Korea Ever-Power · Application Engineering · Access Platforms

Wheel Drive Planetary Gearbox for Access Platforms

A boom lift carries workers to 40 metres — and the wheel drive must move this machine with a centre of gravity that shifts from 1.5 metres (stowed) to 12 metres (extended). Speed must automatically reduce as the platform rises, because the rollover risk at full extension is 5 to 10 times higher than stowed. Every wheel drive decision is a worker-safety decision.

Browse Wheel Drive Planetary Gearboxes →

The most demanding wheel drive application. The telescopic boom extends to 40+ metres, shifting the CG from 1.5 metres (stowed) to 8 to 15 metres (extended with outreach). Machine weight: 12 to 28 tonnes. The wheel drive must handle rough terrain (construction sites, unpaved ground) at the stowed position and must be speed-limited or locked when the boom is extended. Four-wheel drive with individual wheel motors is standard on rough-terrain models.

The articulating boom allows up-and-over reach to access positions behind obstacles. Machine weight: 7 to 22 tonnes. The CG shift during articulation is less predictable than on a telescopic boom — because the boom can be positioned at multiple angles simultaneously. The wheel drive control system must calculate the CG position in real time from the boom angle sensors and adjust the maximum permitted drive speed accordingly.

Vertical-lift platforms that raise the entire work platform straight up. The CG rises with the platform but stays directly above the wheelbase — producing less rollover risk than boom lifts at the same height. Machine weight: 2 to 12 tonnes. Rough-terrain scissor lifts operate on construction sites and require 4WD wheel drives with off-road traction capability. Indoor scissor lifts operate on smooth, level floors and use smaller, non-marking wheel drives optimised for tight turning in confined spaces.

The speed limiting is implemented through the hydrostatic pump displacement control — the machine control system reads the boom angle and extension sensors, calculates the effective CG height, and limits the maximum pump displacement (and therefore the maximum wheel drive speed) to the value permitted for that CG configuration. The wheel drive must respond accurately to these variable speed limits — any overshoot beyond the permitted speed violates the stability calculation and can trigger the machine tilt alarm or, in the worst case, a rollover event.

The smoothness of the speed transition is also safety-critical. When the operator extends the boom while driving (permitted on some machines at low speed), the speed limit decreases progressively. The wheel drive must decelerate smoothly to the new lower limit — without a sudden speed reduction that could jolt the workers on the platform. A 0.5 km/h per second deceleration rate is typically the maximum permitted — slower rates are preferred because the workers standing on the platform experience the deceleration as a longitudinal force that can cause loss of balance.

The braking system must also be height-responsive. At full boom extension on a slope, the parking brake must hold the machine against the gravity component plus a wind load of up to 100 km/h (EN 280 requirement) — while the CG is 8 to 15 metres above the ground. The brake holding torque at full extension can be 3 to 5 times the stowed-position requirement because the high CG amplifies the overturning moment from slope and wind. The wheel drive brake must be rated for this worst-case combination — not just for the stowed-position holding torque.

The dual-control arrangement on most boom lifts adds another wheel drive consideration: the machine can be driven from the ground-level chassis controls OR from the platform controls at height. The ground-level operator has a clear view of the terrain ahead and can anticipate obstacles. The platform operator, 20 to 40 metres above, cannot see the ground surface detail — and relies entirely on the wheel drive speed limiting and tilt protection to prevent driving into a hazard. The wheel drive response to platform-level control inputs must be more conservative (slower acceleration, lower maximum speed, more sensitive tilt cutout) than to ground-level inputs — because the platform operator has less situational awareness of the ground conditions. This dual-control speed mapping is programmed into the machine control system and is enforced through the wheel drive speed-limiting function.

The terrain on construction sites varies dramatically within a single job. The machine may drive from a paved car park (smooth, level, high traction) onto a muddy excavation area (soft, sloped, low traction) within 50 metres. The wheel drive must handle both extremes: enough torque to climb a 25% muddy slope in 4WD low-range, and enough precision to navigate a finished building interior at 0.5 km/h without marking the floor. Some manufacturers offer switchable non-marking tyres and reduced-speed indoor mode — and the wheel drive must accommodate the tyre-change effect on rolling radius (which changes the effective gear ratio and ground speed) without requiring recalibration.

The functional safety requirements for access platform wheel drives are codified in EN 280:2022 (Europe) and ANSI A92.20 (North America). These standards require that the speed-limiting function achieves Performance Level c or d (per ISO 13849) — meaning the probability of a dangerous failure (speed limit not applied when required) must be less than 10⁻6 per hour. Achieving this performance level requires either redundant speed-limiting hardware (two independent controllers, each capable of limiting the drive) or a single controller with diagnostic coverage of 90%+ (self-monitoring that detects and responds to internal faults before they become dangerous). The wheel drive must be designed to interface with both architectures — providing dual speed-feedback signals, redundant brake-engagement circuits, and diagnostic-compatible sensor interfaces.

