5.2 Running Gear, Wheels & Tyre Management

The Foundational Role of Running Gear in MEWP Stability

Mobile Elevating Work Platforms rely entirely upon their running gear—tyres, wheels, hubs, steering linkages, and crawler tracks—to interface with the ground surface. On self-propelled machines (Category 3a and Category 3b), the running gear does not merely facilitate transport; it forms the foundational base of the machine's static and dynamic stability triangle or rectangle.

A typical rough-terrain boom lift has an operating tare weight ranging from 8 tonnes to over 20 tonnes. When the boom is elevated to 20 or 40 metres and slewed across the chassis, a large share of this combined mass is concentrated onto the two tyres on the tipping side of the machine. Any structural failure in a tyre carcass, any significant loss of pneumatic pressure, or any shearing of wheel studs instantly shifts the centre of gravity relative to the tipping line, creating an immediate, unrecoverable risk of catastrophic overturning.

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Tyre Classifications & Engineering Characteristics

MEWPs are equipped with distinct tyre types engineered for specific ground environments, load ratings, and operating speeds. Understanding these differences is essential for accurate pre-use inspection and hazard identification.

Solid Rubber Tyres

Solid rubber tyres consist of a high-density, moulded rubber compound vulcanised directly onto a steel base band, which is then press-fitted onto the wheel hub:

• Engineering Profile: Delivers exceptionally high load-carrying capacity with zero risk of puncture or deflation. The tyre undergoes minimal deflection under heavy elevated loads, providing maximum chassis rigidity.
• Primary Applications: Standard equipment on industrial electric scissor lifts and vertical mast platforms operating on smooth, level concrete slabs in warehouses, factories, and commercial fit-outs.
• Operational Limitations: Solid tyres have zero natural cushioning or shock absorption. Driving a solid-tyred machine over rough asphalt, gravel, or construction debris induces violent chassis vibrations, accelerates mechanical wear on scissor arm bushings, and can dislodge platform occupants.

Polyurethane Cushion Tyres

Polyurethane cushion tyres feature a synthetic polyurethane elastomer tread moulded directly onto a cast steel core:

• Engineering Profile: Characterised by extremely high material hardness (Shore 90A to 95A / Shore D scale), minimal rolling resistance, and exceptional resistance to petroleum-based oils and solvents. They can support enormous compressive wheel loads with an ultra-compact tyre diameter.
• Primary Applications: Fitted to Push-Around Verticals (PAVs), ultra-narrow indoor slab scissor lifts, and internal mast booms navigating polished showroom floors or narrow warehouse aisles.
• Vulnerabilities: Susceptible to catastrophic chunking and tread delamination if driven over expansion joints, metal gratings, or rough outdoor surfaces. Operating polyurethane tyres outdoors on asphalt can shred the tread within hours.

Foam-Filled Pneumatic Tyres

Foam-filled tyres begin life as heavy-duty pneumatic tyre carcasses (typically 12-ply to 16-ply industrial off-road casings). Rather than being inflated with air, the tyre cavity is injected under high pressure with a two-component liquid polyurethane elastomer foam that cures into a resilient, dense synthetic rubber core:

• Engineering Profile: Combines the aggressive, deep-tread traction and large ground contact footprint of an off-road pneumatic tyre with 100% puncture immunity. Even if the tyre casing is slashed by jagged steel rebar, punctured by nails, or penetrated by sharp rock fragments, it cannot lose pressure or collapse.
• Ballast Function: The dense cured foam adds significant deadweight (often 150 kg to 350 kg per tyre assembly) at the lowest point of the chassis, acting as functional ballast that lowers the machine's overall centre of gravity and enhances overturning resistance.
• Primary Applications: Commonly fitted to rough-terrain scissor lifts and rough-terrain articulating and telescopic booms working on construction sites.

Air-Filled Pneumatic Tyres

Air-filled pneumatic tyres utilise conventional compressed air inflation:

• Applications: Predominantly found on highway-towable trailer lifts (Category 1b), vehicle-mounted access platforms (trucks and vans), and older lightweight access machinery. Air inflation is necessary to provide high-speed suspension damping during highway transit.
• Severe Operational Hazards on Self-Propelled MEWPs: Where a self-propelled machine has air-filled tyres, a slow leak, damaged valve or sidewall failure lets the chassis lean towards the soft tyre, using up the slope allowance the stability design depends on. Check pressures as the manufacturer requires and never elevate on a soft or damaged tyre.

