The causes of hydraulic cylinder drift

Hydraulic systems certainly can have their challenges. If your system has stopped working, the first thing you want to do is to ensure your hoses are connected properly and that the couplings engage the way they should.

When problems arise, hydraulic drift – more specifically, hydraulic cylinder drift – is often the culprit. This is the result of unequal pressure created by internal leaks in the cylinder across the piston.

How hydraulic drift happens

The fluid flows from one side of the piston to the other, creating an imbalance. The cylinder becomes hydraulically locked when the rod seal leaks due to pressure around the piston equalizing.It’s this imbalance that causes the cylinder to drift towards either the extend or retract position, unable to hold a load at mid-stroke position.Occasionally, hydraulic fluid contaminated with dirt, grit, water or air is behind hydraulic drift, and when this happens, you’re looking at the possibility of damaged seals or sealing surfaces.Excessive heat can also create hydraulic drift. When heat builds up inside a system, seals have to endure additional wear, hardening prematurely and becoming brittle. Be sure to use the correct seals and keep operating conditions within the recommended temperature.

Why does hydraulic drift matter?

Fluid movements become rough and stop and start abruptly. You can imagine the serious safety risks this creates. There’s also the threat of cylinder failure, caused by the pressure put on one area of the cylinder. When drifting occurs, you know that the cylinders are failing to perform efficiently or as intended. Equipment failure is costly, so stay on top of the situation.

Step 1: Understand the physics

This will help you troubleshoot more effectively. Hydraulicspneumatics.com explains the physics behind drift:

• If a closed-to-actuator or cylinder spool block a double-acting cylinder service port, and the piston seal does bypass, pressure will spread out on each side of the cylinder’s piston.
  • Remember, this is when hydraulic lock occurs, and drift comes to a standstill. That is, unless fluid can seep from the cylinder or the cylinder’s circuit.
• In order to support the same load, the static pressure in the cylinder needs to increase.
  • This is because of pressure acting on the rod-side annulus area due to pressure, brought on by the loss in effective area.
  • Example: say the load-induced pressure on the cylinder’s piston side is 2,000 PSI. On the rod side when the DCV is centered, it’s zero. Assuming no leakage occurs past the spool, and depending on the areas’ ratio, the equalized pressure may be 3,000 PSI.
  • Now let’s say the circuit has a service port relief valve set at 2,500 PSI. The pressure will start to balance itself across the piston seal.
  • The static pressure on the cylinder’s piston side will increase, reaching the cracking pressure of the port relief.
  • When this happens, the cylinder will drift downwards

Step 2: Troubleshooting

Given the nature of how hydraulic drift occurs, you’ll appreciate why a pressure gauge is helpful in diagnosing the cause. You should troubleshoot using pressure testing under controlled conditions.

Piston seal leak
Even if this happens, it’s not always the reason for hydraulic drift. If your ports are unblemished and the cylinder is full of hydraulic oil, the cylinder will maintain its load without a hitch. That is, until a leak happens at the rod seal.

Rod seal leak
When the rod seal is not intact, then expect the negative pressure vacuum on the piston side to dissipate. When that happens, the piston rod will stop working due to pressure equalizing throughout the cylinder.

Pressure gauge readings
When a leak is not obvious, use the pressure gauges on the cylinder. A hydraulic fluid leak reduces the cylinder’s effective area. As the pressure increases on the rod side, so, too, must the pressure in the piston side. This will ensure the same load is maintained.

To calculate the equalization pressure, refer to the piston-side pressure before the leak compared to the pressure on the rod side. To better understand this, let’s say the rod size pressure is zero PSI. On the piston side, pressure is 2000 PSI. Then equalization will take place at 3000 PSI.

Once you achieve equalization, you stop further drift and hydraulic lock from happening again, until more hydraulic fluid leaks occur. Note, some port relief valves prevent a cylinder from reaching equal pressure. This is due to set lower pressure levels. So in the example we just gave, hydraulic drift will continue as overall pressure hits 2500 PSI.

Note, if the cylinder is drifting:
The directional control valve or load control valve could be the problem if, across the piston seal, there is a lack of equalization of pressure.

Preventing hydraulic drift

Your best options:

• Always use a quality hydraulic cylinder.
• Quality seals are also necessary.
• Undertake regular maintenance of your cylinder. Inspect the rod, looking for corrosion and uneven wear and tear.
• Keep your oil clean – high quality breather caps keep oil free from moisture and particle contamination.
• Service your accessories. This includes components such as rod eyes, ball joints or other connections. Not maintaining these components can lead to misalignment and even equipment failure.

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