- Mining Truck
- 794 AC (S/N: HRT1-UP)
- Off-Highway Truck/Tractor
- 794 AC (S/N: MN51-UP; MT51-UP)
Introduction
This Special Instruction contains information for servicing the suspension cylinder for certain Large Off-Highway Trucks.
Do not perform any procedure in this Special Instruction until you have read the information and you understand the information.
Safety Information
The following information is an explanation of various labels that are found in this document.
Warnings
The warning label informs the technician that an injury or death can occur as a result of a condition that may exist.
Notices
A notice informs the technician that component damage can occur as a result of a condition that exists.
Notes
A note contains general information for the technician about the operation that is being performed.
Proper repair is important to the safe and reliable operation of this machine. This document outlines recommended procedures. Some of the procedures require special tools, devices, or work methods.
Before you perform any repairs or before you perform any maintenance, read all safety information. Understand all safety information before you perform any repairs or before you perform any maintenance.
Safety information is provided in this document and on the machine. If these hazard warnings are not heeded, bodily injury or death could occur to you or other persons.
The “Safety Alert Symbol” that is followed by a “Signal Word” identifies a hazard. “DANGER”, “WARNING”, and “CAUTION” are “Signal Words”.
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| Illustration 1 | g00008666 |
The signal word “WARNING” has the following meanings:
- Pay Attention !
- Become Alert !
- Your Safety Is Involved !
The message that appears under the safety alert symbol explains the hazard.
Operations or conditions that may cause product damage are identified by "NOTICE" labels on the machine and in the service information.
The person that services the machine may be unfamiliar with many of the systems on the machine. Use caution when you perform service work. Special knowledge of the systems and of the components is important. Obtain knowledge of the system and knowledge of the component before removing any component or disassemble any component .
Caterpillar cannot anticipate every possible circumstance that might involve a potential hazard. The safety information in this document and the safety information on the machine are not all inclusive. A determination must be made by you that the operation, lubrication, maintenance, and repair procedures will not make the machine unsafe.
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Do not operate or work on this product unless you have read and understood the instruction and warnings in the relevant Operation and Maintenance Manuals and relevant service literature. Failure to follow the instructions or heed the warnings could result in injury or death. Proper care is your responsibility. |
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Failure to follow all safety guidelines prescribed in this document and by governing authorities and regulatory agencies may result in severe injury or death of personnel or machine damage. |
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Do not operate the machine if any guards or covers are missing or inadequately secured. Personnel could be seriously injured or machine damage may occur. |
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Personal injury or death can result from sudden suspension cylinder movement. Sudden movement, up or down, can cause the clearance above your head to change rapidly. Read all warning labels on the suspension cylinders before servicing. Do not check the oil in the suspension cylinder until all the nitrogen pressure has been released. Do not, under any condition, remove valves, cover or plugs from the cylinder unless the rod is fully retracted and all the nitrogen pressure is released. Do not stand under the machine when testing or adjusting the suspension cylinders. |
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Personal injury or death can result from improper lifting or blocking. When a hoist or jack is used to lift any part or component, stand clear of the area. Be sure the hoist or jack has the correct capacity to lift a component. Install blocks or stands before performance of any work under a heavy component. Approximate weights of the components are shown. Clean all surfaces where parts are to be installed. |
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Improper operation, lubrication, maintenance or repair of this product can be dangerous and could result in injury or death. Do not operate or perform any lubrication, maintenance or repair on this product, until you have read and understood the operation, lubrication, maintenance and repair information. |
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Read all Warning labels on the suspension cylinders before servicing. The vehicle must be on level ground before releasing the nitrogen pressure. Do not check the oil in the suspension cylinder until all of the nitrogen pressure has been released. Do not remove valves, covers or plugs from the cylinder unless the rod is fully retracted and all of the nitrogen pressure has been released. Do not stand under the machine during suspension cylinder charging, discharging, or testing and adjusting procedures or while lifting or lowering the machine. Sudden movement upward or downward can cause the clearance above your head to change rapidly. Failure to heed this Warning could lead to injury or death. |
