AU702152B2

AU702152B2 - Shower door pivot assembly

Info

Publication number: AU702152B2
Application number: AU62050/96A
Authority: AU
Inventors: David Aiken Paterson
Original assignee: Dias Aluminium Products Pty Ltd
Current assignee: Dias Aluminium Products Pty Ltd
Priority date: 1995-08-28
Filing date: 1996-08-13
Publication date: 1999-02-18
Anticipated expiration: 2016-08-13
Also published as: AU6205096A

Description

This invention relates generally to pivot assemblies for aluminium-framed, glass-panelled swing doors such as those installed in shower enclosures or recesses as part of shower screens and relates in particular to a shower swing door adjustable pivot assembly for installation by one person, the assembly has two sets of pairs of interchangeable plastic pivot pin and pivot pin bearing blocks; each block has a central pivot pin or pin bearing hole and all blocks have side walls corresponding to inward-facing walls of a groove formed in identical sill and head jamb frames or of a channel in door bottom and top rails for slidable fitment of the blocks into the grooves or 1 channels.

.0.0 According to the invention there is provided swing door pivot '.:04.assembly for pivotally mounting an aluminium-framed, glass-panelled, swing 0 :0 door between a doorway sill fixed channel frame member and a doorway jamb head fixed channel frame member of a shower enclosure, the assembly S including two sets of pairs of blocks comprising a lower set and an upper set, 0 both said sets including a pivot pin block and a pivot pin bearing block either .09 block having a pivot pin or a socket for the pivot pin, one of the blocks of the lower set being adapted to be inserted into and longitudinally slidably and non-rotatably carried within a longitudinal groove formed within the sill frame 2o member, the other lower set block being adapted to be inserted into and longitudinally slidably and non-rotatably carried within a channel of a bottom rail frame member of the swing door, one of the upper set blocks being adapted to be laterally inserted into and slidably and non-rotatably carried within the groove of the jamb head frame member, the other upper set block being adapted to be laterally inserted into and longitudinally slidably and non-rotatably carried within a channel of a top rail frame member of the swing door, all of the blocks being interchangeable and having side walls corres- C 49 ponding in shape to and fitting between side walls of the respective groove of

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2 the sill or head jamb frame members or the respective channel of the door bottom or top rail, a pair of blocks of each set being adapted to be in vertically-opposed alignment with a pin extending therethrough after said pin has been fitted within a respective socket of a vertically-opposed pivot pin bearing block, the arrangement enabling all of said blocks to be inserted into the groove of the doorway sill or jamb head or within the channel of the door top or bottom rail any point therealong for adjustable fixing therein and after the sill or jamb head frame member has been fixed to the doorway.

In one arrangement of the door pivot assembly of the present I 0 invention, the lowermost block of the lower set is a doorway sill frame member pivot pin block with a central pin directed upwardly after the block has been fixed within the groove of the doorway sill frame member, a lower-set door pivot pin bearing block has a central socket for the pin of the lower-set pivot pin block and is adapted to be fixed within the door frame bottom rail channel in l- vertically-opposed relationship with respect to the said lowermost pivot pin block, an uppermost block of the upper set is a doorway jamb head pivot pin bearing block provided with a pivot pin central socket and is adapted to be fixed within the groove of the doorway jamb head frame member, and the door upper-set, pivot pin block is adapted to be fixed within the door top rail channel 20 and below the doorway upper set pivot pin bearing block and is pivotally joined thereto by a pin passed into the socket of the bearing block.

Preferably one of the blocks of each of the abovedescribed sets of blocks has at least one concavely-curved surface with outer edges thereof joined to flat side walls to form a saddle-shaped structure insertable into the door top rail or door bottom rail.

Preferably, all of the said blocks are formed of injection-moulded synthetic polymeric material, but the pins are of steel and are force fitted into O central apertures pre-formed within each pivot pin block.

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5* S In the above arrangement, the doorway lower-set pivot pin block has, when fixed within the groove of the sill frame member a convexly-curved, upwardly-facing surface and the door lower-set pivot pin bearing block has when fixed within the door bottom rail channel a concavely-curved, downwardly-facing bottom surface of corresponding shape to the convex surface of the lower-set pivot pin bearing block to provide a self-closing mechanism for the door when mounted between the upper and lower set pivot pin bearing blocks.

