge end of the switch with an upper camming surface equidistant between and planar parallel to the hinge terminal sides 12a and 12b. A flanged bottom portion of the cam rests on the supporting casting 16a and a flanged rear portion 49b is attached to the cam support spacer 26 by screws and nuts 52 and the interposed rectangular spacer 54. In the illustrated embodiment, the cam is formed from a manganese bronze casting. The cam follower 50 and cam follower side plates 56 are held between the ends of the wrench arms 38a and 38b by cam follower pins 58. When the switch is open, the bottom concave edge of the follower 50 rides the top, convex edge 60 of the cam 49 thereby restraining the collapse of the triangle, that is the downward motion of the arms 36a, 36b and 38a, 38b. When the switch is fully pivoted to the closed position, the convex edge of the follower 50 rides the concave edge 62 of the cam 49, thereby allowing the triangle to collapse into the closed position. During the collapse, the wrench arm pin 40 and the bearing 46 slide in the slot 44, and both the hinge arms 36a and 36b and the wrench arms 38a and 38b rotate about their links with the switch blades 32 and 32a. This rotational motion supplies the clamping force, as described below.
The clamping mechanism of this invention as shown particularly in FIG. 4, includes a clamping bolt 64b, a nut 66, a flanged pressure plate 68, and mated annular camming surfaces and 72 formed in opposed faces of the nut 66 and plate 68, respectively. The bolt 64b passes freely through aligned holes in the pair of blades 32 and 32a, a slot 74 in the terminal side 14b sandwiched between the blades, and the pressure plate 68. The head of the bolt 64b bears inwardly against the blade 32 and the pressure plate bears in the opposite direction against the other paired blade 32a. Both the bolt and the plate are restrained from rotating with respect to the blades by set screws 76. L-shaped brackets 78 hold the set screws 76 in an arm of the bracket overhanging the faces of the bolt head and the pressure plate 68. Bracket mounting screws 80 longitudinally penetrate the other arms of the brackets 78 and thread into the blade. The nut 66 is threaded on the shank of the bolt 64b so that one of its faces 70 bears against an opposing face 72 of the pressure plate. The nut 66 is also connected to the end of the wrench arm 38b.
As illustrated in FIG. 5, the camrned nut face 70 is formed with a series of storage portions 82, ramp portions 84 and flat portions 86. The cammed plate face 72 contains corresponding mating storage portions 820 ramp portions 84a and flat portions 86a. These faces are free to rotate over one another. The storage portions 82 of the nut face 70 all lie approximately in a common plane, but may be of any shape which provides clearance for the flats on the mating cam. Similarly, the flat portions 86 all lie in a common plane perpendicular to the bolt axis. The ramp portions 84 are radially flat, inclined surfaces extending between successive storage and flat portions. A corresponding geometry applies to the portions 82a, 84a, and 86a. The portions 86 and 86a are the only cam portions that require machining to a high degree of flatness since the cammed faces 70 and 72 bear a high pressure only when the flat portions 86 are in contact with the opposed flat portions 86a. The low ramp-to-ramp mating pressure has the additional advantage of reducing the wear on these surfaces.
The downward angular motion of the wrench arm 38b during the collapse of the triangle linkages generates the clamping force. This rotational motion is transmitted to the nut 66 which converts the motion into a linear motion along the axis of rotation through a combination of a screw action of the nut 66 on the bolt threads and a cam action between the faces 70 and 72. These faces and more specifically the ramp portions 84 and 84a are initially in contact (FIG. 5). As the nut rotates, the ramp portions 84 in the nut face slide over the ramp portions 84a in the plate face until the flat portions 86 are in contact with the flat portions 86a. The result of this camming action is that a small angular movement shifts the pressure plate a relatively large distance along the bolt. In turn, the blades 32 and 32a are displaced the same distance since they are caught between the bolt head and the pressure plate. This high degree of lateral movement permits a wide initial blade separation without sacrificing clamping pressure. It should be noted that during the camming action, some lateral motion is also attributable to the screw action. In the illustrated embodiment, each cam portion such as 82, 84 or 86 spans a arc and the slope of the ramp is approximately 0.003 inch per degree of arc. Therefore, 20 of rotational motion by the wrench arm 38b and the nut 66 can achieve a maximum linear motion of 0.06 inch. The achievement of a comparable result with a conventional threaded clamping mechanism alone would require that the clamping bolt have one thread per inch.
