Title of Invention	"LOAD CELL"
Abstract	Load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the deformation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom characterized in that the transition radius assigned to the radial inner limiting wall is bigger by the factor two to ten than the radius assigned to the radial outer limiting wall.

Title of Invention

"LOAD CELL"

Abstract

Load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the deformation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom characterized in that the transition radius assigned to the radial inner limiting wall is bigger by the factor two to ten than the radius assigned to the radial outer limiting wall.

Full Text	This invention relates to a load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the defor¬mation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom. Such a load cell is represented as one of several design examples in DE-PS 1268878. This design which is not realized in actual practice is used to represent a deformation element with square cross section. Geometric relations cannot be taken from this schematic representation where the transition radii of each annular groove which limit the tube sections, are iden¬tical. Almost similar test samples made in accordance with the mentioned representation showed hysteresis effects which could not be tolerated and which excluded a use in legal-for-trade load cells. A reduction of the hysteresis effect could be obtained by a corresponding (over)dimensioning, these designs, however, were of too big size and of unfavorably high material expenditure. The invention is, therefore, based on the objective to produce a reasonably priced, generic load cell characterized by particular compactness which meets all requirements of legal-for-trade load cells. Accordingly, there is provided load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the deformation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom characterized in that the transition radius assigned to the radial inner limiting wall is bigger by the factor two to ten than the radius assigned to the radial outer limiting wall. This problem is solved by the fact that the transition radius assigned to the radial inner limiting wall is approximately bigger by the factor two to ten than the transition radius assigned to the radial outer limiting wall. The solution within the framework of this invention prevents partially plastic deformations causing hysteresis effects by limiting stress increase on the bottom of the groove. In spite of its compact design, the load cell as per invention is, therefore, very robust against overloading. Additional overload stops as they are for instance described in DE-PS 1268878, are not necessary. In spite of extremely small external dimensions, the invention makes it possible to produce a powerful and legal-for-trade load cell made of stainless steel. The men¬tioned limitation of stress increase in the bottom of the groove makes it possible to do without tool steel so that an additional housing for corrosion protection purposes is not necessary. Other favorable features of the invention result from the sub-claims es well as from the following description of a design example of the invention on the basis of the drawing. The drawing shows a half-section of a load cell as per inven¬tion. This one-piece design of the load cell has a central load input plate 1 the outer edge of which passes over to a first tube section axially directed downwards with the wall thickness a. The first tube section 2 connects the load input plate 1 with a deformation ring 3 which radially surrounds the first tube section 2 and also the force input plate by the half. Between the first tube section 2 and the deformation ring 3 a radial distance is provided which is formed by a circumfe¬rential annular groove 4. The annular groove 4 has lateral limiting walls 5 and 6 which run parallelly to the longitudinal axis 7 of the load cell. The bottom of the annular groove 4 is formed by two overlapping radii Rl and R2. The distance between the limiting walls 5,6 is approximately one and a half times the wall thickness. The transition radius Rl assigned to the first tube section 2 is approximately eight times bigger than the transition radius R2 assigned to the deformation ring 3. It is particularly advantageous for the stress behavior in this area if instead of transition radius Rl, a quarter ellipse or a false ellipse is used which is composed by two overlapping radii having the same tangent. The upper end face of the deformation ring 3 is provided with an annular circumferential square groove 8 which