Foot-Pounds per Second to Watts for Lift and Hoist Duty

Taking a US Lifting Duty into an SI Motor Catalogue

A lift schedule written to American practice gives you a car capacity in pounds and a contract speed in feet per minute. Multiply the two and you have the hoisting duty in foot-pounds per second — a genuine power figure, but not one any traction machine, gearless drive or escalator unit is catalogued in. Every European and Asian supplier prints watts and kilowatts, so the duty has to cross over before the enquiry can go out.

Conversion factor: 1 ft·lb/s = 1.3558179 W. A 2 500 lb car at 180 ft/min lifts at 2 500 × 180 ÷ 60 = 7 500 ft·lb/s, which is 7 500 × 1.3558179 = 10 168.6 W, or about 10.2 kW of pure hoisting work before counterweight, gearing and losses are considered.

What Sits Behind the Duty Figure

Rated load times contract speed

The duty is nothing more complicated than that product on a per-second clock. Divide pounds × feet-per-minute by 60 and the answer is already in ft·lb/s, ready to convert.

Lift specs still run in feet per minute

North American contract speeds are quoted as 100, 150, 350 or 700 fpm while the machine that has to deliver them is described in metres per second and kilowatts. The conversion sits right on that seam.

The counterweight comes off afterwards

A traction lift balances the car plus 40–50 % of rated load, so the machine only ever works on the difference. Convert the gross duty first, then take the balance off — never the other way round.

Escalators lift people, not a car

There is no counterweight on a step band, so the duty is the weight of everybody on the incline multiplied by the vertical component of the band speed — the incline length never enters it.

From Rated Load and Car Speed to a Motor Enquiry

Work the schedule line by line. The conversion is the easy part; the discipline is knowing which number you have converted.

Get the gross hoisting duty

Take rated load in pounds, multiply by contract speed in feet per minute, divide by 60. A 6 000 lb freight car at 100 fpm gives 10 000 ft·lb/s. Use rated load, not the car weight — the counterweight already carries the empty car.

Type it in and read the watts

Enter 10 000 on the left and 13 558.179 W appears opposite as you type, with thousands separated by a space so a five-figure duty stays readable. Commas work as decimal points and stray spaces are ignored, which matters when the figure is pasted from a schedule.

Switch the right side to kilowatts

Traction machines are listed in kW, not W. Pick kW in the searchable dropdown and the same duty reads 13.558 kW, which is the number that goes into the enquiry alongside starts per hour and travel.

Reverse when the machine came first

If a supplier has offered an 11 kW machine and you need to know what duty it covers in customary terms, the swap button (↔) runs W → ft·lb/s. The copy button lifts the bare number straight into the schedule spreadsheet.

This is the lifting duty, not the motor rating: the counterweight reduces it, drive and roping efficiency increase it, and acceleration adds a transient on top. Treat the converted watt figure as the starting point for machine selection, never as the nameplate.

Lift and Escalator Duties in ft·lb/s and Watts

Typical vertical-transportation duties written the American way, with the gross hoisting work rate and its SI equivalent. Each watt figure is the ft·lb/s value multiplied by 1.3558179.

Duty	Rated load	Speed	Hoisting rate (ft·lb/s)	Same in watts (W)
Accessibility platform lift	400 lb	20 fpm	133.3 ft·lb/s	180.8 W
Dumbwaiter / service lift	500 lb	50 fpm	416.7 ft·lb/s	564.9 W
Passenger lift, low-rise	2 100 lb	100 fpm	3 500 ft·lb/s	4 745.4 W
Passenger lift, mid-rise	2 500 lb	180 fpm	7 500 ft·lb/s	10 168.6 W
Passenger lift, high-rise gearless	3 000 lb	700 fpm	35 000 ft·lb/s	47 453.6 W
Freight lift	6 000 lb	100 fpm	10 000 ft·lb/s	13 558.2 W
Escalator, 20 ft rise	30 riders ≈ 4 950 lb	50 fpm vertical	4 125 ft·lb/s	5 592.7 W
Inclined moving walk, 12°	40 riders ≈ 6 600 lb	20.8 fpm vertical	2 288 ft·lb/s	3 102.1 W

Speed dominates the column, not capacity: the freight car carries nearly three times the load of the mid-rise passenger lift yet asks for more watts only because both are moving slowly, while the 700 fpm gearless machine needs four and a half times either of them on the same 3 000 lb class of car. Applying a 50 % counterweight halves each passenger figure, then drive and roping efficiency of roughly 0.8 puts most of it back, which is why a 10 kW gross duty commonly ends up as a machine somewhere near 6–7 kW continuous with a much larger transient during acceleration.

Working Through a Vertical-Transportation Schedule

Both ends move as you walk the lift schedule

Retype the duty for each car in the riser and the watt figure follows immediately, so a schedule of eight or ten lifts converts in one sitting without any clearing between lines.

Flip direction when the supplier quotes first

The ↔ button reverses to W → ft·lb/s, the way round you need to test an offered machine against a customary duty already agreed with the client.

Kilowatts one dropdown away from the same duty

Searchable unit lists on both sides cover kW, hp and the rest, so the same hoisting figure can be read in whichever unit the machine catalogue in front of you happens to use.

Clean numbers for the equipment schedule

Results carry up to eight decimals with thousands spaced for legibility, and the copy button hands over the plain value with no unit attached — exactly what a spreadsheet cell wants.

Hoist and Escalator Drive Questions

Why is the traction machine smaller than the hoisting duty suggests?

Because the counterweight is doing most of the lifting. It is normally set at the car's own weight plus 40–50 % of rated load, so with a half-full car the two sides very nearly balance and the machine only has to overcome friction. The gross duty you converted describes the work of raising the payload; the machine sees the out-of-balance part of it, divided by the drive and roping efficiency.

How much does a 50 % counterbalance actually save on a full car?

Roughly half, in the worst case. A fully loaded car going up is out of balance by 50 % of rated load, so the 10 168.6 W duty of a 2 500 lb car at 180 fpm becomes about 5 084 W of useful work, then divides by an efficiency near 0.8 to give roughly 6.4 kW at the shaft. An empty car going down is out of balance by the same amount in the other direction, which is why the drive has to be sized for both cases.

What do I add for acceleration and jerk?

The steady-state duty ignores getting up to speed. Accelerating the whole moving mass — car, counterweight, payload, ropes and the rotating inertia of the machine — at a comfortable 1 m/s² typically calls for 50–100 % more than the running figure for a couple of seconds. Jerk limits, usually held near 1 m/s³ for ride quality, spread that peak rather than remove it, so the drive is chosen on the transient and the motor on the thermal average.

Where does the energy go when a loaded car travels down?

Into the drive, as the machine is driven by the load rather than driving it. Older installations burn that energy in a braking resistor and dump the heat into the machine room, which the cooling load then has to carry. A regenerative drive pushes it back into the building supply instead — typically a fifth to a third of the annual consumption on a busy riser, and it also removes a real heat source from the motor room.

Why do starts per hour matter more than the watt figure?

Because a lift motor spends most of its life stopped and its rating is an intermittent one. Machines are quoted against a duty class with a stated number of starts per hour and a percentage on-time — 180 or 240 starts an hour at 40 % is common on an office riser — and it is the heating over that cycle, dominated by the acceleration peaks, that sets the frame size. Two lifts with identical converted duties can need different machines purely because one runs a far heavier traffic pattern.

Foot-pounds per Second to Watts