CA1230518A - Press with wedge - Google Patents

Abstract

ROGERS, BERESKIN & PARR

Inventor: LADISLAO W. PUTKOWSKI
Title: PRESS WITH WEDGE

ABSTRACT OF THE DISCLOSURE
A press having a pre-stressed frame, to reduce the energy required in each pressing cycle, and in which oppositely acting wedges provide the clamping force. The mold or other structure to be pressed is assembled out-side the press and then is slid on guide rails into the press with very small clearances. The press can also be used for molds or dies fixed to stationary or movable platens.

Description

305i18 This invention relates to a press which employs ; wedges to provide the clamping forces.
Presses are used for numerous operations, such as molding, die casting, hydroforming, vacuum and pressure forming, forging, hobbing, stamping, fine blanking, powder compaction, and others. Many of these processes require very short but high power strokes.
In conventional presses the main clamping device is used not only for opening and closing the mold or other structure to be pressed, but also for the final clamping stroke. This arrangement is uneconomical both initially and in operation. During each cIamping operation the frame must be stretched to the full clamping force, which requires a large amount of energy which is lost ~ 15 after each clamping cycle.
; The invention in one of its aspects provides a press in which the functions of opening and closing, and performing the clamping stroke, are separated. The opening and closing operation is performed outside the press, typi-cally by insert1ng the item or other thing to be clamped within a structure to be compressed. The closed struc-ture is then moved into the press, the frame of which is prestressed, and the clamping forces are applied.
Since the frame is prestressed, preferably to more than the maximum clamping power used, no encrgy is required ~2~0~i18 to stretch the frame during each operation and only very little energ~ is needed to eliminate the sliding gaps between the press structure and the structure to be compressed. Wedges are then used to provide the necessary clamping force.
In one aspect the invention provides a press comprising:
(a) a frame forming a closed loop;
(b) a first pressure surface disposed within said loop;
(c) a movable platen within said loop, the platen having a second pressure surface opposed to and spaced from said first pressure surface to permit the introduction between said first and second pressure surfaces of a structure to be compressed;
(d) means supporting said movable platen for movement thereof towards and away from said first pressure surface; and, (e) means for moving said movable platen towards said first pressure surface to compress a structure disposed between said first and second pressure surfaces;
wherein said frame is pre-stressed and comprises a pa1r of yokes, a pair of compression members mounted between 1230S~L~

said yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a single pair of tensioned elements connecting said yokes and maintained in tension by said compression members, said elements having been tensioned by expanding the elements using heat and fitting the elements to the yokes in a heated condition so that the elements are tensioned upon subsequent cooling.
In another aspect the invention provides a press of the general form defined above but in which the frame comprises a single, pre-stressed annular ring and a plurality of compression members within the ring and maintaining the ring in tension, the ring having been tensioned by extending the ring using heat and fitting the ring to the compression members in a heated condition so that the ring is tensioned upon subsequent cooling.
A further aspect of the invention provides an improvement in a press comprising a frame forming a closed loop and means for producing pressing forces within the loop. The lmprovement resides in that the frame is pre-stressed and comprises a pair of yokes, a pair of compression bars mounted between the yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a single pair of tensioned elements connecting the yokes and maintained i.n tension by the compression members, said . .

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- 4a -elements having been tensioned by expanding the elements using heat and fitting the elements to the yokes in a heated condition so that the elements are tensioned upon subsequent cooling. A pre-stressed frame per se is also provided.
The invention additionally provides a method of making a pre-stressed frame for a press comprising the steps of:
providing a pair of yokes, a pair of compression bars mountea between said yokes at opposite sides of the frame and at -their ends directly abutting the yokes, and a ~single palr of tension elements adapted to be connected to said yokes with said elements in an expanded condition;
heating the elements to said expanded condition;
fitting the elements to the yokes; and, allowing the elements to cool and contract, pre-stressing the frame.
Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings which show various embodiments of the invention, and in which:

