The invention relates to a supporting arm for an object, for example, a lamp fitting, a magnifying glass or similar, comprising a swivel link wherein a cam is incorporated having a cam surface in the rotary plane and 'in spring-actuated contact interaction with a cam follower, each attached to its part of the swivel link.

German Patent Publication No. 521997 makes known a frictional link for a tiltable or pivoting supporting arm. A spring- actuated brake shoe in the tubular supporting arm is pressed into contact with a cam in the form of an evenly curved bracket, which passes through a recess in the tubular supporting arm. The bracket is curved concentrically around the axis of rotation of the supporting arm, and the interaction between the bracket and the brake shoe is thus a constant frictional or braking interaction across the rotary field. The difference in the force which is to be braked - the deflection of the supporting arm gives rise to changes in the torque arm - is taken up by the frictional force, which is dimensioned according to the greatest expected moment of force.

DE 2516142 B2 teaches and describes a rotating-tilting link which is designed such that in all positions there is a sufficient clamping or braking force, for securing the rotating-tilting position. A spherical body on the moveable arm is actuated by a spring-loaded clamp/brake body. Here too, the turning position of the braking part is secured in that the braking mechanism is dimensioned for the greatest anticipated load on the link.

The objective of the invention is to provide a supporting arm which will have a carefully balanced movement in its rotary field, on both sides of the vertical.

According to the invention, a supporting arm is therefore proposed as mentioned by way of introduction, characterised in that the cam surface is so formed (profiled) that the required force of the spring essentially corresponds to its linearly progressive characteristic during the relative movement between the cam follower and cam surface when the supporting arm is rotated about the swivel link.

By means of the invention, a balancing mechanism for a supporting arm is thus provided, which mechanism consists of a spring-loaded cam follower which presses against a cam surface or cam profile, designed and dimensioned with precision. When the supporting arm is turned in the rotary plane, the distance between the centre of rotation and the contact of the cam profile against the cam follower increases.

At the same time there are changes in the angle of incidence between the cam profile and cam follower. These changes can be calculated accurately, so that the required force from the spring essentially will correspond to its linearly progressive characteristic during the relative movement between the cam follower and cam surface when the supporting arm is rotated.

By means of the invention a pivotally mounted supporting arm is obtained where the arm will have a carefully balanced movement on both sides of the vertical (assumed vertical rotary plane) . This makes possible the use of a short and relatively weak spring element relative to the work performed by the spring.

It also makes possible a compact and closed structure, with accompanying hygienic and aesthetic advantages, in particular when the supporting arm is used as a lamp fitting support.

According to the invention, the swivel link may to advantage include a housing which completely encloses the cam and cam follower.

According to the invention, the cam follower may be attached to the supporting arm, said supporting arm being hollow and the spring being positioned in the supporting arm, but the cam follower, according to the invention, may also be attached to the housing, with the spring advantageously positioned in a housing portion formed as the mounting base for the swivel link and thus for the supporting arm.

When the spring is positioned in the supporting arm, the cam follower may to advantage be slideably arranged in the supporting arm.

When the cam is attached to the supporting arm, the supporting arm may to advantage be hollow and accommodate a guide pin on the cam, a shaft journal in the swivel link then advantageously passing through *:he cam.

The invention shall now be explained in detail below in connection with an embodiment example, an account of the parameters and calculations which form the basis for the invention also being given.

With reference to the drawings:

Fig. la and b illustrate the novel balance mechanism in two different positions of the supporting arm,*

Fig. 2 shows a cam profile formed.and dimensioned with the cam follower radius as a reference dimension;

Fig. 3 is a table where the relative force required is calculated for different positions of the arm;

Fig. 4 is a diagram showing the movement of the cam follower relative to the force required;

Fig. 5 is a side view of a supporting arm according to the invention; and

Fig. 6 is a side view of supporting arm embodiment modified in relation to the embodiment in Fig. 5.

Fig. 1 illustrates a supporting arm 1, pivotally mounted in a swivel link, which here is represented by a pivot 2.

The arm 1 is attached to a cam 3, the cam surface or cam profile of which is designated by means of the reference numeral 4 and lies in the rotary plane of the supporting arm 1, as determined by the shaft journal 2. A cam follower 5 rests against the cam surface 4, when actuated by a spring (not shown) symbolised by the arrow F- _j^ respectively F ₂ . The cam follower 5 constitutes here a part of the stationary portion of the swivel link, if the supporting arm 1 is looked upon as the moveable portion of the swivel link.

In Fig. la, the supporting arm is shown in an almost vertical position, with a rotary anglec ₁ relative to the vertical (horizontal shaft journal 2) . In Fig. lb the supporting arm 1 is shown in a rotary position where the supporting arm forms an angle# ₂ relative to the vertical.

Important parameters in the balance mechanism illustrated in Fig. 1 are elastic force F-_ angle of incidence β at the point of contact between the cam follower 5 and the cam profile 4, and the distance 1 between the axis of rotation 2 of the arm 1, and the point of contact between the cam profile and the cam follower.

As mentioned, Fig. 1 illustrates the balance mechanism in two arm positions a and b. The elastic force F- _^ , the angle of incidence β-^ and the distance 1^ in combination will counterbalance the torque of the arm by means of a rotary

movement angleθ-T-_. If the weight to be supported is G, and the length of the supporting arm is L, the necessary elastic force F _n in arm position©.: will be:

G x L x sinøc^

^F n " or l _n x tan/S _n

sino-^

Fn_ = if the constant G x L is omitted (=1) l _n x tan/3 _n

A cylindrical helical spring gives a force or tension which is proportional to the distance it extends or compresses. The characteristic of the spring is rectilinear progressive.