Preferably, one or more of the blocks of each set has or have outwardly-facing shoulders integrally-formed thereon as overhung lateral extensions of tops of the side walls of the blocks to limit inward movement of *..the blocks into the grooves or channels.

Either of the blocks of each set may alternatively, be a pivot pin S: block or a pivot pin bearing block and may have a central pin integrally formed I.S thereon by injection moulding or any of the blocks may be formed with a t to central cylindrical aperture extending therethrough.

The invention according to another aspect provides a method of t on-site installation of a swing door pivot assembly within a shower enclosure -*-using pivot pin blocks with pins inserted into central holes of the aboveo. 20 described pivot pin bearing blocks and which comprises the following steps: 1) inserting a pin into the central hole of a lower set pivot pin block as the lowermost block and placing the block into the doorway sill groove with the pin upstanding from the block.

2) inserting a pin into an upper set pivot pin block with a flat top 2& surface and placing the said upper set pivot pin block into the door top rail channel with the pin upstanding from the block and with the concave bottom of the upper set pivot pin block lowermost and facing into the door top rail channel.

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2b- 3) inserting the narrower concave top of a doubly-concave, tripleholed, bottom set pivot bearing block up into the channel of the door bottom rail and aligning it horizontally with the bottom set pivot pin bearing block and then fixing the pivot pin bearing block into the bottom rail channel.

4) positioning the door in vertical longitudinal alignment with doorway sill and head frame member grooves and then fixing the bottom set pivot pin block into the sill groove, placing a central hole of the bottom set pivot pin bearing block onto the pin of the bottom set pivot pin block.

5) placing the upper set pivot pin bearing block with its convex surface side down upon the flat surface of the upper set pivot block with its concave surface already inserted down into the door top rail channel and inserting the central aperture of the bearing block over the pin of the pivot pin block and the upward-facing saddle side of the bearing block is fitted into the jamb head groove, then levering the door into position and the door and also fitted upper-set blocks are all knocked into correct door-swing plumb position and adjusted for correct swing in a horizontal arc and the upper-set blocks are fixed into a respective channel or groove by fixing screws through holes on either side of the pin or central pin socket hole.

3- A non-limiting example of a practical arrangement and preferred embodiment of the invention will now be described with reference to and illustrated by the accompanying drawings in which: Figure 1 is an isometric top view of a swing door pivot pin block according to the invention.

Figure 2 is an isometric bottom view of the block of Figure 1.

Figure 3 is an end elevation of the block of Figures 1 and 2.

Figure 4 is an isometric bottom view of a swing door pivot bearing block for use in association with the pivot pin block of Figures 1 to 3.

Figure 5 is an isometric top view of the blcok of Figure 4.

Figure 6 is an end elevation of the block of Figures 4 and Figure 7 is an isometric top view of a second type of swing door a. a 1pivot pin block.

Figure 8 is an isometric bottom view of the block of Figure 7.

Figure 9 is an end elevation of the block of Figure 8.

Figure 7A is a similar view to and of another embodiment of the block of Figure 7.

Figure 8A is a similar view to and of another embodiment of the block of Figures 7 and 8.

Figure 9A is a similar view to and of another embodiment of the block of Figures 7, 8 and 9.

Figure 10 is an isometric top view of a pivot bearing block or the block of Figure 1 before insertion of the pin (or without a moulded-in pin).

Figure 11 is an isometric bottom view of the block of Figure Figure 12 is an end elevation of the block of Figures 10 and 11.

Figure 13 is an isometric top and partial view of go*the outer one of a pair of sleeved channel members used for fixed side frame members and sill and door frame jamb head members of a shower enclosure frame assembly.

Figure 14 is an isometric top and partial view of 0*0 an inner member associated with that of Figure 13.

Figure 15 is an isometric top and partial view of members of Figures 13 and 14 assembled.

Figure 16 is a perspective of a shower enclosure incorporating a swing door.

.0 25 Figure 17 is a vertical section partial view on an enlarged scale taken through the door pivot of the door installed in a shower enclosure.

Figure 18 is a vertical sectional partial view on a still larger scale of the bottom of a shower door with a

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pivot bearing block installed within a bottom rail.

Figure 18A is a vertical sectional partial view on the same scale as Figure 18 and to be read in conjunction with it and shows a pivot bearing block within a sill channel which is directly underneath the door bottom rail.

Figure 19 is a horizontal sectional partial view of the doorway frame return jamb and door hand rail portion of the installation.