Non-Marking Tyres

Non-marking tyres are specially formulated for indoor environments where cosmetic floor preservation is paramount (hospitals, schools, aircraft hangars, commercial shopping malls, and gymnasium sports halls):

• Compound Chemistry: Standard black industrial tyres derive their mechanical strength and abrasion resistance from carbon black. Non-marking tyres substitute carbon black with precipitated silica, chalk, and natural rubber resins, preventing black scuff marks when steering across polished concrete or timber.
• Electrostatic Hazard: Carbon black makes standard tyres slightly conductive. Non-marking compounds are better insulators, so a machine on non-marking tyres can build up static charge.
• Antistatic Grounding Straps: Where the manufacturer fits an antistatic strap or chain, check that it is present, undamaged and in contact with the floor.

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Comprehensive Tyre Inspection Criteria & Condemning Limits

During the daily pre-use walk-around, check every tyre around its whole visible circumference against the condemning criteria in the manufacturer's operator's manual. Typical warning signs are:

Tread Wear, Siping & Chunking

• Tread Depth: Rough-terrain tyres need enough tread for traction on mud and slopes. Many solid and cushion tyres carry a wear indicator line on the sidewall; once the tread reaches it, the tyre must be replaced.
• Chunking (Tear-Out): Impacts with kerbs, steel angles or rebar can rip chunks of rubber out of the tread. Large chunks, or any damage reaching the carcass plies or steel base band, mean the tyre must not be used until assessed or replaced.

Sidewall Damage & Bead Separation

• Sidewall Cuts and Gouges: The sidewall is the most vulnerable section of a tyre. Inspect for deep transverse lacerations, torn rubber flaps, and localised bulging. If a sidewall cut is deep enough to expose the underlying nylon cord or steel reinforcement wire, the tyre must be condemned immediately.
• Delamination & Bead Separation: On solid press-on and cushion tyres, inspect the steel base band and rim lip. If the rubber compound has begun to unbond, split, or peel away from the steel band (delamination), dynamic cornering or braking loads will cause the entire rubber tread ring to spin freely or slide off the rim, leading to instantaneous platform tilt.

Inflation Pressure Verification (Pneumatic Tyres)

For any machine equipped with air-filled pneumatic tyres, checking tyre pressure is a non-negotiable daily task:

1. Use a calibrated, industrial tyre pressure gauge with an undamaged chuck.
2. Measure pressure when tyres are cold (before operating or driving the machine).
3. Compare measured pressure against the manufacturer's tyre specification placard or the operator's manual.
4. Inspect valve stems: verify valve stems are straight, not rubbing on the wheel rim aperture, and fitted with a sealed metal or plastic valve dust cap to keep grit and water out of the Schrader core.

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Wheel Rims, Wheel Studs & Fastener Torque Management

Tyres are mounted onto heavy-duty pressed steel discs or cast steel rims secured to the drive hub via high-tensile wheel studs and conical nuts. Wheel assembly integrity is safety-critical.

Wheel Rim Condition

• Circumferential Fatigue Cracking: Inspect the steel rim disc around the bolt hole circle. High cyclic reversing loads from drive motors can induce microscopic fatigue cracks that radiate outward from the stud holes toward the rim edge.
• Bead Flange Distortion: Check the outer rim flange for severe bending or lip distortion caused by striking kerbs or concrete barriers. Severe rim distortion can pinch tyre beads or compromise rim clamping integrity.
• Split Rims (Multi-Piece Wheels): Certain heavy-duty access platforms utilise multi-piece split rims held together by perimeter clamping bolts. Operators must verify that all perimeter rim bolts are present, fully torqued, and show no signs of gap opening between rim halves.

