How to size a hydraulic cylinder: Bore, Stroke & Pressure explained

Picking the wrong hydraulic cylinder to suit an application is among the most costly and frequent mistakes made in power system fluid design. A cylinder with a small size cannot generate enough force. A cylinder that is too big wastes energy, adds weight to the system, and can increase the cost of components. Getting the right size the first time requires a thorough understanding of three key parameters, such as the bore's diameter and stroke's length as well as operating pressure. This guide explains each of these parameters in depth, explaining how these parameters interplay to determine the cylinder's performance.

What is the significance of sizing a cylinder?

Hydraulic cylinders convert the pressure of fluid into mechanical force that is linear. Every decision made in the cylinder—including the bore, the rod's diameter, the stroke, and the seal's configuration—stems directly from the load requirements of the application. Before beginning any calculation, the engineer needs to define what the cylinder will be required to accomplish and how much force it will generate, at what distance, and at how fast.

The cylinder's size is underestimated, which can lead to slowing down the motion and failure of seals due to overloading and rod damage. Oversizing can increase pressure requirements for the system excessively and could result in instability for systems that are controlled from a position. A proper sizing decision is not an issue of safety margins alone, as it directly impacts the efficiency of components, their life span, and the system's ability to respond.

Step 1: Definition of the required load

The initial point of reference for any calculation of cylinder sizing is the output force that must be used. This is the force the cylinder has to move or tug against in its working cycle.

The load requirements are broken into several parts:

A static load refers to the main resistance that a cylinder has to overpower—the weight of the lifting load or cut in presses or the force of clamping in the frame.

Dynamic load is the term used to describe the forces of acceleration when a machine has to move a mass quickly. This is especially relevant for machines that are mobile and automated, in which cycle times are very short.

Friction and back pressure make up the actual load. Friction in slides, guides, and the cylinder generally adds 5-15 percent on top of the load theoretically. The back pressure of the return line should also be taken into account on the rod's side in extension.

Security factor: the majority of industrial cylinders use an effective safety factor of 1.25 up to 2.0 over the load to be calculated, depending on the criticality of the application and the intensity of the duty cycle.

When the force that is effective in all its forms is determined, the bore and pressure can be determined.

Step 2—Calculate bore diameter

The diameter of the bore determines the size of the piston. The area of the piston directly affects the amount of force a given pressure will produce. The connection is easy to understand:

Force = Pressure x Area

Rearranged to find the area required:

Area = Force / Pressure

From this area the bore diameter is determined:

Bore Diameter = (4 x Area / p)

In terms of practicality: if the system is operating in a 200-bar pressure (approximately 2,900 pounds of pressure) with the requirement for force for a cylinder of 50,000 N (roughly 11,240 lbf) The piston area is 25 cm². This area is equivalent to an approximate bore diameter of 56 millimeters. It is typically rounded to the next size standard bore (63 millimeters in a metric series).

It is vital to understand that the calculation of force is different between retraction and extension strokes. When you extend, the entire bore area is used against the load. When retraction is performed, the effective area is diminished due to the rod's cross-section (annular area). In the event that the cylinder is required to produce equally in all directions, then the rod's diameter has to be determined in a manner that is appropriate—as a double-rod cylinder can be thought of.

Standard bore sizes are based on the ISO 6020 or ISO 6022 series for metric applications or SAE standards for imperial systems. Always round to the closest standard bore instead of specifying the diameter of a specific item unless the purpose requires it.

Step 3—Select operating pressure

The diameter of the bore and operating pressure are inextricably linked. For a particular force requirement, the higher pressure permits an enlarged bore, while lower pressure demands a larger bore.

The majority of industrial hydraulic systems run in the range of 140 bar (2,000 psi) to 350 bar (5,000 psi). Mobile equipment and high-performance machinery typically are operating at the higher portion of this range in order to make the dimensions of cylinders smaller. Press and general manufacturing applications are typically operating in between 140 and 210 bars.

The decision to select the operating pressure isn't just an exercise in calculation; it requires considerations at the system level:

Capacity and selection of pumps: higher operating pressures call for valves and pumps that are rated accordingly, resulting in higher costs and causing higher wear rates for seals throughout the entire system.

Seal performance—seals have pressure ratings. Continuously operating at or near the maximum pressure limit of a seal's specifications reduces the time between service and the chance of failure.

