US3840081A

US3840081A - Precision laboratory balance

Info

Publication number: US3840081A
Application number: US00382326A
Authority: US
Inventors: R Brann
Original assignee: Torsion Balance Co
Current assignee: Torsion Balance Co
Priority date: 1973-07-25
Filing date: 1973-07-25
Publication date: 1974-10-08
Anticipated expiration: 1991-10-08

Abstract

A laboratory balance as particularly disclosed herein has a balance beam which is supported for rotational movement on the frame of the balance by means of a central pivot which has high torsional rigidity transverse to the pivot axis and preferably is either a knife and vee bearing or a crossed spring or flexure pivot. One end of the beam is arranged so that beam deflection due to a weight placed on a balance pan associated with the other end of the beam can be measured. The balance pan is carried by a generally vertical member or truss which is suspended from the other end of the balance beam on a second pivot which also has high torsional rigidity transverse to the pivot axis and is preferably a knife and vee pivot or a flexure pivot. The balance beam and the truss together with the frame of the balance and a generally horizontal member or steering link which is pivotally connected at opposite ends to the frame and the truss make up a parallelogrammatic linkage by way of the four pivots. At least one of the pivots associated with the generally horizontal steering link is constituted by an elastic torsion member which is held between two arms of the truss or frame of the balance, the steering link being clamped to the torsion member between said arms. The purpose of the torsion member is that, being of a light construction relative to the two pivots associated with the beam of balance, it has lower torsional ridigity transverse to its pivoting axis than the pivots connected to the beam. Thus, to a significant degree, it constitutes a self-aligning facility and permits the steering link to lie in or move into different vertical planes from the vertical plane in which the balance beam moves without detriment to the performance of the balance.

Description

United States Patent [191 Brann PRECISION LABORATORY BALANCE [75] Inventor: Roger P. Brann, Seaford, England [73] Assignee: Torsion Balance Company, Clifton,

[22] Filed: July 25, 1973 [21] Appl. No.: 382,326

[52] U.S. C1 177/196, l77/D1G. 9, 177/198, 177/202, 177/229 [51] Int. Cl. 601g 1/22, GOlg H24 [58] Field of Search 177/190, 194-196, 177/168-170, 246, 251

[56] References Cited UNlTED STATES PATENTS 2.839188 6/1958 Hadley et al 177/196 2,981,348 4/1961 Hadley 177/246 3,075,597 1/1963 Richardson 177/196 X 3,148,742 9/1964 Givilie 177/46 3,249,164 5/1966 Seed 177/196 X 3,443,653 5/1969 Marshall.. 177/169 X 3,508,624 4/1970 Horanm 177/196 X 3,590,933 1/l97l Forman 177/178 X 3,674,102 7/1972 Bommer et all l77/l78 X FOREIGN PATENTS OR APPLICATIONS 481,717 3/1952 Canada... 177/196 1,228,077 l/l963 Germany 177/246 Primary ExaminerRichard B. Wilkinson Assistant ExaminerVit W. Miska Attorney, Agent, or Firm-John A. Howson [57] ABSTRACT A laboratory balance as particularly disclosed herein has a balance beam which is supported for rotational movement on the frame of the balance by means of a central pivot which has high torsional rigidity transverse to the pivot axis and preferably is either a knife and vee bearing or a crossed spring or flexure pivot. One end of the beam is arranged so that beam deflection due to a weight placed on a balance pan associated with the other end of the beam can be measured. The balance pan is carried by a generally vertical member or truss which is suspended from the other end of the balance beam on a second pivot which also has high torsional rigidity transverse to the pivot axis and is preferably a knife and vee pivot or a flexure pivot. The balance beam and the truss together with the frame of the balance and a generally horizontal member or steering link which is pivotally connected at opposite ends to the frame and the truss make up a parallelogrammatic linkage by way of the four pivots. At least one of the pivots associated with the generally horizontal steering link is constituted by an elastic torsion member which is held between two arms of the truss or frame of the balance, the steering link being clamped to the torsion memberbetween said arms. The purpose of the torsion member is that, being of a light construction relative to the two pivots associated with the beam of balance, it has lower torsional ridigity transverse to its pivoting axis than the pivots connected to the beam. Thus, to a significant degree, it constitutes a self-aligning facility and permits the steering link to lie in or move into different vertical planes from the vertical plane in which the balance beam moves without detriment to the performance of the balance.

