A mobile crane is one of the most mechanically complex machines in construction. An all-terrain crane like the Liebherr LTM 1200 combines a road-legal carrier vehicle with a fully independent lifting superstructure — two separate machines, built on one chassis, each with its own engine, hydraulics, and control systems. The rubber components that isolate, buffer, and seal each system directly affect road performance, lifting accuracy, structural fatigue life, and operator safety.
This guide covers the rubber isolation and buffering components on all-terrain cranes (Liebherr LTM, Grove GMK, Tadano ATF, Manitowoc Grove) and briefly addresses tower crane rubber elements. It is aimed at crane fleet managers, maintenance supervisors, and procurement engineers responsible for keeping these machines serviceable.
The Dual-Engine Challenge
Most all-terrain cranes above 60-tonne capacity use two separate engines: one in the carrier (for road travel) and one in the superstructure (for crane operations — hoisting, luffing, slewing, and powering the outriggers). The Liebherr LTM 1500-8.1 uses a Liebherr V8 diesel in the carrier and a separate Liebherr 6-cylinder in the superstructure. The Grove GMK6400 uses a Mercedes-Benz OM502 in the carrier and a Cummins QSL in the superstructure.
This means two complete sets of engine mounts — each with different duty cycles, different vibration signatures, and different heat environments. The carrier engine runs at variable speed under full road load, producing significant low-frequency vibration transmitted through the chassis to the carrier cab and outrigger system. The superstructure engine typically runs at constant speed during crane operations, producing more predictable vibration but combined with hydraulic pump pulsation from the main hoist and slewing circuits.
Treating both sets of engine mounts as identical components is an error that leads to premature mount failure, increased structural fatigue, and operator vibration exposure that exceeds ergonomic limits.
Carrier Engine Mounts
Function and Load Case
The carrier engine mounts on an all-terrain crane must handle road travel — acceleration, braking, cornering, and rough road vibration — at highway speeds with a fully laden superstructure above. On a Liebherr LTM 1200-9.1 (120-tonne capacity), the carrier engine generates approximately 400 kW and the whole crane weighs up to 72 tonnes in travel configuration. The mounts isolate the engine from the chassis while handling the inertial loads of an engine moving inside that heavy, dynamically active carrier.
Standard carrier engine mount specification for Liebherr LTM series: a combination of four to six individual rubber-metal sandwich mounts arranged around the engine block. Each mount consists of a steel top plate, a vulcanised rubber body, and a steel bottom plate with threaded studs for chassis attachment. The rubber compound is NBR, rated to +120°C, with a dynamic stiffness tuned to isolate the primary engine firing frequency (typically 25–35 Hz at rated speed).
Failure Modes
Rubber body cracking: Thermal fatigue from proximity to the exhaust system. LTM 1090 and LTM 1200 engines sit close to the exhaust system in a confined engine compartment — the rear mounts closest to the exhaust manifold run significantly hotter than the front mounts. Rear mounts on these models typically need replacement at 60–70% of the front mount service life.
Oil contamination: Engine oil leaks from valve cover gaskets or injector seals contacting the rubber body. NBR has reasonable oil resistance, but prolonged oil saturation softens the rubber compound and reduces dynamic stiffness — the mount becomes too compliant, allowing excessive engine movement. This shows up as increased noise and vibration in the carrier cab before the mount physically separates.
Bonding failure: The vulcanised bond between rubber and steel plate separates. This is a progressive failure — the mount initially remains functional as the rubber stays in compression, but loses tensile restraint. Complete separation is sudden and dangerous. Inspect for bond line cracking at every major service interval.
Replacement Intervals
Carrier engine mounts on all-terrain cranes operating in high-utilisation hire fleets: replace proactively at 6,000 engine hours or six years, whichever comes first. In lower-utilisation owner-operator fleets, age-based replacement (six to eight years) is more relevant than hour-based replacement because thermal cycling and ozone degradation progress with calendar time regardless of operating hours.
Superstructure Engine Mounts
The superstructure engine on an all-terrain crane operates under different conditions than the carrier engine. It typically runs at a fixed speed (PTO speed matching the hydraulic system demand), and the superstructure is stationary during crane operations (on outriggers). However, the superstructure engine transmits vibration directly to the crane structure — which means any resonance between engine vibration and superstructure natural frequencies creates structural fatigue at boom foot pins, turntable welds, and luffing cylinder pivots.
