How does force generation differ in pull vs push cylinders?

The push (compression) strokes create greater force over the pull (tension/retraction) strokes within this same hydraulic cylinder since the extend stroke utilizes the entire piston area and the retracting stroke utilizes the smaller annular space remaining after deducting the rod's diameter; that is, the same pressure exerted on a smaller area results in less force. This is asymmetry that is present in the cylinders of every single rod and is directly affecting the way engineers design sequence circuits, cylinders, and troubleshoot systems that are not performing.

The fundamental physics of force equals pressure times the area.

The process of generating hydraulic force is governed by one rule of thumb:

F = P x A

In which F represents force and P is the pressure of the system as well as A being the area that the fluid is acting on. In a double-acting cylinder that is standard, there are two distinct areas of effectiveness that differ depending on which side of the piston is being pressurized, and this is at the heart of all the differences between push and pull, which is explained below.

The piston's full area

When the piston is extended (the pushing stroke), the hydraulic fluid is injected into the cap-end (blind-end) chamber and is pushed against the complete circular face of the piston. The area of effect is as follows:

A(push) = p x (D/2)²

in which D represents the diameter of the piston. Since there is no rod on this side to subtract, the whole face of the piston can be used for force generation. This is why the extension force is the most powerful than the other two figures that appear on the specification sheet for a cylinder.

Pull stroke area: Annular (rod-side) area

In the process of retraction (the pulling stroke), fluid flows into the rod-end chamber rather. The piston rod is part of the chamber's cross-section. So the fluid acts only upon the annular rings in the remaining area:

A(pull) = p x [(D/2)² - (d/2)²]

where the rod's diameter is. The annular surface is always smaller than the entire piston area, and at the same system pressure, the force of retraction is always less than it is in a standard single-rod piston.

What is the significance of the rod-to-bore ratio being so important?

The gap in force between pull and push strokes depends on the size of the rod with respect to bore. A rod that is thin in a cylinder with a big bore reduces the area of the rod's side, which means that pull force remains near to the push force. A large rod used in cylinders that are designed to buckle resistance or for high load sideways—takes away more of the rod's side, which increases the gap.

A rough example: a cylinder that has 4 inches of bore and a 1-inch rod is only losing approximately 6% of its surface on the rod's side, meaning that the pull force is about 94 percent in push. The same bore size of 4 inches with the 2.5-inch rod (common in the long-stroke or heavy-duty application) is losing more than 39% of its area, and the force of pull drops to around 61 percent from push. This is the reason why two cylinders of identical bores can differ in retraction capability solely based on the rod choice.

Velocity trades off against force.

Another effect of the area difference is that it is easy to overlook the fact that cylinder speed is inversely proportional to the area of an unrestricted flow rate. Because the rod's side is smaller and it's the exact same rate of force pushed into the chamber, it will move the piston more quickly in retraction than when it is extended. This is the reason why many hydraulic systems retract significantly faster than they extend, despite the fact the force they pull is less. Circuit designers may employ this technique in a deliberate way—employing a rapid, low-force retraction to cut down on cycle time for return strokes that are not load-bearing—but it also can cause inadvertent speed mismatches when not considered when the sizing process.

Circuits for regeneration: blurring the lines

Certain hydraulic systems employ the regenerative circuit, in which the rod's return flow is diverted back to the cap-end chamber when it is extended instead of being dumped into the tank. This improves the speed of extension but decreases the force available during the push stroke due to it affecting the total space that the pump's flows are creating. Regenerative circuits are an excellent illustration of how pull and push force aren't properties that can be fixed to a cylinder on their own; they're the result of the geometry of the cylinder, along with the way in which the circuit is plumbbed.

Double-rod cylinders: Symmetrical force

There are many applications that do not support the push-pull asymmetry. Double-rod cylinders that run a rod at each end of the piston feature the same annular area for both sides. This allows pulling and pushing the force equally in both directions, but with an extended overall envelope and generally more expensive. They are commonly used in situations such as machine tool tables and meters, where consistent force and speed across both directions are more valuable than smaller packaging.

Practical implications for design of systems

1. The direction of the load is a factor that should influence the cylinder's selection. If the primary (load-bearing) stroke involves a retraction, such as is the case with most presses, lifts, or rams, the typical single-rod cylinder's strength advantage in push is aligned well with the use. If the work stroke is retraction, like clamping a load, pulling it from the outside, or drawing into tooling the cylinder, it needs to be measured so that the smaller annular space provides enough force at the available system pressure. Incorrect sizing is a typical and preventable design flaw.

2. Pressure requirements aren't necessarily symmetrical. To meet a specific force goal for the pull stroke, the pressure of the system on the rod's side will need to be greater than the pressure of the push stroke, especially for rods with large diameters. This can affect the settings of the relief valve along with the pump's selection and the pressure ratings of components throughout the circuit.

3. Force capacity and rod buckling are part of a complete package. The larger rods are typically designed to withstand the buckling caused by compressive (push) loads and columns that are loaded in extension. However, the same rod size decreases the force of retraction. Engineers often have to weigh buckling resistance against pull-force adequacy. The compromise should be addressed at the beginning of sizing, rather than found out during commissioning.

4. Diagnostics should take into account inconsistency. If a cylinder appears to be ineffective on one side but not the other direction, it's not necessarily a problem. Verify that the shortfall falls within the range of the expected push/pull ratio of the bore-and-rod combination before examining the possibility of leaks or worn-out seals or degraded pump performance.

The difference in force between pull and push strokes isn't an issue or flaw within the design of hydraulic cylinders; it's an obvious predictable result of the rod geometry that is based on the same source of pressure. Understanding the relation between rod size and pressure in the system will allow engineers to design the cylinders in a way that is appropriate for the direction that will carry the workload, anticipate the speed variations in strokes, and prevent misinterpreting normal directional asymmetry in the context of an issue with the system.

Why is the force of push always higher than the pull force in a typical hydraulic cylinder?

The push stroke makes use of the complete piston area; the pull stroke makes use of the smaller annular space created through subtracting the rod's cross-section. The same pressure applied to a smaller area will always produce less force.

Is it possible for the hydraulic cylinder to be made to pull the same force that it pushes?

The answer is yes, by using the double-rod piston that has an equally annular area for both ends of the piston, creating symmetrical pull and push forces, at the expense of a larger body of the cylinder.

Does a bigger piston rod reduce the force of retraction?

Yes, within a certain bore size. A rod with a larger diameter takes away some of the rod's side area and reduces the pull force; however, larger rods are usually designed to reduce the resistance to buckling when under compressive loads.

Why do cylinders retract more quickly than they extend at the same flow rate of the pump?

Since the rod-side chamber has a smaller area than the chamber at the cap The identical volumetric rate results in an increased velocity in the retraction stroke, although it creates less force.

In what way do engineers determine the proper size of a cylinder if the load to be carried is retracted?

A: Calculating the available force based on an annular (rod-side) area at the desired system pressure but not the complete piston area. This can be done by increasing the size of the bore or pressure or by reducing rod diameter in the event that the resultant pull force is less than the weight requirement.