In some truck and trailer applications, particularly tippers, it is necessary to dump the suspension during certain operations, such as tipping.
Dumping the suspension lowers the air springs onto the bump stops, resulting in improved stability during dynamic load shifts. However, moving the trailer or raising the chassis while the air springs are deflated could damage the air springs.
The problem occurs because the bellows will collapse onto the air spring piston if the air spring is extended without any air pressure. If the air spring is then compressed while the bellows are in contact with the piston, the rubber can be pinched and damaged.
To prevent this from occurring, a system that ensures a minimum pressure in the air springs, even with the suspension dumped, must be installed into the air circuit. This is not as simple as screwing a residual pressure valve into the exhaust port of the HCV.
Valve manufacturers have purposely designed the valves to prevent people simply adding regulators to the exhaust port because that would inhibit HCV operation.
In most cases, the accessible exhaust ports on HCVs are used only for raise/lower functions and not for dumping. For example, both Haldex and Barksdale valves release air for the dump function through a large, dedicated vent on the side of the valve, not the standard exhaust port.
So, how can residual pressure be maintained without compromising HCV function? There are a few proven solutions.
Separate Pilot Valve System
A method some truck manufacturers use is to bypass the HCV dump function and install a separate pilot valve, which is controlled by the cabin dump switch via a solenoid. A pressure regulator or check valve is then fitted to the exhaust side of the pilot valve, ensuring the air springs retain enough pressure to hold their shape, even when dumped.
Pressure Retention Valve Setup
Another effective method is to use a pair of quick exhaust valves combined with a pressure regulator between the HCV and the air springs. A suitable pressure retention valve is available from Hendrickson as part number AK-136-1.
This system allows the HCV full control over ride height under normal conditions. However, while dumping, air is rapidly expelled via the quick exhaust valves, down to a minimum pressure set by the regulator. This setup not only protects the air springs but can also improve dump response times.
Pressure Setting Considerations
The minimum residual pressure typically ranges from 0.3 to 0.7 bar (4 to 10 psi), depending on the application. For lighter chassis, setting the pressure too high may cause the suspension to lift or float while dumped. An adjustable regulator is preferred over a fixed check valve for this reason, as it allows fine-tuning to suit specific operational needs.
Regardless of the method used, it’s essential that:
- The air springs retain enough pressure to maintain their shape when dumped.
- The system does not interfere with normal HCV operation.
Following these guidelines will ensure that the air springs remain reliable and durable, even in demanding applications like tipper operations.
Read more helpful industry advice from the Hendrickson team.
