Hydraulic Elevator Jack Pressure, Flow, and Bore Change

The two relations are simple and they interact. A larger jack lowers the working pressure -- easier on the cylinder, the packing, and the power unit -- and raises the flow needed for the same car speed, so it wants a bigger pump. A smaller jack does the reverse. A single-stage 12 in bore carrying 14,000 lb is 113.1 sq in of area at 124 psi, and moving that car at 125 fpm takes 734 gpm. NOW THE JACK REPLACEMENT TRAP, and it is three consequences from one dimension change. Fit a 10 in bore instead: the area falls to 78.5 sq in and the working pressure rises 44% to 178 psi, which may exceed the power unit's rating and will certainly change the packing and seal duty. The same pump now moves the car at 180 fpm instead of 125 -- above the contract speed, which is a code compliance problem with a governor and buffer problem behind it. And the relief valve, still set for the old 124 psi, now sits below the new working pressure and lifts during normal service. A jack replacement is not a like-for-like swap unless the bore matches. Telescopic jacks add a wrinkle worth knowing: the effective area changes as stages extend, so the working pressure is not constant through the travel and the highest pressure occurs on the smallest stage. Basic hydraulic relations only. It does not size a jack, which must be checked for COLUMN BUCKLING over its unsupported length -- the governing design case for a long hydraulic jack and one this pressure calculation does not touch -- or for the cylinder wall, head, and packing. It does not size the power unit, evaluate oil viscosity and temperature effects on speed and pressure, or address the pressure switch, low-pressure protection, and the anti-creep and leveling requirements. It does not set the relief valve margins or perform the static and running pressure tests, and it does not address buried jacks, cathodic protection, or the environmental requirements for jack replacement. ASME A17.1 and A17.2, the equipment manufacturer, the elevator authority having jurisdiction, and a licensed elevator mechanic govern.

jack area = pi x bore squared / 4; working pressure = total load / area; pump flow gpm = area x car speed x 12 / 231; car speed from an installed pump = flow x 231 / (area x 12).

The jack pressure and flow relations, by name, with ASME A17.1 named as governing relief settings, pressure testing, and the jack requirements not evaluated here. Column buckling over the unsupported length, not pressure, governs a long hydraulic jack.

Area and flow arithmetic on the user's own jack and pump data; no manufacturer or code table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: bore_in, total_load_lb, car_speed_fpm, pump_flow_gpm, relief_setting_psi, alternative_bore_in, jack_area_sqin, working_pressure_psi, flow_required_gpm, alt_working_pressure_psi, alt_pressure_change_pct, alt_speed_from_pump_fpm

• Bore changes three things at once working pressure, achieved speed with the existing pump, and the relief setting they were matched tohydraulic elevator practice
• Column buckling governs a long jack this pressure relation does not touch the governing design caseASME A17.1
• Telescopic pressure is not constant the effective area changes as stages extend and the highest pressure is on the smallest stagehydraulic elevator practice