Aug
22
Operating a single hydraulic cylinder up and down is a fairly simple procedure, however when you want two, three, four or more cylinders to move up and down at the same time, troubles quickly arise.
Basic hydraulics tells us that hydraulic oil move along the easiest flow path, and if one of the multiple hydraulic cylinders has a small internal leak, the plumbing run is shorter, an extra fitting / adapter is used, one hose is older than another – the results are uneven cylinder extension and retraction.
Known methods of synchronizing hydraulic cylinders are listed here, in their order of increasing complexity, accuracy and cost. include:
- Individual flow controls
- Individual pressure compensated flow controls
- Mechanical Flow Dividing Valves or Assemblies
- Single to multiple hydraulic cylinder.
- Servo / Proportional / PLC Controlled.
Let’s look at them one at a time.
Individual flow controls.
Each cylinder receives it’s very own adjustable needle / flow control valve. These vales literally limit the flow to the cylinder with a simple knob adjustment. However, if the plumbing to and from each cylinder is not exactly the same length, with exactly the same adapters, etc. this rarely works well at all. For a generic log splitting type application, this may work just fine. However, with pressure and temperature fluctuations, the cylinders will never move up and down together well.
Individual pressure compensated flow controls.
Each cylinder receives it’s own pressure compensated flow control. These flow controls maintain exact flow rates, no matter what the temperature or pressure fluctuations are within the system. The plumbing still needs to be virtually the same lengths, and the knob allows anyone to twist it an mess up the settings. Some valves are lockable. A low-cost method, and for 2-3 cylinders, this may work fine. Repeatability (cylinders extending and retracting together) is not very high.
Mechanical Flow Dividing Valves or Assemblies.
All hydraulic flow is passed through a mechanical divider valve or assembly. Each output port (2, 3, 4, 6, 8, etc.) is controlled via=2 0a fixed gear cog. All gear cogs turn at exactly the same rate, due to incoming oil. Outgoing oil is usually 95 – 97% identical, allowing for some very good cylinder synchronization. Using built-in flow relief valves, these style of flow dividers actually “reset” themselves after each extension or retraction cycle. This automatically realigns the cylinders and keeps them in phase with each other. Down sides are usually a 1500 psi maximum operating pressure, and cost.
Single to Multiple Hydraulic Cylinders.
This is a very interesting solution to the hydraulic synchronization problem. All hydraulic oil comes into the extend port of a specially made hydraulic cylinder. As the cylinder extends, multiple cylinder rods extend simultaneously into custom smaller cylinders. The oil that is pressurized out of these smaller cylinders is routed directly to the actual multiple cylinders. However, plumbing runs must be exact for this to work properly. This is a decent solution when running 6+ cylinders at the same time.
Servo / Proportional / PLC Controlled.
Cylinders can be synchronized to +/- 1.0 millimeter using a PLC and closed-loop technology. Each cylinder has it’s own individual linear reporting device. This device sends an analog / digital signal back to the PLC, telling the PLC exactly where the cylinder is along it’s stroke. Each cylinder has it’s own=2 0servo or proportional hydraulic directional control valve mounted directly to the cylinder. The Operator commands the cylinders to move to a certain position. The PLC commands each hydraulic valve to extend or retract the cylinder. The cylinder tells the PLC where it is. As the cylinder gets closer and closer to the needed position, the PLC starts closing down the hydraulic flow, with the cylinder finally coming to a stop as planned. This is an extremely accurate and reliable means to position 4, 6, 8 – 20 hydraulic cylinders in exactly the same spot, each and every time. Cost is high, but accuracy is dead-on.