US3439762A - Beam for a precision balance - Google Patents

Description

April '22, 1969 M. APPIUS 3,439,762

BEAM FOR A PRECISION BALANCE Filed Jime '2, 1967 l N VE N TOR Max Apfc'ws 8 MM 3% United States Patent 3,439,762 BEAM FOR A PRECISION BALANCE Max Appius, Goldach, Switzerland, assignor to Mikrowa Fein-und Schnellwaagen AG., Walzenhausen, Switzerland Filed June 7, 1967, Ser. No. 644,274 Claims priority, application Switzerland, July 6, 1966, 9,802/ 66 Int. Cl. G01g 1/28 US. Cl. 177-235 Claims ABSTRACT OF THE DISCLOSURE A balance beam whose one arm is a rigid plate perpendicular to the upright median plane of the beam, and whose other arm mainly consists of three cylindrical rods fastened to lugs projecting from the plate in different directions, one rod being located in the median plane Background of the invention This invention relates to single-pan precision balances, and particularly to an improved balance beam whose ends respectively carry a pivotally suspended pan and a counterweight, and whose principal pivot connects a central portion of the beam to a stationary support.

Beams of the type described are commonly punched or otherwise press-formed from plate material and arranged on edge so that the length and width of the plate are perpendicular to the principal horizontal pivot axis. Such beams have great flexural strength in a vertical plane, but their resistance to transverse bending stresses is relatively low.

It has been found that the conventional plate-shaped balance beams are subject to transverse, thermal bending stresses in normal operation, and that the stresses cause sufficient deformation of the beams to result in significant weighing errors.

It has heretofore been proposed to overcome lateral beam deformation by the provision of a bimetallic compensating element or by slotting the beam near the principal pivot and by bridging the slot by means of a piece of metal dilfering in its coefiicient of thermal expansion from the primary beam material. The known arrangements have not been fully satisfactory. They either encumber the beam with the weight of supplemental elements, or they reduced the strength and vertical rigidity of the beam.

Summary of the invention The balance beam of the invention includes a rigid plate whose major dimensions of length and width are substantially greater than its thickness. One of the major dimensions extends approximately from the knife or other pivot member on the beam which supports the pan toward the knife which is the member of the principal pivot mounted on the beam. The other major dimension of the plate extends substantially in the direction of the principal pivot axis. The plate thus constitutes one arm of the beam.

The other arm of the beam includes three elongated members. Each member has a first longitudinal portion fastened to the plate and a second longitudinal portion remote from the plate in a direction away from the pan supporting pivot which is fastened to the corresponding second portions of the other longitudinal members by a connecting member. The second portions of the elongated members and the connecting member support the counterweight arrangement.

The coefiicient of thermal expansion of one of the elongated members is different from the corresponding coefficients of the two other elongated members.

The beam of the invention is rigid not only in a vertical plane perpendicular to the axis of the principal pivot, but also resists bending stresses transverse of the vertical plane. More specifically, it does not deform under thermal stresses.

Other features and many of the attendant advantages of this invention will readily be appreciated as the same becomes better understood by reference to the following detailed description of a preferred embodiment when considered in connection with the accompanying drawing.

Brief description of the drawing In the drawing:

FIG. 1 shows a precision balance including the beam of the invention in fragmentary side elevation, and partly in section on the line I-I; and

FIG. 2 shows the device of FIG. 1 in plan view.

Description of the preferred embodiment The stationary supporting structure of the partly illustrated precision balance includes a column 15 on which a bearing 14 constitutes the principal pivot of the balance in cooperation with an agate knife 1 on the balance beam. The beam also supports a nonillustrated hanger and pan on a second agate knife 2. The knives 1, 2 are mounted on short arms 3, 4 projecting from opposite longitudinal ends of a rigid metal plate 5 which practically constitutes one arm of the balance beam.

A fixed lug 6 projects upward from the plate 5 in the vertical median plane of the beam perpendicular to the edge of the knife 1 which extends in the axis of the principal balance pivot. A cylindrical rod 7 extends from the lug 6 toward the end of the beam remote from the pan-supporting knife 2 and supports a scale carrier 8 on which a scale 9 is engraved in a conventional manner to indicate the weight of an object held on the nonillustrated pan.

