U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-05-153
Date: December 2006
Long-Term Pavement Performance Program Falling Weight Deflectometer Maintenance Manual
Chapter 3. Subassembly
The subassembly is a rigid component that bolts directly to the trailer frame with four bolts and nylon lock nuts to secure the bolts. The top of the subassembly, or tower bridge, is bolted into place using eight bolts and washers. Use thread locker (medium-strength blue Loctite® or equivalent) to prevent loosening. Every 3 to 6 months, examine all eight bolts, tighten, and replace any missing bolts).
Figure 25 shows the subassembly and figure 26 shows the subassembly mounted on the trailer.
Figure 25. Freshly painted subassembly with joints reinforced.
Figure 26. Subassembly mounted on trailer.
Conduct a visual examination monthly to ensure no cracks develop. The most common points where cracks may develop are the welds between the channel iron and the hydraulic tank (both sides) and at the opposite ends of the channel. Cracking can be caused if only one side of the channel is welded, rather than both sides. This was a common problem on older units.
If cracks develop, a reputable welding shop needs to make the repairs, observing the following four steps:
Figure 28. Disconnect battery terminals.
Figure 29. Multisignal connection to control box.
Disconnect all electrical components during the welding to reduce the chance of electrical current flowing through the trailer, which might damage sensitive electronics.
After the repairs are made, the affected area can be cleaned and painted. Figures 30 and 31 show the reinforced welds on the subassembly.
Figure 30. Reinforced weld on top of subassembly.
Figure 31. Reinforced weld on bottom of subassembly.
Annual maintenance is recommended on the guide profiles including shimming the rollers for correct alignment. Rails should be checked every 6 months. They may need to be shimmed periodically, depending on the amount of use. Yearly, apply a liberal amount of grease to the guide roller post (figure 32). This requires removing the guide profile rails as seen in figure 25.
Figure 32. Guide locations on a subassembly.
Although the subassembly frame does not require any specific maintenance, it is recommended that all associated fasteners be checked for tightness every 3 to 6 months. Other components also may require periodic maintenance and repair:
Drop Weight Assembly
Minimal maintenance is required on the drop weight assembly:
The hydraulic system is one of the most important parts on the FWD. It can also be the most complicated to assemble and troubleshoot. The following paragraphs describe and illustrate various components of the hydraulic system.
The hydraulic system incorporates the following main items:
The modes of operation of the hydraulic system are to raise and lower the load plate, as well as raise and drop the weights. The modes of operation are best explained in a "truth table," as shown in table 1. It is also useful to install a hydraulic pressure gauge to assist in troubleshooting.
The hydraulic pump is mounted to the top of the hydraulic fluid tank (figures 40 and 41). A single bolt, which runs through the bottom of the tank, holds the pump in place. An O-ring is used to seal the pump to the tank. The pump has proven very reliable. Yearly, remove the hydraulic motor and lubricate the drive coupler with high quality moly-type grease. The only other routine monthly maintenance needed are changing oil and filter, tightening motor to pump bolts, and checking for excess pressure and adjusting if necessary (as explained in the Dynatest Owner’s Manual). A drain plug for replacing or adjusting hydraulic fluid is located on the bottom of the hydraulic tank.
Figure 40. Hydraulic pump, motor, and fluid tank.
Figure 41. Hydraulic pump.
Semiannually check all steel line clamps. These clamps are used to keep the steel lines from rubbing against each other and from contacting areas where a hole could result. Check the clamps every 6 months and apply a medium-strength thread locker to fasteners.
The hydraulic motor mounts directly to the pump (figure 42). The 12 V-motor is a brand that is equipped with inspection covers, and it has replaceable brushes and brush retainers. Brushes and retainers (figure 43) should be checked every 3 months.
Figure 42. Hydraulic motor.
Figure 43. Brushes and retainer.
The manifold block is attached to the pump housing (figure 44). A gauge helps in troubleshooting pressure-related problems. If one is not installed, it is recommended to install one.
Figure 44. Manifold block and A/B valve mounted to hydraulic pump.
NOTE: Rubber O-rings are used to seal the manifold block to the pump and the A/B valve to the manifold (figure 45).
Figure 45. Components of the A/B valve and manifold block.
PS1 and PS2 are pressure sensitive switches (figures 46 and 47). PS1 is a pressure sensitive switch with a normally closed contact that opens when the pump outlet pressure exceeds approximately 40 bar (34.47 to 4,136.85 kPa (5–600 psi)). PS1 is used for excess pressure detection; thus, the (red) PS1 light emitting diodes (LED )must turn off at excess pressure.
PS2 is a pressure sensitive switch with a normally open contact, which closes from the pressure in the return oil during catch lowering (setting approximate 2 bar (25–30 psi)). PS2 must stay on while the catch is moving downward, and it must come off when the catch stops at its bottom position. If any of these operations fail, check to see if oil is leaking from the end of the switch. A leak is a good indication the switch has failed.