A

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Fig. 1 is a longitudinal sectional view of a press according to the invention;
Fig. 2 is a view taken along lines 2-2 of Fig. 1;
Fig. 3 is a view taken along lines 3-3 of Fig. l;
Fig. 4 is a side view showing the wedges and movable platen of the Figs. 1 to 3 press with rollers therebetween;
Fig. 5 is a perspective view of the set of wedges of the Fig. 1 press;
Fig. 5a is a side view showing a modification of Fig. 5;
Fig. 6 is a side view of the wedges of Fig. 5 and showing a driving arrangement therefor;
Fig. 7 is a top view of the wedges and driving arrangement of Fig. 6;
Fig. 8 is a side view of a modified driving arrange-ment for the wedges of Fig. 5;
Fig. 9 is a top view of the driving ar_angement of Fig. 8;
Fig. 10 is a perspective view of an alternative form of prestressed frame module for the press of Fig. l;
Fig. lla and llb are side views showing teeth for the frame of Fig. 10;
Fig. 12 is a graph showing load distributions for the Figs. lla and llb teeth;

~230S~L8 Fig. 13 is a front View 6howin~ a module frame for the press of Fig. l;
Fig. 14 is a ~ectional view along line 14-14 of Fig. 13;
Fig. 15 is a front view of an alternative frame module;
Fig. 16 is a sectional view along line 16-16 of Fig.
15;
Fig. 17 is a perspective view of another variant of the frame module;
Fig. 18 is a top view partly in section, of a further modified press according to the present invention; and Fig. 19 is a sectional view, taken along line 17-17 of Figure 18.
Reference is first made to Figures 1 to 3, which show a press 10 according to the invention and intended for molding or die forming operations. The press lO includes a frame 12 which, in principle, comprises top and bottom yokes 14a, 14b held together by side members 16 as best shown in Fig.

2. The frame 12 is pre-stressed, as will be described shortly, to more than the maximum clamping power of the press so that no energy is required to stretch the frame during each clamping operation.

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In this particular embodiment, the frame is of a modular construction as indicated diagrammatically in Figs.
1 and 3. The frame is shown as comprising six modules, which are indicated by the reference numeral 150. The modules may be of the form shown in Figs~ 13 and 14; 15 and 16; or 17(to be described). As best shown in Fig. 2, each module comprises respective portions 14am, 14bm of the yokes 14a, 14b and portions 16m of the side members lS.
These portions 14am, 14bm and 16m are secured together to form a closed loop, so that the respective modules lS0 can be handled as separate units. To assemble the frame 12, the modules 150 are assembled side by side. For con-venience, they are assembled together in a support frame (not shown) which ensures correct alignment of the modules lS0. Then, the other components of the press (described below) are assembled in the frame 12 formed of these modules 150. The modules are accurately formed so that, in use, the load applied to the frame 12 is evenly distributed be-tween the modules 150.
This modular design enables the size of the frame 12 to be readily changed to accommodate different sizes o~
articles that are to be pressed. It is simply necessary to assemble together the required number of modules 150.
The lower surface 18 of the upper yoke l~a con-stitutes a first pressure sur~ace (a separate platen fixed to the upper yoke 14a can also be used to provide this - 8 - ~ ~30~8 pressure surface). A movable platen 20 is located we]l below the upper yoke 14a and has an upper surface 22 facing the first pressure surface 18 and constituting a second pressure surface.
The movable platen 20 i5 guided for vertical movement on the side members 16 by ~lots 24 in its sides.
The movable platen 20 also has two sloping lower surfaces 26 and 27 which are supported on correspondingly inclined upper surfaces 28 and 29 of ac~uating wedges 30 and 31 which are movable back and forth horizontally. Both wedges 30 and 31 have respective horizontal lower surfaces 32 and 33 which are supported on the horizontal upper surface 35 of a stationary member 36. Stationary member 36 is in turn fixed to the lower yoke 14b, and distributes any applied force to the modules 150. Of course, the ~edges need not act directly on platen 20; an intermediate member having surfaces 26, 27 may be employed. -The wedge 30 is formed as a U-shaped wedge which slides to the right as shown in the drawings to produce a clamping force. The U-shaped w~dge 30 includes a base 60 and a pair of spaced tapered wedge arms 62 one located on each side of the other wedge 31. The wedge arms 62 and the wedge 31 are supported on the horizontal upper surface of member 38, with the wedge 31 located between the wedge arms 62. The inclined upper surfaces 28, 29 like the surfaces 26 and 27 of the platen 20 have the same though opposite slopes so that uniform upward movement of the movable platen 20 will be achieved when wedges 30, 31 are properly aligned and move at the same speed in opposite 9 ~2~

