Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual

Chapter 4. Strike Plate, Load Cell, Raise/Lower Assembly

Figure 90 shows the strike plate, load cell, and the raise/lower assembly.

Figure 90. Load cell mounted to the strike plate.

STRIKE PLATE

The strike plate (figure 91) requires routine cleaning and semiannual visual inspection of the welds. The following components should also be checked annually or as indicated:

• Load cell and mounting bracket.
• Center cylinder mounting nut.
• Hydraulic line mounting clamps.
• Rear raise/lower bar shaft and mounting bracket.
• Guide profile rails. Check cap screws every 3 to 6 months.

Figure 91. Strike plate.

The load cell swivel and retaining plate are located on the bottom of the strike plate (figure 92). Figure 88 shows a different view of the raise/lower car mounting bracket. Check bolts annually and apply medium-strength thread locker. Annually remove the load cell swivel and clean.

Figure 92. Bottom view of load cell swivel and retaining bracket on strike plate.

LOAD CELL

The load cell attaches to the strike plate, and the load cell swivel is pressed in to fit in the strike plate, making a snug fit. The following illustrations show the steps taken during a major overhaul of a load cell:

1. Place the load cell base down (figure 93).
   

   Figure 93. Load plate (base down).

   
2. Attach the mounting brackets (figures 94 and 95). The brackets should be checked annually. Make sure all bolts are in place and the rubber is not torn.
   

   Figure 94. Load plate with mounting brackets.

   

   Figure 95. Load cell with brackets mounted.

   
3. Attach the load cell to the strike plate by using a guide tool to center the load cell on the swivel collar, which is essential for correct alignment (figure 96). Grease the load cell swivel monthly. It does not require a large amount of grease; one or two pumps from a hand-held grease gun with high-grade grease are sufficient. Failure to grease the load cell swivel can result in load cell calibration errors.
   

   Figure 96. Mounting brackets and lock flange attached to strike plate.

   
4. Assemble the rubber pad and PVC disk before installing them on the load cell (figure 97). Use a strong adhesive to secure the rubber pad to the PVC disk before screwing the PVC disk to the load cell. Secure the PVC disk and rubber pad to the bottom of the load cell (figure 98).

   NOTE: Photos here are part of a major overhaul. It is recommended that if a load cell replacement is necessary that the rubber pad and PVC disk be put together before installing on the load cell. After replacing a load cell, conduct a full, absolute calibration.

5. Attach the rear raise/lower bar guide to the front of the strike plate. The bottom screw tends to work loose; check it annually. Use a high-strength thread locker to secure the screw. The bottom screw can be difficult to remove. It may be necessary to apply heat to the bolt to remove it.
   

   Figure 97. Strike plate with attached load cell and pad (shown inverted).

   

   Figure 98. Adhesive applied to PVC disk.

   

RAISE/LOWER BAR

The raise/lower bar guide raises and lowers the geophone rail to the ground. The raise/lower box sleeve in the front of the trailer guides the front end (figure 99).

Figure 99. Raise/lower bar guide location before assembly(rear).

The rear end of the raise/lower bar attaches to the rear lift nut (figure 100).

Figure 100. Raise/lower bar guide during assembly.

A steel cable and pulleys raise and lower the bar (figure 101). Lubricate the front raise/lower pulleys monthly and check the cable every 3 to 6 months for damage. Replace it if damaged.
Note: The bottom screw tends to work loose so it is a good idea to check annually. Use high strength lock tight to secure the screw.

Figure 101. Raise/lower bar guide in location.

Figure 102 shows the raise/lower bar removed from the FWD, and figure 103 shows the center lift nut and attaching bolts. Little maintenance is required. Yearly, remove the attaching bolts on the rear, clean, and reapply with medium-strength thread locker.

Figure 102. Raise/lower bar.

Figure 103. Center lift nut and attaching bolts.

