GB2331795A - Springs - Google Patents

Abstract

A spring 10 comprises two pivot portions 14. The pivot portions 14 are connected at their respective ends by part circular resiliently compressible portions 12. A method of making springs comprising stamping a blank 20 from a sheet of material, which blank has two pivot portions 14 and a pair of part-circular compressible portions 12 joining the ends thereof; and folding the part-circular portions 12 into a desired position relative to the pivot portions 14 is also disclosed.

Description

65877.597 2331795 Springs This invention relates to springs and more

particularly, but not exclusively, to springs for providing an overcentre bias for pivotally mounted bistable lever mechanisms such as those used in thermally responsive controls for liquid heating vessels such as kettles and hot water jugs, for example the Applicant's well known R72 control.

Traditionally, such lever mechanisms employ a C, spring which is formed by bending a rectangular strip of metal into a curve. The spring is mounted between a fixed seat and a seat on the lever in such a way that the distance between the two seats is at a minimum at a position intermediate the end points of the lever's permitted travel. The spring is compressed as the lever is moved from either end of its travel, and so biases the lever back towards the respective end of its travel until it passes over the centre point, at which point it will be biased towards its other end point with a snap action.

The spring should bias the lever into its end positions with sufficient "up,' force to retain it there under normal operating conditions, but not so great a force that the mechanism cannot be moved from those positions by an actuator, such as a bimetallic actuator. An up force of between 509 and 100g is typical. Also, a movement typically of.75 mm is needed to move the mechanism from its end position to its overcentre position, this movement being typical of that produced in a bimetallic actuator.

A typical 'Cl spring for generating that size of up force and degree of movement would be relatively long, and would also generate significant axial forces in the spring. Indeed, a spring of say 12-13mm in length would typically generate an axial load in the region of 1.5kg is to produce an up force of 85g. Such a force is transmitted to the spring mountings, which typically are of plastics. These mountings must, therefore, be substantial. Furthermore as these mechanisms are often used in high temperature environments such as under the base of a heating vessel, thermal creep may occur under these forces. Accordingly, temperature resistant plastics must be used, which is expensive.

A further problem with such arrangements is that the spring moves through a relatively small angle (typically < 1.50) in producing the requisite movement, which means that there must be very accurate alignment between the mounting points to ensure satisfactory operation. This tight tolerancing adds to manufacturing costs.

Further, the length of the spring itself is significant, and it may mean that a compact design may not be easily achievable.

The present invention seeks to provide a spring which overcomes the above problems, and from a first aspect provides a spring comprising two pivot portions connected at respective ends thereof by resiliently compressible arcuate portions.

In accordance with the present invention the separation of the two pivot portions, which are resiliently movable towards each other, is not constrained by the length of the resiliently compressible portions, which are located substantially away from the pivot portions themselves. The separation between the pivot portions may therefore be significantly reduced, making for a more compact construction. Furthermore, the reduction in this spacing between the pivots also means that for a given up force, the axial force in the spring and thus acting on the supports is very significantly reduced, meaning that the mountings need not be produced in specialised plastics materials. This also means that for a given actuating movement, the actual pivotal movement of the springs is greater. A larger designed movement makes tolerancing of mounting components less stringent, which further reduces cost.

Springs in accordance with the invention may by of any suitable resilient material e.g. plastics, and may comprise more than one material. Preferably however the spring is integrally formed from metal, most preferably steel.

The spring may be formed from wire. However, if formed from wire, the pivot portions will tend to rotate in their pivotal seats, thereby producing wear.

Accordingly, the spring is preferably formed from sheet or strip material, preferably having a thickness in the range 0.1-0.15 mm. This is advantageous in that the edges of the pivot portions engaging in their seats will act as knife edges, reducing friction and thereby improving operation as compared to a wire spring. The spring can most easily be stamped out as a blank from sheet or strip material.

Preferably the resilient arcuate portions each comprise a single turn connecting the respective ends of the pivot portions. The arcuate portions preferably are substantially circular in shape, extending over an arcuate angle typically of over 27C.

The arcuate portions may lie in the same plane as the pivot portions, but preferably they are arranged at an angle of between 50 and 950 with respect to the pivot portions, more preferably 300 to 90' and most preferably 450 to 800. By 'folding up, the resiliently compressible portions the width of the spring is reduced allowing it to be used in confined spaces.

In the preferred embodiment, the arcuate portions are slightly domed prior to being folded up to their operative positions. This is done to ensure that the arcuate portions will deform in the same sense as they are compressed during use.

is The preferred spring geometry is therefore a relatively complex 3dimensional geometry which will involve forces bending and torsional forces as opposed to just pure bending forces in the case of a 'Cl spring.