directions. The upper sur~ace 28 of the wedge 30 is forrned from the combined area of the upper surfaces 64 of wedge arms 62 and is preferably equal to the area of the upper surface 29 of we~ge 31 so that the forces needed to move each wedge 30, 31 will be as nearly the same as possible.
Th~ actuating wedges30 and 31 are propelled horizontally back and forth by a conventional hydraulic cylinder 40. The pi~ton rod 41 of cylinder 40 is connected at 42 to the wedge 31. The cylinder 40 is itself mounted on a support 44 having arms 45. The arms 45 are fixed to the wedge arms 62.
- Appropriate lubrication -may be-provided between the surface 35 of the stationary member 36 and the surfaces 32 and 33, and also between the surfaces 26-~9. Since the lower surfaces 32, 33 of the wedges 30, 31 are horizontal, no lateral force can be applied to the assembly comprising the wedges 30, 31 and the platen 20. In use, to load the press, the cylinder rod 41 is drawn into the ~ylin~er 40, thereby moving the wedge 30 to the left and the wedge 31 to the right, as viewed in Figure 1. The slots 24 in the platen 20 ensure that it stays aligned in the press. The press can then be loaded with a mold, as des~ribed below.

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Fig. l also ~hows a pair of mold ox die halves 48a, 48b assembled together and ready to be inserted into the space 50 between the first and second pressure surfaces 18, 22. Mold or die halves 48a, 48b are con-structed so that they can be separated to receive or discharge any desired item or thing to be clamped or molded, and then assembled together into a single mold structure as shown. The details of the mold or die, which can e.g. be an injection mold or which can receive and transmit pressure to an item, are not shown since they are conventional.
The lo~er mold ox die half 48b is supported, by . means of slots 52 in its sides, which have a sliding fit on a pair of rails 54 which extend through the space 50.
lS The rails 54 are secured to the side members 16. The arrangement is dimensioned so that when the mold or die halves are slid into the space 50, there are only small clearances between the top of the upper mold or die half 48a and the first pressure surface 18, and between the bottom of the lower mold or die half 48b and the ~ second pressure surface 22 [assumlng that wedges30, 31 ; ' ~ have been withdrawn (as described above~] .
" In operation, the material or other item to be molded or pressed is placed within the mold or die :

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halves 48a, 48b which are then assembled togethèr and closed outside the press. The mold or die halves 48a, 48b are then slid along the rails 54 into the space 50 in th~ press. (If the mold is an injection mold, it is filled with hot plastic or other material after it has been inserted into the space 50 and clamped.) At this time the upper surfaces 56 of the slots 52 in the lower mold or die half 48b rests on the tops of the rails 54.
The movable platen 20 and the wedges 30, 31 are as indicated dimensioned to allow a small clearance between the bottom of the lower mold or die half 48b and the second pressure surface 2? at this time.
When the mold or die halves are in position, . ..
cylinder 40 is actuated to drive the actuating wedge 30, 31 towards one another, i.e. the wedge 30 moves to the right and the wedge 31 mov~s to the left as shown in Fig. 1, closing the clearances and lifting the movable platen 20 and the mold or die halves so that the rails 54 are approximately centered in the slots 52. (As shown, the rails 54 are of lesser height than the slots 52 in this position.) This ensures that the rails will not ~e stressed during the clamping operation. As the wedges 30, 31 continue to be driven in their respective and opposite directions, clamping force is exerted on the mold or die halves 48a, 48b to perform the required molding or other . .