If the boom-position sensor fails (sending a false stowed signal), or if the speed-limiting software malfunctions, the machine may permit full stowed-speed driving (6 to 8 km/h) with the boom fully extended. At this speed with a CG at 12+ metres, even a small ground irregularity (a 50 mm kerb edge, a pot hole, a slight slope change) can produce a lateral acceleration that exceeds the tipping threshold — resulting in machine rollover with workers at height. This is a single-point-of-failure scenario that the EN 280 standard addresses through redundant sensor requirements (two independent boom sensors, each capable of independently limiting the drive speed).

The parking brake must hold the fully loaded machine on the maximum rated slope (typically 5 to 25% depending on the machine class) with the boom extended and a wind load of up to 100 km/h. If the brake pad wears below the minimum thickness, the holding torque decreases below the required level — and the machine may creep downhill when parked on a slope with extended boom. This creep may be imperceptible at first (1 to 2 mm per minute) but accelerates as the slope-holding equilibrium is lost. By the time the operator or the workers on the platform notice the movement, the machine may have reached a speed that is difficult to arrest — especially if the engine is off and the hydraulic braking is unavailable.

Construction-site surfaces contain potholes, ruts, debris, and uneven compaction. When a wheel drive propels the machine over a pothole at even low speed (2 to 3 km/h), the wheel descent produces a sudden tilt of 2 to 5 degrees — amplified at the platform height by the moment arm. At 30 metres platform height, a 3-degree base tilt displaces the platform by 1.6 metres laterally — enough to throw an unrestrained worker against the guardrail or, in extreme cases, over it. The wheel drive must provide smooth, jerk-free propulsion with active tilt monitoring — automatically stopping the drive if the chassis tilt exceeds the EN 280 limit (typically 3 to 5 degrees depending on the boom configuration).

How does an access platform wheel drive differ from other mobile equipment drives?

The wheel drive is safety-critical — governed by EN 280/ANSI A92 standards that mandate redundant speed limiting, height-dependent maximum speed, tilt monitoring with automatic drive cutout, and brake performance verification at annual intervals. No other wheel drive application has this level of regulatory control because no other application carries personnel at height. Every wheel drive design decision (gear quality, brake sizing, speed response, sensor redundancy) is subject to functional safety analysis and third-party certification.

What is the typical service life?

5,000 to 10,000 hours for the planetary gearbox. Brake pads: 2,000 to 4,000 hours depending on the frequency of slope operation. The annual EN 280 inspection includes brake holding-torque verification, speed-limiting function test, and tilt-sensor calibration — all of which verify the wheel drive safety functions. Any component that fails the annual inspection must be replaced before the machine returns to service. Rental fleet machines accumulate 1,000 to 2,000 hours per year — reaching the gearbox end-of-life within 3 to 5 years. Owner-operated machines typically accumulate 300 to 800 hours per year — extending the gearbox life to 7 to 15 years. The rental fleet duty cycle also includes more frequent transport loading/unloading, which imposes additional shock loads on the wheel drive during ramp driving and tie-down stressing. Rental companies typically maintain wheel drives on a time-based schedule (every 6 to 12 months regardless of hours) because the utilisation varies widely between rental periods. Owner-operators can use hour-based scheduling (every 1,000 to 2,000 hours) because the usage pattern is more predictable. Both approaches must verify the brake holding-torque and speed-limiting functions at every service — because these are the safety-critical functions that protect workers at height.

What gear ratio is typical?

30:1 to 80:1. Maximum travel speed (stowed) is 6 to 8 km/h for rough-terrain models, 4 to 6 km/h for indoor models. The higher ratios provide smooth, jerk-free low-speed control at the critical 0.5 to 1.5 km/h extended-boom operating speed — where the drive smoothness directly affects the worker safety and comfort on the elevated platform.

Does Korea Ever-Power supply wheel drives for access platforms?

Yes. Korea Ever-Power manufactures wheel drive planetary gearboxes for self-propelled boom lifts, articulating lifts, and rough-terrain scissor lifts from 3,000 to 25,000 Nm with spring-applied failsafe brakes, DIN Class 6 gears for platform-smooth low-speed drive, height-dependent speed-limiting-compatible hydraulic interfaces, and EN 280/ANSI A92 compliance provisions. Provide the platform manufacturer, model, maximum working height, terrain type, and whether the machine will be used indoors or outdoors for a complete specification that addresses both the safety requirements and the operational environment.

Korea Ever-Power provides access platform wheel drives from 3,000 to 25,000 Nm with EN 280 compliance, failsafe braking, and height-dependent speed control.