Why the RT Scissor Lift Needs a Different Wheel Drive Than a Boom Lift
A boom lift can reach a work position from 10 to 20 metres away — the boom extends horizontally to bridge the gap between the machine position (on firm ground) and the work point (above an obstacle or across a gap). A rough terrain scissor lift has zero horizontal reach — the platform rises vertically, directly above the wheelbase. This means the machine must drive to the exact position beneath the work point, regardless of the ground conditions at that position.
This fundamental difference transforms the wheel drive planetary gearbox requirement. A boom lift wheel drive must handle rough terrain during transit only — the machine parks, deploys outriggers, and the boom does the reaching. An RT scissor lift wheel drive must handle rough terrain at the work position — because the machine IS the work position. The ground surface under the scissor lift when the platform is raised to 15 metres is the same unprepared construction-site surface that the machine just drove across. There are no outriggers to stabilise the machine (on most RT scissor models) — the wheel drive and tyres provide both the traction for driving and the stability base for the elevated platform.
The stability advantage of the scissor lift configuration is that the CG stays centred over the wheelbase as the platform rises — unlike a boom lift where the CG moves forward and outward. This means the static rollover angle decreases more slowly with height on a scissor lift than on a boom lift: a scissor lift at 15 metres has approximately 70 to 80% of its stowed rollover margin, while a boom lift at 15 metres with full outreach has only 30 to 50% of its stowed margin. This stability advantage allows the RT scissor lift to drive (at reduced speed) with the platform partially or fully raised — a capability that most boom lifts do not permit.
The no-outrigger design places the entire stability responsibility on the wheel drive and tyre system. When the platform is raised and the machine is parked on a slope, the only things preventing rollover are: (1) the machine wheelbase geometry (track width and wheelbase length), (2) the tyre contact patches (which must not slide on the surface), and (3) the wheel drive parking brake (which must hold the machine stationary against the gravity component). If any one of these three elements fails — the tyres lose grip on a wet surface, the brake loses holding torque from pad wear, or the machine is driven onto a slope that exceeds the stability limit for the current platform height — the machine will overturn with workers at height. This triple-dependency makes the RT scissor lift wheel drive the most safety-critical wheel drive application after the boom lift.
4WD Gradeability — Climbing Construction-Site Slopes with an Elevated Platform
RT scissor lifts are rated for gradeability of 25 to 45% (14 to 24 degrees) in the stowed position — the steepest slope they can climb under their own power. This gradeability requirement is governed by the construction-site terrain: ramps to elevated work areas, slopes around building perimeters, and unfinished grading that creates temporary slopes across the site.
| Class | Weight (t) | Height (m) | Grade % | Drive Torque |
|---|---|---|---|---|
| Compact RT (4–6 t) | 4–6 | 8–12 | 30–40% | 3,000–6,000 Nm |
| Standard RT (6–10 t) | 6–10 | 12–18 | 25–35% | 6,000–12,000 Nm |
| Heavy RT (10–14 t) | 10–14 | 15–20 | 25–30% | 10,000–18,000 Nm |
The gradeability requirement directly sizes the wheel drive torque. On a 35% slope with an 8-tonne machine, the gravitational traction demand is approximately 27.5 kN — before adding rolling resistance on the unprepared surface (0.05 to 0.10 coefficient = 4 to 8 kN additional). The total drive force of 31.5 to 35.5 kN must be delivered continuously while climbing the slope at 3 to 5 km/h — and the wheel drive must hold the machine stationary on the same slope when the operator stops to raise the platform.
The 4WD system is essential for slope climbing on loose surfaces. On gravel or compacted earth, the traction coefficient is 0.4 to 0.6. On a 35% slope, the gravitational component alone consumes 85 to 95% of the rear-axle traction — leaving virtually no margin for rolling resistance or acceleration. Four-wheel drive distributes the traction demand across all four wheels — each wheel needing only 40 to 50% of its available traction instead of 85 to 95% — providing the margin needed for reliable slope climbing with a loaded platform.
The descending pass is equally critical. On a 30% downhill slope, an 8-tonne machine generates a gravitational force of approximately 23.5 kN pulling the machine downhill. The wheel drive must provide continuous retarding torque — either through the hydrostatic motor back-pressure or through a service brake — to maintain a controlled descent speed. The braking energy during a 100-metre descent at 4 km/h is approximately 23.5 kN x 100 m = 2,350 kJ — enough to overheat a small dry brake if the descent is continuous. Wet-disc or oil-cooled brakes are preferred for RT scissor lifts that regularly operate on slopes above 15%.
