DE29820285U1 - Device for weighing loads - Google Patents

Description

BUSE MENTZEL LUDEWIG Patent Attorneys

Authorized representative before the European Patent Office Dipl.-Phys. Buse

PO Box 201462 Kleiner Werth 34 Dipl.-Phys. Mentzel

D-42214 Wuppertal D-42275 Wuppertal Dipl.-Ing. Ludewig

⁷⁶ Wuppertal-Wuppertal,

Keyword: "Torsion-free load introduction"

Company Runzheimer Sensortechnik GmbH, Holthauser Straße 65,
42369 Wuppertal

Device for weighing loads

The invention relates to a device for weighing loads, which is particularly suitable for use in mobile weighing systems and lightweight weighing systems.

Every direct weighing method is fundamentally based on the principle that a weighing bridge loaded with the load to be weighed is connected to a fixed base via measuring cells. A wide variety of designs of base, measuring cells, load introductions and weighing bridge have been implemented.

Due to their robustness, compact design and the direct generation of an electrical signal that can be easily digitized,

TELEPHONE (0202)55 70 22 COMMERZBANK AG CREDIT- AND VOLKSBANK EG POSTAL OFFICE VAT No. TELEX 8591606 wpald WUPPERTAL 4 129 805 WUPPERTAL103 490014 COLOGNE 72634-506 DE 121035988 TPI FFAY imim &sfgr;&tgr;&igr;^&eegr;&igr; Kl 7 '!"!ndnnni Rl 7 &EEacgr;&Ogr;&Lgr;&Pgr;&Pgr; OR

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In the industrial sector, measuring cells with strain gauges have become established.

In general, efforts are made to construct the base and weighing bridge as stably as possible, whereby "stable" in this context means that the tensions and elastic deformations of the structure that occur when the loads intended for weighing are applied are minimized as much as possible. This requires special stiffening measures, particularly for large loads of several 100 to several 1,000 kg, as often occurs in the industrial sector, which are material and space-intensive and cause considerable costs due to the necessary development work.

Particularly in the area of mobile weighing technology, these measures run counter to the primary goal of realizing light and flexible constructions. This is particularly evident in the area of pallet truck technology. Conventional pallet trucks, which can have a sufficiently stable chassis for the installation of conventional weighing devices, only offer a lift of around 12 cm. This may not be sufficient, especially when used in the area of loading ramps, work tables, etc. There are pallet trucks, so-called scissor lift trucks, which can achieve lifts of 80 cm with the help of lifting scissors. However, this advantage generally comes at the expense of stability. For example, the chassis of a scissor lift truck is often designed as a hollow profile construction, with sliding or roller bearings for the movement of the lifting scissors being arranged in the hollow spaces that can be provided for stabilization structures in conventional pallet trucks. Additional difficulties are caused by the different support point of the lifting forks through the scissors depending on the lifting height. As a consequence, when the weighing bridge, which is designed as a weighing shoe, is loaded with the

Buse · Menfzel ·* Ludew*g*

The load to be weighed causes significant bending and torsion effects on the lifting forks. This effect is even stronger the more decentralized the load is placed.

The bending of the lifting forks has two effects:

On the one hand, the linear distance between the tip and the base of the lifting fork is shortened, and on the other hand, the support surface of the lifting fork takes on different angles in relation to the weight of the load to be weighed at different positions. In this context, "torsion" is understood to mean the inward or outward rotation of the lifting fork elements.

The three deformations mentioned mean that the weight no longer acts as a normal force on the measuring cell(s) used, which subjects the cells to additional torsional, bending and tensile forces. This is where the otherwise particularly advantageous strain gauges show considerable weaknesses. Even minimal tension in the measuring cells can lead to significant, non-reproducible errors in the measurement. Since measuring cells that work according to a different principle are not suitable for the industrial use described for various reasons, only inadequate solutions are currently available, particularly in mobile weighing technology.

The use of forklifts and high-lift trucks with weighing devices can only partially solve the problem and is also much less costly considering the associated costs.