~3~8 operation. Since the frame 12 is prestressed and no energy is required to stretch it, only a small quantity of energy is required to eliminate the sliding gaps between the mold or die halves 48a, 48b and the pressure ~urfaces 19, 22 and to supply the necessary clamping forces.
Although only one mold or die has been shown, preferably two molds or dies will be used so that while one is being loaded or unloaded, the other can be processed in the press.
Preferably, the lower surfaces 26,27 of the platen 20 are symmetrical, to aid substantially parallel movement of the movable platen 20 under maximum power and under unbalanced loading conditions. The use of two opposed wedges 30, 31 ensures that no lateral force is applied to the platen 20.
These features are extremely difficult to obtain in conventional presses without very high cost.
It will be appreciated tha~ the movement of the actuating wedges 30, 31 can be programed as required for structural foam molding, hobbing, or other processes. The sliding wedge surfaces can be treated with anti-friction materials as required, or they may be equipped with rollers to reduce friction. Such rollers are diagrammatically indicated in Fig. 4, which shows elongated rollers (i.e.
roller bearings) 55,56, 57 held in elongate cages 55a, 56a, 57a respectively. Two sets of rollers 55 and two corresponding -13~ 3~5~

cages 55a are provided~ These are loca~ed be~ween the upper surfaces 64 of the wedge arms 62 and the surface 26 of the platen 22. The rollers 56 and associated cage 56a are located between the upper surface 29 of the wedge 31 and the surface 27 of the platen Z0. Thus, the rollers 55,56 permit relative movements between the wedges 30, 31 and the platen 20. The rollers 57 are located between the wedges 30, 31 and the stationary member 36, to permit horizontal movement of the wedges 30, 31 relative to the stationary member 3Ç. The rollers 57 are provided in three parallel sets or rows, each of which has its own associated cage 57a. Two outer rows of rollers 57 are located between the wedge arms 62 and the stationary member 36. A third row of rollers 57 is located between these two rows, and between the wedge 31 and the stationary member 36. This permits the wedges 30, 31 to travel in opposite horizontal directions.
If desired, each of the wedge arms 62 and the wedge 31 can comprise two or more actuating wedges of the same or different taper, one sliding on top of the other, in order to increase the effective vertical stroke~ Each wedge can be actuated at different speeds and under a different program if required. Multiple action presses can also be built using different sets of wedges for each action, each set of wedges having the opposing double action of wedges 30, 31. The wedges can be mounted on the yokes 14a, 14b. The wedges can into the die or mold.

~ 14 - 1Z3~518 It is also possible for each of the wedge arms 62 and wedge 31 to comprise two or more wedges, arranged in a series.
By way of example, Fig. 5a shows an arrangement in which each of the wedge arms 62 and the wedge 31 comprises three wedges S denoted 62a and 31a respectively. The front wedge arm 62 has been omitted from Fig. Sa for clarity.
Various types of drives may be used to propel wedges 30, 31. As discussed above, a hydraulic cylinger 40, having a piston rod 41, can be used. The cylinder 40 is fixed to be base of a support 45 whose side arms 44 are secured to wedge arms 62. The cylinder rod 41 protrudes through the base and is connected to the wedge 31. When the piston rod 41 retracts, wedge 31 moves to the right and wedge arms 62 of wedge 30 move to the left, and when the piston rod 41 extended the wedges 30, 31 again move in opposite directions. As shown in Figure 6, if high accuracy is desired, then rack teeth 69a, 69b, 70a, 7Ob are provided at the opposed sides of the respective wedges 30, 31 and engage idler pinions 71a, 71b rotatably mounted on shafts 72a, 72b fixed to the frame 12. The upper ends of the pinions 71a,71b or pinion shafts are accommodated in slots 72c in the movable platen 20. The pinions 71a, 71b prevent unequal movement of the wedges 30, 31 and ensure that wedge arms 62 move to the same extend in one direction as wedge 31 moves in the other direction. A variant of the wedge 30 is shown in Figure 7. In this variant, the wedge arms 62 are separate and are not joined by a base 60 as previously shown.