3 Claims, 3 Drawing Figures FIELI) OF THE INVENTION This invention relates to precision laboratory balances, and in particular, but not exclusively, to precision top-loading balances.

BACKGROUND OF THE INVENTION In all toploading precision balances a balance beam is supported for rotational movement on a main centre pivot which has high torsional rigidity transverse to the pivot axis. A knife and vee arrangement is most commonly used as the centre pivot but the use of a flexure pivot or crossed spring pivot is also well known. A graticule or some graduated measuring device, which may be electronic, is fixed to one end of the beam such that when the beam deflects due to weights on the balance pan, the small angle of rotation can be measured. In some balances, this graduated device may be replaced by an electronic arrangement which produces a force which always maintains the beam in a horizontal or null position. The other end of the beam is provided with a balance pan supporting member or truss which usually hangs vertically from the beam by a second pivot which again has high torsional rigidity transverse to the pivot axis and may be a knife and vee arrangement or a flexure pivot. In use, movement of the truss is such that the balance pan is always maintained in a horizontal plane regardless of loading.

U.S. Pat. Specifications 2,812,935 and 3,412,818

show two prior examples of top-loading balances in which a balance pan supporting member or truss maintains a balance pan in a horizontal plane by means of a steering link and additional lower pivots which together with the two upper pivots set up a parallelogram of four pivots. In both of these prior U.S. patents a knife and vee arrangment is used as the centre pivot bearing of the balance beam and the pan carrying vertical member is suspended from the balance beam also by a knife and vee bearing. In U.S. Pat. No. 2,812,935, however, the lower pivot bearings which make up the parallelogram linkage are ball-bearing universals while in U.S. Pat. No. 3,4l2,8l8 the lower pivots are short knife bearings which have flexible tension wires stretched between them. Such arrangements of the lower bearings have been designed to overcome problems of non-alignment with the upper knife and vee bearings which have very great torsional stiffness transverse to their pivot axes because it is practically impossible to manufacture a balance having a rigid parallelogram linkage of, say, knife and vee bearings which will always remain properly aligned during operation of the balance. These prior arrangements are, however, fairly expensive to manufacture and also considerable adjustment of the lower bearings is required so that they can align themselves effectively with the upper pivots in use.

SUMMARY OF THE INVENTION According to the invention there is provided a precision laboratory balance having a frame, a balance beam pivoted on the frame for rotational movement in a generally vertical plane about a first horizontal pivot, a generally horizontal weighing pan, a truss pivoted to the beam adjacent to one end thereof about a second horizontal pivot, the weighing pan being carried by the truss, and a steering link pivoted to the truss about a third horizontal pivot and to the frame about a fourth horizontal pivot such that the four horizontal pivot points are located to effectively define the corners of a parallelogram, the result being that the weighing pan is maintained in a generally horizontal plane on pivotal movement of the balance beam, at least one of the third and fourth pivots being constituted by an elastic torsion member which has low torsional rigidity both along its pivot axis and transverse thereto.

Preferably both of the steering link pivots are constituted by torsion bands each torsion band extending between two spaced apart projections on the truss and frame respectively and the respective ends of the steering link being secured to the torsion bands between the two projections. It will be appreciated however, that the spaced apart projections may, alternatively, form part of the steering link in which case the torsion band would be carried by the steering link and an extension of the truss or frame would be secured to the torsion band between the projections on the steering link.

It is essential in the construction of the torsion pivots that the torsion members be of suitable size to permit angular distortion in all planes transverse to the pivoting axes of the members. The first and second pivots of the mechanism are constituted by devices which are virtually rigid in all planes transverse to their pivoting axes. The angular flexibility of the torsion pivots in the planes described allow the steering link to align itself automatically in planes dictated by the first and second pivots.

