Piston/Rod Differential Area
Differential area is the ratio of effective areas between the piston side and the rod side of a cylinder, and it explains why force and speed are not equal in the two directions. On an 80/45 mm cylinder this ratio is about 1.46. On the rod side force drops by that ratio and speed rises by the same ratio.
Piston area is found from the square of the bore, and rod side area by subtracting the rod cross section from the piston area. An 80 mm piston gives about 50.3 cm², a 45 mm rod about 15.9 cm²; the rod side is left with 34.4 cm². The ratio is 50.3 / 34.4 ≈ 1.46.
- Force: at 200 bar, extend force is about 10,200 kgf while retract force is about 7,000 kgf.
- Speed: at the same 20 l/min flow rate, extend gives about 6.6 cm/s and retract about 9.7 cm/s.
- Return flow: during extension the oil returning to tank is less than what goes in; during retraction it is more. Return line size must be chosen accordingly.
This asymmetry is the answer to the most common "one direction is faster" question in proportional control. When electronic lever control lets you define separate speed limits for the two directions, this difference can be balanced out; otherwise it continues to exist physically.
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