Referring to the drawings generally, there is mainly shown door pivot pin and door pivot pin bearing block components of a door pivot assembly for on-site installation by a single person of an aluminium-framed, glass-panelled pivoting swing door 45 of a glass-panelled, 15 aluminium extrusion shower enclosure. Figures 1 to 3 on •"drawing SHEET 1 show a door pivot pin block 10 which may be injection moulded of suitable synthetic polymeric thermoplastic material such as those sold under the registered trade marks CELCON or NYLON and into which a steel 20 pivot pin 11 has been force fitted it will however be understood that the pin and block could be integrallymoulded of plastic material. The block is loaf-shaped with flat bottom 12 and side walls 13, 14 and end walls and with a convexly curved top wall 15 which overhangs the side walls. On either side of the pin 11 there is provided screw fixing holes 16, 17 which have a wide-diameter top opening and bore joined internally to a narrow diameter bore with a narrow-diameter bottom opening 16a, 17a, so that after fixing any screw heads are concealed and recessed. This -6pivot pin block 10 is adapted to be slidably and adjustably inserted into and then fixed within the upwardly-facing channel 29a of the outer channel member 29 used as the door sill (refer to Figures 13 and 15 drawing SHEET 6).

The inner channel is labelled 30. The two overhanging shoulders 18, 19 prevent the block 10 from being inserted too far into the sill channel and also facilitate sliding movements whilst adjusting position of the block during door alignment.

Figures 4 to 6 show a pivot pin bearing block which has flat side walls 21 and 22, flat end walls and concavely-curved top and bottom wall surfaces 23, 24 respectively. A central hole 25 provides a socket for .00.

rotation of the pin 11 of Figure 1 placed therein and o .o.o 15 screw fixing holes 26, 27 similar to those of Figures 1 to 3 are provided. The pivot pin bearing block 18 is adapted to be slidably and adjustably inserted into a channel 31a in the door bottom rail 31 as shown in Figures 17 and 18 on Sheets 7 and 8 of the drawings. The 20 pivot pin bearing block 18 is retained or is prevented from being forced too far into the channel 31a by means of the two overhanging edges 28, 28a which also facilitate sliding movements of the block within the channel. The 0* complementary, opposed, widely-curved surfaces 15, 23 respectively of the pivot pin block 10 and the pivot bearing block 20 enable nesting self-closing and locking movements of the shower enclosure swing door Figures 7 to 9 on drawing Sheet 3 show an upper pivot pin block 32 which is adapted to be slidably and -7adjustably inserted and then after adjustment fixed by screws within the channel 43a of a shower swing door top rail 43 (as at the top of Figure 17). The upper pivot block 32 is of elongated rectangular box-like form with a central steel pin 33 force-fitted into a central hole extending therethrough and has a top 34, flat side walls 35a and end walls 35b, 35c, but a concavely-curved bottom surface 36. Three adjustment and fixing screw apertures extend through the block near the ends, one is a longitudinal, elongate wider diameter slot 37 which extends partly through the block and is stepped down to a narrower-diameter oblong slot 37a. Adjacent the opposite end there is a wider-diameter oblong slot 38 extending from the flat top surface partly through the block and it is 15 then stepped inwardly into two narrow-diameter holes 39, 40 extending through the remainder of the thickness of the block to emerge through the curved bottom surface 36.

Figures 7A, 8A and 9A on drawing sheet 4 show another embodiment of the upper pivot pin block of Figures 7, 8 and 9, but with lateral shoulder extensions 145, 145a of the top wall which facilitate sliding of the block along the top edges of the top rail channel during adjustment and keep the bottom surface clear of the long central fixing groove and screw at the base of the channel.

Figures 10 to 12 on sheet 5 of the drawings show an upper pivot bearing block 41similar in structure to the pivot pin bearing block 10 of Figures 1 to 3 and installed in door top rail 43, but wherein the central aperture 42 does not have a steel pin force-fitted into it or integrally moulded -8thereon, but acts as a socket for the pivot pin 33 when the upper pin bearing block 32 is inserted into the channel 44a of the shower door head channel 44.