Once the camming action is complete and the flat portions of the faces 70 and 72 are in contact, further clamping pressure is provided by the screw action alone. If a relatively fine pitch thread is used, its mechanical advantage allows a high final clamping pressure with a low torque. A pitch of seven threads per inch yields good results and is less expensive to manufacture than coarser pitches. The low slope of the ramps 84 and 840 also yield a good mechanical advantage combined with high speed operation during the camming phase of the clamping.
Although the clamping mechanism is described above with reference to the jaw terminal 14b, the same mechanism operates to simultaneously clamp the blades 32 and 32a to the jaw terminal 14a, and at the pivotal end of the switch, to the hinge terminals 12a and 12b. Terminals 12a and 14b, however, employ right-hand threaded bolts 33a and 64b, whereas terminals 12b and 14a employ left-hand threaded bolts 33b and 64a. Since the wrench arms 38a and 38b in the preferred embodiment are located between the hinge arms 36a and 36b, these arms attach to the nuts 66 at different locations on the nut (FIG. 1), and the locations of the slots 44 and pin 40 are reversed from their locations in prior art switches of this type. This reversal of locations results in a higher clamping pressure with a lower maximum torque and a more uniform operating torque.
In the full sequence of operation, the blades are closed from the open position (FIG. 3) by pivoting them about the contact bolts 33a and 33b at the hinge end. The switch is closed when the bolts 64a, 64!) at the jaw end are fully seated in the slots 74 (FIG. 2). The top edges of the slots 74 are rounded and highly chamfered to provide a smooth transition as the blades approach the jaw terminal sides 14a and 14b. With the switch closed, the collapse of the triangle linkage causes the camming action to force the blades together in firm contact with the interposed terminal sides. In the final screw phase, the clamping pressure is rapidly increased by the screw action. Opening the switch is the reverse of these cycles. The clamping pressure is reduced by the nut unthreading; the blades separate, through the camming action; and finally, the blades pivot clear of the jaw terminals.
These cycles are operated either manually or mechanically. In either case, the operating force is applied to the ends of the hinge arms 36a and 36b at a point beyond the slots 44. In order to have a single handle or operating arm act on both hinge arms, an arched hinge arm link 88, mounted to the arms by shoulder screws 90 and cap screws 91, joins the arms. The handle is then attached to the link between the arms by suitable screw means, both of which are not shown to increase the clarity of the drawings. Suitable operating means are described in US. Pat. No. 3,244,827, dated Apr. 5, 1966, to Kussmaul.
A significant advantage of the clamping mechanism of this invention is that it allows convenient, independent adjustment of both the final clamping force and the initial blade opening. With reference to FIGS. 4 and 5, adjustment of the final clamping pressure is made by rotating the clamping bolt 64b when the cam flats 86 and 86a are in contact. With the cam faces in this position, the pressure is dependent solely on the screw action. Rotation of the bolt threads the nut until the desired degree of pressure is reached. The bolt is then secured by a set screw 76 being tightened against the head of the bolt.
To adjust the initial or maximum blade separation, the blades 32 and 32a are fully unclamped. Then, the rotation of the pressure plate 68 against the blade 320 changes the relative position of the cam portions and therefore the initial blade separation. For this adjustment, either the ramp portions 84 and 84a are initially in contact as shown in FIG. 5, or the cam surfaces 70 and 72 are initially in full contact (FIG. 4), the blades being then at the maximum separation. A narrower separation is achieved by rotating the plate 68 so that the cams are in partial contact (FIG. 5). When the desired degree of blade opening is reached, the plate 68 is secured by another set screw 76. This adjustment is independent of the clamping pressure adjustment since the bolt position is not changed and since the flats are a surface of revolution about the bolt axis. In addition, these adjustments provide a novel means for compensating for wear in the cam portions of the threads.