On the basis of this characteristic and with the aid of the above formula, the cam profile 4 can be formed and dimensioned as shown in Fig. 2, where the reference dimension is the punch radius r.

As documentation of the correctness of the dimensions and also to show how critical these are, we shall now refer to Fig. 3, which is a table where the relative force required (G x L omitted) has been calculated in arm positions from 0° to 60°. In this graphical representation, Fig. 4, the X axis constitutes the movement S of the cam follower 5, which is actuated by the arm rotationoT, whilst the Y axis represents the force required F.

it can be seen that the two parameters 1 and β cause the necessary elastic force F to increase in rectilinear proportion to the cam follower movement (and the angle°C) so that it coincides with the linear progressive characteristic of the spring. Thus, balance is achieved in the supporting arm.

The dimensions of an extension spring or a compression spring can be calculated on the basis of a desired spring characteristic. The formulae for this are drawn up as follows:

^F max ^{x R} f x d x 132 ^F max ^s P ^r --- ⁿ 9 movement d = 3 i ⁿ = - _> ^{f =}

0.1963 x K R ^J x F _m ma, _v x F _m ma, _v x - Fmm■,_,

wherein d = wire thickness, R = the mid radius of the spring, stipulated, K = the slide resistance of the material 80 kg/mm ^*4 , n = number of turns, f = spring extension incl. initial tension. All measurements are in mm.

If the punch radius is set at 5 mm, the arm length at 400 mm and the weight it is to support at 1 kg, the spring will, for example, require the following dimension: d = 2.1 mm, R = 4 mm, n = 13.

Fig. 5 illustrates an example of a practical utilisation of the invention. In Fig. 5, the supporting arm 1 forms a component in a supporting structure consisting of two interarticulated arm parallelograms 7, 8, 9, 10 and 1, 6, 9, 12. The link portion 10 is, for example, intended to support a lamp base or lamp attachment which is not shown. The portion 12 has shaft journal 13 for pivotal mounting in a table clamp or wall fitting which is not .shown in the drawing.

In a known way per se the arm parallelogram 7, 8, 9, 10 is balanced by means of a helical spring 14, one end of which is attached to the arm 8 and the other end of which is attached to a steel wire portion 15 that is hook-shaped at the ends, the second end of which is hooked into the end of the bar 7.

The supporting arm 1 is hollow and accommodates a spring 16, one end of which is attached to the supporting arm 1 and the second end of which is secured to the cam follower 5, which is slideably arranged in the hollow supporting arm 1 and interacts with a cam surface 4. This cam surface 4 is attached to the part 12, which made be made in the form of a housing which encloses the lower ends of the supporting arm 1 and the bar 6. The cam surface 4 and the cam follower 5 interacting therewith on spring actuation are thus well protected in the housing 12 and in the hollow supporting arm 1, which is mounted in the housing 12 by means of a shaft journal 2. The shaft journal 2 may be divided, or it may pass through the slideable cam follower 5, said cam follower being provided with a corresponding longitudinal through-going opening (not shown) .

When the supporting arm 1 is turned towards the right or the left in Fig. 5, the cam follower 5 will move relative to the cam profile 4 (see Figs. 1 and 2) . The cam profile is made in such a way that the force required from the spring 16 essentially will correspond to its linear progressive characteristic during the relative movement between the cam follower and the cam surface. The supporting arm 1 and the supporting structure of which this forms an essential part, will thus be mounted pivotally such that a precisely balanced movement to both sides of the vertical (horizontal pivot pin 2) is achieved.

The modified embodiment illustrated in Fig. 6 differs from the one shown in Fig. 5 in that the cam 3 is attached to the supporting arm 1 and the cam follower 5 is now attached to the fixed part of the swivel. The spring 16 is now located in the hollow pin 13, which constitutes a part of housing 12, and tensions the cam follower 5 against the cam 3 having cam surface 4 mounted in the supporting arm 1.

The cam 3 has a guide pin 17. This guide pin passes into the

hollow supporting arm 1. The cam 3 is secured in the supporting arm 1 by a pivot pin 2 extending through the cam.

Before the supporting arm 1 is rotated in the paper plane, i.e., about the pivot pin 2, the interaction between the cam surface 4 and the cam follower 5 will ensure that the force required from the spring 16 essentially will correspond to its linear progressive characteristic during the relative movement between cam follower and cam surface.

In the embodiment in Fig. 5, the spring 16 is an extension spring. In the embodiment in Fig. 6, the spring 16 is a compression spring, but the reverse could also be the case, by designing the respective cam profiles as cam grooves.

With reference to Fig. 5 and Fig. 6, an example is shown of a practical application together with interarticulated arm parallelograms. However, it would also be possible to use the supporting arm 1 with the swivel link according to the invention directly connected to a supporting surface for, e.g., a television, a monitor or similar without an arm parallelogram.

The supporting arm 1 respectively the shaft journal portion 13 in Fig. 5 would then form the part connected to, e.g., a supporting surface, and the respective second part, the supporting arm 1 or, for instance, be fixed.