GB2263775A

GB2263775A - Strain measuring device

Info

Publication number: GB2263775A
Application number: GB9201436A
Authority: GB
Inventors: James Edward Oakley
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-01-23
Filing date: 1992-01-23
Publication date: 1993-08-04
Anticipated expiration: 2012-01-23
Also published as: GB9201436D0; GB2263775B

Abstract

A strain measuring device is attached with clamp screws 2 to the surface of a structure, which is thereafter subjected to a compressive stress. The separation of the screws along the longitudinal axis of the device defines a gauge length 4 which compresses in sympathy with the surface. Contraction of the gauge length is accommodated by an equivalent extension in a region 6 of reduced thickness and defined by U-shaped slot 5. This region has a strain amplification function and offers reduced resistance to an applied stress in comparison to both the bulk of the device and the structure itself. Strain gauges 8 located in the region 6 detect the strain. Jacking screws 9 enable adjustment to be made to compensate for bending strains introduced in the device when it is attached to a non-flat surface. The reduced thickness region 6 may be made of material with a lower modulus of elasticity than that of the frame. <IMAGE>

Description

A STRAIN MEASURING DEVICE The invention relates to a strain measuring device.

The use of a strain link device is known to be a simple and convenient way of sensing the load in a structure. For example, when secured to the metallic frame of a container such as a silo, the device senses the generally compressive strain induced in the structure due to the weight of material inside. However, these known devices must usually be installed on a precision surface.

If the surface is rough or dirty, the devices will detect unwanted, extraneous strains even when under zero load.

The object of the invention is to provide a strain measuring device of simple, economical and compact construction, which can be installed even on rough, largely unprepared surfaces, and which reduces the buckling tendency which is always present in sensitive equipment undergoing compression.

In accordance with the present invention there is provided a strain measuring device, for attachment to the surface of a structure which is to undergo a compression, comprising first and second members adapted for attachment to the said surface at spaced positions defining a gauge length, the first and second members being connected such that contraction of the gauge length produces an extension in a region of the second member, and the first and second members being so constructed that the percentage extension of the said region of the second member is greater than the percentage contraction of-the gauge length; the device further comprising apparatus for detecting said extension in the second member.

In one embodiment, jacking screws enable adjustment of the strain measuring device to compensate for bending strains introduced in the device when it is attached to a non-precision surface. In practice, therefore, a metal surface need only be brushed to remove any debris before the device is secured in position.

The invention will be understood clearly from a reading of the following description made with reference to the accompanying drawings in which: Figure 1 is a plan view of the strain measuring device; and Figure 2 is a cross-section along the longitudinal axis (I-I) of the strain measuring device secured to the surface of a structure.

The strain measuring device according to the invention comprises a frame 1, which is in the shape of a rectangular plate, having a triangular arrangment of clamping screws 2 to secure the device to the surface of a structure 3. The separation of the screws along the longitudinal axis (I-I) of the device defines a gauge length 4. At one end of the device, a clamp screw 2 is partially isolated from the bulk of the frame 1 by a U-shaped slot 5. A region 6 of the plate, enclosed by the U-shaped slot 5 is reduced in thickness in relation to the remainder of the frame 1. The dimension of region 6 along the longitudinal axis is length 7. Since the width of the region 6 is also smaller than the width of the plate, the cross sectional area of this region 6 is significantly smaller than that of the plate. Strain gauges 8 are located in the region 6.The other end of the device is secured by a pair of clamp screws 2 which are disposed equidistantly either side of the longitudinal axis (I-I). Two jacking screws 9 are arranged along the longitudinal axis (I-I) either side of the pair of clamp screws 2.

In operation, the strain measuring device is secured to the surface of a structure 3, which is to undergo a compression, with the clamping screws 2. Preferably, washers 10 are used to stand the device away from the surface. The gauge length 4 contracts in sympathy with the surface of the structure 3. Contraction of the gauge length 4 is accommodated by a significant extension in the region 6. The region 6 exhibits no tendancy to buckle since it is subjected to a tensile force, even though the structure is compressed.

The device has a strain amplification function. The diminished cross sectional area of the region 6 implies that this region offers a smaller resistance to an applied force than the rest of the frame. The rest of the frame is essentially rigid by comparison and can transmit a significant proportion of any force imposed across the gauge length 4 to the region 6. In addition, the length 7 of the region 6 is smaller than the initial gauge length 4. Thus if the change in the gauge length is converted directly by the stiff incompressible frame into an extension of the region 6 by the same amount, the percentage extension in the region 6 is greater than the percentage contraction of the gauge length 4. The strain gauges 8 are located so as to detect the amplified extension.

If the strain measuring device is secured to a rough surface, spurious strains may be introduced in the device. These would be highlighted by taking readings with the strain gauges 8.

The jacking screws 9 can be adjusted to increase or decrease the reading to zero. These are then locked with locking compound and the device covered against the elements.

Another embodiment (not shown) provides an alternative method of producing a strain sensitive region in the device. The region 6 of the plate, reduced in thickness, could be replaced by a member composed from a material of lower modulus of elasticity than that which constitutes the frame. A judicious choice of the dimensions of this member would ensure that the member offers a considerably smaller resistance to an applied force than the rest of the frame, and that the device provides a strain amplificiation function.

Claims

CLAIMS:

1. A strain measuring device, for attachment to the surface of a structure which is to undergo a compression, comprising first and second members adapted for attachment to the said surface at spaced positions defining a gauge length, the first and second members being connected such that contraction of the gauge length produces an extension in a region of the second member, and the first and second members being so constructed that the percentage extension of the said region of the second member is greater than the percentage contraction of the gauge length; the device further comprising apparatus for detecting said extension in the second member.

2. The device of claim 1 wherein the dimension of the region of the second member which extends as the gauge length contracts is shorter than the gauge length.

3. The device of claim 1 or 2 wherein the extensible region of the second member has a reduced cross-sectional area in relation to the first member.

4. The device of any of claims 1 to 3 wherein the extensible region of the second member has a lower modulus of elasticity in relation to the first member.

5. The device of any of claims 1 to 4 wherein the apparatus for detecting the extension in the second member is a strain gauge located in the extensible region of said member.

6. The device of any of claims 1 to 5 wherein bending strains introduced in the device through attachment to the surface of a structure are counteracted by a jacking screw which communicates with the device and the surface.