SOW-and-description-of-services-1095-2016-06-29.docx

BLOCK GAGE STATEMENT OF WORK

June 8, 2016 Photographic description of block gages (GFE)

1. The 4”-block gage is a single block of 17-4PH stainless steel machined into two plates connected by 4 flexures. Each plate has 4 threaded holes, all 8 of which are the same thread size. To one plate is attached a housing for a coil and from the other extends an mount for a plunger made of magnetic material. The housing or the plunger mount could be used as a possible attachment for the cabling extending from the strain gaged element.

1. Each flexure is approximately 1.0 in. by 0.1 in. and each is precisely machined. Figure 1 shows two of the flexures on the front face of the block gage. In some of photographs a ruler with an inch scale is included.

Figure 1.

Figure 2 shows the block gage rotated 90 degrees from the position shown in Figure 1. Figure 3 shows the block gage rotated 180 deg from the position shown in Figure 1. Figure 4 shows the block gage rotated 270 deg from the position shown in Figure 1 and the coil housing. Figure 5 and 6 show the other two faces of the block gage. Figures 7 and 8 are closer views of the facing sides of the flexures. This is where the strain gages will be attached.

Plunger mount

Figure 2.

Figure 3.

Coil housing

Figure 4.

Figure 5

Figure 6.

Coil housing

Figure 7.

Strain Gage Installation.

One gage shall be placed at the base of each end of each flexure: a total of 8 gages. The gages and the gage locations shall be numbered as in Figure 8. Locations 1 through 4 are on the near (two flexures nearest the camera) flexures. Location 5 is behind 1 in the view shown in Figure 8, 6 behind 2, 7 is behind 3 and 8 is behind 4. Locations 5-8 are on the far flexures therefore.

Face e

Figure 8.

Each gage shall be placed with the solder tabs facing its opposite at the other end of the same flexure. Each gage shall be placed equidistant from each face nearest to the gage as shown in Figure 8. This dimension from the gage center to the face is denoted as dimension F, see Figure 9. Dimension F shall be chosen so that the gage substrate is not on the radius between the flexure and the face, but as close to the radius as practical. This is expected to be that the edge of the epoxy substrate is <0.01 inches from the end of the radius. For this reason, small as practical, 350 ohm gages will be used. The gages shall have epoxy backing for low hystresis and creep.

Radiused portion Radiused portion F

D

Figure 9. Flexure schematic showing gages.

The Wheatstone bridge shall be wired as indicated below, where E is the excitation and V is the

V

E output voltage. Each gage will be on an interior face of the flexure facing a gage on the opposing flexure: 1 facing 2, 3 facing 4, etc. Each gage shall be located equidistant from the edge of a flexure, this dimension, dimension D, is the same for all gages and will result in the center of the gage being located at the center of each flexure.

This wiring scheme is done to provide a force reading due to the shear force in a direction depicted by the green arrows in Figure 8.

The tolerance on dimensions F and D from gage to gage shall be .004 inch or smaller.

Each gage will be epoxied to the flexure square to each flexure, so the principal axes of the gage are aligned with each flexure. All strain gages shall be of the same type model number and lot.

Balance resistors will be used so that V under no load and either 5 or 10 V excitation is as low as practical.

The contractor shall apply waterproofing to the gages. And shall provide proof to navy representatives that the gage is waterproof to a depth of 5 ft in tap water. Waterproofing shall include as a minimum a waterproofing such as beeswax, and a tough covering such as a polysulfide coating. The coating shall be applied such that waterproofing is ensured, but the coating shall not be so thick as to cause noticeable hystresis or creep to occur in the measurements. The contractor shall provide output V vs time measurements before waterproofing is applied as proof of not causing hystresis.

The contractor shall create the Wheatstone bridge as shown above with materials typically used for that purpose in high quality strain gage sensor applications. Surface preparation of stainless steel surface of the block gage shall be according to manufacturer’s recommendations for all materials to be applied to the surface.

The signal and excitation shall be provided to the gage using the 6-pin Spacecraft SCPC02A10-6P (www.spacecraft.com, Las Vegas NV) male connector. This type of connector was formerly sold under the name of Amphenol. The connector shall be mounted and potted in the hole of the coil housing shown in Figure 4. The contractor shall pick a means of mounting the connector that is convenient to the company and meets all specifications in this document.

The contractor shall provide a 50 ft cable twisted pair cable with a sheath, able to withstand waterproofing. Such a cable is Belden 8434. One end shall mate to the block gage connector and therefore is a female connector. The other end of this cable shall be terminated with the same model female connector.

All contractor applied materials and parts, when completed, shall not extend beyond the 4 in. cube wall boundaries. That is, if a straight edge is run across any face of the block gage, it would not interact with any contractor applied material or part. This is done to prevent damage to the waterproofing and connector, primarily due to handling.

After the strain gages and all electrical circuits including the connector have been applied and constructed, but before any waterproofing is done, with the exception of a clear polyurethane coating (if part of the waterproofing method used) the contractor shall demonstrate to government representatives that the strain gages is wired appropriately, that the strain gages are placed with accuracy according to specifications, and that the gage will register a voltage when load is applied as specified. A load of ½ the rated load of the block gage will be applied (250 lb). The contractor shall calibrate the block gage in house using at least 7 loads up to the rated load. Loading shall be applied upwards and downwards for a total of 13 loads total. Zero offset shall be recorded. An accuracy of 0.5% or better is expected.

The block gage shall be immersed in water for a period of 8 hours before the calibration in the above paragraph is attempted.

Deliverables

1. A single strain gaged block gage that is waterproofed.

2. A written statement before beginning work that the wiring diagram shown will provide a signal of the force indicated in Figure 8.

3. Written calibration record with stated accuracy.

4. Written record of water immersion.

5. Schedule of when the block gage will be visible for inspection prior to waterproofing. This shall be provided to the government 14 calendar days or greater prior to the date.

6. A means for government personnel to check the accuracy of strain gage placement before wateproofing, such as a microscope or powerful magnifying glass (with means of holding it in place) and a dial caliper gage accurate to 0.001 inches.

7. A statement of how the connector will be attached to the block gage that will be reviewed by the government within 10 calendar days.

Government Furnished Equipment A single block gage will be furnished by the government for this work.

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