Alternatively, as shown in Figures 8 and 9, the wedges 30, 31 may be powered directly by a pair of pinions, 73a, 73b engaging teeth 74a, 74b, 75a, 75b on the side faces of the wedges 30, 31~ The pinions 73a, 73b are fixed to gears - 5 76a, 76b rotatably mounted in the base member 36. The gears 76a, 76b are driven by a common driving pinion 80 powered by an electric 9 hydraulic or air motor (not shown). This ensures that equal displacement of the wedge arms 62 of the wedge 30 and of the wedge 31 occurs. Again, in Figures 8 and 9, the we~ge arms 62 are shown separate from one another.
In each of the embodiments of Figs. 6, 7 and Fiqs. 8, 9, the movable platen is located for vertical movement by pins 81 which extend downwardly between the wedges and are received in guides tnot shown) in the fixed platen or in the frame.
Fig. 10 shows an alternative form of prestessed frame which can be used for a one-piece or modular frame construction.
As shown, the yokes 14a, 14b have teeth 86a, 86b respec-tively at their ends/ which engage corresponding teeth 88a, 88b in tie bars 90. The tie bars 90 are prestressed (stretched) so that they are under a tension force which exceeds the maximum clamping force expected in the press. The tie bars 90 are maintained in their stretched condition by compression bars 92 which extend between the respective yokes 14a, 14b and hold the yokes apart. The space 94 between the yokes 14a, 14b ànd compression bars 92 accommodates the platen or platens, wedges and guide rails as described above. As shown, ends of the tie bars are tapered. The top of the yoke 14a comprises a flat portion and two sloping edge portions. The bottom yoke 14b is adapted to receive the tie rods 90 and includes feet 95 provided with holes for securingthe frame 12 in position.

~L~3C~18 To assemble a frame of the kind shown in Fig. 10, the yokes 14a, 14b and the compression bars 92 are assembled together, and then the tie bars 90 are heated to a temperature sufficient to extend them to the required desree. The S heated tie bars go are then fitted to the yokes and cooled, placing them under the required tension. Either the compression bars or the tie bars can then be used for mounting the mold slidiny rails 54. Because the tie bars are prestressed, the energy that would otherwise be ~ required to stress them during each clamping operation is saved. The conventional keys and screws used to locate and hold the parts togeth~r are not shown, for simplicity.
In a frame such a~ that shown in Fig. lO, it is desirable that the load on each mating pair of the teeth 86a, 86b, 8~a, 88b be kept as uniform as possible. If the teeth were of constant pitch, as shown in Fig. lla (which shows the teeth mating prior to prestressing), then the load~would be hi~hest on the first tooth and would be much less on each succeeding tooth, as shown in curve 95 in Fig. 12. Failure would then tend to begin at tooth 96 in Fig. lla, as indicated by crack 97, and could eventually result in complete ~ailu~e of the ~ie - bar 90.

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To avoid this difficulty, a tooth design as shown in Fig. llb is preferred, in which the pitch or dis-tance between teeth decreases or each tooth on the tie bar prior to prestressing, the yoke having constant pitch teeth. As shown in Fiq. llb, in the non-prestressed condition the gap at 98 is zero; the gap at 100 is the amount of expansion of one tooth under full tensioning or prestressing; the gap at 102 is the amount of expansion of two teeth under full tensioning, etc. The amount of expansion of each tooth is constant. Then, when the tie bars 90 are prestressed, each of ~he gaps 100, 102 etc. becomes substantially zero, provided that ~uch gaps have ~een properly dimensioned for the particular prestress load to be applied. The stress distribution ; 15 for the teeth is then shown at 104 in Fig. 12 and will be seen to be substantially uniform. This tooth design can also be used for frames which are not prestressed, in which case, when the frame is stressed (i.e. when clamping is occurring) the teeth will be approximately evenly loaded, and at other times there will be gaps between all the mating teeth except the first pair.
The tie bars can have constant pitch and the yokes or plates variable pitch if necessary.

