Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-HRT-05-153
Date: December 2006

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.

Photograph shows a load cell mounted to the strike plate. Arrows indicate the locations of the strike plate, manual transport locks, guide rails, and hydraulic line mounting clamps.

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.

Photograph shows a strike plate. Arrows indicate the locations of the holes for raise and lower guide rails, hole for side cylinder, holes for load cell attaching brackets, hole for center cylinder, rear raise and lower bar mounting bracket, and where to mount the center cylinder.
 

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.

Photograph shows the bottom view of a load cell swivel and retaining bracket on strike plate. Arrows indicate the locations of the strike plate, holes for bolts to secure retaining plate open parenthesis use medium-strength thread locker close parenthesis, load cell swivel open parenthesis remove and clean annually close parenthesis, and retaining 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).

    Photograph shows a load plate with the base turned down. Arrows indicate the locations of the four holes for attaching brackets, three holes for PVC plate under load cell, collar for attaching to strike plate, and load DIN plug connector.
     
  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.

    Photograph shows a load plate with mounting brackets. Arrows indicate the locations of the attaching bolts open parenthesis use medium-strength thread locker close parenthesis, four brackets, and load cell.

    Figure 95. Load cell with brackets mounted.

    Photograph shows a load cell with brackets mounted. Arrows indicate the correct position of the lock collar applied after load cell is pressed into the swivel and mounted to the strike plate.
     
  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.

    Photograph shows mounting brackets and lock flange attached to strike plate. Arrows indicate the locations of the six screws open parenthesis use a medium-strength thread locker close parenthesis, four brackets open parenthesis use a medium strength lock-tight close parenthesis, lock collar, and load cell swivel grease fitting open parenthesis use a high-quality grease once a month, but only one to two pumps with a grease gun close parenthesis.
     
  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).

    Photograph shows a strike plate with attached load cell and pad open parenthesis shown inverted during an overhaul, same steps can be followed without removing the strike plate close parenthesis. Arrows indicate the locations of the rubber pad, P V C disk, load cell, bolts that attach P V C disk, attaching brackets, and strike plate.

    Figure 98. Adhesive applied to PVC disk.

    Photograph shows an adhesive that has been applied to the surface of the P V C disk. Arrows indicate the location of the adhesive on the 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).

Photograph of the rear view of a raise and lower bar guide assembly and location. Arrows indicate the locations of the raise and lower guide rail open parenthesis front close parenthesis, center cylinder, hydraulic lines, hydraulic lines and clamp open parenthesis check every 3 to 6 months close parenthesis, hole for raise and lower bar guide assembly, four bolts open parenthesis must be checked annually close parenthesis, top attaching bolt and washer open parenthesis apply medium-strength thread locker annually close parenthesis, guide shaft open parenthesis lubricate weekly close parenthesis, raise and lower bar attaching nut, and bottom screw open parenthesis use high-strength thread locker and check this fastener once a year; remove and clean threads on bolt and reapply high-strength thread locker close parenthesis.
 

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

Figure 100. Raise/lower bar guide during assembly.

Photograph shows a raise/lower bar during assembly. Arrows indicate top attaching bolt and washer open parenthesis use medium-strength thread locker close parenthesis, guide shaft open parenthesis lubricate weekly close parenthesis, raise/lower bar attaching nut, and bottom screw open parenthesis check fastener yearly, remove and clean threads on bolt, reapply high-strength thread locker close parenthesis.
 

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.

Photograph shows a raise/lower bar in location. Arrows indicate the locations of the strike plate, load cell, P V C disk, ribbed rubber sheet, center cylinder hydraulic lines, center lift nut, and tab and bottom screw open parenthesis use high-strength thread locker and check every 6 months; this screw has a tendency to work loose; it should be removed and cleaned, and then have an application of high-strength thread locker, and then be replaced close parenthesis.
 

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.

Photograph shows raise/lower bar. Arrows indicate the locations of the guide rod open parenthesis lubricate weekly close parenthesis, raise/lower bar, center nut, and attaching bolts open parenthesis use thread locker annually close parenthesis.

Figure 103. Center lift nut and attaching bolts.

Photograph shows a closeup view of a center lift nut and attaching bolts. Arrows indicate the hole in the bar that needs weekly lubrication.
 

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.

    Photograph shows a closeup view of a cable connection. Arrows indicate the locations of the guide rod open parenthesis goes through the steel sleeve on the raise/lower box close parenthesis, raise/lower cable, rails, pulley open parenthesis note: lubricate rollers monthly; every 6 months, check pulleys for damage or excessive wear close parenthesis, cable routing, and set screw.

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

    Photograph shows the raise/lower bar, guide rod, and attaching cable. Arrows indicate the locations of the stainless steel cover, guide rod, pulley, and raise/lower rails.
     