Superstructure engine mount specification emphasises isolation efficiency at the running speed rather than road travel durability. Softer mounts (lower dynamic stiffness) are generally specified for superstructure engines. The Tadano ATF 220G-5 superstructure uses elastomeric mounts with a lower stiffness rating than the corresponding carrier engine mounts.
When replacing superstructure engine mounts, verify the stiffness specification from the service manual — do not substitute carrier engine mounts into superstructure positions, even if the physical dimensions match.
Outrigger Pad Isolation Elements
Function and Load Capacity
Outriggers extend from the crane carrier on four corners, placing steel outrigger pads on the ground to distribute the crane’s working load. On a crane lifting at capacity, each outrigger can carry 40 to 120 tonnes of ground reaction force, depending on the lift configuration and crane capacity.
Between the outrigger beam and the steel pad, many crane designs use a rubber isolation element — either a moulded rubber pad or a rubber-metal sandwich. The function is not vibration isolation in the conventional sense; the outrigger is stationary during lifting. The rubber element serves to:
- Distribute the point load from the outrigger beam tip more evenly across the pad
- Compensate for minor angular mismatch between the outrigger beam and the ground surface
- Absorb dynamic loading from sudden load swings or wind-induced boom motion
The rubber compound in outrigger isolation elements must carry very high static compressive loads — 40 to 120 tonnes over a contact area of typically 0.04 to 0.08 m². This corresponds to compressive stresses of 5 to 30 MPa. Standard elastomers rated for these stresses are high-hardness natural rubber (70–80 Shore A) or polyurethane compounds.
Inspection and Replacement
Outrigger isolation elements rarely fail in dramatic ways. The failure mode is gradual compression set — the rubber permanently deforms under repeated high compressive loading, losing thickness and compliance. An element that has taken compression set appears visually similar to a new element but is significantly stiffer and shorter.
Check element thickness at each major service against the manufacturer’s minimum specification. On Grove GMK series cranes, the specification is typically documented in the outrigger maintenance section of the carrier service manual.
A Crane Fleet Manager’s Experience: Liebherr LTM 1090, Netherlands
Pieter van den Berg manages a fleet of six all-terrain cranes for a Dutch heavy lift contractor, including three Liebherr LTM 1090-4.2 machines. He identified a pattern of accelerated rear carrier engine mount wear across all three machines after a period of intensive work on a motorway project involving frequent road travel at maximum carrier gross weight.
“The rear mounts were cracking at the rubber body after about 3,500 hours — much earlier than the front mounts. We checked the exhaust temperatures with a thermal camera and found the rear mounts were running at 140 to 160°C during prolonged highway travel. The rated temperature for those mounts is 120°C.”
He worked with a parts supplier to source mounts with a high-temperature NBR compound rated to +150°C for the rear positions. “We’ve now gone past 5,500 hours on the first set of upgraded mounts with no cracking. The front mounts are still the standard compound — no reason to change those.”
This targeted approach — different specifications for different mounting positions on the same engine — is more cost-effective than specifying the highest-rated compound across all positions.
Slewing Ring Wear Pads
The slewing ring is the large-diameter bearing that connects the superstructure to the carrier, allowing the crane to rotate (slew). On most modern all-terrain cranes, the slewing ring is a bolted ball or roller bearing race. Around the circumference of the slewing ring, a series of polymer or rubber-composite wear pads bear against the ring outer diameter to limit lateral play and absorb side loads during slewing.
These wear pads are a maintenance item — they wear progressively and require replacement when wear exceeds the manufacturer’s specified limit. Excessive wear allows lateral movement of the superstructure, which increases dynamic loading on the slewing ring teeth and, in severe cases, produces perceptible oscillation during slewing movements.
On the Liebherr LTM 1500-8.1, slewing ring wear pads are polymer (UHMWPE) rather than rubber. On smaller cranes (Tadano ATF 90G-4, Grove GMK4100L), rubber-composite pads are more common. Check the crane service manual for material specification and replacement intervals — these are not interchangeable.
Counterweight Buffer Elements
When the superstructure rotates, the counterweight sweeps a large arc at the rear of the crane. Buffer elements — rubber or polyurethane blocks — absorb the end-stop impact if the slewing motion reaches the mechanical stop (either by operator error or control system fault). These elements also absorb the dynamic load from counterweight oscillation during sudden slewing deceleration.
