By Josh Cosford, Contributing Editor
Drift is the unintentional movement of a cylinder when it’s meant to be held in place by directional, PO check or counterbalance valves. The objective of many cylinder applications is to move a load to a predetermined position and maintain that position for various lengths of time. For example, a bucket lift used by utility providers to work on power lines absolutely requires a cylinder to lift to the desired position and stay there. Therefore, a drifting cylinder is disconcerting at best and dangerous at worst.
A cylinder may drift for various reasons, most of which offer us explanations for a drifting bucket lift application. First, assuming we start with a cylinder facing rod upwards in our lift application, we know pressurized fluid must remain in the cylinder’s piston side (cap side) until directed to exhaust by the operator. So long as the fluid is contained under pressure in the cylinder’s piston side volume, the cylinder is stable.
Should any fluid leave or leak from the piston side volume, the cylinder will lower (or drift) unintentionally. The cause of this drift may be directly attributed to the path the fluid takes to exit. The most likely culprit of cylinder drift is the valve located closest to the cap side port. In most cases where the safe control of cylinders is required, a counterbalance valve is that closest valve.
A counterbalance valve is essentially a pilot-operated relief valve with a reverse flow check valve. As cap pressure exerts its force upon the port of the valve, its spring holds the valve closed until load pressure increases above the spring value. However, only in rare circumstances does a counterbalance open directly from load pressure. If such is the case, you need a higher pressure valve or a larger actuator. A counterbalance valve should remain shut until it receives a pilot signal from the opposing work port, piloting the valve open to allow the cylinder to retract. Should any leakage occur in the counterbalance valve, fluid may pass through to either the downstream or opposite work ports allowing the cylinder to drift.
Stable cylinders are critical in lifting applications, where hydraulic cylinder drift could pose a risk.
Leaking piston seals may also allow a cylinder to drift … but sometimes not. With the above application using a single counterbalance valve, it’s recommended to use a float or open center spool in the directional valve, allowing the work ports to drain to tank in neutral. Opening the work ports to the tank ensures the counterbalance valve’s spring chamber remains drained and does not allow pressure additive to the spring value. With the above configuration, leaking piston seals will allow the cylinder to drift down as fluid leaks from the piston to the rod side of the cylinder.
However, if your cylinder is held aloft with just a closed center directional valve, you might assume that a cylinder with leaking piston seals could still drift downward. Fluid should travel from the cap side volume to the rod side volume, right? Not at all, actually. Because the fluid volume in the rod side of the cylinder is smaller than the volume in the cap side of the cylinder, fluid has nowhere to go. You could literally remove the piston seals entirely, and the cylinder will only drop a fraction of an inch as the pressure equalizes inside the entire volume of the cylinder.
If your cylinder application employs a rod-down configuration, it could absolutely drift to the bottom in some cases. This is because fluid quickly moves from the rod side of the cylinder to the piston side above when seals leak. This scenario may occur whether you have a closed center valve, counterbalance valves or even dual PO check valves, and is a sure sign your piston seals are shot.
The final example of cylinder drift occurs during a more unassuming situation. There are circumstances where hydraulic fluid leaks into a cylinder. Rare cases involve a positioning cylinder operated with no other valves than simply a directional valve with a closed center operated by a pressure compensated pump. Should pressure in the tank line increase due to some restriction, fluid from the pressure compensated pump may leak towards the cylinders and expose the work ports to that fluid. Because of the differential area of the cylinder, the force acting upon the piston side overcomes the force on the annular area of the rod side, and the cylinder may drift forward.
Preventing drift
So we know how cylinder drift may occur, so let’s discuss how to prevent or remedy drift. The first step to prevent cylinder drift comes at the design stage. Going back to our first example with the counterbalance valve, you must understand how the valve interacts with the cylinder. Because a counterbalance valve is essentially a relief valve, there is always the chance it could crack open slightly if load pressure is too close to the valve’s spring setting.
A cylinder too small for the application may experience periods of load-induced pressure exceeding the maximum value of the spring inside the counterbalance valve. It’s unlikely the load will drop catastrophically, but the poppet or spool in the valve may begin to crack open, allowing the cylinder to drift downward. Ensure that you choose your cylinder bore and counterbalance valve pressure range far enough apart never to experience pressure overlap.
Choosing the correct cylinder and seal package for your application is also essential. Well-engineered and well-manufactured cylinders offer tighter clearances between their piston outside diameter and the cylinder barrel inside diameter. This tighter gap helps seals better prevent leakage not only when new but especially when worn. As well, higher quality cylinders are offered with better quality finishes, such as a chromed and honed finish to the barrel ID.
The seal selection makes a difference because not all seal types work as effectively at sealing, strangely enough. Some seals, such as U-Cup or lip seals, offer low-friction designs better suited to high velocity or low friction applications requiring little or no static friction. As a result, they’re designed to break away from a stop position more quickly and will “chatter” less in most applications. Under low pressure, a lip seal also has a better chance of leaking since they count on pressure pushing the lips out against the wall surface, thereby improving their sealing.
An interference fit seal, such as a T-seal or crown seal, offers a superior guard against leakage, especially at lower pressure. However, it comes with more friction and the resulting reduction in maximum cylinder velocity. However, many of the interference fit seal options are considered “leak-free” and will hold a load indefinitely should it be asked. And when these seals fail, you can get by in a pinch by replacing them with O-rings supported with backup rings.
It goes without saying that a cylinder must be adequately maintained to ensure the sealing material is always fresh and ready to do its job. If you neglect maintenance duties, such as proper cooling and filtration, you can expect your seals to fail prematurely. Even the most well-maintained machines will still see their cylinder seals wear over time. To prevent leakage and drift from old, tired seals, ensure you have in stock replacement seals to execute a quick repair should cylinder drift become an issue.
Poppet-style counterbalance valves feature a cone and seat design, to offer the lowest leakage rate for directional or pressure valves. Always keep in mind that leakage in any form could result in drift.
Image courtesy of Danfoss
Lastly, preventing leakage that causes cylinder drift comes down to choosing and maintaining the correct valves, whether directional or pressure. Although it’s not recommended to hold a load using only a closed port directional control valve, cylinder drift may still occur in applications, as discussed previously. Select high-quality spool valves which are machined with tighter clearances that are less likely to leak across any of its ports. If you’re ever curious about the quality of your directional valve, plumb a 5-gallon accumulator to a work port charged to system pressure and then see how long it takes to pressure to decay. The faster the pressure decays, the more leakage it allows.
If you rely on counterbalance valves to hold a load, you may want to select only poppet-based valves. Spool valves are inherently leaky, and any sustained pressure at their work ports will result in some leakage. Most reputable valve manufacturers publish the leakage rate of their valves, so compare valves to ensure you’re getting the lowest leakage valve possible. However, due to their cone and seat design, poppet valves offer the lowest leakage rate for directional or pressure valves. Always keep in mind that leakage in any form could result in drift.
Filed Under: Cylinders & Actuators, Engineering Basics