In all top loading precision balances, the mechanism must be capable of supporting loads placed eccentrically on the weighing pan. Those placed eccentrically in a vertical plane containing the axis of the second pivot are supported by the transverse rigidity of the first and second pivots, and any resultant forces are not transmitted to the third and fourth pivots. However, loads placed eccentrically on the pan in a plane parallel to the beam of the balance induce forces which act normal to the torsion members of the third and fourth pviots and lie substantially in the plane containing those members. Thus, the torsion bands which constitute those pivots must be capable of withstanding these forces without undue linear deflection. One means of achieving this is to make the torsion members from a material having asymmetric cross-sectional area, the major axis of the cross section being arranged to lie in the plane containing both members. A typical cross section is rectangular, the major axis of the rectangle being substantially greater than the minor, that is to say, a flat band. In such a system the self-aligning facility described above is achieved whilst linear rigidity is maintained within design limits.

In very low capacity highly precise laboratory balances small weighing pans are used. This, combined with the small weights that can be placed on the pan alleviate the necessity to compensate for eccentric loading. In such cases the torsion pivots may be constituted by members having circular or symmetric cross section without loss in performance.

In accordance with a preferred embodiment of the invention, the third and fourth pivot points are located vertically belowthe second and first pivot points respectively, and the weighing pan is supported by the truss above the second pivot in the manner of a toploading balance.

The torsion members are preferably made of beryllium copper alloy although stainless steel can be used quite successfully.

In very light extremely precise balances, the steering link may with advantage be pivotally connected to the or each torsion member in such a manner as to permit relative rotation about an axis which is transverse to the pivot axis of the torsion member. This may be achieved by use of ball bearings associated with the respective pivot or pivots.

An advantage of torsion pivots is that they can be designed to have low torsional stiffness both transverse to the pivot axis and along the pivot axis which means that in use they are effectively self-aligning and that their contribution to the overall sensitivity of the balance is minimal. A further advantage associated with the use oftorsion pivots in a balance according to the invention is that such pivots are comparatively simple in design and therefore make the manufacture of the balance relatively more economical.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred forms of laboratory balance in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic perspective view of such a balance in which the balance beam is carried by a knife and vee type pivot;

FIG. 2 is a view similar to FIG. 1 but showing a modified form of balance in which the balance beam is carried by a flexure type pivot; and

FIG. 3 is a view similar to FIG. 1 but showing only the lower portion of a balance having a modified arrangement for connecting torsion bands to a steering link.

Referring to FIG. 7, the balance beam is shown at as supported on the frame 12 of the balance by a knife and vee type centre pivot 14. One end of the balance beam carries some device for indicating the deflection of the beam. As shown, this device is in the form of a graticule 16 which in cooperation with a light source (not shown) enables the beam deflection to be read off. The other end of the beam is provided with a second pivot 18, in the form of a knife edge. A vertically extending member or truss hangs from the knife edge 18 as shown. A weighing pan 22 is rigidly mounted on the top end of the truss 20.

The lower end of the truss 20 carries two spaced apart

arms

24 and 26 between which a torsion strip 28 extends under light tension. A similar torsion strip 30 extends under light tension between two spaced apart projections 13 on the frame 12. A rigid steering link 32 is clamped adjacent each of its ends to the torsion strips 28 and 30 respectively. For example, as shown in FIG. 1 the torsion strip 28 extends transversely across the adjacent end of the steering link 32 and is clamped to the link by a clamping plate 27 secured by screws 29. A similar clamping plate 33 with screws 35 is provided to clamp the opposite end of steering link 32 to the torsion strip 30.

The four pivot points constituted by the

pivots

14, 18, 28 and 30 all provide pivoting movement about the horizontal. The pivot 30 is vertically below the pivot point 14 while the pivot 28 is vertically below the pivot point 18. Thus, the four pivots constitute the corners of a parallelogram. It can be seen that when the balance beam pivots about the pivot point 14, the truss 20 is restrained to move in a vertical direction and hence the weighing pan 22 remains horizontal. By virtue of their size the torsion strip pivots have low torsional rigidity both along their pivot axes and transverse thereto, whereby the lower pivots are able to align themselves properly with the upper pivots when the balance is in use.