The preferred geometry will produce a spring with a lower spring rate than a traditional 'C' spring.

Preferably the pivot portions are provided with respective flange portions which act to provide a stop when they touch thereby preventing overstressing of the spring, particularly during installation. They may also be used to facilitate the installation of the spring. They also act to rigidify the knife edges. Preferably the flange portions extend substantially at right angles to the pivot portions, and they may most easily be formed simply by folding over respective regions of the pivot portions.

As stated above, springs in accordance with the present invention are advantageously formed from sheet or strip material and bent into the desired configuration.

When viewed from a second aspect therefore the invention provides a method of making a spring comprising stamping a blank from a sheet of material, said blank having two pivot portions and a pair of compressible arcuate portions joining the ends of the pivot portions, and folding the arcuate portions into a desired position relative to said pivot.

The blank may be scored to facilitate this bending. The arcuate portions may be domed, as discussed above so as to control, their deformation.

Preferably the method of the present invention comprises forming a bend in the pivot portions to form a flange portion. Again, this bend may be scored to facilitate bending.

It will be appreciated that the invention also extends to a switch mechanism incorporating a spring in accordance with the invention and to a thermally sensitive control for a liquid heating vessel incorporating such a switch.

is A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a side view of a prior art C spring mounted in a lever mechanism for comparison purposes;

Fig. 2 is a force diagram for the arrangement of Fig. 1; Fig. 3 is a perspective view of a spring in accordance with the present invention; Fig. 4 is a force diagram for the spring of Fig. 3; and Fig. 5 is a view of a blank for forming the spring of Fig. 3.

Turning to Fig. 1 there is shown a typical prior art C spring over-centre lever arrangement for comparison with a preferred embodiment of the present invention. The lever 102 pivots about a knife edge 104 between an upper and lower position. In this example, the lever forms part of an electrical control for a liquid heating vessel. The distal end of the lever 102 is therefore acted upon by the movable portion of a bimetallic actuator 112 e.g. as part of a dry switch-on protection device. An overcentre C spring 106 is located between the distal end of the lever 102 and a mounting post 108. The edges 110 of the spring 106 form knife edges to allow the spring to pivot. For the bimetallic actuator 112 to move the lever 102 from its upper to its lower position, it must press the lever 112 in order to move it past the centre point shown, thereby compressing the spring 102 slightly. The resilience of the spring which tends to try to re-extend it, therefore biases the spring 102 into its upper or lower position accordingly.

Fig. 2 shows a simplified force diagram which will is be used to explain a particular example. In this example the movement required for the end of the lever from its upper position to the centre point, d = 0.7 mm. This length of movement may be chosen e.g. to ensure that a set of electrical contacts (not shown) is opened when the bimetallic actuator operates on the lever 102. The force with which the lever is biassed into its upper position must be sufficient to hold it there during normal operation, but must not low enough that it may be overcome by the actuation force of the bimetallic actuator 112. The vertical biassing force required in this example, Fv = 0.875N. The length of the spring (distance between its pivots) at that position L, = 12 mm. The axial force F on the spring 102 in order to develop the necessary vertical bias at the angle e from the centre-line, is given by F, = F1d L, F, = FvL, d 0.875x12 = 15N 0.7 Thus the lever 102 and mounting post 108 must be able to withstand a constant force of at least 15N (equivalent to a weight of approx. 1.5 kg) during normal use. Where the spring is in a high temperature environment such as a liquid heating vessel, only expensive high performance plastics will be able to withstand such a force.

It would appear from the above equation that in theory the axial force could be reduced whilst maintaining the same biasing force as long as a shorter spring were used. However this is not possible in practice since a shorter C spring would either be even stiffer or would have to be deformed past its elastic limit, rendering it unusable.

Turning to Fig. 3 however, there is shown a spring 10 according to the present invention. The device comprises two resiliently compressible arcuate portions in the form of generally frusto-conical loops 12. The loops 12 join respective ends of two pivot portions 14. Part of each pivot portion 14 is bent upwardly to form two laterally facing flanges 16. As the two pivot portions 14 are moved together the radius of the loops 12 is decreased and therefore their conical angle will tend to increase. The natural separation of the two flanges 16 is chosen to be large enough for the compression required during use and also to allow the device to be compressed sufficiently to install it. The flanges 16 however prevent excessive compression of the device, particularly beyond its elastic limit. The central part 18 of each pivot portion is used as a knife edge so as, in use, to pivot in a corresponding V groove with minimum friction. The knife edges 18 are separated by 6 mm in this embodiment.

A second example will now be given with reference to Fig. 4 in which the spring 10 has been pivotally mounted by means of its knife edges 18 between the distal end of the lever 102 and the mounting post 108 as in Fig. 1. The separation of the two pivot knife edges 18, i.e. L2 = 2. Smm.

