Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
1999-12-22
2002-04-23
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
C073S472000
Reexamination Certificate
active
06374682
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a force-measuring apparatus, particularly a weighing cell, with a load receiver serving to receive the force that is to be measured. The load receiver is restrained to a mode of parallel motion in relation to a stationary part by two guides that extend in parallel planes and are rigid in all directions within the planes but movable in the direction transverse to the planes. Each of the guides is connected at one end to the load receiver and at the opposite end to the stationary part. The apparatus includes an adjuster device by which the position of at least one of the attachment areas where one of the guides is attached to the stationary part can be adjusted in the direction transverse to the planes of the guides. The adjuster device has an arm that extends approximately parallel to the planes of the guides and is rotatably connected to the stationary part so that the arm is rotatable about a pivotal axis that is approximately parallel to said planes and approximately perpendicular to the lengthwise direction of the arm. The arm includes the attachment area as well as an adjustment area for an adjusting member that serves to tilt the arm about its pivotal axis.
Weighing cells of this kind have the purpose of generating a position-independent measuring signal for the weight of a load that is placed on a weighing pan or load platform attached to the load receiver, i.e., the signal for a given load has the same magnitude no matter where the load is placed on the weighing pan. The parallelism of the guides is critical for the position-independent property of the cell, which requires that the weighing pan or platform move in a strictly parallel mode in response to a load. Particularly with high-precision balances, it is therefore necessary to provide an adjuster device that allows the parallel alignment of the guides in relation to each other to be precisely adjusted.
A weighing cell with an adjuster device of this kind is proposed, e.g., in DE 86 15 750.7 U1. The weighing cell described there has one end of the guides connected to the stationary part through an additional parallel-guiding linkage. The upper portion of the additional parallel-guiding linkage carries the attachment area for attaching the guides. A bottom-end surface of the additional parallel-guiding linkage and an upward-facing fixed surface of the stationary part enclose between themselves an approximately wedge-shaped gap. Inserted in this gap is a bolt that extends parallel to the planes of the guides. An adjustment screw runs through the bolt, transverse to the longitudinal axis of the latter, and is held in a threaded hole of the stationary part. The adjustment screw serves to move the bolt in or out within the wedge-shaped gap where the bolt bears against the bottom-end surface of the additional parallel-guiding linkage on one side and against the upward-facing fixed surface on the other side. This arrangement allows the upper portion of the additional parallel-guiding linkage with the attachment area, and thus the attached ends of the guides, to be adjusted up or down in the direction transverse to the guides. This adjustment serves to align the guide that is attached to the upper portion of the additional parallel-guiding linkage to be exactly parallel to the other guide. However, this adjuster device is expensive to manufacture because, in addition to the parallel-guiding system for the load receiver, an additional parallel-guiding linkage is required that carries the attachment area for attaching one of the guides to the stationary part.
In a weighing cell of the kind named at the beginning and known from DE 87 08 485.6 U1, the attachment area that is adjustable with the adjuster device and serves for the attachment of one of the guides to the stationary part is formed on an arm that is connected to the stationary part through a pivotal flexure. In the weighing cell according to the aforementioned reference, the arm is formed by a separating slit that extends into the stationary part about parallel to the plane of the guides and opens to the rear of the weighing cell. Enclosed between the dead end of the slit and a top surface of the stationary part is a narrow bridging portion of material that forms the pivotal flexure. The adjustable attachment area is arranged on the topside of the rear portion of the arm, i.e., at the opposite end from the pivotal flexure. To adjust the position of the attachment area in the transverse direction in relation to the plane of the guides, the weighing cell of the aforementioned reference has an adjusting member in the form of a differential screw. One threaded section of the differential screw is engaged in a threaded hole through the rear portion of the arm, while the other threaded portion (of different pitch) is engaged in a threaded hole of the body of the stationary part. The rear portion of the arm can be precisely adjusted up or down by turning the differential screw.
However, the small angular movement of the arm that is produced by turning the differential screw of this weighing cell also causes a slight tilting of the threaded hole in the arm in relation to the threaded hole in the body of the stationary part. Thus, by turning the differential screw, transverse forces are introduced into the arm, which can lead to an undesirable distortion of the arm. To eliminate this problem, an insert is placed in the slit of the known weighing cell. The inserted part contacts the underside of the arm and the adjacent body surface of the stationary part along straight contact lines that are parallel to each other.
Although this adjuster device which comprises the pivotally connected arm, the differential screw and the insert, as just described, has significant technical advantages over the adjuster device with the additional parallel-guiding linkage, the manufacturing process of the adjuster device is still expensive because, in addition to cutting the internal threads to receive the sections of the differential screw in the arm and the body of the stationary part, it is also necessary to manufacture and install an insert with two exactly parallel contact lines.
SUMMARY OF THE INVENTION
In view of the aforementioned problems inherent in state-of-the-art devices, it is therefore the object of the present invention to provide a force-measuring apparatus that belongs to the kind described at the beginning but offers the advantage of simplified construction while assuring a high degree of measuring accuracy.
According to the invention, this goal is accomplished through a design advancement of the known force-measuring apparatus. The essential characteristic features of the inventive design are that in the lengthwise direction of the arm, the distance from the adjustment area for the adjusting member to the attachment area is at least exactly the same as the distance from the attachment area to the pivotal axis so that, when the adjustment area is moved up or down by a given amount, e.g., through the action of a suitable adjusting member, this causes the attachment area to move by a lesser amount.
The invention is based on the observation that the arm, which is pivotally connected to the stationary part so as to provide an adjustable attachment area, can also be used as a lever to effect a reduction of the amount of displacement-travel at the ratio of the respective distances of the adjustment area and the attachment area from the pivotal axis. The lever reduction allows the position of the attachment area to be precisely adjusted through the use of a standard adjustment screw with a single fine-pitched thread. The adjustment screw is engaged in a corresponding thread in the body of the stationary part and applies its adjusting force to the adjustment area of the arm or, alternatively, the adjustment screw is engaged in a corresponding thread in the adjustment area of the arm and pushes against the body of the stationary part. Because an adjustment screw of this kind is engaged only in one mating thread, the screw can
Burkhard Hans-Rudolf
Hess Luzi
Fuller Benjamin R.
Kueffner Friedrich
Martir Lilybett
Mettler-Toledo GmbH
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