is used for the accommodation of an annular strain gauge 9 to be applied on the bottom of the square groove. The cross section of the lower upper end of the deformation ring 3 is designed in the form of a truncated cone tapering in downward direction, with the ring area 10 which is on the radial plane and which is used for the application of the lower annular strain gauge 11, being precisely below the annular ring groove. The second tube section 12 which is directed downwards paral-lelly to the longitudinal axis 7, is attached at the conically designed external convex surface of the deformation ring 3, approx. in the same height as the groove bottom of the annular groove 4. The conical external surface 22 and the second tube section 12 form an angle of approx. 55°. Between the internal convex surface of the first tube section 2 designed parallelly to the longitudinal axis 7 and the annular area 10, there is a conical section 13 with a helix angle of 45° to the longitu¬dinal axis. The second tube section 12 ends in the load cell base 14 which is designed as reinforced ring when compared with tube section 12. At its lower end face, the load cell base 14 is provided with a circumferential force input surface 15 designed as rounded collar or as level surface the radius of which cor¬responds to the second tube section 12. Therefore, no relative movements between load cell and founda¬tion occur during loading what results in a positive effect on the hysteresis characteristics. The load cell base 14 has a step 16 at its internal convex surface where the p.c. board 17 which includes the electronic control is fixed. In an additional step 18, the cover 19 is arranged which is welded to the load cell base 14. An additional, annually formed cover 20 is fixed on the top of the deformation ring 3 for sealing and protects the strain gauge 9. The deformation ring 3 in the annual groove area 4 has at least one longitudinal borehole used as cable duct in order to be able to realize a wiring of the strain gauge 9 with the p.c. board 17. The transversal borehole 21 in the load cell base 14 is used to push through the cables for the supply voltage and for the output signal. The load cell base radially exceeds the 2nd tube section 12 in external direction. Also the external convex surface of deformation ring 3 has this external dimension in its upper area. Approx. in the prolongation of the above described conical external surface 22, the external dimen¬sion tapers to the external diameter of the 2nd tube section 12 the wall thickness of which corresponds approx. to that of the 1st tube section 2. The tube sections 2, 12 acting as elastic joints guarantee a deformation without hysteresis of the deformation ring 3. If the load cell is loaded with the load F via a load input plate 1 the strain gauge 9 is subject to compression while stain gauge 11 is extended. On the p.c. board 17, the strain gauges are switched to a Wheatstone bridge where the output signal is proportional to the applied load F. WE CLAIM:- 1. Load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the deformation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom characterized in that the transition radius (Rl) assigned to the radial inner limiting wall (5) is bigger by the factor two to ten than the radius (R2) assigned to the radial outer limiting wall (6). 2. Load cell as claimed in claim 1, wherein the annular strain gauges (9,11) are applied to the end faces of the deformation ring (3) axially pointing in opposite directions with the strain gauges (11) pointing in the direction of the base plate (14) having a smaller annual diameter than the strain gauges (9) pointing in the direction of the load input plate (1). 3. Load cell as claimed in any one of the previous claims, wherein the transition radii (R1,R2) are overlapping directly. 4. Load cell as claimed in any one of the previous claims, wherein the transition radius (Rl) assigned to the radial inner limiting wall is bigger than the wall thickness (a) of the first tube section (2). 5. Load cell as claimed in any one of the previous claims, wherein the annual diameter of the strain gauges (11) pointing in the direction of the base plate (14) is smaller than or similar to the diameter of the annual groove. 6. Load cell as claimed in any one of the previous claims, wherein the base plate (14) has a defined annular load input surface (15) the diameter of which corresponds to the diameter of the second tube section (12). 7. Load cell as claimed in claim 6, wherein the load input surface (15) has a rounded contour. 8. Load cell substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Full Text