Wheel Studs & Nut Torque Verification

Wheel fasteners endure extreme shear and bending moments when a MEWP turns, brakes, or travels across slopes:

• Missing or Sheared Studs: Every wheel stud and nut must be present; a machine with a missing wheel nut must not be used. Losing one nut overloads the neighbouring studs and can lead to progressive failure and detachment of the wheel.
• Rust Weeping: Look for fine orange rust streaks radiating outward from beneath wheel nuts across the rim paint. Rust weeping indicates micro-movement: the nut has loosened, allowing moisture to enter and generate fretting corrosion between the stud and rim hole.
• Torque Indicators & Witness Marks:
  • Torque Seal Paint Lines: A distinct line of brightly coloured inspection lacquer drawn across each tightened wheel nut and onto the wheel rim. If the paint line is cracked, misaligned, or broken, the nut has rotated and backed off.
  • Wheel Nut Movement Indicators (Checkpoint Indicators): Fluorescent yellow or green plastic arrow tags fitted over each wheel nut. The arrows are set up to point directly at one another in pairs (tip-to-tip). If any arrow rotates out of alignment, the operator has immediate, unmistakable visual proof that the fastener has loosened.

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Track-Drive Systems on Crawler MEWPs & Spider Booms

Tracked platforms (crawler booms and spider lifts) replace wheels with continuous rubber or steel track assemblies. These systems provide exceptional traction over deep mud, loose sand, and steep embankments while distributing machine weight across a large surface area.

Track Tension Verification & Adjustment

Correct track tension is crucial. A loose track can come off (de-track) when turning or slewing on slopes, while an over-tightened track wears rollers and overloads the drive motors. At the pre-use check, look for obvious slack, damage or misalignment; tension is measured and adjusted to the manufacturer's procedure (often a sag measurement between the sprocket and idler, adjusted with a grease tensioner), which is normally a maintenance task.

Sprockets, Rollers & Rubber Track Belting

• Drive Sprockets: Inspect the cast steel drive sprocket teeth. Sprocket teeth must have symmetrical, flat-topped profiles. Condemn sprockets exhibiting "hooked" teeth (worn into sharp wave shapes), broken teeth, or severe side-face wear.
• Idlers & Bottom Rollers: Check that all lower guide rollers and upper carrier rollers rotate freely. Look for seized rollers, flat spots, and leaking grease/oil seals. Ensure the roller guide channels are scraped clear of packed, hardened clay or construction debris.
• Rubber Track Carcass: Inspect the continuous rubber belt:
  • Check internal rubber drive lugs: missing or torn drive lugs cause the sprocket to slip under load.
  • Inspect the outer tread cleats for cuts and chunking.
  • Steel Cord Exposure: Continuous rubber tracks are reinforced internally by high-tensile, wound steel tension cables. If cuts or gouges in the rubber expose the internal steel cords to the atmosphere, the track must be condemned immediately. Exposed cords corrode rapidly upon contact with moisture, leading to sudden tensile snapping and catastrophic track shedding on gradients.

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Ground Clearance, Undercarriage Architecture & Pothole Protection Skirts

The underside of a MEWP chassis houses safety mechanisms that prevent overturns and enable emergency recovery:

Pothole Protection Systems (PHP Skirts)

On Category 3a scissor lifts, ground clearance presents an engineering paradox: the chassis requires sufficient clearance (75 mm to 100 mm) to navigate ramps while stowed, but requires minimal clearance (15 mm to 20 mm) when elevated to prevent overturning if a wheel drops into a floor depression.

During pre-use checks, the operator must verify:

• Mechanical Linkages: Ensure the mechanical drop-bars and scissor-actuated linkages beneath the chassis sides are straight, clean, and free of bends or weld fractures.
• Obstruction Free: Ensure that hardened mortar, gravel, or construction debris is not lodged behind the skirts, which could prevent them from deploying fully or jam them in the down position.
• Electrical Interlock Limit Switches: As the platform rises from the stowed position, the skirts must swing down and close limit switches that permit continued operation. If a skirt strikes an obstruction, the switch stays open, an alarm sounds and drive (and on many models further elevation) is prevented.

Free-Wheel / Manual Brake Release Mechanisms

MEWPs are equipped with Spring-Applied, Hydraulically Released (SAHR) safety brakes. When the machine is stationary, powerful internal springs force multi-plate friction discs together, locking the drive wheels. When the operator commands drive, hydraulic pump pressure overcomes the spring pack to release the brakes.

In emergency breakdown scenarios where a dead machine must be winched onto a recovery trailer, a manual brake release mechanism (a hydraulic hand pump or mechanical screw release bolts) allows the operator to override the spring brakes:

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MEWP Running Gear & Tyre Comparison Matrix