A 60 t/h, 30 m elevator needs about 7.08 kW at the motor shaft under the stated margin assumptions, while its loaded-start torque requirement can exceed the steady running torque by 2.0–2.3 times rated torque [S1].
Motor selection must therefore combine lifting power with breakaway torque, gearbox service factor, backshaft arrangement, and the consequence of a power interruption. The bucket elevator drive train must be specified for the loaded-start case, not merely for steady material lift.
Separate net lifting power from drive requirements
Net lifting power is calculated from mass throughput and vertical lift, using P = Q × H / 367 kW when Q is in t/h and H is in metres. This produces 1.63 kW for 30 t/h over 20 m, 3.41 kW for 50 t/h over 25 m, and 7.63 kW for 80 t/h over 35 m [S1].
Those figures exclude digging resistance, belt or chain friction, bearing losses, bucket losses, and gearbox efficiency. One published sizing method combines those allowances as P motor ≈ Q × H / 367 × 1.3 / 0.9, giving 2.36 kW, 4.92 kW, and 11.02 kW for the same three cases [S1]. The 1.3 digging-and-friction factor and 0.9 gearbox divisor are assumptions, not universal constants.
An independent ideal-power check uses P_lift = Q × H × 9.81 / 3600. For 100 t/h at 30 m, this gives approximately 8.18 kW for material lift alone, explicitly excluding startup, duty, traction, take-up, and mechanical losses [S3]. The motor and shafts must not be selected from that lower-bound energy value.
Bucket selection also affects power demand. Centrifugal elevators commonly use AA or AC buckets for digging and scooping, while continuous designs use MF, HF, or SC buckets for gentler discharge. Continuous and chain-type elevators have fixed bucket spacing, which removes one adjustment option during commissioning [S4].
Convert power and start ratio into shaft torque
Motor-shaft torque is calculated as T = 9,550 × P / n, with P in kW and n in rpm. A 7.5 kW motor at 1,460 rpm develops approximately 49 Nm rated torque; the corresponding 7.08 kW shaft load is about 46.3 Nm [S1].
For the same motor, direct-on-line starting produces approximately 98.2 Nm, or 2.0 times rated torque, leaving about 52 Nm above the calculated load before accounting for acceleration and other drivetrain effects. Published starting-torque ratios across six 4-pole motor frames span 2.0–2.3, producing example starting torques from 22.8 Nm to 215.2 Nm [S1].
Torque at the elevator head shaft is not the same as motor torque. Gear ratio, gearbox efficiency, and shaft loading determine the available output torque. The gearbox selection must therefore be checked against the required breakaway torque at the output, not against motor horsepower alone.
The physical start condition remains the governing input. A stopped centrifugal elevator can retain a fully loaded leg, so the drive must both support the load and accelerate the complete rotating mass. A soft starter or variable-frequency drive can limit electrical current, but neither changes the fundamental mechanical breakaway requirement [S5].
Compare starting methods against loaded-start torque
Direct-on-line starting is the reference case because it exposes the motor's full locked-rotor capability. For a 7.5 kW, 1,460 rpm motor with rated torque near 49.1 Nm, starting torque is about 98.2 Nm. The calculated 46.3 Nm running-load torque leaves a 2.12-times-load torque margin before acceleration allowance [S1].
A star-delta start reduces torque in the star step to roughly one third, producing about 32.7 Nm in the worked example. That is below the 46.3 Nm load, so the motor can hum without accelerating the elevator until an overload device trips [S1]. This approach is therefore unsuitable when the elevator must restart from a full-load standstill.
Reduced-voltage starting is not a universal solution, either. A VFD can control acceleration and lower current, but the drive still needs enough peak torque to break the loaded train away from rest. Torque-limited acceleration, thermal duty, repeated-start cycling, and gearbox service factor must be checked together [S5].
The drive-train comparison is straightforward. Direct-on-line starting provides the highest starting torque and fewer controlled components, but it produces the highest inrush current. Star-delta reduces current and star-stage torque, making it unsuitable for this loaded-start example. A VFD offers controlled acceleration and reduced current, but requires explicit breakaway-torque and thermal-duty verification. Soft starters provide controlled voltage, but the final mechanical torque requirement still remains [S1] [S5].