NOTE: If PS1 or PS2 must be replaced, use Teflon® tape on the threads to ensure a secure, leak-free seal. Take care to not over tighten to the point of breaking the fitting or causing thread damage. Over tightening may also damage the manifold block.
Figure 46. PS1 and PS2 switches.
Figure 47. Threads on the PS1/PS2 switch.
The hydraulic motor solenoid switch requires no routine maintenance (figure 48). Periodically check to make sure the connections are tight.
Figure 48. Solenoid mounted to hydraulic motor.
Periodically check the wiring carefully for loose connections (figures 49 and 50). Be careful not to over tighten connections, as damage can occur to terminal lugs. As part of the monthly maintenance, remove A/B wiring plugs and clean with contact cleaner. Also, remove and clean coils on each end of the A/B valve. Remove and clean the stems on C/D valve assembly.
Figure 49. Wiring for Motor and Solenoid.
Figure 50. Electrical wiring for the hydraulic motor.
The main cylinder is made up of four major components:
Figure 51shows the main cylinder partially disassembled. Damaged seals could result in leaking hydraulic fluid or loss of hydraulic pressure.
Figure 51. Main cylinder components.
The center cylinder shaft is used to raise and lower the weight package catch assembly. The shaft is composed of a piston, two O-rings, seal, guide and wiper seals, and three attaching bolts. The main cylinder gland guides the shaft. The piston moves up and down inside the inner tube Figure 52. Center cylinder and components. The shaft should be straight and free of nicks and pitting. The piston, shaft, seals, and O-rings should (figure 52).
Figure 52. Center cylinder and components.
The shaft should be straight and free of nicks and pitting. The piston, shaft, seals, and O-rings should be examined if there is a problem with piston operation. Figure 53 shows a completed center cylinder after all seals and O-rings have been replaced. Figure 54 shows the oil hole port through which the hydraulic fluid flows to into the cylinder.
Figure 53. Center cylinder.
Figure 54. Oil hole port.
Main Cylinder Gland
The main cylinder gland is screwed into the main cylinder tube to guide the center cylinder. It is also equipped with a bleed screw to bleed the hydraulics of excess air. Numerous O-rings and seals may need replacement
Figure 55 shows the main cylinder gland and all necessary seals used in this component. Replacement of these seals is relatively simple. First, remove the old seals. Second, install the O -ring (A), then the internal seals (B) at the same location. Third, install the glide ring seal. (It may be necessary to form this seal first.) Fourth, put the outer O-ring into place. Fifth, install the dust seal. Finally, install the bleed screw and sealing ball (figure 56).
Figure 55. Main cylinder gland and components.
Figure 56. Assembled main cylinder gland.
Catch Flange and Plunger Piston
The catch flange is screwed into the center cylinder and the release plunger freely floats until hydraulic pressure builds up to the release pressure. The upper catch assembly attaches to the catch flange. After the weight package is raised to the target height, the release plunger is forced up and releases the inner catch collar. Figure 57 shows the O-rings and seals required for this component. Figures 58 through 60 show various views of the catch flange.
Figure 57. Catch flange, release piston, and plunger shaft.
Figure 58. Top view of catch flange.
Figure 59. Assembled catch flange and release piston (bottom view).
Figure 60. Assembled catch flange and release piston (top view).
When installing this component, do not use a thread locker. Setscrews are used to secure this flange. A pipe wrench can be used to tighten flange to the center cylinder; however, great care should be used to avoid marring or damaging the outer surface. This component does not require excess force to tighten.
NOTE: Always install a new outer O-ring if this component has been removed. Also, before reinstalling this component, a fine metal file should be used to remove any dimples caused by the setscrews on the cylinder. This will help ensure a flush surface and reduce the chance of hydraulic leaks.
Bench Assembly Method
To bench-assemble the main cylinder components, follow these steps:
To assemble the main cylinder on the unit, follow these steps, which are similar to those when assembled on a bench:
Figure 65. Main cylinder attached to strike plate.
Upper Catch Assembly
The upper catch assembly grabs the weight package lift collar so that the weight package can be raised and dropped. The upper catch is equipped with numerous moving components that will wear with use. (figures 66 and 67). It is important to keep all parts lubricated and free of debris, which could cause binding or excessive wear. Clean and inspect the moving components annually. Replace worn components.
NOTE: This repair can be made on the FWD without removing the cylinder shaft.
Figure 66. Upper catch shell and interior components.
Figure 67. Upper catch shell.
After all components are clean, begin reassembly. Following are the steps to assemble the upper catch:
Two side cylinders (front and rear) raise and lower the load/strike plate (figure 86). They are assembled similar to the main cylinder; therefore, a detailed description of assembly is not necessary.
Figure 86. Side cylinder and components.
To install the side cylinders after they have been assembled, follow these steps:
Topics: research, infrastructure, pavements and materials
Keywords: research, infrastructure, pavements and materials,Falling weight deflectometer, FWD, pavement deflection, maintenance, Dynatest, pavement testing equipment
TRT Terms: research, facilities, transportation, highway facilities, roads, parts of roads, pavements