In an extension ram specification, the pulling force is approximately what fraction of the pushing force?

• A. 1/2
• B. 1/4
• C. 2/3
• D. 1

Answer: (A)

When a double-acting ram extends, the jaw-dropping factor is the difference in effective area between extending and retracting. Pushing uses the full piston area, so the pushing force is pressure times A. Pulling, however, uses the rod end, where the rod intrudes into the piston face, so the effective area becomes A minus the rod cross-sectional area Ar. The pulling force is then pressure times (A − Ar). The ratio of pulling to pushing forces is (A − Ar) / A = 1 − (Ar / A). Because the rod takes up a substantial portion of the piston face in many designs, this ratio is often about 1/2. So the pulling force is typically roughly half of the pushing force. The exact value depends on the rod size relative to the piston, but 1/2 is a common approximation.