However, measuring cells based on strain gauges with special load introduction are known from the field of aircraft construction or container weighing technology. Such measuring cells consist of a body, a measuring head, which can be connected in one piece to the body, and

Buse · Menfeel •"Ludewig*

·

on which the strain gauges are located and a bearing block for load absorption and introduction. The bearing block is connected to the measuring head via a bearing axis attached to it. It is mounted in such a way that it can be moved axially and has a degree of freedom of rotation around the bearing axis mentioned. A spherical bearing is also provided, which allows additional degrees of freedom of rotation around all three spatial directions, whereby the rotations around the axes perpendicular to the bearing axis are of course angle-limited.

These degrees of freedom should be named as follows:

&Dgr; denotes the axial translation of the bearing block,
φ is the angle of rotation around the bearing axis,

&THgr; the tilt angle around the axis perpendicular to the bearing axis and perpendicular to the
Weight of the load to be weighed and

&psgr; the tilt angle around the axis parallel to the weight of the
load to be weighed.

Such load cells are used, for example, in aircraft construction, where particular emphasis is placed on light, albeit sometimes very different materials, which are also exposed to extreme temperature fluctuations. Here, there can be significant differences in the expansion lengths due to different expansion coefficients during temperature changes. In this area, the axial degree of freedom Δ is used in particular. In container weighing, where containers of different sizes are filled while hanging on a frame, such cells are provided at the suspension points on the frame. The suspension devices, such as chains or hooks, take different angles to the weight force depending on the size of the container to be weighed. These differences

Buse · Menfzel ·* Ludewig

can be compensated in particular via the rotational degrees of freedom Θ and ψ.

The aim of the invention is to use the load introductions known from completely different areas to advantage in the field of mobile weighing technology and thus to find weighing devices that meet the requirements for lightness and flexibility as well as for measuring accuracy. At the same time, the use of special stabilization devices should be largely dispensed with, which are labor- and material-intensive and cannot be used in space-critical applications. An example of this is again the pallet truck, which is drastically limited in its possible height by the standard for the Euro pallet.

These objects are achieved by the features of the characterizing part of claim 1, which have the following meaning in detail:

The aim is to arrange several measuring cells in such a way that all tensions and deformations can be absorbed by the degrees of freedom of the load introduction. To do this, the measuring cells are first arranged in such a way that an imaginary polygon, the corners of which are defined by the positions of the cells projected onto the plane of the weighing bridge, largely encloses the load-bearing surface of the weighing bridge. This ensures that the non-normal forces, i.e. those force components that do not act in the direction of the weight force exerted by the load to be weighed, are distributed at least approximately evenly across the various measuring cells. At least three measuring cells are required to define such a polygon.

Buse · Menfeel ·* Ludewig

Furthermore, it must be ensured that the measuring cells are arranged in such a way that they can compensate for the non-normal forces acting on them. Finally, it must also be ensured that all load introductions are mechanically in a position before the load is applied that allows the compensation of tensions and deformations in any direction that may occur, i.e. the bearing blocks of the measuring cells must not be "at the stop".

This latter goal is achieved by the second feature of the characterizing part of claim 1. If the bearing block of one of the measuring cells is blocked in its axial degree of freedom Δ or at least severely restricted, the position zero point is thereby specified. The other measuring cells are then arranged accordingly. In this way, it is prevented that the load introductions are already in a deflected position even without a load, for example by driving on an inclined plane or the action of force in a collision.

However, the degree of freedom Δ blocked in a measuring cell must not be blocked for the entire weighing device. Therefore, according to the third feature of the characterizing part of claim 1, at least one second measuring cell is arranged at least approximately on the same axis line. The alignment can be parallel or antiparallel to the first cell. The background to this measure is that changes in length caused by the load, which cannot be compensated by the first, blocked load introduction, are now compensated by a correspondingly larger axial displacement of the bearing block of the second cell. In order to avoid tension here, it is important that the second measuring cell is in axial extension of the first.

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The at least necessary third measuring cell is aligned according to the fourth feature of the characterizing part of claim 1 so that it is neither parallel nor anti-parallel to the first measuring cell. In this way, a change in length that occurs perpendicular to the bearing axes of the first two measuring cells can also be achieved by axially displacing the bearing block of the third measuring cell. No further special measure for determining a zero point is required here, since the first two measuring cells, whose load introductions do not have a translational degree of freedom perpendicular to their bearing axis, form a corresponding position zero point.