It should also be noted that the adjustment at each bolt is independent. For example, the bolts 33a and 33b at the pivot end can be adjusted for a close initial separation to provide good blade guidance and to reduce dirt contamination between the blades 32 and 32a and the hinge terminal sides 12a and 12b. At the same time the bolts 64a and 64b at the jamb end can be adjusted for wide initial blade separation to allow for blade warping and misalignment.
To facilitate these adjustments, the bolts 33a, 33b, 64a and 64b each have four spanner holes 92 drilled through the bolt head in a direction parallel to the shank, the spanner holes being adapted to receive a spanner wrench. Likewise each pressure plate 68 has six radial holes 94 in its edge, these holes receiving a suitable adjusting tool. Also, a spring 96 located be tween paired blades 32 and 32a urges them to their initial separation as the clamping pressure is released. This has the advantage of reducing the friction between the blades and the terminations.
In the illustrated embodiment, the rotational movement of the wrench arm during the clamping cycle is approximately 30. The cammed faces have six segments, each spanning 60, that are further divided into storage, ramp and flat portions each spanning 20. Thus the clamping mechanism can be adjusted to vary from 20 of ramp travel and 10 of flat travel, to 10 of ramp travel and of flat travel. It is possible, however, to achieve a wide variety of operating characteristics by using a different number of cam segments, cam portions of different angular length, or flat portions that are surfaces of revolution or are slightly ramped. The illustrated arrangement, however, has four principal advantages. First the slope of the ramps (0.003 inch/) provides a wide maximum blade separation. Second, it reduces the wear on the cams since (i) the moving parts are in area contact except during the transition from ramp contact to flat contact when they are in line contact (ii) the pressure between the cam faces is low during the camming phase of the clamping operation. Third, the fine pitch screw threads give a high mechanical advantage and a resultant low torque necessary for the clamping. Fourth, the flat portions are easily machined both initially and after use to compensate for wear.
The switch operating characteristics can also be varied by changing the configuration of the collapsible triangle linkage from that of an isosceles triangle to that of a scalene triangle. With the latter configuration, different amounts of angular motion will be applied to the pivot and jamb ends as the linkage collapses.
The wide blade opening provided by this invention is highly advantageous since it permits the blades to have an unusually high degree of misalignment, warp, or other variations from flatness without affecting the performance of the switch. All of these conditions when present in other switches increase the frictional wear on the blades as they contact the terminals and increase the level of torque required to close and clamp the switch. A wide blade opening also eliminates the cost, time consumption and difficulty of machining the blades to an extremely high degree of flatness. In addition, machining copper blade surfaces removes portions of the work hardened skin and may result in excessive or nonuniform wear.
All of these blade problems are accentuated when a switch is designed for an extremely high current capacity. High currents, in the range of 30,000 to 66,000 amperes, require that the switch blades have a large crosssectional area. This requirement in turn requires a longer and larger blade in order to give adequate clear ance when the switch is open and a large contact area when the switch is closed. Blade alignment and manufacturing problems increase disproportionately with the blade size. Therefore the alignment and adjustment advantages of this invention, describe above, are essen tial in producing a reliable, low cost switch for extremely high current applications FIG. 6 illustrates an alternative embodiment of this invention in which the position of the bolt and nut are reversed. In this arrangement, a bolt 98, corresponding to the bolt 64b, has a head 100 that corresponds in shape and location to the nut 66 shown in FIG. 4. A nut 102, corresponding in shape and location to the bolt head shown in FIG. 4, is threaded on the shank of the bolt 100. In the cam phase of the operation, rotation of the wrench arm 38b rotates the attached bolt head 100 causing the cam surface 70 formed on the inwardly directed face of the head to rotate over the mating cam surface 72 of the pressure plate 68. In the screw phase, the flat portions of the surfaces 70 and 72 are in contact and further rotation of the bolt head 100 causes the bolt 98 to thread in the nut 102 which is secured against rotation by the set screw 76.'The pres sure plate 68 is also secured against rotation relative to the switch blade 32 by another set screw 76..