Two lugs 10 project downwardly and laterally in opposite directions from the plate 5. Respective endS of two cylindrical rods 11 are attached to the lugs 10 and diverge toward the remote end of the beam symmetrically relative to the upright median plane of the beam. Their far ends are connected by a transverse bar 12. The bar 7 passes through a central bore 13 in the beam 12 and is fixedly soldered to the beam, the solder joint not being visible in the drawing. The rod 7 consists of an aluminum alloy (Al-Si-Mn-Mg) having a coefficient of thermal expansion of 11.7 10- F., the rods 11 of an aluminum alloy (Al-Mg-Si) having a corresponding coefficient of 12.8 10- F.

The ends of the bar 12 carry two downwardly open, cup-shaped receptacles 16 whose upright axes are approximately aligned with the axes of the rods 11 in the direction of elongation of the latter. As is best seen in FIG. 1, the upright internal walls of the receptacles 16 are relatively heavy so that the receptacles function as counterweights, and convexly and concavely curved about the principal pivot axis so that nonillustrated vanes mounted on the stationary support structure of the balance may move inward and outward of the receptacles during pivoting movement of the beam without touching the walls, yet leaving 'but a narrow open gap along the walls which impedes movement of air into and out of the receptacle, thus providing a damping effect. The terminal part the rod 7 is different from the coefficient of the rods 11,

the center of gravity of the beam assembly remains at a practically fixed location relative to the principal pivoting axis of the beam on the knife 1. The sensitivity of the balance and the zero position of its scale are therefore virtually unaffected by the usual changes in ambient temperature.

The rods 7 and 11 are connected to form a lattice girder of great flexural and torsional rigidity due, in particular, to the divergence of the rods 11 from the plate toward the bar 12. Cylindrical rods have been found to be particularly advantageous because they are readily made free from asymmetrical internal stresses.

What is claimed is:

1. In a precision balance having a support; a beam equipped with a pivot member for supporting a weighing pan at one end, and a counterweight at the other end; and a principal pivot connecting a portion of said beam inter-mediate said ends to said support, said principal pivot including a pivot member fixedly mounted on said intermediate beam portion, the improvement in the beam which comprises:

(a) a rigid plate member having major dimensions of length and width substantially greater than the thickness of said plate member, one of said major dimensions extending substantially in a direction from one of said pivot members toward the other pivot member, and the other major dimension extending substantially in the direction of the axis of said principal apivot,

(1) said pivot members being fixedly mounted on said \plate member;

(b) three elongated members having first longitudinal portions fastened to said plate member and second longitudinal portions spaced from said first portions in a direction away from said pan-supporting pivot member; and

(c) a connecting member fastened to said second portions and connecting the same,

.(1) said second portions and said connecting member constituting the other end of said beam,

(2) the coefiicient of thermal expansion of one of said elongated members being different from the coefficients of thermal expansion of the other two elongated members.

2. In a balance as set forth in claim 1, one of said elongated members extending in a median plane of said beam perpendicular to said axis, said plate-member extending in a plane transverse to said median plane.

3. In a balance as set forth in claim 2, the other two elongated members being oppositely spaced from said median plane and substantially symmetrical relative to said median plane.

4. In a balance as set forth in claim 3, said other two elongated members diverging in a direction from said plate member toward said connecting member.

5. In a balance as set forth in claim 3, said other two elongated members having the same coefficient of thermal expansion.

6. In a balance as set forth in claim 3, said counterweight including two weight members mounted on said connecting member and longitudinally approximately aligned with said other two elongated members.

7. In a balance as set forth in claim 3, each of said weight members constituting an element of a damping mechanism including a receptacle member and a vane member, the members of each damping mechanism being respectively mounted on said support and on said beam for movement of the vane member in the receptacle member when said beam moves on said principal pivot.

8. In a balance as set forth in claim 3, said one elongated member having a free terminal part extending longitudinally beyond said connecting member, and a scale carrier on said terminal part.

9. In a balance as set forth in claim 3, said elongated members being of substantially circular cross section.

10. In a balance as set forth in claim 3, three fixed lug members projecting from said plate member in different directions, said first portions being respectively fastened to said lug members.

References Cited UNITED STATES PATENTS 3,019,846 2/ 1962 Baur 177246 3,112,808 12/ 1963 Macurdy 177261 FOREIGN PATENTS 147,715 8/ 1952 Australia.

ROBERT S. WARD, JR., Primaly Examiner.

L. HAMBLEN, Assistant Examiner.

US. Cl. X.R.

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