This invention relates to a load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the defor¬mation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom.
Such a load cell is represented as one of several design examples in DE-PS 1268878. This design which is not realized in actual practice is used to represent a deformation element with square cross section. Geometric relations cannot be taken from this schematic representation where the transition radii of each annular groove which limit the tube sections, are iden¬tical. Almost similar test samples made in accordance with the mentioned representation showed hysteresis effects which could not be tolerated and which excluded a use in legal-for-trade load cells. A reduction of the hysteresis effect could be obtained by a corresponding (over)dimensioning, these designs, however, were of too big size and of unfavorably high material expenditure.
The invention is, therefore, based on the objective to produce a reasonably priced, generic load cell characterized by particular compactness which meets all requirements of legal-for-trade load cells.

Accordingly, there is provided load cell with axially symmetric measuring spring equipped with a central load input plate which is connected via a first tube segment with a deformation ring carrying strain gauges which is arranged concentrically between the first and a second tube section which connects the deformation ring with a base plate, the first tube section being surrounded at least in part by the deformation ring and separated by an annular groove with lateral limiting walls which mainly run parallelly to the longitudinal axis of the load cell and with two transition radii in the groove bottom characterized in that the transition radius assigned to the radial inner limiting wall is bigger by the factor two to ten than the radius assigned to the radial outer limiting wall.
This problem is solved by the fact that the transition radius assigned to the radial inner limiting wall is approximately
bigger by the factor two to ten than the transition radius assigned to the radial outer limiting wall.
The solution within the framework of this invention prevents partially plastic deformations causing hysteresis effects by limiting stress increase on the bottom of the groove. In spite of its compact design, the load cell as per invention is, therefore, very robust against overloading. Additional overload stops as they are for instance described in DE-PS 1268878, are not necessary. In spite of extremely small external dimensions, the invention makes it possible to produce a powerful and legal-for-trade load cell made of stainless steel. The men¬tioned limitation of stress increase in the bottom of the groove makes it possible to do without tool steel so that an additional housing for corrosion protection purposes is not necessary.
Other favorable features of the invention result from the sub-claims es well as from the following description of a design example of the invention on the basis of the drawing.
The drawing shows a half-section of a load cell as per inven¬tion. This one-piece design of the load cell has a central load input plate 1 the outer edge of which passes over to a first tube section axially directed downwards with the wall thickness a. The first tube section 2 connects the load input plate 1 with a deformation ring 3 which radially surrounds the first tube section 2 and also the force input plate by the half. Between the first tube section 2 and the deformation ring 3 a radial distance is provided which is formed by a circumfe¬rential annular groove 4. The annular groove 4 has lateral limiting walls 5 and 6 which run parallelly to the longitudinal axis 7 of the load cell. The bottom of the annular groove 4 is formed by two overlapping radii Rl and R2. The distance between the limiting walls 5,6 is approximately one and a half times the wall thickness. The transition radius Rl assigned to the first tube section 2 is approximately eight times bigger than the transition radius R2 assigned to the deformation ring 3. It
is particularly advantageous for the stress behavior in this area if instead of transition radius Rl, a quarter ellipse or a false ellipse is used which is composed by two overlapping radii having the same tangent.
The upper end face of the deformation ring 3 is provided with an annular circumferential square groove 8 which is used for the accommodation of an annular strain gauge 9 to be applied on the bottom of the square groove.
The cross section of the lower upper end of the deformation ring 3 is designed in the form of a truncated cone tapering in downward direction, with the ring area 10 which is on the radial plane and which is used for the application of the lower annular strain gauge 11, being precisely below the annular ring groove.
The second tube section 12 which is directed downwards paral-lelly to the longitudinal axis 7, is attached at the conically designed external convex surface of the deformation ring 3, approx. in the same height as the groove bottom of the annular groove 4. The conical external surface 22 and the second tube section 12 form an angle of approx. 55°. Between the internal convex surface of the first tube section 2 designed parallelly to the longitudinal axis 7 and the annular area 10, there is a conical section 13 with a helix angle of 45° to the longitu¬dinal axis.
The second tube section 12 ends in the load cell base 14 which is designed as reinforced ring when compared with tube section 12. At its lower end face, the load cell base 14 is provided with a circumferential force input surface 15 designed as rounded collar or as level surface the radius of which cor¬responds to the second tube section 12.
Therefore, no relative movements between load cell and founda¬tion occur during loading what results in a positive effect on the hysteresis characteristics.
The load cell base 14 has a step 16 at its internal convex surface where the p.c. board 17 which includes the electronic control is fixed. In an additional step 18, the cover 19 is arranged which is welded to the load cell base 14.
An additional, annually formed cover 20 is fixed on the top of the deformation ring 3 for sealing and protects the strain gauge 9. The deformation ring 3 in the annual groove area 4 has at least one longitudinal borehole used as cable duct in order to be able to realize a wiring of the strain gauge 9 with the p.c. board 17. The transversal borehole 21 in the load cell base 14 is used to push through the cables for the supply voltage and for the output signal. The load cell base radially exceeds the 2nd tube section 12 in external direction. Also the external convex surface of deformation ring 3 has this external dimension in its upper area. Approx. in the prolongation of the above described conical external surface 22, the external dimen¬sion tapers to the external diameter of the 2nd tube section 12 the wall thickness of which corresponds approx. to that of the 1st tube section 2. The tube sections 2, 12 acting as elastic joints guarantee a deformation without hysteresis of the deformation ring 3.
If the load cell is loaded with the load F via a load input plate 1 the strain gauge 9 is subject to compression while stain gauge 11 is extended. On the p.c. board 17, the strain gauges are switched to a Wheatstone bridge where the output signal is proportional to the applied load F.