Although this invention has been described in the context of a double pole switch, it is readily adaptable to a single pole switch and multi-pole switches with one or more throws. In particular, two or more switches of the type described herein may be placed in a juxtaposed parallel alignment. The resulting four pole switch can then be operated with a single operating handle if a common yoke or bar is connected across the arm links 88 of the component switches.
Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.
Having thus described my invention, what is claimed is:
l. Means for clamping together a plurality of members, comprising, in combination,
a bolt having a head on one side of the members and a threaded shank on the other side thereof,
a nut threaded on the shank to form a bolt and nut pair,
a pressure plate situated between said members and one of the inwardly directed faces of the bolt and nut pair, said plate having a first cam surface engaging a second cam surface formed on said inwardly directed face, and
means to rotate the bolt and nut relative to one another and simultaneously to rotate the first and second cam surfaces relative to one another.
2. Means according to claim 1 in which each of the cam surfaces further comprises at least one segment having storage, ramp and flat portions in annular sequence.
3. Means according to claim 2 in which said flat portions comprise a surface of revolution.
4. Means according to claim 1 in which the pressure plate and one element of the bolt and nut pair are secured against rotation relative to the members.
5. Means according to claim 2 in which the sense of the pitch on the nut is opposite to the slope in the axial direction of the first cam surface.
6. A switch comprising, in combination,
blade means including a pair of conductive members with facing surfaces thereof relatively movable in a direction transverse thereto for making currentcarrying contact with a fixed conductive memben forming a terminal and a first cam surface portion facing in said direction outwardly from said pair of members and being restrained from rotation relative thereto,
bolt means including a bolt portion having an axis extending in said direction, a nut portion threaded on the bolt portion and a second cam surface portion in rotatably slidable contact with said first cam surface portion, said bolt and nut portions being adapted upon relative rotation to vary the pressure of said contact between said pair of members. one of said bolt and nut portions being restrained from rotation about said axis and the other being restrained frorn rotation relative to said second cam surface portion, and
clamping means for rotating about said axis said other of said bolt and nut portions.
7. A switch according to claim 6, in which at least one of said cam surface portions has an annular path about said axis, said path including a ramp portion having a first slope in said direction and a second portion contiguous with said ramp portion and having a second slope substantially lower than said first slope in said direction, the other of said cam surface portions being adapted to contact said ramp and second portions consecutively upon rotation relative thereto.
8. A switch according to claim 7, in which said second portion lies substantially in a plane normal to said axis.
9. A switch according to claim 7, in which the clamping. means are adapted for rotation about a first angle during contact of said cam surface portions at said ramp portion and a further angle during contact of said cam surface portions at said second portion.
10. A switch according to claim 7, in which said bolt and nut portions are threaded at a pitch angle substantially smaller than said first slope.
11. A switch according to claim 6, in which said clamping means include an arm secured to said othcr of said bolt and nut portions, and a mechanism operable to rotate said arm through a predetermined angle and to produce relative displacement of said pair of conductive members.
12. A switch according to claim 11, in which said arm and mechanism form a collapsible triangular linkage having an apex in said axis.
13. A switch according to claim 12, in which the angle in said linkage at said apex is acute and decreases upon movement of said pair of conductive members into current-carrying contact.
14. A switch according to claim 6, in which the clamping means are rotatably adjustable in relation to said other of said bolt and nut portions.
15. A switch according to claim 9, in which the clamping means are rotatably adjustable in relation to said other of said bolt and nut portions to vary the relative magnitudes of said first and further angles.