Further advantages arise from the subclaims, the specific description and the drawings. They show:

Fig. 1 View of the long side of a measuring cell,

Fig. 2 View of the front face of a measuring cell,

Fig. 3 Top view of a measuring cell,

Fig. 4 Scissor lift truck in loaded and unloaded condition

(Side view long side),

Fig. 5 Scissor lift truck in loaded and unloaded condition

(Front view),

Fig. 6 inventive arrangement of the measuring cells in a

Scissor lift truck in top view,

Fig. 7 Fastening of the measuring cells to the shield of a

Scissor lift trucks,

Buse · Menial ·· Ludewig· ··· ··* *'

Fig. 8 Fastening of the measuring cells to the shield of a

scissor lift truck,

Fig. 9 Mounting of the measuring cells on the chassis of a

scissor lift truck.

Figures 1 to 3 show various views of the measuring cells 10 used in the context of the invention. The 3-part structure of body 11, measuring head 12 and bearing axis 13 with bearing block 14 is clearly visible. Body 11 and measuring head 12 are connected to one another in one piece in the embodiment shown. The bearing block 14 is held on the bearing axis with the aid of a locking ring 15. However, four degrees of freedom of movement are retained. The axial, translational degree of freedom Δ and the degree of freedom of rotation about an axis perpendicular to the bearing axis 13 and perpendicular to the normal force Θ are shown in Figure 1. The degree of freedom of rotation φ about the bearing axis 13 is shown in Figure

2. The third rotational degree of freedom ψ around an axis parallel to the normal force can be seen in Figure 3.

The four degrees of freedom are preferably realized in that the bearing block 14 is mounted for a joint bearing (not visible in the drawings), which in turn is mounted rotatably and axially displaceably on the bearing axis 13.

The measuring head 12 comprises the actual sensor, which is based on the principle of double strain gauges. Two blind holes 16 lying opposite one another leave a narrow web 17 in the middle, a few millimetres wide, on the walls of which

Buse · Menftfet ·" Ludewig· ···

Double strain gauges 18 are attached. In order to protect the sensitive strain gauges and their electrical leads 19 from mechanical damage or damage caused by moisture, dirt, etc., the blind holes 16 are hermetically sealed to the outside with metal membranes 20.

In a particularly advantageous embodiment, the measuring head 12 is separated from the body 11, which is fixed to the support 22, by a groove 21. This very precisely defines a point that serves as a reference for the bending to which the measuring cell 10 is exposed when a load acts on the bearing block 14.

The bridge circuit required for the measurement and other electronic devices such as displays, digitizers, etc. can either be integrated into the body 11 or at least partially relocated to more distant control units that are connected to the sensor via the cable feed 23.

Figures 4 and 5 illustrate the main problem for which the present invention offers a solution. A scissor lift truck 24 is shown in its lowest position (dash-dotted line) and in a position with the lifting scissors 25 at least partially extended. The different positions of the lifting scissors 25 in both positions can be seen from the position of the roller 26. In the upper position, the lifting fork 27 is shown once in an unloaded state (solid line) and once in a state loaded with a load 28 (dashed line). As can be clearly seen in Figure 5, the lifting fork 27 consists of two fork elements connected by a shield 29. When the lifting fork 27 is loaded with the load 28, the bending and torsion effects shown exaggeratedly in Figures 4 and 5 occur. Figure 4 clearly shows that

Buse · Mentrei »*Ludewig· ··· ·· *··

When the load is applied, the two ends of the lifting fork are bent in and upwards, which shortens the linear distance between the shield 29 and the fork tip 30. At the same time, a right angle between the fork plane and the normal force is only maintained at one point under the load. Figure 5 clearly shows the additional torsional effect of the individual fork elements. It is obvious that the deflection and torsional effects depend on the mass of the load and its positioning on the lifting fork 27.

It becomes clear what problems arise for weighing technology when strain-sensitive load introductions are used in sensors.

