Hermetically sealed load cell

Details Hermetically sealed load cell Hermetically sealed load cell

2002-10-01

2004-09-14

Lefkowitz, Edward (Department: 2855)

Measuring and testing

Dynamometers

Responsive to force

C073S862637, C177S211000, C177S229000

Reexamination Certificate

active

06789435

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the provision of a hermetic seal for the strain gages of a load cell.
BACKGROUND INFORMATION
A conventional load cell of the bending beam type includes a live end block, a dead end block, and plural bending beam elements extending between and interconnecting the live end block and the dead end block. Foil strain gages or the like are affixed, e.g. adhesively mounted, on the thin sensing sections of the bending beam elements. The strain gages are connected by wires to the sensing electronics, e.g. a weighing bridge circuit. The dead end block is bolted to a stationary support, while a live load is introduced into the live end block. The live load, i.e. the load to be weighed, causes the bending beam arrangement to deflect, thereby inducing corresponding strains in the bending beam elements. The strains of the bending beam elements are sensed and measured by the strain gages, whereby the weight of the applied live load can ultimately be determined.
Such bending beam load cells are often used in so-called single point applications, in which a single load cell is provided to weigh an entire load. For example, the entire load of a weighing platform is introduced into the live load introduction end of the bending beam load cell, in order to weigh the live load on the weighing platform.
In most applications, it is necessary to seal and protect the delicate strain gages from environmental influences which include mechanical abrasion or other mechanical damage, corrosion and etching by harsh chemicals and the like, oxidation, and other undesirable influences that destroy or damage the strain gages or interfere with the proper functioning thereof. It has become known to apply a polymeric seal layer, such as a layer or film of polybutylene over the strain gages. It has further become known to seal, embed, or pot the strain gages in a sealing mass of room temperature vulcanizing (RTV) silicone or the like.
While such known sealing methods provide an effective environmental seal, such measures are temporary and not very robust. Namely, such polybutylene, silicone, or other polymeric seal layers have been found to leak, breakdown, or peel off under harsh environment applications, such as in the food production and preparation industry, and in the chemical processing and handling industry. In such harsh environment applications, the load cells, and particularly the seal areas provided over the strain gages, are exposed to harsh or severe chemicals, as well as frequent cleaning, for example using solvents and the like, and using mechanical scrubbing or pressure washing procedures. Under such severe conditions, conventional polymeric seals on the strain gages of load cells have been found to be unsatisfactory due to a short reliable operating lifespan before leakage or peeling of the seal material occurs.
To provide a longer-term, more-durable hermetic seal, in comparison to the above mentioned polymeric seals, it has also become conventionally known to encapsulate or seal the strain gages with a stainless steel seal member. A problem that arises when trying to use a metal seal member, is that the seal member itself takes up some of the stress and thus influences the development of the strain in the sensing sections of the bending beam arrangement. The particular conventional load cell configuration that has become known for addressing the above problems is schematically illustrated in a simplified manner in present FIG.
1
.
As shown in
FIG. 1
, a conventional bending beam load cell
1
′ having a stainless steel hermetic seal is a rather complicated triple beam arrangement
2
′ including an upper beam element
5
′, a lower beam element
6
′, and a central beam element
7
′, respectively extending and connected between the live load introduction end
3
′ and the dead end
4
′. The entire bending beam arrangement
2
′ is machined from a single monolithic block of stainless steel. The complicated configuration as shown in
FIG. 1
results in a rather high machining effort and cost. Particularly, the upper and lower beam elements
5
′ and
6
′ are substantially straight beam elements, while the central beam element
7
′ is a circular ring element. The upper and lower beam elements
5
′ and
6
′ maintain a parallelogram configuration, and passively handle off-center load application moments. On the other hand, the central beam element
7
′ including the circular ring element is the active bending element that takes up the load to be measured.
For measuring the strain of the circular ring element and thereby measuring the applied live load, strain gages
9
′ are applied on the inner circumferential surface of the ring element of the central bending beam element
7
′. In
FIG. 1
, the strain gages
9
′ are merely schematically indicated as a dashed line. Actually, the strain gages
9
′ are not visible from the outside, because they are encapsulated and hermetically sealed by a cylindrical sleeve or tube
10
′ of stainless steel that is arranged in the interior of the ring element of the central bending beam element
7
′. The hermetic seal tube or sleeve
10
′ is welded around the edges to the ring-shaped element of the central bending beam element
7
′, to achieve the hermetic seal with a complete stainless steel enclosure. Thus, only a stainless steel surface is exposed to the environment, and the strain gages are hermetically sealed therein. An electrical cable
12
′ is connected and sealed into the dead end
4
′ to conduct the weighing signals provided by the strain gages
9
′ and pre-processed by electronic circuitry in the load cell
1
′.
While the conventional hermetically sealed stainless steel bending beam load cell schematically illustrated in
FIG. 1
provides an effective durable hermetic seal and is suitable for use in harsh or extreme environmental conditions, it also suffers several disadvantages. The machining required for the complex configuration of the load cell results in rather high machining efforts and cost. Also, the complex configuration with several interior surfaces, corners, notches, grooves, and the like forms spaces in which liquids will puddle. This is a disadvantage in the food processing and chemical processing industries, in which the load cells are frequently exposed to various liquids during use and during cleaning procedures. The puddling and accumulation in the “nooks and crannies” of the complex configuration of the load cell make it difficult to keep the load cell clean, and make higher demands on the long term corrosion resistance and hermetic sealing.
Also, the complex triple beam configuration including a ring-shaped strainable element necessarily leads to a rather large profile height for a given beam length and load capacity. Moreover, this triple beam arrangement, and the use of a cylindrical internal stainless steel sleeve or tube to provide the hermetic seal, make it difficult to achieve a low load capacity. Namely, due to the influence of the hermetic seal tube or sleeve
10
′ and due to the arrangement of three bending beam elements
5
′,
6
′ and
7
′, a certain minimum load is required to sufficiently strain the central bending beam
7
′ for an accurate weighing result. The minimum capacity for such load cells is typically about 20 kg, although claims of a capacity down to about 6 kg have been noted.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide a load cell with a hermetic seal for the strain gages thereof, while achieving a lower capacity range, a simpler configuration, a reduced machining effort, a reduced cost, a lower profile height, and an easy retrofit capability to replace previously existing load cells. The invention further aims to provide a hermetic seal configuration for a load cell, that achieves improved separation or isolation of the strain from the seal eleme

Affiliated with

Also associated with

Rating

Hermetically sealed load cell does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Hermetically sealed load cell, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Hermetically sealed load cell will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3196240