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Dry nitrogen is the only gas approved for use in the suspension cylinders. Charging the suspension cylinders with oxygen gas will cause an explosion. This danger will not happen if nitrogen cylinders with standard CGA (Compressed Gas Association, Inc.) Number 580 Connections are used. When you order nitrogen gas, be sure that the cylinders are equipped with CGA No. 580 Connections. Do not use color codes or other methods of identification to tell the difference between nitrogen cylinders and oxygen cylinders. Never use an adapter to connect your nitrogen charging group to a valve outlet used on both nitrogen, oxygen or other gas cylinders. Be sure you use dry nitrogen (99.8% purity). |
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When applying an oil charge or a nitrogen charge with the machine on the ground, the machine may suddenly move due to internal sticking of the suspension cylinders. |
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High Pressure Cylinder. Do not remove any grease fitting, valve or plug until the truck is supported and pressure is relieved or the truck may drop suddenly, resulting in personal injury. Suspension groups should only be serviced by properly trained personnel who have read and understand the service manual procedure to relieve pressure and to charge cylinders. |
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Care must be taken to ensure that fluids are contained during performance of inspection, maintenance, testing, adjusting, and repair of the product. Be prepared to collect the fluid with suitable containers before opening any compartment or disassembling any component containing fluids. Refer to Special Publication, PERJ1017, "Dealer Service Tool Catalog" for tools and supplies suitable to collect and contain fluids on Cat® products. Dispose of all fluids according to local regulations and mandates. |
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Failure to charge the suspension cylinders properly can lead to damage to the frame. |
Always observe the list of basic precautions that follows:
Safety Signs
Safety information is presented by the following: signs, information plates, and decals. Read all “Safety” signs on the machine before operating, lubricating, or repairing the machine. Understand all “Safety” signs on the machine before operating, lubricating, or repairing the machine. Replace any safety signs that are in the conditions that follow: damage, unreadable and missing.
Protective Equipment
When you work around the machine, always wear protective equipment that is required by the job conditions. Protective equipment includes the items that follow: hard hat, protective glasses, and protective shoes. In particular, wear protective glasses when you use a hammer or when you charge the suspension cylinders. Use the appropriate protective equipment that is required by the job conditions. Protective equipment for welding includes the items that follow: gloves, welding hood, goggles, and apron. Do not wear loose clothing or jewelry that can catch on parts of the machine.
Mounting and Dismounting
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| Illustration 2 | g00037860 |
Use steps and handholds whenever you mount the machine. Use steps and handholds whenever you dismount the machine. Before you mount the machine, clean the step and the handholds. Inspect the step and handholds. Make all necessary repairs.
Face the machine whenever you mount the machine and whenever you dismount the machine. Maintain a three-point contact with the step and with handholds.
Note: Three-point contact can be 2 feet and 1 hand. Three-point contact can also be 1 foot and 2 hands.
Do not mount a moving machine. Do not dismount a moving machine. Never jump off the machine. Do not try to mount the machine when you carry tools or supplies. Do not try to dismount the machine when you are carrying tools or supplies. Use a hand line to pull equipment onto the platform. Do not use any controls as handholds when you enter the cab or when you exit the cab.
Specifications for Cables, Chains, and Lifting Devices
Use approved cables, chains, and lifting devices to lift components. Refer to the manufacturer's weight specifications to determine the application when you select the following items: cable, chain, and lifting devices. When you lift a component, the lift angle is critical. Refer to the Illustration 3 to view the effect of the lift angle on the working load limit.
Note: The lifting devices illustrated in this publication are not Caterpillar parts.
Note: Ensure that the hooks are equipped with a safety latch. Do not place a side load on the lifting eyes during a lifting operation.
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| Illustration 3 | g00629745 |
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Lift angles for lifting slings. (A) The load capacity is 100% of the working load limit for the sling. (B) The load capacity is 86% of the working load limit for the sling. (C) The load capacity is 70% of the working load limit for the sling. (D) The load capacity is 50% of the working load limit for the sling. | |
Hot Fluids and Parts
To avoid burns, be alert for hot parts on machines which have been stopped and hot fluids in lines, tubes and compartments.
Be careful when you remove filler caps, breathers, and plugs on the machine. Hold a rag over the cap or plug to prevent being sprayed by pressurized liquids. When the machine has been stopped, the danger of hot fluids is greater.
Corrosion Inhibitor
Corrosion inhibitor contains alkali. Avoid contact with the eyes. Do not allow corrosion inhibitor to contact the skin for extended periods of time. Avoid repeated contact with the skin. Do not drink corrosion inhibitor. If there is contact, immediately wash skin with soap and water. For contact with the eyes, flush the eyes with large amounts of water for at least 15 minutes. Seek medical attention.