The thickness of the cylinder's wall and loads for mounting—greater pressure requires walls that are thicker and sturdier end caps. Mounting stresses resulting from deflection in the cylinder's body are also increased when pressure is increased.

An ideal approach is to choose the operating pressure that will keep the cylinder within 70 to 80 percent of its nominal pressure capacity in peak load conditions. This gives room for system transients without putting strain on components beyond their limits.

Step 4—Determine stroke length

The length of the stroke is the total linear distance that the cylinder has to produce starting from fully retracting until fully expanded. The process of determining stroke is typically easier than pressure and bore calculations, but it can have significant structural implications.

Determine the travel you need to make first. Find the distance between the extended and retracted positions within the machine. Add any necessary overtravel to ensure end-of-stroke cushioning or safety clearance.

Account for mounting geometry. When mounting pivots, the stroke of the cylinder must be calculated based on the design of the linkage or pivot arm and not only the distance that is linear between the endpoints. Inconsistencies in geometry calculations could lead to significant stroke sizing errors.

Verify the slenderness. Long-stroke cylinders that have smaller bore diameters are prone to buckling of the column under the pressure of compressive loads. It is recommended that the Euler Buckling Formula (or simplified hydraulic chart of buckling cylinders) be used when the ratio of stroke to bore is excessive. In general, in cases where the stroke is about ten times the bore's diameter, it is necessary to conduct a buckling study. A bigger bore, stop tube, or alternative mounting arrangement could be required to tackle this.

Take into consideration the length that is retracted. The length that is collapsed of the cylinder has to fit into the machine's available envelope. For extremely long strokes within restricted space, telescopic cylinders might be the only option.

Step 5: Verify the rate of flow and speed of the cylinder.

After bore and stroke have been established, the flow rate to reach the desired speed for a cylinder can be determined:

Flow Rate (L/min) = Piston Area (cm²) x Velocity (cm/s) x 0.06

If a 63 mm bore cylinder has to be extended at 200 mm/s, then the required flow will be approximately 37 L/min over the entire stroke. This amount must be within the limits of the pump and the directional control valve. If the flow required is greater than the capacity of the system, the cycle time has to be extended or the bore diameter needs to be reviewed.

The speed of the motor also influences heat generation along with pressure drops across the valve. Applications with high speeds require care with regard to the Cv rating of the valve and line sizing in order to avoid loss of pressure from dominating the system.

Making it work: An sizing checklist

Before you finalize a cylinder specification, make sure to check the following:

• The total effective load is calculated based on back-pressure, friction, and the safety factor.
• Bore is measured to the closest standard dimension to the minimal amount that has been calculated.
• Operating pressure is within 70 to 80 percent of the cylinder's capacity at peak load
• The length of the rod is used to account for the mounting geometry and also includes an the overtravel allowance
• A buckling analysis has been carried out in cases where the ratio between stroke and bore exceeds 10:1.
• The required flow rate must be within the system pump and valve capacity
• Rod diameter is sufficient to meet the requirements for retraction force and column stability
• Material and seal specifications are compatible with the type of fluid and the temperature range, as well as the duty cycle

Sizing mistakes that are common to avoid

The rod's back pressure is not being considered on its side. It is commonly ignored in force calculations and causes cylinders to be unable to extend under back-pressure and combined load.

Making use of nominal pressure instead the actual pressure in the system Some systems are operating lower than their nominal pressures in normal operation. The cylinder's size should be based on the actual work pressure. Not just the pressure listed on the nameplate of the pump.

Not paying attention to mounting loads, side loading, and misalignment are major reasons for rod and seal wear. The correct choice of mounting style (clevis or trunnion) must be in line with the direction of load for the application.

Avoiding thermal effects In high-cycle applications, the fluid temperature increases affect viscosity and sealing performance. Consider temperature variations in pressure drop as well as the seal's choice of material.

The proper size of a hydraulic cylinder is a method that links load requirements to physical dimensions using the fundamentals that govern fluid power. Bore stroke, pressure, and bore are not inseparable choices; they are connected variables that have to be matched to meet the requirements that the system has to meet. Engineers who approach the sizing of cylinders as a methodical calculation, not a rule-of-thumb exercise, design systems that operate smoothly, last longer, and operate consistently throughout their operational range. No matter if the device is an industrial press, portable boom, or high-precision fixing fixture for clamping, the principles of sizing are the same: start by determining the weight, determine the pressure, then calculate the bore. Examine the stroke and verify the flow.