viii Features are student-focused learning tools designed to help you get the most out of your studies. This visual guide high- lights the features designed for the textbook. Features of the Textbook Copyright Goodheart-Willcox Co., Inc. Heavy Power Trains and Systems 484 Adjusting the Dual Servo Pistons Dual servo piston pumps have threaded servo caps installed over both of the servo pistons. As the servo caps are installed, a depth micrometer or a dial indicator is used to measure the swash- plate. Both sides of the swashplate are measured. Eaton states that the two measurements should be within 0.0005″ (0.0127 mm) of each other. It is critical to mark the caps before removal to aid the assembly process. See the Eaton dual-servo pis- ton pump in Figure 14-9. When rebuilding this pump on a workbench, most technicians can cor- rectly adjust the servo pistons and will not have to perform a fine tune adjustment of the servos when the pump is being test run on the pump test stand. Adjusting the Pump DCV Neutral Position The pump’s direct control valve (DCV) uses a cen- tering spring to hold the control spool in a neutral position. Similar to the electronic null adjust- ment, the control spool needs to be adjusted so that the servo control oil pressures are balanced. Some transmissions are designed to have zero forward and reverse servo pressures while in neutral. Other transmissions have positive but balanced forward and reverse servo pressures, such as 40 psi forward and 40 psi reverse, while in neutral. A cap commonly covers the DCV centering spring. The centering spring has an adjustment screw for placing the spool in a neutral position. See Figure 14-10. Depth micrometer Servo caps Goodheart-Willcox Publisher Figure 14-9. The installation of the servo caps adjusts the pump’s swashplate angle. Prior to removing the caps, mark their location with a permanent marker. A depth micrometer or a dial indicator is used to center the swashplate while installing the servo caps. Case Study Uncommanded Actuation of a Hydraulic Pump An MT 845 Challenger uses a hydraulic steering pump and motor (essentially a hydrostatic transmission) as the hydraulic steering input for the differential steering system (covered in Chapter 24, Track Steering Systems). The machine uses a permanent-magnet speed sensor to detect rotation in the hydraulic steering motor. During engine startup, the tractor would steer without command. Interestingly, the machine sensed that the steering motor was rotating even though the operator was not turning the steering wheel. This caused several reactions: • The ECM de-energized the steering pump solenoids. • The machine stopped steering. • A fault code was recorded on the monitor. • The tractor could not be moved or steered. The steering pump’s servo piston and null adjustment were both checked and found to be centered. The steering pump solenoid wires were also checked for shorts. When the steering pump solenoid coils were removed and the machine’s engine was started, the machine would not actuate the steering pump without command. The repair required a new ECM. Although rare, it is possible for an ECM to cause an uncommanded actuation of a hydraulic pump. Copyright Goodheart-Willcox Co., Inc. 485 Chapter 14 ∣ Hydrostatic Drive Service and Diagnosticss Shaft Run-Out Hydrostatic pumps and motors are mounted in different configurations. If the mount or adaptor is incorrectly machined or not flat, the shaft splines can wear prematurely. Some manufac- turers provide detailed instructions for checking the pump mount surface and the shaft drive for run-out. The measur- ing tool of choice is a dial indicator. If the shaft’s splines fail prematurely, the pump or motor can be disassembled and the shaft replaced. The problem that caused the shaft to fail prematurely must be fixed. Installing a new shaft is typically cheaper than purchasing a new pump or motor. If the failure is prematurely worn shaft splines, most technicians will sim- ply replace the shaft, even if the hydro- static transmission is a closed-loop drive, because the rotating group is usually unrelated to the worn shaft splines. Note With proper maintenance, service, and contamination control, a pump can often last well over 10,000 hours. Startup (Commissioning) of a Hydrostatic Drive Many mistakes can be made during the installation of a new pump or motor. The Interna- tional Fluid Power Society (IFPS) labels the process of returning a system to service follow- ing pump or motor replacement as commissioning the hydraulic system. If the component is not installed correctly, the life of the component can be drastically reduced. Be sure to follow the manufacturer’s startup procedures when installing a new hydrostatic pump or motor, even if the component was removed only to install a new shaft. The following is an example of one manufacturer’s startup procedure: 1. Prevent the machine’s engine from running by removing a key or pulling a fuse. 2. Fill the case (of the pump and/or motor) with oil through the case drain plug. 3. Install a pressure gauge in the charge pressure test port. 4. Place the multi-speed range gearbox in the neutral position. (Manufacturer’s instructions differ regarding whether or not to also release the brake and chock the machine’s wheels.) The startup procedure should be designed so that the hydrostatic transmission has no load on the motor’s shaft. 