Figures 13 and 14 on sheet 6 of the drawings show fixed frame channel members used when sleeved for the sill, sidejambs, and jamb head member of the shower enclosure; the inner channel member 30 is pop-rivetted to walls or floor and the outer channel member 29 is pop-rivetted onto the inner channel. The outer and wider channel member 29 has a glazing channel 29a therein for fixed panels and the also serves as a sill recess for mounting therein the lower pivot pin block 10 and the upper pivot pin bearing block 32 in the head jamb.

ooeo 1 Figure 16 shows the pivot door assembly as installed 15 in a shower enclosure.

Figure 17 shows the shower door pivot assembly of the invention after the door 45 has been completely .",installed within the shower enclosure and additionally .moo shows vinyl glazing gaskets 46, 47 which were pre-inserted 20 into door glazing channels 31a and 43a. Additionally there is shown upper and lower fixing screws 49, 50 respectively fixing block 49 into channel head member 44 and block into door sill outer channel member 29. The door head inner channel member is labelled 44b.

In Figures 18 and 18a there is shown the partial assembly disposition of the glass door panels 45 mounted within the door bottom rail 31 with a pivot pin bearing block 20 inserted within the channel 31a of the bottom rail 31. The fixing screw 50 of Figure 17 which passes through block 20 into fixing channel 31c of bottom rail 31 has been omitted. Immediately below the partial assembly of Figure 18, there is shown in Figure 18a a pivot pin block slidably positioned within the channel 29a of upper or outer channel member 29 prior to fixing by block 10 onto the channel by drilling and screwing down through the block end aperture(s).

Figure 19 also shows a glazing strip 48 and also shows a door grip stile 51 (combination door hand rail and glazing channel member) which has one strip 52 of a magnetic catch, the other strip 52a being on the door return jamb 53.

The swing door pivot block assembly or system of this invention enables one person to easily install a 15 swing door in a shower enclosure by enabling infinite or lateral or horizontal and vertical sliding adjustment of the block in relation to each other and thus the door and plumb alignment of the door in relation to fixed frame members of the shower enclosure doorway. The design of the pivot block system also incorporates in the lower set of blocks an automatic self-closing mechanism comprising V. S S ooo o an outer (and downwardly-facing) concave surface which parks over an outer (and upwardly facing) convexly-curved surface. The convexly-curved lower pivot pin block (with bottom saddle-shaped structure seated within the sill channel) has a top surface flush with the domed upper surface of the channel vinyl infill buffer strip used to trim the sill channel.

The invention also includes a method of on-site assembly of the door and its four-pivot-block system into four respective channels and which comprises the following steps: 1) inserting the bottom pivot pin block with its steel pin into the central aperture (or already moulded *into it) is placed into the sill channel groove, 2) the door top rail or upper pivot pin block with steel pin already factory inserted is placed into the door top rail channel and with the block's concave bottom saddle structure facing down into the channel, 3) the narrower saddle top of doubly-concave, 20 triple-holed, bottom pivot bearing block is inserted up into the channel of door bottom rail and horizontally aligned with the bottom pin block and the pin block is fixed into the bottom rail channel, 4) the door is positioned in vertical longitudinal alignment with sill and top rail channels and the bottom pin block is fixed into the sill channel and bottom bearing block is placed onto the pin of the bottom pin block. (It will be noted that the outer holes in the blocks on either side of the pin and or pin bearing socket holes are spaced at a distance -1 1which permits fixing screw access in either side of a door when in open positions), the upper pivot pin bearing block is placed with its convex surface side down upon the flat surface of the upper pivot pin block (its saddle side already inserted down into the top channel) by inserting the central aperture of the bearing block over the pin of the pin block and the upward-facing saddle side of the bearing block is fitted into the head channel, the door is then levered into position and the door and also fitted top blocks are all knocked into correct door swing plumb position and adjusted for correct swing in a horizontal arc and the top blocks are fixed into respective channels by fixing screws through 1 holes on either side of the pin or central pin socket hole.

15 Either or both of the two top blocks can be adjusted simply and simultaneously when once the door is in a laterally square position clearing the sill. Lateral adjustment can be made to achieve a preferred position of a door relative to the door frame for instance the door 20 can be moved in closer or further away from the frame so S. that the preferred clearance is achieved.

According to the invention it is a one man operation to fix the pivot blocks both to the door frame and to the door and to fix the door within the doorway frame during installation of a shower enclosure. The elongate upper pivot pin block provides adequate clearance for access to the fixing screws when the door is in place and also enables the pin block to be slid from side to side with door attached between sill and head channel during any -1 2re-alignment operations required due to mis-alignment caused by movement of foundations.

