Elevator Counterweight Balance and Unbalanced Load

The overbalance fraction is a compromise between two worst cases. At 50 percent the empty-car and full-car unbalanced loads are equal and the machine sees the same demand in both directions. Below 50 the full-car case governs and the machine works hardest lifting a full car up; above it, the empty case governs. Forty to forty-five percent is common because a full car going up is the loading that matters for comfort and because it keeps traction favourable. An 8,000 lb car with a 3,500 lb capacity at 45% overbalance wants a 9,575 lb counterweight, and the two ends of the range are 1,575 lb unbalanced empty against 1,925 lb full -- the FULL car governs, which is what 45% buys. At 50% both would be 1,750 lb. TRACTION is the constraint that bounds the choice. A traction machine drives the ropes by friction in the sheave grooves, and the ratio of the tensions on the two sides has to stay within what the groove profile and the wrap angle can hold. Too much counterweight or too little and the ropes slip, which on an elevator is a serious event and is why a traction calculation accompanies any counterweight change. The field consequence is diagnostic: a car that runs well loaded and struggles empty, or the reverse, is reporting its balance, and the balance point is MEASURED -- by loading the car until the machine draws the same current in both directions -- rather than assumed. That is the check that follows any change to the car, and the trap is that it usually is not run. Replace the flooring with something 400 lb heavier and the balance point moves from 45% of capacity to 34% without anyone touching the counterweight, and the machine now works harder lifting a full car than it was designed to. This does not evaluate traction itself, account for compensation ropes or chains that offset the changing rope weight over a tall travel, size the machine, motor, or brake, or address the counterweight's own guide rails, safeties, and clearances -- and hydraulic elevators have no counterweight at all. ASME A17.1 and A17.2, the equipment manufacturer's data, the state or local elevator authority, and a licensed elevator mechanic govern.

counterweight = car weight + (overbalance / 100) x rated capacity; unbalanced load = the absolute difference between the loaded car and the counterweight at each end of the range; balance point = actual counterweight - car weight.

The counterweight overbalance relation as standard traction-elevator practice, by name, with ASME A17.1 named as governing traction and a licensed elevator mechanic named. Traction itself -- the ratio of tensions the sheave grooves can hold -- is a separate required check and is not evaluated here.

Arithmetic on the user's own car, capacity and counterweight weights; no manufacturer or code table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: car_weight_lb, rated_capacity_lb, overbalance_pct, actual_counterweight_lb, added_car_weight_lb, counterweight_required_lb, unbalanced_empty_lb, unbalanced_full_lb, balance_point_lb, overbalance_actual_pct, modified_overbalance_pct

• Fifty percent equalises the two ends below it the full car governs, above it the empty cartraction elevator practice
• Traction bounds the choice the tension ratio across the sheave must stay within what the grooves holdASME A17.1
• Added car weight moves the balance point new flooring or fixtures change it without anyone touching the counterweightelevator maintenance practice