Buffer element replacement is often deferred until visible damage is present. The correct approach is to replace them at each major service regardless of visible condition — they are moderate-cost, high-consequence components.
Cab Mounts: Carrier and Superstructure
All-terrain cranes have two cabs: the carrier cab (for road travel) and the superstructure cab (for crane operations). Both use rubber cab mounts to isolate the operator from chassis/structure vibration.
Carrier cab mounts: Similar to truck cab mounts, these carry the cab weight plus dynamic loads from road travel. Cab mounts on Liebherr and Grove carriers are typically four-point or six-point rubber-metal mount systems. Failure from oil contamination and fatigue cracking is the primary concern.
Superstructure cab mounts: These isolate the operator from hydraulic pump vibration and slewing mechanism noise. Softer specification than carrier cab mounts in most designs. On Tadano ATF cranes, the superstructure cab mount system includes progressive-rate elements that stiffen at higher displacement — this limits cab sway when the crane is working in windy conditions.
Tower Crane Rubber Elements
Tower cranes use rubber elements in two locations relevant to this guide:
Mast anchor pads: Where the mast base frame sits on the foundation or building structure, rubber anti-vibration pads reduce structure-borne noise transmitted to the building. These are typically flat rubber-metal sandwich pads, 40–60 Shore A, sized to the mast base bolt pattern. Replacement is driven by calendar age (five to seven years) rather than operating hours.
Slewing mechanism isolation: The slewing drive unit on tower cranes (typically planetary gearbox plus electric motor) mounts on rubber elements that isolate gear noise from the mast structure. These are small, high-load rubber mounts requiring periodic inspection for compression set.
Contact Babacan Group for supply of outrigger isolation elements, engine mounts, and cab mounts across the Liebherr LTM, Grove GMK, and Tadano ATF crane families.
Recommended Maintenance Intervals Summary
| Component | Crane Type | Inspection | Replacement |
|---|---|---|---|
| Carrier engine mounts | All-terrain | Every 2,000 hours | 6,000 hours / 6 years |
| Superstructure engine mounts | All-terrain | Every 2,000 hours | 6,000 hours / 8 years |
| Outrigger isolation elements | All-terrain | Every major service | At compression set limit |
| Slewing ring wear pads | All-terrain | Every 500 slewing hours | At wear limit |
| Counterweight buffer elements | All-terrain | Every major service | Every major service |
| Carrier cab mounts | All-terrain | Every 1,000 hours | 4,000 hours / 5 years |
| Superstructure cab mounts | All-terrain | Every 2,000 hours | 6,000 hours / 6 years |
| Mast anchor pads | Tower | Annually | 5–7 years |
For sourcing and cross-reference support on crane rubber parts, request a quote from Babacan Group with crane model, serial number, and component location.
A Tower Crane Maintenance Case: Rotterdam Construction Site
Jan Kowalski is the site maintenance coordinator for a high-rise project in Rotterdam using three Potain MDT 389 tower cranes. He noticed progressive structure-borne noise in the floors adjacent to one mast — a low-frequency hum audible during slewing operations.
“We checked the slewing drive mounts and found two of the four mounts had compressed to about 40% of their original thickness. They were still holding the drive unit in position, but they weren’t isolating anything. Replaced all four mounts, noise dropped to inaudible. Total job was two hours on a Saturday morning.”
The mounts had been in service for nine years — two years past the recommended replacement interval. The progressive nature of compression set failure meant there was no sudden breakdown event, just a gradual loss of isolation that nobody noticed until the structure-borne noise became a complaint.
Key Takeaways
- Dual-engine all-terrain cranes require two independent sets of engine mount specifications — carrier and superstructure mounts have different duty cycles, thermal environments, and vibration signatures, and should not be treated as interchangeable.
- Carrier engine rear mounts run hotter than front mounts on Liebherr LTM series due to exhaust proximity — specifying higher-temperature compound for rear positions only is a targeted, cost-effective solution.
- Outrigger isolation elements carry 40–120 tonnes of compressive load and fail by compression set rather than fracture; measure thickness against specification at each major service.
- Counterweight buffer elements and slewing ring wear pads are moderate-cost, high-consequence items that should be replaced at each major service interval regardless of apparent condition.
- Cab mounts on both carrier and superstructure deteriorate with both calendar age and operating hours — use whichever limit is reached first as the replacement trigger.
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