Certain further features preferably are incorporated in the balance, in particular to counteract manufacturing errors. Thus the mounting of the torsion strip 30 may be adjustable in a vertical direction and the effective length of the rigid steering link beam 32 may be adjustable. Moreover the balance may also include provision for adjusting the tension in the torsion strips 28 and 30. Their tension affects the torsional stiffnes of the parallelogram linkage and hence the sensitivity of the balance.

The provisions just referred to may be incorporated in any suitable manner and illustrative manners of such incorporation as shown in rather diagrammatic form in all figures of the drawings. They will be described in connection with FIG. 1.

Thus, in FIG. 1 the torsion strip 30 is clamped at its opposite ends on feet 13 of frame members 12 by means of clamping plates 15 and screws 17. To select the proper vertical level of the torsion strip 30 on the frame shims 19 are selected and positioned beneath both ends of the strip 30. Screws 17 as then inserted through the clamping plates 15 and shims l9 and threaded into the feet 13. It will be understood that the torsion strip 28 is clamped to the underside of the lower ends 24 and 26 of truss 20 by clamping plates and screws (not shown) similar to those just described. It is not necessary to provide shims for the strip 28 since vertical adjustment of the strip is not required.

For adjustment of lengthwise tension on either of the torsion strips 28 or 30 a simple expedient is to clamp one end of the strip at the rearward or left-hand end as viewed in FIG. 1 and let a free end of the strip protrude beyond the still loosely assembled clamping plate on the forward end, as is shown in FIG. 1. By grasping the protruding end and applying the desired degree of tension to the strip and holding it under such tension while the adjacent clamping plate is tightened and desired result is achieved.

For adjustment in effective length of the steering link 32 the link may be made up of two aligned

sections

21 and 23 joined by a plate 25 fixedly screwed to the section 23, for example, and extending over the gap between

sections

21 and 23. The plate 25 is provided with a pair of slots 27 elongated in the direction of length of link 32. A screw 29 is provided to extend through each slot 27 and to be threaded into the section 21 thereby to securely hold plate 25 in desired position of adjsutment as affored by slots 27.

The torsion balance shown in FIG. 2 differs from that which has just been described only in that the balance beam 34 is supported on the frame 36 of the balance by a flexure pivot 38 rather than a knife and vee pivot. This flexure pivot may be similar to that shown in U.S. Pat. No. 2,939,694, for example. As shown, a flexure pivot 40 is also used for the connection between the balance beam 34 and the vertically extending truss 42. F lexure pivots have the advantage that they are better at withstanding possible shocks in transit and rough usage. Knife and vee pivots may come apart or become loose with time. The lower beam and torsion band con struction of the balance illustrated in FIG. 2 is similar to that described above in reference to FIG. 1 and the components have therefore been identified by the same reference numerals prined.

The flexure pivots 38 and 40 shown in FIG. 2, like knife bearings have great resistance to torsion in directions transverse the pivotal axis but, unlike knife bearings which have substantially no torsional resistance to pivoting about the pivot axis, the flexure bearings have a definite resistance to such axial pivoting. This resistance is due to the fact that the flexure strips must be flexed as an incident to such pivoting. However, the extent of such resistance can be reliably determined and thus taken into account in the design of the balance. The lower steering link torsion bands 28' and 30', because of the factors already discussed have very lgiht torsional resistance to axial pivoting, much lighter than that of the flexure pivots 38 and 40, whereby the characteristics of the parallelogram are dictated by the flexure pivots and are not appreciably affected by the reaction of the torsion bands in the performance of their self aligning functions.

The modification shown in FIG. 3 is to the manner in which the lower rigid beam or steering link 32 (32') is clamped to the torsion strips 28 (28') and 30 (30). As shown in FIG. 3, a pair of miniature ball bearings 44 and 46 is positioned at the opposite ends, respectively of the steering link 32 (32). The ball bearings 44 and 46 are arranged to rotate freely about a vertical axis. One relatively rotatable element of each ball bearing is secured to the adjacent end of the link 32 (32'). The other rotatable element of each ball bearing is secured to a lower clamping plate 48 lying beneath each of the strips 28 (28) and 30 (30). The strips are each clamped to the respective plate 48 by a screw clamping plate 50 similar to the plates 33 described above in connection with FIG. I. The ball bearings 44 and 46, thus installed, do not permit relative pivoting movement of the steering link on an axis which is parallel to the strips; they do however permit relative rotation about an axis transverse to the torsion strips, in particular about the vertical. The result of this is to render the balance less affected by manufacturing tolerances. This modification is thus particularly suited for use in very light extremely precise laboratory balances.