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Figs. 13 and 14 show a frame module 151 which m~y be used in a frame of the form shown in Figs. 1 to 3. The module comprises an integral annular ring 106 which at its top and bot-tom runs through slots 10~a, 108b in upper and lower yokes llOa, 110b. The yokes 110a, 110b are held apart by compression bars 112. To prestress the frame, the yokes 110a, 110b are ~orced apart by any desired expansion means, such as hydraulic jacks, and stressing spacers 114 are inserted above the compression bars 112 to eliminate the spaces created by the stretching of the ring 106.
The compression bars 112 and stressing spacers 114 are provided with slots corresponding to the slots 108a, 108b. The expansion means is then removed.
An alternative form of module is shown in Figs. 15 and 16 and is designated 152. In this case there are no slots in the yokes or in the compression bars. A ring 116 is employed which is wider and thinner than the ring 106 of Figs. 13 and 14. The ring 116 extends across the full width of this module 152. For this reason, the yokes 118a and 118b and compression bars 120 are provided with plain outer surfaces. To assemble this module~ the yokes 20 118a, 118b and compression bars 120 are laid on a horizontal sur-face. Then, the annular ring 116 is heated to expand it. It is then dropped over the yokes and compression bars and allowed to cool. Once it is cool, the ring 116 is prestressed to the correct tension.
An alternative form of module is shown in Fig. 17 and is designated by the reference 153. It comprises a top yoke 122a, a bottom yoke 122b and compression bars 123. Around the top yoke 122a and the compression bars 123 U-shaped tension bars or strips - lg~ 3~

124 are positioned. As shown, two tension strips 124 are provided although just one stronger strip 124, or 3 or more tension strips 124, could be provided. Each end of each tension strip 124 is provided with a tooth 125, which extends across the width of the strip. These teeth 125 engage corresponding teeth 126 of the bottom yoke 122b. Again, each strip 124 could be provided with two or more teeth. This construction is assembled similarly to the frame shown in Flgure 10. The yokes 122a, b and the compression bars 123 are assembled together, either vertically or horizontally. Then, the strips 124 are heated and placed in position. On cooling, the strips 124 contract to set up the required tension in each strip 124.
Finally, reference is made to Figs. 18 and 19 which show an embodiment of the invention which uses an annular prestressed frame or ring 125 held in a stressed condition by four identical truncated sector shaped compression members 126. To assemble the structure of Figs. 18 and 19, the compression members 126 are assembled together and, as before, the ring 125 is then heated to expand it. The ring 125 is then placed o~er compression members 126, cooled, and screwed in place (with fasteners not shown).
The flat truncated surface 128 of each compression member support wedges 130, 131 which in turn-support a movable platen 132. Ihe wedges 130, 131 generally correspond to the wedges 30, 31 described above. The drive means for these wedges 130, 131 and their exact mode of operation is not described here as it is similar to that described above.

30S~L8 ~ 20 -The platens 132, each of which is similar to the movable platen 20, are guided for radial movement inwardly and outwardly by slots 134 in circular frame side plates 136. One side plate 136 is mounted on each side of the ring 125 and compression members 126. Each wedge 131 is located between two oppositely directed wedges 13n.
The wedges 130 extend through openings 133 in the top side plates 136 (Figure 19), whilst the wedges 131 extend through openings 138 in the bottom side plates 136.
In operation, a mold or die 140 to be compressed is suspended (eg. by a hook 142 on its top) and is lowered through a hole 144 in the upper side pla~e 136 to a position between the movable platens 132. The wedges 130, 131 are then driven in opposite directions inwardly with respect to the whole press (by any desired means as previously discussed). This forces all four movable platens 132 radially inwards against the faces of the mold ~ or die 140 to provide uniform comprPssion.
: The press described can also be used for molds, dies or other structures to be compressed, fixed to stationary or movable platforms. In all of the embodiments shown, the movable platens may be provided with springs (not shown) to bias them to a withdrawn position when the wedges are retracted.
Any number of compression members-can be used.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A press comprising:
(a) a frame forming a closed loop;
(b) a first pressure surface disposed within said loop;
(c) a movable platen within said loop, the platen having a second pressure surface opposed to and spaced from said first pressure surface to permit the introduction between said first and second pressure surfaces of a structure to be compressed;
(d) means supporting said movable platen for movement thereof towards and away from said first pressure surface; and, (e) means for moving said movable platen towards said first pressure surface to compress a structure disposed between said first and second pressure surfaces;
wherein said frame is pre-stressed and comprises a pair of yokes, a pair of compression members mounted between said yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a single pair of tensioned elements connecting said yokes and maintained in tension by said compression members, said elements having been ten-sioned by expanding the elements using heat and fitting the elements to the yokes in a heated condition so that the elements are tensioned upon subsequent cooling.

