Elevator Buffer Stroke and Impact Speed

That 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 whose governor trips mechanically at 575 fpm strikes at 9.58 ft/s and needs 17.1 in of stroke at a one-gravity average; double the car speed to 1,000 fpm, tripping at 1,150, and the stroke goes to 68.5 in. THE SPEED THAT MATTERS IS NOT THE CONTRACT SPEED. A car reaching the buffer has already overspeeded past the governor's mechanical trip, so the buffer is sized on the GOVERNOR tripping speed -- which is why raising a governor's setting by 10 percent to stop nuisance trips raises the required stroke by 21%, and the buffer that was correct is now short. A change that looked like a governor adjustment has invalidated equipment two floors below, which is why governor trip, safety type, and buffer stroke 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.

impact speed = governor mechanical tripping speed; stroke = impact speed squared / (2 x retardation x 32.174); the retardation an installed buffer imposes = impact speed squared / (2 x rated stroke) / 32.174.

The kinematic stroke relation at the governor tripping speed, by name, 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.

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

Estimate. AHJ and licensed professional govern.

Field names used by the API: contract_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 the governor trip, not contract speed the car has already overspeeded before it reaches the bufferASME A17.1
• The retardation limit is a human one about one gravity average with a bounded peak, so occupants survive the stopASME A17.1