Elevator Buffer Stroke and Impact Speed

ScopeThat square law is what separates buffer types. A slow car striking a buffer at a modest speed needs a short stroke and a spring will do; double the speed and the required stroke QUADRUPLES, which quickly exceeds what a spring can practically provide and pushes the design to an oil buffer that dissipates the energy rather than storing it. The code reflects this by permitting spring buffers only up to a stated car speed. A 500 fpm car is sized to strike at 115% of its rated speed, 575 fpm or 9.58 ft/s, and needs 17.1 in of stroke at a one-gravity average, which ASME A17.1 Table 2.22.4.1 prints as 17.00 in; double the car speed to 1,000 fpm and the stroke goes to 68.5 in. The governor may trip above that 115% -- up to 625 fpm at 500 fpm rated -- and a strike at that speed would need 20.2 in, which is why the governor trip, safety type, and buffer are designed as a set. The retardation limit is a human limit rather than a structural one: the buffer could stop the car in a much shorter distance and the code does not allow it, because the occupants have to survive the stop. An average of about one gravity with a bounded peak is what sets the stroke, and it is why an oil buffer's orifice profile matters -- a buffer that stops the car in the right distance with a spike at the start of the stroke fails the peak criterion even though the average is correct. A kinematic calculation only. Buffer selection is governed by the code, which sets permitted types by car speed, required strokes, retardation limits, and testing; oil buffers have a rated striking speed and a rated load range, and one applied outside that range does not produce its rated retardation. It does not address counterweight buffers, pit depth and clearances, or buffer support and mounting. ASME A17.1 and A17.2, the buffer manufacturer's ratings, the elevator authority having jurisdiction, and a licensed elevator mechanic govern.

Formula or tablestriking speed = 1.15 x rated speed (ASME A17.1 2.22.4.1.1); stroke = striking speed squared / (2 x retardation x 32.2), to the quarter inch at one gravity (Table 2.22.4.1); the retardation an installed buffer imposes = striking speed squared / (2 x rated stroke) / 32.2.

Source and editionThe kinematic stroke relation at 115% of the rated speed, per ASME A17.1 2.22.4.1.1 by section, with the ASME A17.1 average retardation limit of about one gravity and its bounded short-duration peak named. Buffer type, required stroke and rated striking speed come from the code tables and the manufacturer.

Free accessConstant-acceleration kinematics on speeds and a stroke the user supplies; no code table or manufacturer rating is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namescontract_speed_fpm, governor_trip_fpm, permitted_retardation_g, buffer_rated_stroke_in, buffer_rated_speed_fpm, impact_speed_fps, stroke_required_in, stroke_at_1g_in, retardation_installed_g, max_speed_for_buffer_fpm

• Stroke goes as the square of speed doubling the impact speed quadruples the stroke requiredkinematics
• Size on 115% of the rated speed A17.1 2.22.4.1.1 and Table 2.22.4.1 (500 fpm -> 575 fpm -> 17.00 in)ASME A17.1
• The retardation limit is a human one about one gravity average with a bounded peak, so occupants survive the stopASME A17.1