The traction management during slope climbing is complicated by the weight transfer between axles. On a 30% uphill slope, approximately 60 to 70% of the machine weight transfers to the rear axle — increasing the rear wheel traction but reducing the front wheel traction proportionally. If the front wheels lose grip on a loose surface, the machine cannot steer — even though the rear wheels still have traction. The wheel drive must include a traction-limiting function that prevents the front wheels from spinning uselessly while the rear wheels push the machine forward — because a spinning front wheel on a slope loses all steering authority and the machine can drift sideways toward the edge of the ramp or slope.
Driving with Elevated Platform — The Capability That Defines the RT Scissor Lift
Unlike most boom lifts (which prohibit driving with the boom extended), many RT scissor lifts permit driving with the platform partially or fully raised — at reduced speed. This capability allows the workers to reposition the machine along a building facade without lowering the platform, climbing down, driving, parking, and raising again — saving 3 to 5 minutes per repositioning and increasing the productive time by 20 to 30% on tasks that require frequent short moves.
The wheel drive must support this elevated-drive mode with the same height-dependent speed limiting as boom lifts — but with the additional constraint that the platform is carrying workers and tools during the drive. Any sudden acceleration, deceleration, or torque pulsation from the wheel drive planetary gearbox is transmitted directly to the workers standing on the 4 to 8 m2 platform at 10 to 15 metres height. The EN 280 standard limits the maximum platform acceleration during elevated drive to 0.5 m/s2 — approximately one-twentieth of the gravitational acceleration — to prevent workers from losing balance.
The maximum permitted drive speed with elevated platform is typically 1.0 to 2.5 km/h — set by the tilt-sensor system based on the current platform height and the ground slope. The wheel drive must maintain this speed precisely on the uneven construction-site surface without exceeding the 0.5 m/s2 acceleration limit — a combination of speed accuracy (±0.2 km/h) and smoothness (no cogging or torque spikes) that requires DIN Class 6 gears and proportional hydraulic control with sub-0.3-second response time.
The pothole and obstacle hazard is more severe for RT scissor lifts than for boom lifts during elevated drive. A boom lift operator at 30 metres height cannot see the ground directly beneath the machine — but the machine travels slowly and the boom flexibility absorbs some of the terrain irregularity. A scissor lift operator at 15 metres can see the ground more clearly, but the rigid scissor mechanism transmits every terrain irregularity directly to the platform — there is no boom flexibility to absorb the impact. A 50 mm terrain step that produces a barely perceptible lurch on a boom lift produces a sharp jolt on a scissor lift that can dislodge tools from the platform guardrail and startle workers into unsafe reactions. The wheel drive speed during elevated repositioning must therefore be conservative (1.0 to 1.5 km/h maximum) — even though the stability margin would theoretically permit higher speeds.
Three Failure Modes Specific to RT Scissor Lift Wheel Drives
A 10-tonne RT scissor lift descending a 200-metre ramp at 25% grade must dissipate approximately 49 kJ of braking energy — equivalent to the kinetic energy of a car at 40 km/h. If the operator uses the service brake continuously (rather than the hydrostatic retard), the brake temperature rises by 80 to 150 degrees C during the descent. On a dry-disc brake with a fade-onset temperature of 300 degrees C, a second consecutive descent (without cooling time) can push the brake into the fade zone — where the friction coefficient decreases with increasing temperature and the machine accelerates despite the brake being applied. Brake fade on a loaded scissor lift on a construction-site ramp has resulted in serious accidents — making the brake thermal capacity the most safety-critical specification of the wheel drive.
Construction sites contain nails, rebar stubs, wire offcuts, and sharp debris that can puncture the scissor lift tyres. A sudden tyre deflation while driving with an elevated platform produces an immediate tilt of 3 to 8 degrees (depending on the tyre size and the platform height) — enough to destabilise workers on the platform and potentially exceed the tipping threshold on a cross-slope. Unlike a gradual leak (which the tilt sensor can detect and respond to), a sudden blowout produces the full tilt in less than 0.5 seconds — faster than the tilt-sensor response time on many machines. The wheel drive cannot prevent this failure, but the brake must engage immediately on tilt-alarm to prevent the machine from driving further with a flat tyre and an elevated platform.
RT scissor lifts operate on construction sites where standing water, mud, and wet concrete slurry are routine. The wheel drives are typically positioned at the lowest point of the machine — 150 to 300 mm above ground level — and are frequently submerged in puddles, mud, and slurry up to the hub centre. This continuous immersion subjects the shaft seal to hydrostatic pressure (in addition to the normal dynamic sealing duty) and forces contaminated water through any seal imperfection. Wet concrete slurry is particularly damaging: the calcium hydroxide (pH 12 to 13) attacks standard NBR seal material, and the cement particles (when dried) form a hard, abrasive crust on the shaft surface that grinds through the seal lip from the outside.
Часто задаваемые вопросы
Korea Ever-Power provides RT scissor lift wheel drives from 3,000 to 18,000 Nm with 4WD gradeability, elevated-drive smoothness, and construction-site seal protection.
Редактор: Cxm