Following are the steps to assemble the raise/lower bar:

1. Install the center lift nut over the guide shaft. Place the guide shaft between the strike plate and the lower tab as seen in figure 101. After installing this component, assemble the raise/lower bar. Align the hole in the rear bar with the hole in the rear lift nut. Apply medium-strength thread locker to the bolt. Repeat on the opposite side.
2. Attach the cable-pulley configuration to the trailer frame (figures 104). Attach a steel cable and run it through two pulleys (figure 105). Lubricate the pulleys monthly with silicone spray. Periodically remove the cover and inspect components for damage or excess wear, and repair or replace as necessary.
   

   Figure 104. Closeup of cable connection.

   

   Figure 105. Raise/lower bar, guide rod, and attaching cable.

   
3. Place stainless steel cover over guide rod (figure 106). Make sure the pin is in place during transport.
   

   Figure 106. Raise/lower cover.

   
4. Attach the cable to the top of the rails with straps (figure 107).
   

   Figure 107. Attached cable.

   
5. Attach the tension spring (figure 108). The tension spring helps take up cable slack on the top of the raise/lower bar when the bar is fully raised.
   

   Figure 108. Mounted geophone holders to raise/lower bar.

   
6. The geophone holders are screwed into specific locations along the rails, although the locations can be adjusted. Mount the geophone holders to the raise/lower bar and adjust position (figures 109 and 110).
   

   Figure 109. Completed assembly of raise/lower bar.

   

   Figure 110. Rear view of raise/lower assembly.

   

GEOPHONE HOLDER

The geophone holder attaches to the raise/lower bar on the FWD. The purpose of the geophone holder is to provide a means of adjusting the geophones to various distances from the load plate.

Figure 112 shows geophone holder components. Upon inspection, if holder springs are weak or damaged replace only as a set, not individually, to maintain proper seating pressure.

Figure 111. Geophone holder components.

Geophones are preassembled. Figure 112 shows an assembled geophone holder for an 80 mil (2,032 microns) standard geophone.

Figure 112. Assembled geophone holder.

Figure 113 shows the components for the geophone.

Figure 113. Geophone components.

Use the following steps to assemble your own geophone rather than using a preassembled one:

1. Mount the ground ring to the geophone (figures 114 and 115).
   

   Figure 114. Ground ring and geophone.

   

   Figure 115. Mounted ground ring.

   
2. Prepare the geophone cable for assembly. Use a sharp art knife to split the two-strand wire and shield. Separate the wires to solder them onto the geophone (figures 116 and 117).
   

   Figure 116. New geophone cable.

   

   Figure 117. Separated wire.

   
3. Twist the wire shield and attach to the ground lug (figure 118).
   

   Figure 118. Attached ground lug.

   
4. Solder the positive and negative wires to the appropriate prongs on the geophone (figure 119). Screw the ground lug into the side to the ground ring.
   

   Figure 119. Geophone cable soldered to geophone.

   
5. Place the geophone in the geophone housing and fill the internal area with electronics-grade silicone (figure 120). Install the strap.
   

   Figure 120. Silicone-filled housing.

   
6. Place the plastic base cover on the geophone (figure 121).
   

   Figure 121. Base cover on housing.

   
7. Attach the steel base plate, which will secure the magnet to the geophone housing (figure 122).
   

   Figure 122. Geophone with base plate attached.

   
8. Attach the magnet with a screw and medium-strength thread locker (figures 123 and 124).
   

   Figure 123. Attached magnet.

   

   Figure 124. Completely assembled geophone.

   
9. Pay attention to the DIN (Deutsches Institut fur Normung eV) plug connector at the other end of the geophone cable (figure 125). Attach the DIN plug housing to the end of the cable.
   

   Figure 125. Raw cable and DIN plug housing.

   
10. Solder wires to the internal component of the DIN plug (figure 126).
    

    Figure 126. Internal view of the DIN plug.

    
11. Attach the second half of the DIN plug outer housing to the plug (figure 127).
    

    Figure 127. Completed DIN plug.

    

After the geophone is assembled, plug the geophone into the correct DIN connector and run a drift screen check to ensure they are functioning properly). Follow by conducting a reference calibration.