No matter how much attention is given to motion controller selection and programming, hydraulic system performance may be limited by incorrect sizing or physical location of components. A common design oversight is the use of undersized cylinders. Designers sometimes intend to increase piston rod speed by specifying a cylinder with a smaller bore — based on the assumption that for a given amount of oil flow, a smaller cylinder will produce quicker accelerations and higher velocities.

However, this only works for very light loads. For actuators moving moderate to heavy masses, acceleration, velocity, and deceleration are limited by the available force — not by oil flow. Because cylinder bore determines the force it can produce, if the bore is too small, the cylinder may be incapable of attaining the desired speeds or cycle times required by the application.

A real world example
We were recently asked to diagnose problems with a system operating incorrectly. After obtaining specific application data, we performed a hydraulic system simulation. Our mathematical model suggested-that more force was necessary. The simulation was changed to evaluate cylinders with larger bores. The dramatic results, graphed in Figures 1 and 2, suggested replacing the existing 2-in. bore cylinders with 3¼-in. bore cylinders and replacing the hydraulic valves with appropriately larger ones.

Figure 1 shows how an undersized cylinder may not provide enough force to quickly decelerate a large load when extending. The cylinder cavitates (pressure goes to zero) on the cap end, and pressure on the rod end exceeds system pressure. The system is out of control because oil on the rod side is going back into the oil supply, thereby actually decreasing deceleration!

When the cylinder diameter is increased to 3¼-in., Figure 2, the piston rod accelerates the load at the same rate, but the pressure doesn't change as much because of the greater piston surface area. The cylinder pressures throughout the cycle are in the middle of the pressure range, with plenty of pressure drop across the piston to maintain positive control.

Increasing the cylinder bore increases the natural frequency (stiffness)of the system, allowing the motion controller to manage faster accelerations and decelerations, and yielding higher system performance when properly tuned. However, larger cylinders require larger valves and higher flow. Very large cylinders increase cost. And because large valves tend to be slower, at some point increasing valve size will no longer increase system response.