Hydraulic cylinder extend and retract speed

Technical article

Understand Hydraulic cylinder extend and retract speed

This calculator finds the extend and retract speed of a differential cylinder from bore and rod diameter and supplied flow rate. It shows why such a cylinder retracts faster than it extends at the same flow.

What does this quantity describe?

On extension, oil acts on the full piston area Ap = π·D²/4, giving extend speed vext = Q/Ap. On retraction, only the smaller annular area Aann = π·(D²−d²)/4 — reduced by the rod — is active, so at the same flow vret = Q/Aann. Because Aann is smaller than Ap, vret is always greater than vext.

Picture two garden hoses with the same water flow, one fitted with a narrower nozzle: water shoots out faster through the narrower opening. In the same way, the smaller annular area on retraction forces the same flow through an effectively smaller area, raising the speed.

Formula and variables

vext = Q/Ap; vret = Q/Aann

vext = Q/Ap, Ap = π·D²/4
vret = Q/Aann, Aann = π·(D²−d²)/4
vret/vext = Ap/Aann

Symbol / input	Meaning
Bore diameter D	Cylinder bore inner diameter; sets the piston area Ap.
Rod diameter d	Piston rod diameter; reduces the effective area during retraction.
Flow rate Q	Flow rate delivered to the cylinder.
Extend speed vext	Rod speed while extending, acting over the full piston area Ap.
Retract speed vret	Rod speed while retracting, acting over the smaller annular area Aann.
Speed ratio vret/vext	Ratio of retract to extend speed; equals the area ratio Ap/Aann.

Choose the inputs correctly

Bore diameter D and rod diameter d set the two active areas; flow rate Q is the oil supplied to the cylinder. For the sizing direction — what flow or diameter is needed for a target speed — Q or D can also be chosen as the target.

How to use the calculator

Select the target quantity. For extend and retract speed, enter D, d and Q. To find the flow or diameter needed for a target speed instead, choose that quantity as the target and enter the speed as a known value.

Worked example

With D = 80 mm, d = 45 mm and Q = 40 L/min, Ap ≈ 5,027 mm² and Aann ≈ 3,436 mm². This gives vext = 40 L/min / 5,027 mm² ≈ 0.133 m/s extending and vret = 40 L/min / 3,436 mm² ≈ 0.194 m/s retracting — a speed ratio of about 1.46.

Understand the result and units

A speed ratio of 1.46 means the same cylinder retracts nearly half again as fast as it extends at unchanged flow. Equal extend and retract times need either throttling the faster motion (usually retraction), or a cylinder with a different rod-to-bore ratio.

Diameters are usually given in mm, flow rate in L/min and speed in m/s; the areas are computed internally from the diameters.

Why does flow set speed and pressure set force?

In a hydraulic cylinder, speed and force are governed by two independent quantities: the supplied flow Q sets how fast the piston moves (v = Q/A), while pressure rises to exactly whatever level is needed to overcome the external load (F = p·A). A higher-flow pump makes the cylinder faster, not stronger; a higher system pressure rating raises the maximum possible force, not automatically the speed.

Typical applications

The relationship is used to size cycle times for presses, lifts and feed axes, to select proportional or servo valves that control speed via flow, and to check whether an existing pump can meet a required cycle time.

Assumptions, limits and common mistakes

The model assumes an ideally sealed piston without internal leakage; real cylinders typically reach about 90–95% of this speed extending and 85–90% retracting because a small share of the flow leaks past the piston seal. Friction, breakaway effects and the end-of-stroke cushioning built into many cylinders above certain speeds are not included; for the exact dimensions of a specific differential cylinder, historical DIN-ISO dimension series listed in the 1999 Dubbel (e.g. DIN ISO 3320) should be checked against the currently valid standard.

Common mistake: A common mistake is using the same area Ap for both directions of motion — the annular area on retraction is always smaller than the full piston area. It is also easy to overlook that a higher speed at the same load needs more hydraulic power (P = Δp·Q) and therefore a larger pump, not just a different valve.

Frequently asked questions

Why does the cylinder retract faster than it extends?

Because retraction only has the smaller annular area available, reduced by the rod cross-section. At the same flow, a smaller active area gives a higher speed.

Does a higher flow rate also increase cylinder force?

No. Flow sets speed; force comes from the pressure that develops to match the external load, multiplied by the active area.

How do I get equal extend and retract times?

Either throttle the faster motion (usually retraction), use a valve circuit that adjusts flow accordingly, or choose a cylinder with a different rod-to-bore diameter ratio.

Why are real speeds lower than calculated?

Because a small share of flow leaks internally past the piston seal instead of contributing to motion; typical real efficiencies are about 90–95% extending and 85–90% retracting.

What about a through-rod (double-rod) cylinder?

With a rod on both sides, the active area is the same annular area in both directions, so extend and retract speed are equal at the same flow — this calculator instead assumes the common single-rod differential cylinder.