What is claimed is:

l. A precision laboratory balance having a frame, a balance beam pivoted on the frame for rotational movement in a generally vertical plane about a first horizontal pivot, a generally horizontal weighing pan, a truss pivoted to the beam adjacent to one end thereof about a second horizontal pivot, the weighing pan being carried by the truss, and a steering link pivoted to the truss about a third horizontal pivot and to the frame about a fourth horizontal pivot such that the pivoting axes of the four horizontal pivots are located to effectively define the corners of a parallelogram, the result being that the weighing pan is maintained in a generally horizontal plane on pivotal movement of the balance beam, said first and second horizontal pivots each being substantially torsionally rigid in all planes transverse to their pivoting axes, at least one of the third and fourth pivots being constituted by an elongate flexible metal torsion band extending horizontally lengthwise between spaced mounting means to define the pivoting axis of said at least one of said third and fourth pivots and the associated end of the steering link being secured to the torsion band between and spaced from said spaced mounting means whereby said at least one of said third and fourth pivots has torsional rigidity in all planes transverse to its pivoting axis which is substantially lower than that of the first and second pivots.

2. A balance according to claim 1 wherein the third and fourth pivots are both constituted by torsion bands of asymmetric cross section with the major axis of cross section of each of. said torsion bands lying in a plane containing the pivoting axes of said third and fourth pivots.

3. A balance according to claim 2 wherein each of said first and second pivots is constituted by a plurality of crossed flexure bands definingthe horizontal pivoting axis of each of said first and second pivots, said flexure bands having a predetermined torsional resistance to pivoting of said first and second pivots about the pivoting axes thereof, and wherein the torsional resistance of the torsion bands constituting said third and fourth pivots to pivoting of said third and fourth pivots about the pivoting axes thereof is substantially lower than said predetermined torsional resistance of said first and second pivots.

Claims

1. A precision laboratory balance having a frame, a balance beam pivoted on the frame for rotational movement in a generally vertical plane about a first horizontal pivot, a generally horizontal weighing pan, a truss pivoted to the beam adjacent to one end thereof about a second horizontal pivot, the weighing pan being carried by the truss, and a steering link pivoted to the truss about a third horizontal pivot and to the frame about a fourth horizontal pivot such that the pivoting axes of the four horizontal pivots are located to effectively define the corners of a parallelogram, the result being that the weighing pan is maintained in a generally horizontal plane on pivotal movement of the balance beam, said first and second horizontal pivots each being substantially torsionally rigid in all planes transverse to their pivoting axes, at least one of the third and fourth pivots being constituted by an elongate flexible metal torsion band extending horizontally lengthwise between spaced mounting means to define the pivoting axis of said at least one of said third and fourth pivots and the associated end of the steering link being secured to the torsion band between and spaced from said spaced mounting means whereby said at least one of said third and fourth pivots has torsional rigidity in all planes transverse to its pivoting axis which is substantially loWer than that of the first and second pivots.

2. A balance according to claim 1 wherein the third and fourth pivots are both constituted by torsion bands of asymmetric cross section with the major axis of cross section of each of said torsion bands lying in a plane containing the pivoting axes of said third and fourth pivots.

3. A balance according to claim 2 wherein each of said first and second pivots is constituted by a plurality of crossed flexure bands defining the horizontal pivoting axis of each of said first and second pivots, said flexure bands having a predetermined torsional resistance to pivoting of said first and second pivots about the pivoting axes thereof, and wherein the torsional resistance of the torsion bands constituting said third and fourth pivots to pivoting of said third and fourth pivots about the pivoting axes thereof is substantially lower than said predetermined torsional resistance of said first and second pivots.