5. Start and run the engine at low idle while monitoring charge pressure. If pressure fails to build to specification in less than 15 seconds, shut off the machine and diagnose the cause. 6. If charge pressure meets specification, increase the engine speed to 1500 rpm to purge air through the system. 7. Slowly move the propulsion lever halfway forward and allow it to operate for 4 to 5 minutes. Charge pressure should drop a little due to the flushing relief when the hydrostat is rotating. Cemtering spring End cap Goodheart-Willcox Publisher Figure 14-10. An Eaton dual-servo pump can contain a manual control valve that is centered with a spring. The end cap is removed to gain access to the centering spring adjustment. Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 480 For electronically controlled pumps, remove the pump control valve solenoid coils to see if still in If still a or a failed is not at Single-Servo Cradle Bearing Pump Newer HST pumps use a single-servo piston to control a cradle bearing swashplate. The the to and - sion. The pump requires adjustments for centering the pump to • Centering the servo piston. • DCV null adjustment. Centering the Servo Piston A Bosch Rexroth single-servo pump includes a threaded screw that is used for placing the piston in an neutral See Figure 14-1 . Note that a jam nut holds the threaded rod in position. As the threaded rod is the of swashplate. See Figure 14-2. Prior to adjusting servo a technician loop the two servo together. jumper hose is attached to test port and the reverse port, S1 and S2. balances the servo to reverse servo Warning Note that as this adjustment is made, the machine will move. Always follow the manufacturer’s instructions for lifting and blocking the to or Figure 14-3 shows a pump on a stand, not a running transmission. The photo also shows the S1 and S2 setup the servo piston’s position can be Gauges are installed to measure pressure at drive service of for measuring the two pressures. Because most manufacturers recommend having jack it Goodheart-Willcox Publisher Figure 14-2. As the threaded adjustment is turned, it causes the swashplate to The rod is adjusted until the swashplate is centered in a the to the swashplate. It is pinned to the swashplate and the of Goodheart-Willcox Publisher Figure 14-1. A single-servo cradle bearing hydrostatic pump an end plate a for the Jam nut H y EEquipment quipment Sy C pter S o with servo piston. Copyright Goodheart-Willcox Co., Inc. 479 Chapter 14 Hydrostatic Drive Service and Diagnostics Objectives After studying this chapter, you will be able to: ✓ List the steps required to adjust the neutral position on two styles of hydrostatic pumps. ✓ List the steps for starting a hydrostatic transmission after replacing a pump or motor. ✓ Describe the process for troubleshooting the following hydrostatic transmission symptoms: ✓ Overheating. ✓ Low power in both directions. ✓ Low power in just one direction. ✓ Machine will not move. A s discussed in Chapter 13, Hydrostatic Drives, hydrostatic transmissions (HSTs) do not coast or by If a is misadjusted, the pump not a true but instead creep either or of pump is a cradle style or a dual-servo trunnion there are types of for setting pump to a neutral One is to set pistons to neutral, and adjustment is to set the control valve to Pro Tip Always adjust the position of the servo piston(s) before adjusting the control valve. If the servo piston(s) out of adjustment, the control valve will be incorrectly to the to the misadjusted servo piston(s). This misstep will require another adjustment to center the control valve after the servo piston(s) are centered correctly. Centering Adjustments Safety precautions must be taken prior to making any will the to turer’s safety procedures when making pump adjustments. Many manu- to place on stands to prevent or equipment run over. If the pump is controlled, may to more speed to prevent error codes uncommanded machine In addition to a misadjusted pump, a misadjusted pump linkage or a can cause to of must be considered a for a transmis- sion. In some manually controlled, dual-path hydrostatic drives, such as steers older the the dual-pump can be The manufacturer’s service lit- procedures for both Learning Objectives clearly identify the knowledge and skills to be obtained when the chapter is completed. Pro Tips provide you with advice and guidance that is especially applicable on-the-job. Warnings alert you to potentially dangerous materials and practices. Step-by-Step Procedures are presented throughout the textbook to provide clear instructions for hands-on service activities. Notes tips help you develop critical thinking, diagnostic and troubleshooting skills needed in the workplace today. Case Studies describe real-life situations encountered by technicians in the field to help you understand what you can anticipate and expect in the workplace.