2. A press according to claim 1, wherein said tensioned elements comprise a pair of tie bars connecting said yokes at opposite sides of the frame, and wherein said tie bars and yokes have mating teeth connecting said tie bars and said yokes together, the pitch of the teeth on said tie bars being non-uniform under zero tensioning of said tie bars and being proportioned so that under said tensioning of said tie bars, said teeth of said tie bars engage the teeth of said yokes with substantially no gaps at any tooth so that each tooth is under substantially the same stress as each other tooth.

3. A press according to claim 1, wherein said tensioned elements are provided by a ring which encircles the yokes and compression members, with the compression members maintaining said ring in tension.

4. A press according to claim 3, wherein the yokes and the compression members include external grooves, in which the ring is seated.

5. A press according to claim 3, wherein the yokes and compression members have plain exterior surfaces and the ring is of the same width as the yokes and the compression members.

6. A press as claimed in claim 1, wherein the frame comprises a plurality of individual modules, each of which modules comprises a pair of said yokes, and a pair of said compression members between said yokes, wherein said tensioned elements are provided by a ring encircling the yokes and compression members, with the compression members maintaining said ring in tension.

7. A press comprising:
(a) a frame forming a closed loop and comprising a single pre-stressed annular ring, and a plurality of compression members within said ring and maintaining the ring in tension, said ring having been tensioned by expanding the ring using heat and fitting the ring to the compression members in a heated condition so that the ring is tensioned upon subsequent cooling.
(b) a first pressure surface disposed within said loop;
(c) a movable platen within said loop, the platen having a second pressure surface opposed to and spaced from said first pressure surface to permit the introduction between said first and second pressure surfaces of a structure to be compressed;
(d) means supporting said movable platen for movement thereof towards and away from said first pressure surface; and, (e) means for moving said movable platen towards said first pressure surface to compress a structure disposed between said first and second pressure surfaces.

8. In a press comprising a frame forming a closed loop and means for producing pressing forces within said loop;
the improvement wherein said frame is pre-stressed and comprises a pair of yokes, a pair of compression bars mounted between said yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a single pair of tensioned elements connecting said yokes and main-tained in tension by said compression members, said elements having been tensioned by expanding the elements using heat and fitting the elements to the yokes in a heated condition so that the elements are tensioned upon subsequent cooling.

9. A pre-stressed frame for a press, comprising a pair of yokes, a pair of compression bars mounted between said yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a pair of tensioned ele-ments connecting said yokes and maintained in tension by said compression members, said elements having been tensioned by expanding the elements using heat and fitting the elements to the yokes in a heated condition so that the elements are tensioned upon subsequent cooling.

10. The improvement claimed in claim 8, wherein said tensioned elements are formed by portions of a single annular member encircling said yokes and compression bars.

11. A frame as claimed in claim 9, wherein said ten-sioned elements are formed by portions of a single annular member encircling said yokes and compression bars.

12. A frame as claimed in claim 9, wherein said tensioned elements form part of a U-shaped tension strip extending over an upper one of said yokes and outwardly of said compression bars and coupled at respective ends thereof to the other said yoke.

13. A method of making a pre-stressed frame for a press comprising the steps of:
providing a pair of yokes, a pair of compression bars mounted between said yokes at opposite sides of the frame and at their ends directly abutting the yokes, and a single pair of tension elements adapted to be connected to said yokes with said elements in an expanded condition;
heating the elements to said expanded condition;
fitting the elements to the yokes; and, allowing the elements to cool and contract, pre-stressing the frame.