A more complete finish is obtained by insertion of sealing strips into exposed channels.

It should be understood that any of the blocks of this invention described by the relative terms "upper" or "lower" are interchangeable and that all pivot pin bearing blocks without pins can be converted to pivot pin blocks with pins by force-fitting steel pins into the central cavity of the pivot pin bearing blocks either at a factory or on-site. Alternativly, all pivot pins of the pivot pin blocks could be moulded-in with the blocks by injection moulding or could be machined by fabricating the blocks either from plastic or resin or from metal and alternatively again the pins could be glued or centrally om bonded onto blocks which could also be of solid plastic and the fixing holes could be drilled into and through blocks.

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a 14-- 2. The assembly according to claim 1 wherein a lowermost block of the lower set is a doorway sill frame member pivot pin block with a central pin directed upwardly after the block has been fixed within the groove of the doorway sill frame member, a lower-set door pivot pin bearing block having a central socket for the pin of the lower-set pivot pin block which is adapted to be fixed within the door frame bottom rail channel in vertically-opposed relationship with respect to the said lowermost pivot pin block, an uppermost block of the upper set is a doorway jamb head pivot pin bearing block provided with a pivot pin central socket and is adapted to be fixed within the groove of the doorway jamb head frame member, the door upper-set, upper pivot pin block is -adapted to be fixed within the door top rail channel and below the doorway upper set pivot pin bearing block and pivotally joined thereto by a pin passed a.o.

into the central socket of the bearing block.

3. The assembly according to claim 1 or 2 wherein one of the blocks of each set has at least one concavely-curved surface with outer edges thereof joined to flat side walls to form a saddle-shaped structure insertable into the door top rail or door bottom rail.

4. The assembly according to any one of claims 1 to 3 wherein all of thesaid blocks are formed of injection-moulded synthetic polymeric material, S: but the pins are of steel and are force fitted into central apertures pre-formed within each pivot pin block.

The assembly according to any one of claims 2 to 4 wherein the doorway lower-set pivot pin block has, when fixed within the groove of the sill frame member a convexly-curved, upwardly-facing surface and the door lower-set pivot pin bearing block has when fixed within the door bottom rail channel a concavely-curved, downwardly-facing surface of corresponding shape to the convex surface of the lower-set pivot pin bearing block to provide 13-- The claims defining the invention are as follows: 1. A swing door pivot assembly for pivotally mounting an aluminiumframed, glass-panelled, swing door between a doorway sill fixed channel frame member and a doorway jamb head fixed channel frame member of a shower enclosure, the assembly including two sets of pairs of blocks comprising a lower set and an upper set, both said sets including a pivot pin block and a pivot pin bearing block, either block having a pivot pin or a socket for the pivot pin, one of the blocks of the lower set being adapted to be inserted into and longitudinally slidably and non-rotatably carried within a longitudinal groove formed within the sill frame member, the other lower set block being adapted to be laterally inserted into and longitudinally slidably and non-rotatably carried within a channel of a bottom rail frame member of the swing door, one of the upper set blocks being adapted to be laterally inserted into and slidably and non-rotatably carried within the groove of the jamb head frame member, the other upper set block being adapted to be laterally inserted into and longitudinally, slidably and non-rotatably carried within a channel of a top rail frame member of the swing door, all of the blocks being interchangeable and having side walls corresponding in shape to and fitting between side walls of the respective groove of the sill or head jamb frame members or the respective channel of the door bottom or top rail, a pair of blocks of each set being adapted to be in vertically-opposed alignment with a pin extending therethrough after said pin has been fitted within a respective socket of a vertically-opposed pivot pin bearing block, the arrangement enabling all of said blocks to be inserted into a groove of the doorway sill or jamb head or within the channel of the door top or bottom rail at any point therealong for adjustable fixing therein and after the sill or jamb head frame member has been fixed to the doorway.