Pressurized Items
- Always use a board or a piece of cardboard when you check for a leak. Leaking fluid under pressure can penetrate body tissue. Fluid penetration can cause serious injury and possible death. A pin hole leak can cause severe injury. If fluid is injected into your skin, you must get treatment immediately. Seek treatment from a doctor that is familiar with this type of injury.
- Relieve all pressure in air, oil, or water systems before any lines, fittings, or related items are disconnected or removed. Always make sure that all raised components are blocked correctly. Be alert for possible pressure when you disconnect any device from a system that utilizes pressure.
- Fuel lines that are damaged and fuel lines that are loose can cause fires. Lubrication lines that are damaged and lubrication lines that are loose can cause fires. Hydraulic lines, tubes, and hoses that are damaged can cause fires. Hydraulic lines, tubes, and hoses that are loose cause fires. Do not bend or strike high-pressure lines. Do not install lines which have been bent or damaged. Check lines, tubes, and hoses carefully. Do not use your bare hand to check for leaks. If fluids are injected into your skin, you must get treatment immediately. Seek treatment from a doctor that is familiar with this type of injury.
- Pressure air or water can cause personal injury. When pressure air or water is used for cleaning, wear a protective face shield, protective clothing, and protective shoes. The maximum air pressure for cleaning purposes must be below
205 kPa (30 psi) . When you use a pressure washer, keep in mind that the nozzle pressures are high. The nozzle pressures are frequently above13790 kPa (2000 psi) . Follow all the recommended practices that are provided by the manufacturer of the pressure washer.
Conditions for Servicing
- Suspension cylinder shows signs of oil leakage.
- Increased deflection with a normal ride height (indicator of oil loss).
- Frequent changes in ride height that require recharging with nitrogen gas.
- The suspension cylinder travel is limited.
- Harsh or bouncy ride indicating possible repeated operating at the end of the suspension travel stroke.
Machine Preparation
- Verify that the dump body is empty and free of any residual material.
- Ensure that all machine fluid levels are full.
- Park the machine on a level surface.
- Turn off the machine and switch off the battery disconnect switch. Chock the wheels.
Visual Inspection
Clean the suspension cylinders. Visually inspect cylinders for evidence of wear, damage, or leakage. Pay special attention to the exposed portion of the inner tube and the charging valves. Repair or replacement may be required if damage is found.
Check for evidence of damaged or loose retaining hardware. Immediate repair or replacement is required if damage is found.
Check each suspension cylinder for nitrogen leakage. A nitrogen leak is difficult to detect. A solution that consists of soap and of water can be used to detect leaks.
Required Tools
| Tool Chart | |||
| Item | Part Number | Description | Qty |
| A | Oil Refill Pump(1) Or Tool Group Nitrogen Fill- Alternative(2) | 1 | |
| B | Nitrogen Charging Group(1) | 1 | |
| C | Nitrogen Charging Fitting Gp(1) | 1 | |
| D | Lift Stands(3) | 2 | |
| (1) | When you work with the |
| (2) | The additional part number is an FT drawing Should a dealer want to fabricate, the dealer can use the |
| (3) | Match the correct lift stand (D) with the weight of the truck that is being serviced. |
Reference: Special Instruction, NEHS0742 for assembly instructions for tool (B) and tool (C).
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| Illustration 4 | g00295778 |
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Tool ( 1) Shutoff valve for the regulator (2) (3) Shutoff valve (4) Charging hoses | |
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| Illustration 5 | g01058817 |
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Tool ( 2) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) | |
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| Illustration 6 | g01433219 |
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Tool ( 2) (15) (16) (17) (18) (19) (20) (21) | |
175-7410 Nitrogen Charging Fitting Gp (C) is used with 175-5507 Nitrogen Charging Group (B) to charge the suspension cylinders. 8T-0859 Pressure Gauge (9) is used to check the pressure of the cylinder before the charging procedure is performed.
Oil Charging on the Machine
- Raise the machine until the suspension cylinders are fully extended. Secure in position with suitable cribbing.
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Illustration 7 g06311303 Side view of rear suspension cylinder.
(A) Cover
(B) Valve - Remove protective covers (A) over charge valves (B) and slowly open both valves. Carefully release the gas charge.
- Connect a vacuum pump to one of the charge valve assemblies and close the other.
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NOTICE Do not allow liquid oil to be drawn from the suspension.