  3. Place stainless steel cover over guide rod (figure 106). Make sure the pin is in place during transport.

    Figure 106. Raise/lower cover.

    Photograph shows a raise/lower cover. An arrow indicates the location of the transport pin.
     
  4. Attach the cable to the top of the rails with straps (figure 107).

    Figure 107. Attached cable.

    Photograph shows a cable that has been attached to the rails with straps.
     
  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.

    Photograph shows mounted geophone holders to raise/lower bar. Arrows indicate the locations of the rails, geophone holder, tension spring, and mounting bracket.
     
  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.

    Photograph shows a completed assembly of a raise/lower bar with all components in the correct positions.

    Figure 110. Rear view of raise/lower assembly.

    Photograph shows the rear view of raise/lower assembly. An arrow indicates the location of the grease fitting open parenthesis grease monthly close parenthesis. Note the position of this geophone. It is for L T P P testing. Geo number 3 is located 305 millimeters from the center of the load cell to the center of the holder.

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.

Photograph shows the components of a geophone holder. Arrows indicate the locations of the bolts to hold geophone holder to rail, clamping bolts, geophone housing, foam guide open parenthesis lubricate weekly close parenthesis, geophone holder top bracket, geophone holding springs, geophone clamping disk open parenthesis check monthly and clean with a wire brush if necessary close parenthesis.
 

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

Figure 112. Assembled geophone holder.

Photograph shows an assembled geophone holder. Springs should be checked monthly. open parenthesis Weak or damaged springs can cause the geophone to not have a consistent set pressure. This can cause variability in the geophone readings. close parenthesis The interface between the geophone clamping disk and the geophone holder should be lubricated monthly with low-viscosity lubricant.
 

Figure 113 shows the components for the geophone.

Figure 113. Geophone components.

Photograph shows the components of a geophone laid out individually. Arrows indicate the locations of the geophone, geophone housing, electrical cable, clamping ring magnet, magnet screw, screws, ground lug with screw and nut, D I N plug, D I N plug lock ring, exterior base plate, interior base, spacer ring, strap/nut, and cable grommet.
 

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.

    Photograph shows a ground ring and geophone. Arrows indicate the locations of the ground ring and the geophone.

    Figure 115. Mounted ground ring.

    Photograph shows a ground ring that has been mounted on a geophone. An arrow indicates the location of the opening for geophone cable.
     
  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.

    Photograph shows a new geophone cable. Arrows indicate the locations of the grommet and the geophone cable.

    Figure 117. Separated wire.

    Photograph shows a separated wire. Arrows indicate the locations of the negative wire, shield, positive wire, and prongs for wire.
     
  3. Twist the wire shield and attach to the ground lug (figure 118).

    Figure 118. Attached ground lug.

    Photograph shows an attached ground lug. An arrow indicates the location of the 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.

    Photograph shows a geophone cable soldered to geophone. Arrows indicate the locations of the two prongs positive/negative, geophone, ground lug attached with screw, ground ring, and cable and grommet.
     
  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.

    Photograph shows a housing filled with electronic grade silicone sealant/adhesive. A tube of silicone adhesive also appears in the photo. Arrows indicate the locations of the applied silicone, cable, strap, and housing.
     
  6. Place the plastic base cover on the geophone (figure 121).

    Figure 121. Base cover on housing.

    Photograph shows a plastic base cover positioned on the silicone filled housing. An arrow indicates the location of the base cover.
     
  7. Attach the steel base plate, which will secure the magnet to the geophone housing (figure 122).

    Figure 122. Geophone with base plate attached.

    Photograph shows a geophone with base plate attached. Arrows indicate the locations of the spacing ring, center nut to secure magnet, steel base plate, screws open parenthesis require a high-strength thread locker close parenthesis, and housing.
     
  8. Attach the magnet with a screw and medium-strength thread locker (figures 123 and 124).

    Figure 123. Attached magnet.

    Photograph shows an attached magnet. Arrows indicate the locations of the screw open parenthesis requires thread locker close parenthesis, magnet, and housing.

    Figure 124. Completely assembled geophone.

    Photograph shows a completed geophone with all components properly assembled.
     
  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.

    Photograph shows a raw cable and DIN plug housing. Arrows indicate the locations of the outer housing open parenthesis first piece close parenthesis and raw cable.
     
  10. Solder wires to the internal component of the DIN plug (figure 126).

    Figure 126. Internal view of the DIN plug.

    Photograph shows an internal view of the DIN plug. An arrow indicates the location of the soldered wire.
     
  11. Attach the second half of the DIN plug outer housing to the plug (figure 127).

    Figure 127. Completed DIN plug.

    Photograph shows a completed DIN plug. An arrow indicates the location of the outer housing open parenthesis second piece close parenthesis.

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.

Previous | Table of Contents | Next

 


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration