Drive machines, gearboxes and main drive chains

Brackenridge E2 had started to growl. The mall's operations manager heard it from the food court and called Lena, who sent Nell and Tamsin on the next night shift. With E2 locked out and the upper landing plate lifted, the noise was gone, of course; it only happened under load. Nell ran the unit up on inspection with the plate off and the drive guarded, put her hand on the gearbox housing, then on the motor, then on the main drive chain guard, and listened with a mechanic's stethoscope. "Gearbox input bearing," she said. "You can hear it rumble at motor speed, not at chain speed." This lesson covers the machinery that turns electricity into the motion of the step band: the motor, the gearbox, the main drive chain or coupling, the main drive shaft and the brakes that sit on them.

Drive arrangements

Most escalators in North American buildings use a top-mounted drive. The drive machine sits inside the truss at the upper landing, under the landing plate. It is an electric motor coupled to a gearbox, and the gearbox output turns the main drive shaft, either through a main drive chain or through a direct coupling. The main drive shaft carries the two step chain sprockets and the handrail drive sprockets, so one machine drives the whole escalator. The machine has its own brake, the machine brake, usually on the motor shaft or the gearbox input.

Other arrangements are used where a single top machine would be too large or hard to reach. Some makers build the motor and gearbox into the main drive shaft itself, with a planetary gearbox inside the sprocket hub. Long transit escalators and long moving walks sometimes use more than one drive, placed along the incline, with each drive pulling a section of the step band. Older units, like the original Brackenridge machines, use a worm gearbox, which is compact and quiet but less efficient; newer machines use helical or planetary gears, which waste less energy as heat. The arrangement determines where the brakes must be and how the drive is serviced, so Nell made Tamsin draw each site's drive from memory before she let her work on it.

The motor

Escalator motors are almost always three-phase induction motors, the same family Tamsin studied in ELV-210. They run at a nearly constant speed, so the step speed is set by the gear ratio and the size of the step chain sprockets rather than by the motor control. On older units the motor is started across the line or in star-delta to limit the starting current; newer units use a variable-frequency drive, which starts the escalator gently and allows the energy-saving modes described in lesson 9. The motor is sized for the heaviest passenger load the escalator is designed to carry upward, plus the friction of the step band, handrails, chains and gears. On a down escalator carrying passengers, the load drives the motor rather than the other way around, and the motor acts as a generator; a drive with regeneration can return that energy to the building.

The power needed to lift passengers is the weight of the passengers on the incline multiplied by the vertical component of the step speed. Escalator makers use a design load per exposed step on the incline for this calculation, and the friction of the empty escalator is added as a fixed amount found by testing similar units. Dividing by the efficiency of the gearbox and drive chain gives the motor power, which is then rounded up to a standard motor size.

import math
# Motor power and drive ratio for an escalator (illustrative design figures)
rise, angle = 18.0, 30.0            # ft, degrees
step_depth = 1.3125                    # ft
load_per_step = 265                 # lb design load on each exposed incline step
speed = 100                           # rated speed, ft/min
friction = 3.4                       # hp to run the empty escalator
efficiency = 0.85                    # gearbox and drive chain
incline = rise / math.sin(math.radians(angle))
steps = int(incline / step_depth)
lift = steps * load_per_step * speed * math.sin(math.radians(angle)) / 33000
motor = (lift + friction) / efficiency
sizes = [10, 15, 20, 25, 30]
chosen = next(s for s in sizes if s >= motor)
print(f"exposed incline steps: {steps}, design load {steps * load_per_step:,.0f} lb")
print(f"lifting power {lift:.1f} hp, plus friction {friction:.1f} hp")
print(f"motor power needed {motor:.1f} hp, standard size {chosen} hp")
# drive ratio: motor speed to main shaft speed
pitch, teeth, motor_rpm = 5.25, 16, 1460
sprocket = pitch / math.sin(math.radians(180 / teeth))
shaft_rpm = speed / (math.pi * sprocket / 12)
print(f"step chain sprocket pitch diameter {sprocket:.1f} in, main shaft {shaft_rpm:.2f} rpm")
print(f"overall reduction from the motor: {motor_rpm / shaft_rpm:.0f} to 1")

exposed incline steps: 27, design load 7,155 lb
lifting power 10.8 hp, plus friction 3.4 hp
motor power needed 16.8 hp, standard size 20 hp
step chain sprocket pitch diameter 26.9 in, main shaft 14.19 rpm
overall reduction from the motor: 103 to 1

E2's original motor is rated 20 hp, which matches the calculation with some margin. The overall reduction of about 100 to 1 between the motor and the main shaft is shared between the gearbox and the main drive chain, whose sprockets give the final reduction and let the maker fine-tune the step speed. A change in sprocket size, or a gearbox replaced with one of a different ratio, changes the step speed, and the step speed must then be measured and the handrail drive checked, because the handrails must still move with the steps.

Gearboxes

The gearbox is the heart of the drive and the part that most often decides when a drive must be overhauled. Worm gearboxes have a steel worm on the input shaft driving a bronze wheel on the output; the bronze wheel wears slowly, the wear shows as increasing backlash at the output, and the oil carries bronze particles that show up as a gold sheen. Helical and planetary gearboxes have hardened steel gears that wear very little but can suffer from bearing failure or pitting of the tooth surfaces. In every type, the bearings and the oil are where trouble starts. Bearings announce themselves with noise and heat: a rumble at motor speed comes from the input shaft bearings, a slower knock from the output. Oil must be at the right level and the right grade, and it must be changed at the maker's interval or when an oil sample shows wear metals or water.

At E2, the stethoscope and a vibration reading confirmed the input bearing. The gearbox maker's procedure for the original worm box allowed the input bearing to be replaced on site, so the job was booked for two nights: one to remove the motor and input shaft, one to fit the new bearing and seal, refill and run in.

The main drive chain

On most top-mounted drives, the gearbox output sprocket turns the main drive shaft through a main drive chain. This is a heavy roller chain, often duplex or triplex, which carries the full driving force of the escalator. Like any roller chain, it stretches with wear as its pins and bushes wear, and it must be kept at the correct tension and lubricated. A slack drive chain runs noisily and can jump teeth; a tight one wears quickly and loads the bearings. The chain is checked for elongation, side wear and damaged rollers, and the sprocket teeth are checked for the hooked shape that shows they are worn.

The main drive chain is a single point of failure: if it breaks, the main drive shaft is no longer connected to the machine brake, and a loaded down escalator could run away. That is why the code requires a broken drive chain device that stops the escalator if the chain breaks, and why taller escalators and those whose machine is connected to the main shaft by a chain also need an auxiliary brake acting directly on the main drive shaft. Lesson 7 covers the device and lesson 8 the brakes. Heavy-duty transit guidelines go further, ruling out belt drives between the motor and the gearbox and asking for low-oil sensors on gearboxes and long design lives for the bearings.

The main drive shaft and its bearings

The main drive shaft runs across the upper end of the truss and carries the two step chain sprockets, the handrail drive sprockets or wheels and, where fitted, the auxiliary brake disc. It runs in two large bearings, one on each side of the truss. Because the step chains on both sides must stay in step, the two sprockets are fixed to the shaft so that their teeth line up, and the shaft must be square to the truss. If a sprocket slips on its shaft or the shaft goes out of square, one step chain leads the other, the steps run crooked, and they begin to rub the skirt on one side. Tamsin learned to check the shaft bearings for heat and noise and to look at the alignment marks Nell's crew paints across each sprocket hub and its shaft, which show at a glance whether anything has moved.

Checking drive chain tension

Main drive chain tension is checked with the escalator locked out, by measuring the sag of the slack side of the chain midway between the sprockets with a straightedge and a rule, and comparing it with the maker's figure, often given as a small fraction of the distance between the sprocket centers. The chain is tightened by moving the drive machine on its base or by an idler, never by removing links unless the chain has stretched beyond its adjustment. At the same time, the sprocket alignment is checked with a straightedge across both sprocket faces, because a misaligned drive chain wears the sides of its links and the sprocket teeth. On E2, the drive chain sag was within the maker's range, but the chain guard had been bent inward at some time and was touching the chain, which explained a polished stripe on the guard and part of the noise that had led to the call.