- Draw a vacuum on the suspension cylinder. Monitor the vacuum process until all of the gas has been extracted. A smoother, darker suspension oil is an indicator that all of the gas has been removed.
Note: This process typically takes 15 to 30 minutes.
- Stop the vacuum process.
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Table 2 Rear Oil Charge Distance Sales Model Pin to Pin Measurement D1 794 AC 1514.6 ± 13 mm (59.63 ± 0.51 inch) Show/hide table
Illustration 8 g06310399 Rear suspension cylinder oil charge height. - Remove the cribbing and slowly lower the truck frame until a pin to pin measurement (D1) of suspension cylinders is reached. Refer to Table 2.
- Release any residual gas pressure in the suspension.
- Install the appropriate charging adapter. Open the port and observe if oil flows from the vent hole in the adapter. If oil does not flow from the hole, the oil level in the suspension cylinder is too low.
- Attach the oil charging kit to this port.
- Connect the suction line of the charging pump to a supply of quality oil.
- Add oil to the suspension cylinder until oil flows out of the drain hose. Stop the pump.
Note: During the filling operation, loosen the charge valve body slightly to remove pressure and to check the oil level. Oil and gas under minimum pressure will escape through the vent hole.
- Once oil level is achieved, remove the charging kit and adapters.
Oil Charging Off the Machine
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The following procedures are used for the initial oil filling after a suspension cylinder has been assembled or serviced only. The suspension cylinder must be removed from the machine for this procedure. |
- Secure the suspension cylinder in a vertical position. Ensure that no pressurized gas is in the cylinder.
- Remove the protective cover over the top charge valves and open both valves.
- Attach oil charging kit to one of the charging valves.
- With the cylinder completely retracted, pump oil into the unit until oil begins to come out of the open charge valve and stop the pump.
- Use appropriate crane or hoist to raise the suspension cylinder to full extension and lower to collapsed position 3 times.
- Extend cylinder to pin to pin measurement (D1). Refer to Table 2 and Illustration 8.
- Pump oil into the unit until it again comes out of the charge valve.
- Stop the pump and close the charge valve. Close the remaining charge valve and remove the oil charging kit.
Alternative Method:
- Fill suspension cylinder with oil while fully extended.
- Compress suspension cylinder (while exhausting oil) to pin to pin measurement (D1). Refer to Table 2 and Illustration 8.
- Close valves and remove oil charging kit.
Nitrogen Charging
- Remove the charging valve covers.
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Illustration 9 g06384941 Typical example of charging kit installation
(B) ValveShow/hide tableNOTICE Both rear suspension cylinders must be charged at the same time. Ensure that charging kit is installed in such a way that the pressure is equalized between the cylinders during nitrogen charging process.
- Connect the gas charging kit to the charge valves on the suspension cylinders. Ensure the shut off valves on the kit are closed.
- Adjust the pressure regulator to approximately
4137 kPa (600 psi) - Charge the suspension cylinders:
Note: When charging, nitrogen should be added slowly and periodically stopped to allow the suspension to equalize. The exact process will vary slightly with equipment, material, and operator.
Show/hide tableTable 3 Rear Nitrogen Charge Distance Sales Model Pin to Pin Measurement D2 794 AC 1571.75 mm (61.88 inch) - Open the shut off valves before opening the gas pressure regulator valve.
- Add nitrogen to the cylinders until an empty machine ride height pin to pin measurement of (D2) is achieved. Refer to Table 5 and Illustration 13.
Note: If the machine will be operating in ambient temperatures that are significantly colder than the charging location, it is recommended to increase the charge height
12 mm (0.5 inch) to25 mm (1 inch) . This will compensate for the effects of the reduced temperature on the nitrogen gas.
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Illustration 10 g06310932 Rear suspension cylinder empty ride height - Close the individual shut off valves and then the gas pressure regulator valves.
- Remove the charging equipment and check for leaks using a soap solution.
- Reinstall the protective covers and torque to
5 ± 0.5 N·m (44.25 ± 4.43 lb in) - Operate the machine for approximately 24 hours, and then repeat the empty ride height check. Adjust as required.
Oil Charging on the Machine
- Raise the machine until the suspension cylinders are fully extended. Secure in position with suitable cribbing.
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Illustration 11 g06311393 Side view of front suspension cylinder
(C) Cover
(D) Valve - Remove protective covers (C) over charge valves (D) and slowly open both valves. Carefully release the gas charge.
