How do you bleed a hydraulic ram?
Air trapped in a hydraulic ram can cause sluggish movement, jerky operation, and incomplete extension or retraction. Bleeding the ram removes that air, restoring smooth, predictable performance. The process generally involves depressurizing the system, locating the bleed point, venting air through a bleed screw or valve, topping up fluid, and cycling the ram to purge any remaining air.
Understanding why bleeding matters
In a hydraulic system, fluid transfers force. When air is present, it compresses and expands differently than the fluid, creating delays and inconsistent motion. Bleeding is essential after maintenance, component replacement, or detectable soft or unresponsive operation to ensure the ram operates at its designed speed and force.
Preparation and safety
Before you begin, shut down the equipment and ensure it is fully depressurized. Wear appropriate PPE, including eye protection and gloves, and keep a tray or pan ready to catch leaking fluid. Use the correct hydraulic fluid for the system and work in a clean area to prevent contamination.
Follow these steps in order to bleed the ram safely and effectively.
- Power down the system and relieve all pressure. If your unit has a manual relief valve, operate it slowly to release stored pressure.
- Move the ram to a neutral position if possible and secure it to prevent unexpected movement.
- Identify the bleed point(s). Common locations include a dedicated bleed screw or valve on the cylinder end cap or at the valve block feeding the ram.
- Place a drain pan under the bleed point and prepare clean rags and a flashlight to inspect for bubbles and leaks.
- If a bleed screw or valve is present, attach a clear hose if available to observe air escaping more clearly.
- Slowly loosen the bleed screw/valve to begin venting air. Do not tighten fully until you are sure air is no longer present.
- Continue until a steady stream of hydraulic fluid (with no visible air bubbles) exits the bleed point. Then snug the bleed screw/valve firmly to seal it.
- Top up the reservoir or sump to the manufacturer-recommended fluid level and replace the cap or plug securely.
- Cycle the ram slowly through its stroke in both directions to purge any remaining air from the lines and the cylinder interior.
- Inspect all fittings for leaks and ensure the system maintains the correct fluid level after cycling.
When you finish the initial bleed, recheck the operating characteristics. If the ram still feels spongy or hesitates, you may need to repeat the bleed process or inspect for leaks, faulty seals, or blocked lines.
Bleeding by cylinder type
Single-acting hydraulic ram
Single-acting rams rely on hydraulic pressure for extension and typically rely on a spring or gravity to retract. Bleeding focuses on the circuit feeding the ram. If the rod returns sluggishly, check the return path and any check valves or bypass passages for air entrapment.
Follow these steps to bleed a single-acting ram:
- Isolate and depressurize the system as described above.
- Identify the bleed point on the ram or valve block feeding the ram.
- Bleed the feed line first, watching for air until a steady flow is observed.
- Cycle the ram to purge any remaining air and ensure proper extension under normal load.
After completing the steps, verify that the ram extends smoothly under load and retracts as designed when pressure is released.
Double-acting hydraulic ram
Double-acting cylinders have two independent flow paths (one for extension, one for retraction). Bleeding should purge both ends of the cylinder and associated lines. Start with the end that usually moves fastest or is most accessible, then address the opposite end.
Follow these steps to bleed a double-acting ram:
- Depressurize the system completely and secure the ram.
- Bleed the cap-end line (the side driving the piston toward extension) first, then the rod-end line (the side driving retraction).
- Allow air to escape from each end by loosening the respective bleed screws/valves slowly until fluid runs clear and bubble-free.
- Closely monitor the reservoir level and top up between bleeding steps if necessary.
- Cycle the ram through multiple full strokes in both directions to purge air from both circuits.
Once both ends are bled, recheck for leaks and ensure the system remains at the correct fluid level after cycling.
Post-bleed checks and maintenance
After bleeding, perform a thorough check of the system: inspect all fittings for leaks, verify the fluid type and level, and ensure return lines are functioning correctly. Dispose of any used hydraulic fluid according to local environmental regulations. If air reappears or performance does not improve, consult the equipment manual or a qualified technician, as the issue may indicate a faulty seal, a clogged line, or a damaged component.
Summary
Bleeding a hydraulic ram is a practical maintenance task that restores smooth, predictable operation by removing air from the hydraulic circuit. Key steps include depressurizing the system, locating bleed points, venting air slowly, topping up fluid, and cycling the ram through its range of motion. The exact procedure can vary by cylinder type, so consult the manufacturer’s manual for device-specific instructions. With proper safety practices and careful attention to fluid quality and fittings, bleeding helps prevent performance issues and extends the life of hydraulic equipment.