Claims

6. The assembly according to any preceding cliam wherein one or more of the blocks of each set has or have outwardly-facing shoulders integrally formed thereon as overhung lateral extensions of tops of the side walls to limit inward movement of the blocks into the grooves or channels.
7. The assembly according to any preceding claim wherein either of the blocks of each set may be a pivot pin block or a pivot pin bearing block.
8. The assembly according to any preceding claim wherein any of the :blocks has a central pin integrally formed thereon by injection moulding. S: 9. The assembly according to any one of claims 1 to 7 wherein any of the blocks are formed with a central cylindrical aperture exending therethrough and a separate pin is force-fitted into the aperture to form a pivot pin block.
10. The assembly according to any preceding claim wherein the bottom block of the upper set is a pivot pin block of elongated box-like form with flat side walls, a flat top, flat end walls and a concavely-curved bottom surface, a central aperture in the flat top having a steel pivot pin force-fitted therein and, adjacent each end, an oblong fixing screw slot formed in the flat top and extending vertically partly through the block, the slot at one end being stepped down to a narrower oblong slot which emerges through the concavely- curved bottom surface.
11. The assembly according to claim 10 wherein adjacent the other end opposite the said one end, the slot is stepped inwardly into two fixing-screw holes extending through the remainder of the thickness of the block to emerge through the concavely-curved bottom surface.
12. A method of on-site installation of a swing door pivot assembly within a shower enclosure using pivot pin blocks with pins inserted into central holes of pivot pin bearing blocks according to any one of claims 1 to 11 and which comprises the following steps: 1) inserting a pin into the central hole of a lower set pivot pin block as the lowermost block and placing the block into the doorway sill groove with the pin upstanding from the block. 2) inserting a pin into an upper set pivot pin block with a flat top surface and placing the said upper set pivot pin block into the door top rail channel with the pin upstanding from the block and with the concave bottom of the upper set pivot pin block lowermost and facing into the door top rail channel. S 3) inserting the narrower concave top of a doubly-concave, triple- holed, bottom set pivot bearing block up into the channel of the door bottom rail *..and aligning it horizontally with the bottom set pivot pin bearing block and then Sfixing the pivot pin bearing block into the bottom rail channel. 4) positioning the door in vertical longitudinal alignment with doorway sill and head frame member grooves and then fixing the bottom set pivot pin block into the sill groove, placing a central hole of the bottom set pivot pin bearing block onto the pin of the bottom set pivot pin block. placing the upper set pivot pin bearing block with its convex surface side down upon the flat surface of the upper set pivot block with its concave surface already inserted down into the door top rail channel and inserting the central aperture of the bearing block over the pin of the pivot pin block and the upward-facing saddle side of the bearing block is fitted into the jamb head groove, then levering the door into position and the door and also fitted upper-set blocks are all knocked into correct door-swing plumb position and adjusted for correct swing in a horizontal arc and the upper-set blocks are fixed into a respective channel or groove by fixing screws through holes on either side of the pin or central pin socket hole.
17- 13. A swing door pivot assembly for a shower enclosure substantially as hereinbefore described with reference to the accompanying drawings. 14. A method of on-site assembly of a swing door and door pivot assembly for a shower enclosure substantially as hereinbefore described with reference to the accompanying drawings. Dated this 18th day of September, 1998 DIAS ALUMINIUM PRODUCTS PTY.LTD. By Our Patent Attorney JOHN L.DAVIES CO. *e Se ABSTRACT A swing door pivot assembly has two pairs of inter- changeable plastic blocks (10, 20) for pivotally mounting a shower door (45) between a doorway sill (29) and door framehead member one block of each pair (10, 32, 41, 132) forms a pivot pin block, the other (20) forms a pivot pin bearing block; all blocks fit slidably within a channel (31a, 43a) of door top and bottom rails (31, 43) and in a correspondingly-shaped channel (29a) of door sill (29) or head jamb frame member (44) of mating pairs of channel frame members an inner channel member (30) is fixed to a floor or shower base and is concealed within and fixed to an outer channel member pivot blocks are placed into and slide along a respective channel (29a, 44a, 31a, 43a) for on site positioning during installation of swing door all blocks have moulded-in, screw-fixing holes (16, 17, 26, 27, 39, 40) and some have adjustment slots (37, 38). In one form block (10) of each pair has an outward-facing convex surface (15) and its mating block has an outward-facing concave surface (23) of corresponding shape to form a self-locking mechanism and in most forms blocks have shoulders (18, 19, 28, 28a, 141, 142) to prevent them entering too deeply into the channels. An inner concave surface of blocks enables clearance over fixing screw heads (49, 50). Any of the blocks can be moulded with a central pin hole (25, 42) and steel pivot pin (11, 33, 133) is forced-fitted into it, or pin and block are moulded in one piece.