This can be remedied by using the measuring cells described at the beginning with torsion-free load introduction. However, the special arrangement of the measuring cells is of central importance here. The lifting fork 27, which essentially forms the chassis 31 of the pallet truck, is designed as a hollow profile according to Figure 9. A weighing platform above it, which consists of two weighing shoes 32 connected to one another via the weighing plate 33, is connected to the chassis by means of the four measuring cells MZ 1, MZ 2, MZ 3 and MZ 3', as can be seen in Figure 6. In the embodiment shown in Figure 6, the axial degree of freedom Δ of the bearing block 14 is blocked. As already explained at the beginning, this defines a mechanical position zero point. The necessary axial play is, however, ensured by the unblocked bearing block 14 of the second measuring cell MZ 2. This is arranged on the same axis line as MZ 1. A particularly advantageous way of fixing the measuring cells is shown in Figures 7 and 8. A support 21a welded to the plate 29 provides the foundation on which the body 11 of a measuring cell is fixed, preferably by means of screws. The weighing plate 33 is attached to the bearing blocks 14 of the measuring cells MZ 1 and MZ 2.

Buse · Men«** »»Ludewig· ··· ··'

In Figure 7 and Figure 8 it can be clearly seen that the measuring cells MZ 1 and MZ 2 are arranged in the extension of their respective axis lines. In the embodiment shown, the arrangement is antiparallel. Of course, the effect of MZ 1 taking over the deflection by MZ 2 would also be achieved with a parallel alignment. For reasons of symmetry, however, the embodiment shown is preferable.

In order to take into account the translational degree of freedom that is perpendicular to the axis line of MZ 1 and MZ 2, at least one additional measuring cell MZ 3 must be provided that is neither parallel nor antiparallel to the axis line mentioned. MZ 3 is preferably aligned perpendicular to MZ 1 and MZ 2. This can be seen in Figure 6. Due to the special symmetry of a forklift truck, it is advisable to arrange an additional measuring cell MZ 3' near the second fork tip 30. Figure 9 shows a particularly advantageous type of attachment. A support 22b is welded into the chassis 31 of the scissor lift truck 24, which is designed as a hollow profile. The body of the measuring cell MZ 3 or MZ 3' is attached to this, preferably by means of screws. The connection to the weighing shoe 32 is made through a recess 34 in the chassis 31. This can be designed as an opening or, as shown in Figure 6, as a slot. The weighing shoe 32 is then preferably attached to the bearing blocks 14 of the measuring cells MZ 3 and MZ 3' using screws. Blocking a degree of freedom of the load bearings of MZ 3 or MZ 3' is not necessary, since the measuring cells MZ 1 and MZ 2, which are perpendicular to it and do not have a translational degree of freedom perpendicular to their axis line, define a sufficiently stable position zero point.

The advantageous arrangement of the measuring cells shown ensures that all torsions and tensions that occur are compensated by the load absorption of the measuring cells, so that the

Buse · Mentetei ·» LudeWig* ···

Strain gauges are only subjected to the shear force exerted by the normal component of the weight. This ensures reliable and reproducible measurement even when the lifting truck is asymmetrically loaded. At the same time, stabilizing or stiffening devices can be dispensed with in the hollow profile 31, which makes it possible to run the bearings of the lifting scissors 25 in this hollow profile. In this way, it is possible to construct a forklift truck with a weighing device that on the one hand does not exceed the low maximum construction height (pallet clearance height) specified by the Euro pallet standard, but which nevertheless allows high lifts and ensures reliable weighing of the loads. Due to the lack of stiffening measures, such a scissor lift truck with a weighing device is significantly more cost-effective than a forklift truck or high-lift truck with similar performance characteristics.

Of course, it is also possible to transfer the advantageous arrangement shown using the example of the scissor lift truck to other areas. This is particularly true of stationary weighing technology, where the arrangement can be used to advantage in that stiffening measures previously considered necessary can be partially omitted, which leads to significantly more cost-effective production. The arrangement must obviously be adapted to the respective geometries of the base and weighing bridge. The number of load cells used is also variable. In this respect, the embodiments shown serve to illustrate the invention and are not intended to limit its scope of protection in any way.

