Strain-gauged washer for measuring bolt preload
First Claim
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1. A strain-gauged washer for measuring a preload in a bolt, said washer comprising:
- a washer body having an inner diameter (ID) sized to fit around a shank of the bolt, an outer diameter (OD) based on a size of a head of the bolt, and a thickness which is standard for a flat washer with the ID and the OD, said washer body including one or more slots which are holes drilled completely through the thickness of the washer body and located between the ID and the OD of the washer body; and
one or more strain gauges affixed within each of one or more slots in the washer body, where the strain gauges are oriented through the thickness of the washer body to detect compressive strain, and where the strain gauges provide an output signal corresponding to the preload in the bolt when the bolt is tightened.
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Abstract
This invention installs multiple miniature strain gauges inside a regular dimensioned bolt washer to accurately measure bolt preload. To enhance the strain gauge sensitivity, an alloy with low elastic modulus and high yield strength is selected to fabricate the metal washer. In addition, multiple gauges are connected in series to multiply the effective gauge length and enhance measurement sensitivity. Further, the stain gauges are encapsulated in the middle of the washer as opposed to on the external surface which offers much improved sensitivity and physical protection of the strain gauges.
33 Citations
19 Claims
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1. A strain-gauged washer for measuring a preload in a bolt, said washer comprising:
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a washer body having an inner diameter (ID) sized to fit around a shank of the bolt, an outer diameter (OD) based on a size of a head of the bolt, and a thickness which is standard for a flat washer with the ID and the OD, said washer body including one or more slots which are holes drilled completely through the thickness of the washer body and located between the ID and the OD of the washer body; and one or more strain gauges affixed within each of one or more slots in the washer body, where the strain gauges are oriented through the thickness of the washer body to detect compressive strain, and where the strain gauges provide an output signal corresponding to the preload in the bolt when the bolt is tightened. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A strain-gauged washer for measuring a preload in a bolt, said washer comprising:
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a washer body having an inner diameter (ID), an outer diameter (OD), and a thickness which are selected from a family of washers having a NAS1587 specification, said washer body including two or more slots which are holes drilled completely through the thickness of the washer body, where the slots are equally spaced in a circular pattern concentric with and located between the ID and the OD of the washer body; and at least one strain gauge affixed within each of the two or more slots in the washer body, where the strain gauges are oriented through the thickness of the washer body to detect compressive strain, and where the strain gauges provide an output signal corresponding to the preload in the bolt when the bolt is tightened. - View Dependent Claims (14, 15, 16, 17)
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18. A system for measuring a preload in a bolt, said system comprising:
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a strain-gauged washer including a washer body having an inner diameter (ID) sized to fit around a shank of the bolt, an outer diameter (OD) based on a size of a head of the bolt, and a thickness which is standard for a flat washer with the ID and the OD, and two or more strain gauges affixed within one or more holes formed completely through the thickness of the washer body, where the strain gauges are oriented through the thickness of the washer body to detect compressive strain, the holes are located between the ID and the OD of the washer body in order for the strain gauges to detect a maximum strain field in the washer body, and the strain gauges are connected in series in order to increase an effective strain gauge length and improve output signal sensitivity, and where the strain gauges provide an output signal corresponding to the preload in the bolt when the bolt is tightened; a signal wire leading from each of the strain gauges through channels formed in a face of the washer body to an outer periphery of the washer body, said signal wire terminating in a connector, said signal wire carrying the output signal from the strain gauges; and a measurement instrument attached to the connector and receiving the output signal, where the instrument displays the preload in the bolt in real time as the bolt is tightened. - View Dependent Claims (19)
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Specification