Assuming the same vertical bias, F2 and vertical travel to the centreline, d as the previous example, the axial force F is given by:

F2 = RL2 d = 0.875x2.5 = 3.2N 0.7 Thus it will be seen that a much lower axial force on the spring is required than for the C spring and therefore there is a correspondingly lower requirement on the strength of its mountings.

A comparison of the angle through which the two springs move from one end position to the overcentre position gives:

Sin E), = _d = 0.7 L, 12 - E), = 3' but Sin e2 = d = 0.7 L2 2.5 - e2 = 15 0 Thus the spring 10 moves through a much greater angle than does the C spring 106.

Typically, a 'C' spring would have to be compressed by about 1.5mm to produce a 15N spring force, ie about 1ON/mm. However, with the described embodiment of the invention, the amount of movement of the pivot portions of the spring towards one another to produce 3N is about 2.5mm, ie about 1.2 N/mm. Accordingly, this allows a greater tolerance in the manufacture of the lever 102 and mounting post 108 as say a difference in position of say.25 mm would produce a much smaller difference in spring force in a spring of the invention than in the prior art arrangement.

The potential problem of increased friction with a larger pivot angle is minimised by forming the knife edges 18 as thinly as possible to minimise the contact area between the edges 18 and their mounting grooves.

A method of making a spring in accordance with the invention will now be described with reference to Fig.

5. This Figure shows a spring blank 20. This blank 20 is formed by stamping it out of a flat sheet of steel.

In contrast to the steel used to manufacture C springs, this steel will never be subject to very high loads and will consequently operate far from its elastic limit. This means that a lower grade of steel may be used without compromising the likelihood of the spring failing and therefore that the cost of the spring can be reduced.

As well as stamping the blank 20 from the planar sheet, a press is used, either as part of or separately from the stamping tool, to dome spherically the two arcuate portions 12 and give them a generally frustoconical shape. This ensures that the direction in which the loops bend can be preselected rather the two loops for example bending in opposite directions. This doming is not however essential and the two loops may be instead be left substantially planar without preventing the spring from operating properly.

The dotted lines 22,24 are bend lines which may or may not be preweakened, e.g by the stamping or pressing tool. These lines are not required to give resilience to the spring and therefore may be preweakened without weakening the action of the spring. To form the blank 20 with the domed arcuate portions 12 into the spring 10 shown in Fig 3, firstly the two flanges 16 are bent upwardly along the bend lines 22. The two loops 12 are then bent downwardly along bend lines 24.

It will be appreciated that a lever arm mechanism as described above need not move symmetrically about the overcentre position. For example, while in one end position the lever may be say.7mm from the centre position, in the other it maybe as much as 3mm. This being the distance by which a set of contacts must be opened to be regarded as 'off,. In that situation, with a spring of the invention, the angle rotated by the spring to that latter position will be even greater, so the spring force acting on the spring mountings in that situation will be even smaller than in the other end - position. In the case of a 'Cl spring, this angle will still be relatively small, so there will not be such a significant reduction in spring force.

Fro the above, it will be seen that the invention provides a low rate spring mechanism which reduces forces acting on spring supports during use, allowing less expensive materials to be used. A more compact arrangement may also be achieved.

Claims (13)

Claims

1. A spring comprising two pivot portions connected at respective ends thereof by resiliently compressible arcuate portions.

2. A spring as claimed in claim 1, wherein said arcuate portions are formed integrally with said pivot portions.

3. A spring as claimed in claim 1 or 2 wherein said spring is of metal.

4. A spring as claimed in claim 1, 2 or 3 which is formed from a sheet of strip material.

5. A spring as claimed in any preceding claim wherein said arcuate portions comprise a single turn.

6. A spring as claimed in claim 5 wherein said arcuate portions are substantially circular.

7. A spring as claimed in claim 5 or 6 wherein said arcuate portions extend over an arcuate angle of greater than 2700.

8. A spring as claimed in any preceding claim wherein said arcuate portions are arranged at an angle of between 50 and 950 with respect to the pivot portions.

9. A spring as claimed in any preceding claim wherein said pivot portions are provided with flange portions which act as stops to prevent overstressing of the spring.

10. A method of making a spring comprising stamping a blank from a sheet of material, said blank having two pivot portions and a pair of compressible arcuate portions joining the ends of the pivot portions; and folding the arcuate portions into a desired position relative to said pivot portions.

11. A method as claimed in claim 10 comprising scoring said blank to facilitate said folding.

12. A method as claimed in claim 10 or 11 comprising bending the pivot portions to form flange portions.

13. A method as claimed in claim 10, 11 or 12 comprising doming said arcuate portions prior to folding them.