Select the gearbox around surge and back-running
Bucket elevator drives differ from ordinary belt conveyor drives because starting torque can be several times steady running torque, the loaded side is gravity-driven, and the gearbox often mounts directly on the head shaft. Starting torque dominates the selection, while a holdback or backstop is required where power loss could let the loaded leg run backward [S5].
Published service-factor bands provide a preliminary comparison: 1.4–1.6 on running power for centrifugal belt elevators, 1.5–1.8 for continuous chain elevators, an explicitly specified breakaway multiple for high-lift fully loaded restarts, and an increased factor for frequent-stop cycling [S5]. These are selection practices, not substitutes for the elevator manufacturer's load data.
Repeated starts impose cumulative fatigue on couplings, keys, gear teeth, shafts, and bearings. A coupling rated for steady operation may still be unsuitable if its peak torque and repeated-start cycle rating are not confirmed [S5]. In addition, a torque limiter should be adjusted to the minimum required setting, with little or no slipping during startup under normal full load [S6].
The head-shaft interface also has to be resolved before manufacture. A hollow-shaft gearbox eliminates an output coupling and separate pedestal bearing, but the gearbox bore, shaft diameter, torque arm, and support arrangement must match. The Series II manual specifies adjustment of the motor mount and torque arm to suit the drive package, with the tensioning bolt positioned as instructed [S2].
Prevent reverse motion and manage stored energy
A loaded elevator leg creates a reverse-torque condition after a power cut, because gravity drives the loaded side toward the boot and can rotate the motor. A correctly selected backstop or holdback must restrain this motion without slipping under normal startup [S1] [S5].
Backstop selection is tied to drive arrangement and torque direction, not simply to motor power. The drive engineer should verify the worst reverse torque, mounting orientation, permitted shock load, and the torque arm's reaction capacity. The gear reducer's output torque, shaft diameter, bearing capacity, and backstop interface must be checked as one assembly [S5].
Isolation and lockout are essential because a loaded leg can move after power is removed. A qualified electrician should verify wiring and rotation, and maintenance requires isolation and lockout before work begins [S1]. A load switch or other protective device cannot substitute for a mechanically suitable backstop and controlled isolation procedure.
Bolted bucket attachments also form part of the safe operating envelope because loose fasteners change retention and create imbalance. One installation table lists recommended maximum torque of 72 in-lb for a 1/4-inch bolt, 132 in-lb for a 5/16-inch bolt, and 11 ft-lb for a 3/8-inch bolt [S7]. These values are installation limits, not drive-torque substitutes.
Use manufacturer data for final selection
Final motor and gearbox selection belongs on the elevator manufacturer's duty calculation, not on a generic web calculator. The calculation must state throughput, lift height, material, elevator type, speed, bucket arrangement, starting condition, duty cycle, gearbox ratio, losses, service factor, and backstop arrangement. [S3]
The operating envelope in the Series II manual includes a 500 lb maximum load limit, while its equipment-information page captures model, serial number, rpm, head-pulley diameter, discharge height, and horsepower [S2]. Those nameplate values should be reconciled with any replacement motor and reducer before commissioning.
Shaft selection also requires more than motor power. The headshaft and bootshaft must be assessed for combined bending and torsion, fatigue, deflection, and bearing load. Bucket size, spacing, speed, and chain or belt configuration all influence the load path [S3].
The elevator design sequence for a new installation should therefore proceed from material and capacity to bucket type, calculated lift power, loaded-start torque, gearbox rating, backstop, shaft checks, and commissioning. For a retrofit, retain the original nameplate and manufacturer data, identify any restart condition, and compare it with the proposed motor's locked-rotor torque and the gearbox's service rating.
Two trackable signals determine whether the selected drive is adequate: the calculated full-load breakaway torque at the gearbox output, including the approved service factor, and the measured current or torque demand during repeated loaded starts. If the starting stage cannot accelerate the elevator within its current or thermal limits, the solution is a correctly sized motor, reducer, backstop, or starting method, not a longer stalled attempt.