WE CLAIM:-
1. Load cell with axially symmetric measuring spring equipped with a
central load input plate which is connected via a first tube segment
with a deformation ring carrying strain gauges which is arranged
concentrically between the first and a second tube section which
connects the deformation ring with a base plate, the first tube
section being surrounded at least in part by the deformation ring
and separated by an annular groove with lateral limiting walls
which mainly run parallelly to the longitudinal axis of the load cell
and with two transition radii in the groove bottom characterized in
that the transition radius (Rl) assigned to the radial inner limiting
wall (5) is bigger by the factor two to ten than the radius (R2)
assigned to the radial outer limiting wall (6).
2. Load cell as claimed in claim 1, wherein the annular strain gauges
(9,11) are applied to the end faces of the deformation ring (3)
axially pointing in opposite directions with the strain gauges (11)
pointing in the direction of the base plate (14) having a smaller
annual diameter than the strain gauges (9) pointing in the
direction of the load input plate (1).
3. Load cell as claimed in any one of the previous claims, wherein the
transition radii (R1,R2) are overlapping directly.
4. Load cell as claimed in any one of the previous claims, wherein the
transition radius (Rl) assigned to the radial inner limiting wall is
bigger than the wall thickness (a) of the first tube section (2).
5. Load cell as claimed in any one of the previous claims, wherein the
annual diameter of the strain gauges (11) pointing in the direction
of the base plate (14) is smaller than or similar to the diameter of
the annual groove.

6. Load cell as claimed in any one of the previous claims, wherein the
base plate (14) has a defined annular load input surface (15) the
diameter of which corresponds to the diameter of the second tube
section (12).
7. Load cell as claimed in claim 6, wherein the load input surface (15)
has a rounded contour.
8. Load cell substantially as hereinbefore described with reference to
and as illustrated in the accompanying drawings.

Documents:

721-del-1997-abstract.pdf

721-del-1997-claims.pdf

721-del-1997-correspondence-others.pdf

721-del-1997-correspondence-po.pdf

721-del-1997-description (complete).pdf

721-del-1997-drawings.pdf

721-del-1997-form-1.pdf

721-del-1997-form-13.pdf

721-del-1997-form-19.pdf

721-del-1997-form-2.pdf

721-del-1997-form-3.pdf

721-del-1997-form-4.pdf

721-del-1997-form-6.pdf

721-del-1997-gpa.pdf

721-del-1997-petition-137.pdf

721-del-1997-petition-138.pdf

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Patent Number

215565

Indian Patent Application Number

721/DEL/1997

PG Journal Number

11/2008

Publication Date

14-Mar-2008

Grant Date

27-Feb-2008

Date of Filing

21-Mar-1997

Name of Patentee

SCHENCK PROCESS GMBH.

Applicant Address

LANDWEHRSTRASSE 55, D-64293 DARMSTADT, GERMANY.

Inventors:

#	Inventor's Name	Inventor's Address
1	BRUNE BISCHOFF	TRAUBENWEG 12, D-64342 SEEHEIM, GERMANY.

PCT International Classification Number

G01L 1/26

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	196 11 484.5	1996-03-25	Germany