Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
2002-01-30
2004-02-24
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
Reexamination Certificate
active
06694829
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a load cell, and particularly to a load cell provided with a strain body and a sensor for detecting a displacement of the strain body caused by a load.
2. Background Information
In general, a load cell used in a measuring device or the like has a strain body (i.e., a body to be strained) having a movable portion and a fixed portion, and a sensor for detecting a displacement of a strain body caused by a load applied thereto. A result of the detection is converted into a weight of a measuring target object, i.e., an object to be measured.
The movable portion of the strain body of the load cell is fixed to a support member supporting the measuring target object, or is fixed to a connection member, which connects the support member to the strain body, and will be referred to as a “measuring target side member” hereinafter. The fixed portion of the strain body is fixed to a fixed base or a connection member, which connects the fixed base to the fixed portion of the strain body, and will be referred to as a “fixed base side member” hereinafter. When a load of the measuring target member acts on the strain body via the measuring target side member, the movable portion of the strain body is displaced downward relatively to the fixed portion. This displacement is detected by a sensor such as a strain gauge, and is converted into a weight.
The strain body of the load cell is often made of aluminum or aluminum alloy primarily in view of demand for high precision and workability. The measuring target side member and the fixed base side member are primarily made of a material such as steel or stainless steel.
Accordingly, a connection between the strain body and the measuring target side member as well as a connection between the strain body and the fixed base side member are in such a state that dissimilar metal materials are electrically in contact with each other. Therefore, if an atmosphere containing oxygen and/or moisture is present around these connections, electricity flows between the dissimilar metal materials to cause galvanic corrosion so that corrosion expands rapidly to reduce a lifetime of the load cell.
For overcoming the above disadvantages, the strain body may be made of stainless steel. However, it is difficult to ensure a high weight detection precision, and particularly, it is difficult to use the stainless steel instead of aluminum from the viewpoint of a processing or working cost.
For preventing the galvanic corrosion, the strain body may be coated with a molded material such as silicone or polyurethane. However, the molded material, which is present between the member on the measuring target side or fixed base side and the strain body, deteriorates a weight detection precision due to viscoelastic properties of the molded material. If a molded material having a thickness from hundreds of micrometers to several millimeters is present between the member on the measuring target side or fixed base side and the strain body, this lowers the degree of fixing between the measuring target side member and the strain body as well as the degree of fixing between the fixed base side member and the strain body. Also, deformation of the molded material caused by the load deteriorates the weight detection precision. Accordingly, molding is effected on the strain body, which is already attached to the measuring target side member and the fixed base side member. However, this requires a complicated work for molding, and increases a working cost.
The molding may be effected on the strain body alone while masking the movable portion and fixed portion. However, this cannot prevent the galvanic corrosion.
Further, for preventing the corrosion of the strain body, the strain body may be covered with a box made of stainless steel after attaching the measuring target side member and the fixed base side member to the strain body. However, this requires a devised structure for preventing restraint on movements of the measuring target side member and the movable portion of the strain body, resulting in disadvantages relating to the cost and size.
As another manner for preventing the galvanic corrosion, a film having an insulating property may be interposed between dissimilar metal materials. In this manner, however, water enters a fine space between the strain body and the film to cause corrosion. As still another manner, a member made of a relatively soft material may be interposed between dissimilar metal materials. In this manner, however, the load of the measuring target object is not transmitted 100% to the load cell so that an error occurs in a result of measurement.
If a scale including the strain body is used, e.g., in hot and humid surroundings or in a place exposed to salt-laden moisture of seawater or the like, corrosion occurs on the contact surfaces of dissimilar metal materials, and is also liable to occur on other surfaces.
In view of the above, there exists a need for a load cell which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
SUMMARY OF THE INVENTION
An object of the invention is to provide a load cell, which can suppress galvanic corrosion in a contact portion between the strain body and the member attached or connected to the strain body, and also can ensure a sufficient degree of fixing between the strain body and the member attached to the strain body so that a high weight detection precision can be achieved.
Also an object of the invention is to provide a load cell, which has improved resistance to water, rust and corrosion, and can achieve high weight detection precision.
According to a first aspect, a load cell includes a strain body, a bridge circuit and a coating film. The bridge circuit is formed of a strain gauge arranged on the strain body. The coating film is formed on the strain body, covers at least a portion of the strain body attached to a member made of a metal material dissimilar to the strain body, includes a coating layer made of resin having a glass transition temperature of 40° C. or more, and has an electrical insulating property.
According to the load cell, when a load on a measuring target side acts on the strain body, a strain gauge detects a displacement of the strain body caused by the load. Since the degree of displacement of the strain body depends on the magnitude of the load applied by a measuring target object, the weight of the measuring target object is obtained from the result of detection of the strain gauge.
In the above structure, the coating film having the insulating property covers the portion of the strain body, to which the dissimilar metal member (i.e., the member made of a metal material dissimilar to the strain body) is attached, so as to prevent galvanic corrosion in such a case that the dissimilar metal member is in contact with the strain body. Thus, in the structure having the dissimilar metal member attached to the strain body, the coating film having the insulating property is interposed between the dissimilar metal member and the strain body. Therefore, an electricity hardly flows between the dissimilar metal member and the strain body, although these are made of different kinds of metal, respectively.
The dissimilar metal member attached to the strain body may be a fixing member for fixing a portion of the strain body to the fixed base, a movable member for transmitting a load of the measuring target object to another portion of the strain body, a stop or limiter member such as a screw or a metal piece for restricting deformation of the strain body caused by an excessive load, and/or a board provided, e.g., with a line terminal portion between the strain gauge and external wiring and/or an amplifier for amplifying a strain gauge output.
In contrast to the above, if a coating film were not present between the dissimilar metal member and the strain body, the strain body wou
Chimura Yukiko
Wakasa Yukio
Ishida Co. Ltd.
Noori Max
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