- Connect a vacuum pump to one of the charge valve assemblies and close the other.
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NOTICE Do not allow liquid oil to be drawn from the suspension.
- Draw a vacuum on the suspension cylinder. Monitor the vacuum process until all of the gas has been extracted. A smoother, darker suspension oil is an indicator that all of the gas has been removed.
Note: This process typically takes 15 to 30 minutes.
- Stop the vacuum process.
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Table 4 Front Oil Charge Distance Sales Model Pin to Pin Measurement D3 794 AC 1593.85 mm (62.75 inch) Show/hide table
Illustration 12 g06311381 Front suspension cylinder oil charge height. - Remove the cribbing and slowly lower the truck frame until a pin to pin measurement (D3) of suspension cylinders is reached. Refer to Table 4.
- Release any residual gas pressure in the suspension.
- Install the appropriate charging adapter. Open the port and observe if oil flows from the vent hole in the adapter. If oil does not flow from the hole, the oil level in the suspension cylinder is too low.
- Attach the oil charging kit to this port.
- Connect the suction line of the charging pump to a supply of quality oil.
- Add oil to the suspension cylinder until oil flows out of the drain hose. Stop the pump.
Note: During the filling operation, loosen the charge valve body slightly to remove pressure and to check the oil level. Oil and gas under minimum pressure will escape through the vent hole.
- Once oil level is achieved, remove the charging kit and adapters.
Oil Charging Off the Machine
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The following procedures are used for the initial oil filling after a suspension cylinder has been assembled or serviced only. The suspension cylinder must be removed from the machine for this procedure. |
- Secure the suspension cylinder in a vertical position. Ensure that no pressurized gas is in the cylinder.
- Remove the protective cover over the top charge valves and open both valves.
- Attach oil charging kit to one of the charging valves.
- With the cylinder completely retracted, pump oil into the unit until oil begins to come out of the open charge valve and stop the pump.
- Use appropriate crane or hoist to raise the suspension cylinder to full extension and lower to collapsed position 3 times.
- Extend cylinder to pin to pin measurement (D3). Refer to Table 4 and Illustration 12.
- Pump oil into the unit until it again comes out of the charge valve.
- Stop the pump and close the charge valve. Close the remaining charge valve and remove the oil charging kit.
Alternative Method:
- Fill suspension cylinder with oil while fully extended.
- Compress suspension cylinder (while exhausting oil) to pin to pin measurement (D3). Refer to Table 4 and Illustration 12.
- Close valves and remove oil charging kit.
Nitrogen Charging
- Remove the charging valve covers.
- Connect the gas charging kit to the charge valves on the suspension cylinders. Ensure the shut off valves on the kit are closed.
- Adjust the pressure regulator to approximately
4137 kPa (600 psi) - Charge the suspension cylinders:
Note: When charging, nitrogen should be added slowly and periodically stopped to allow the suspension to equalize. The exact process will vary slightly with equipment, material, and operator.
Show/hide tableTable 5 Front Nitrogen Charge Distance Sales Model Pin to Pin Measurement D4 794 AC 1755 ± 13 mm (69.1 ± 0.5 inch) - Open the shut off valves before opening the gas pressure regulator valve.
- Add nitrogen to the cylinders until an empty machine ride height pin to pin measurement of (D2) is achieved. Refer to Table 5 and Illustration 13.
Note: If the machine will be operating in ambient temperatures that are significantly colder than the charging location, it is recommended to increase the charge height
12 mm (0.5 inch) to25 mm (1 inch) . This will compensate for the effects of the reduced temperature on the nitrogen gas.
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Illustration 13 g06311398 Front suspension cylinder empty ride height - Close the individual shut off valves and then the gas pressure regulator valves.
- Remove the charging equipment and check for leaks using a soap solution.
- Reinstall the protective covers and torque to
5 ± 0.5 N·m (44.25 ± 4.43 lb in) - Operate the machine for approximately 24 hours, and then repeat the empty ride height check. Adjust as required.
Payload Monitoring
When any work has been performed on the suspension cylinders, the payload monitor must be recalibrated.
Note: Payload monitoring can be set only after cylinders have been properly charged and empty ride height verified. The machine must be fully assembled, unloaded, and the dump body must be free of any debris.
Refer to Testing and Adjusting, UENR3957, "Payload System - Calibrate" to calibrate the payload monitoring system.