Counterweights & Load Balancing

How counterweights are sized and why they save energy

The counterweight is one of the most energy-saving elements in a traction elevator. Understanding how it's sized and how it affects motor requirements is a frequent exam topic — and it's exactly the kind of specific knowledge that separates a prepared candidate from a guesser.

Purpose of the Counterweight

The counterweight reduces the energy the motor must expend. Without one (as in a hydraulic elevator), the motor must lift the full weight of the car plus passengers every single trip up.

With a counterweight, the system can be balanced at the most common load point — typically 40–45% of rated capacity — so that at that load, the motor theoretically needs to do no lifting work at all (only overcome friction).

The counterweight also stores potential energy: when a loaded car descends, it raises the counterweight and recovers some energy. Regenerative drives can convert this to electricity and feed it back to the building power grid.

How Counterweights Are Sized

Standard formula: CWT weight = Car weight + (40% to 50% of rated load capacity).

Industry standard in North America: 40–45% of rated capacity.

Example: Car weighs 3,000 lbs. Rated capacity is 2,500 lbs. CWT = 3,000 + (0.40 × 2,500) = 3,000 + 1,000 = 4,000 lbs.

At the balance point (car carrying 1,000 lbs = 40% of 2,500 lbs), the loaded car weighs 4,000 lbs — exactly equal to the counterweight. The motor only needs to overcome friction.

Motor Load at Different Car Weights

Empty car (0% load): Car weighs 3,000 lbs, CWT weighs 4,000 lbs. CWT is heavier by 1,000 lbs. Motor must work to raise the car — but this is the same work it would do lowering a full car.

Balance point (40% load): Car + passengers = 4,000 lbs. CWT = 4,000 lbs. Balanced. Near-zero motor work.

Full load (100%): Car + passengers = 3,000 + 2,500 = 5,500 lbs. CWT = 4,000 lbs. Motor must lift the 1,500 lb difference. This is the peak motor demand — and it's much less than lifting 5,500 lbs without a counterweight.

Key insight: the motor sees its hardest load going up at 100% capacity, and going down empty (when the light car must overcome the heavy counterweight pulling upward).

Counterweight Construction

A steel frame (similar in design to the car sling) with stacked cast iron or concrete filler weights. Fillers are added or removed during installation and load testing to achieve the correct total weight.

The CWT travels on its own guide rails — either on the opposite side of the hoistway from the car or alongside it, depending on the hoistway design.

Key Takeaways

✓CWT = car weight + 40% of rated capacity. This is the standard — memorize it.
✓At the balance point, motor work approaches zero.
✓Motor peaks: going up at 100% load, OR going down empty.
✓Hydraulic elevators have NO counterweight — a common exam trap.
✓CWT always travels in the opposite direction from the car.

Key Terms

Counterweight (CWT)

Weighted frame that travels opposite the car to reduce motor load.

Rated load / capacity

Maximum passenger or cargo weight the elevator is designed to carry.

Balance point

Car load at which system is in equilibrium; typically 40–45% of rated capacity.

Filler weights

Cast iron or concrete weights added to the CWT frame to reach the target total weight.

Car weight

Weight of the empty cab plus the car frame, doors, and fixtures.

Regenerative drive

Drive system that recovers energy during descent and returns it to the power grid.

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