BUSE MENTZEL LUDEWIG Patent Attorneys

Authorized representative before the European Patent Office Dipl.-Phys. Buse

PO Box 201462 Kleiner Werth 34 Dipl.-Phys. Mentzel

D-42214 Wuppertal D-42275 Wuppertal Dipl.-Ing. Ludewig

¹ ° Wuppertal,

Keyword: "Torsion-free load introduction"

Company Runzheimer Sensortechnik GmbH, Holthauser Straße 65, 42369 Wuppertal

List of reference symbols:

10 Measuring cell

11 Corpus

12 Measuring head

13 Bearing axis

14 Bearing block

15 Retaining ring

16 Blind hole

17 Bridge

18 strain gauges

19 electrical supply line

20 Membran

21 Throat

22 supports

22a Support on

22b Support in

23 Cable feed

24 scissor lift trucks

25 Scissor lifts

26 Role

27 Lifting fork

28 Load

29 Shield

30 Fork tip

31 Chassis

32 Weighing shoe

33 Weighing plate

34 Recess in

TELEPHONE (0202)55 70 22 COMMERZBANK AG CREDIT- AND VOLKSBANK EG POSTAL OFFICE VAT No. TELEX 8 591606 wpatd WUPPERTAL4129805 WUPPERTAL103 490014 COLOGNE 72634-506 DE 121035988 FAX (0202) 571501 Bank code 33040001 Rl.7 HdÄnnq« RiV &Tgr;7&Pgr; mn cn

Claims (9)

BUSE MENTZEL LUDEWIG Patent Attorneys Authorized representative before the European Patent Office Dipl.-Phys. Buse PO Box 2014 62 Kleiner Werth 34 Dipl.-Phys. Mentzel D-42214 Wuppertal D-42275 Wuppertal Dipl.-Ing. Ludewig Wuppertal-Wuppertal, 70 Keyword: Torsion-free load introduction Runzheimer Sensortechnik GmbH, Holthauser Strasse 65, D-42369 Wuppertal Protection claims 1. Device for weighing loads, consisting of a base and a weighing bridge, the base is connected to the weighing bridge via several load cells, whereby the load cells each consist of a body, a measuring head with strain gauges, whereby the measuring head can be connected to the body in one piece, and a load-bearing device mounted on the measuring head, wherein the load-bearing device is designed as a bearing block which is arranged to be axially movable on a bearing axis connected to the measuring head and which has at least two degrees of rotational freedom, namely both about a first axis of rotation formed by the bearing axis and about a second axis of rotation which is perpendicular to the first axis and perpendicular to the weight of the mass to be weighed, characterized in that at least three load cells are arranged in such a way that the load-bearing surface of the weighing bridge is largely enclosed by an imaginary polygon, the corners of which are defined by the vertical projections of the measuring cells into the plane of the weighing bridge TELEPHONE (0202)55 70 22 COMMERZBANK AG CREDIT- AND VOLKSBANK EG POSTAL OFFICE VAT No. TELEX 8 591606 wpald WUPPERTAL 4 129805 WUPPERTAL103 490014 COLOGNE 72634-506 DE 121035988 FAX (0202) 57)5 01 Bank code 33040001 Bank code 33060098 Rl 7 170 inn ^n Buse · MentzeJ · and that the axial degree of freedom of the bearing block of at least one first measuring cell is largely restricted and that at least one second measuring cell is arranged parallel or antiparallel at least approximately on the same axis line as the first measuring cell and that at least a third measuring cell is aligned neither parallel nor antiparallel to the first measuring cell. 2. Device according to claim 1, characterized in that the third and/or further measuring cell(s) is/are aligned perpendicular to the first measuring cell. 3. Device according to one of claims 1 and 2, characterized in that the measuring head is separated from the body of the measuring cell by a groove. 4. Device according to one of claims 1 to 3, characterized in that supports are attached to the base, on which measuring cells are fixed with their body and the associated bearing blocks are connected to the weighing bridge above. 5. Device according to one of claims 1 to 4, characterized in that the base is designed as a hollow profile construction in which supports are fastened, on which measuring cells are fixed with their body in such a way that the associated bearing blocks protrude through openings or slots in the profile roof and are connected to the weighing bridge above. 6. Device according to one of claims 1 to 5, characterized in that the base is designed as the chassis of a mobile lifting truck, in particular a scissor lift truck. 7. Device according to claim 6, characterized in that the weighing bridge is designed as two interconnected weighing shoes, wherein the weighing shoes at least partially encompass the forks of the lifting truck chassis. Buse · MentEel »iLudewig »·* 8. Device according to one of claims 6 and 7, characterized in that the lift truck is a scissor lift truck. 9. Device according to one of claims 1 to 5, characterized in that the base is designed as a substructure of a stationary weighing platform.