Hydraulic Cylinder Flow Rate Calculator
Calculate required GPM for specific cylinder speeds
Calculation Results
Understanding Hydraulic Flow Rate
In hydraulic systems, the flow rate (measured in Gallons Per Minute or GPM) determines how fast a cylinder will extend or retract. Matching your pump capacity to your cylinder requirements is critical for achieving the desired cycle times in industrial machinery, mobile equipment, and wood splitters.
The Core Formula
The flow rate required for a specific cylinder speed is calculated using the cross-sectional area of the piston and the desired velocity. The standard formula for flow in GPM is:
Extension vs. Retraction
It is important to note that a cylinder requires different flow rates for extension and retraction to maintain the same speed:
- Extension: Fluid fills the entire bore diameter. This requires more volume per inch of stroke.
- Retraction: Fluid fills the "annulus" area (the bore area minus the rod area). Because the rod takes up space, less fluid is needed to move the piston at the same speed.
Example Calculation
If you have a 4-inch bore cylinder with a 2-inch rod and you want it to move at 100 inches per minute:
- Bore Area: π × (2²) = 12.57 sq in.
- Extension Flow: (12.57 × 100) / 231 = 5.44 GPM.
- Annulus Area: 12.57 – (π × 1²) = 9.42 sq in.
- Retraction Flow: (9.42 × 100) / 231 = 4.08 GPM.
Frequently Asked Questions
Why is the number 231 used in the formula?
There are exactly 231 cubic inches in one US gallon. This constant converts cubic inches per minute (the product of area and speed) into Gallons Per Minute.
What happens if my pump GPM is lower than calculated?
If your pump provides less flow than the calculated requirement, the cylinder will move slower than your desired speed. Velocity is directly proportional to flow.