Slow Down or Stop – Managing Cylinder End of Stroke
Let’s start with a joke. (Courtesy of Chris Hurley, RHK Hydraulics)
A cop sees Chris slowly roll through a stop sign. He pulls him over and says, “You failed to come to a complete stop at that stop sign back there.” Chris shrugs his shoulders and says, “So I didn’t stop, I slowed down – what’s the difference?” The cop takes out his nightstick and starts beating Chris with it, then asks him, “Do you want me to slow down, or stop?”
No matter what the application is, hydraulic cylinders have an expected actuation speed. In most cases that speed is linked to productivity and is as quickly as safely possible. This means that we must be able to maintain safe control over moving the load and stopping the load – at high speed. A load that is in motion is a form of kinetic energy, and this energy must be absorbed either mechanically or hydraulically as it comes to a stop.
Allowing the cylinder to come to a hard stop mechanically sends shock waves throughout machine members and the cylinder itself. This causes fatigue stress, cracking, and premature failure. Absorbing the kinetic energy hydraulically by incorporating a cushion within the cylinder can provide a smooth dissipation of the kinetic energy, but will create a pressure spike within the cylinder. This pressure spike is not visible outside of the cylinder, and will not register on the system gauges. The cylinder must be designed to withstand these pressure spikes with heavier construction and, in some cases, special seals.
There are a variety of cushion designs used in hydraulic cylinders, but they all work off of the same basic principal; squeezing the oil through an orifice at the end of the stroke. The graphic below shows a conical cushioning bush attached to the piston, which chokes off the flow of oil as it enters the seat in the cylinder cap. The adjustable orifice shown at the bottom allows for fine tuning the cushion as required.
If a cushion is to be installed on the annular (rod end) of a hydraulic cylinder the ratio of the cap end to rod end must be considered. The piston rod takes up a certain amount of area within the bore, which reduces the effective area of the annular side. Because of this, any restriction of the oil exiting the retract side of the cylinder will cause some degree of intensification. If the ratio is high enough, the intensification may cause the cylinder to fail. For example, a cylinder with a ratio of 3:1 operating at 3000 PSI would cause the pressure to reach 9000 PSI if a cushion was installed in the annular side.
An alternative to cushioning a cylinder is to use position feedback to command the directional control valve to meter the flow to the cylinder just before the end of the stroke. This requires a proportional control valve, electronic feedback, and a programmable logic control (PLC) to take one signal and provide another. The programmed ramp time is then adjusted until the desired performance is achieved. This method has several advantages: it keeps the pressure spike on the upstream side of the directional control valve where it can be managed by system pressure controls (reduces pressure spikes within the cylinder); it prevents unauthorized personnel from adjusting the actuation speed (must be programmed); and it can be utilized on any cylinder with position feedback – regardless of the ratio (prevents intensification). The obvious disadvantage is system complexity and cost.
Regardless of the chosen method, managing deceleration of hydraulic cylinders at the end of stroke has a direct impact on the service life of the cylinder and overall machine performance. Selecting the most effective and cost practical solution requires a clear understanding of the application as well as proven design techniques. As Chris recently found out, there is a big difference between slow down and stop! If you’re searching for “hydraulic cylinders Canada,” you can stop your search. As leading hydraulic cylinder manufactures, RHK Hydraulic Cylinder Services Inc. can create a custom hydraulic cylinder for you. Contact us today to learn more.