Hitachi 9250478 ZAXIS450-3 ZAXSI470-3 ZAXSI500-3 ZX450 ZX470 ZX490 JD450 Carrier Roller Assembly/Heavy duty undercarriages manufacture
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Product Specifications
Technical Description: Hitachi ZX450/ZX470 Hydraulic Excavator Carrier Roller Assembly/OEM source manufacture
1.0 Introduction and Functional Overview
The Carrier Roller Assembly, also referred to as the upper roller or support roller, is a pivotal component within the undercarriage system of the Hitachi ZX450 and ZX470 hydraulic excavators. Mounted on the upper deck of the track frame, its functions are critical for the stability, efficiency, and longevity of the entire track system.
Upper Track Strand Support: It carries the weight of the track chain's upper (return) strand as it travels from the front idler back to the sprocket, preventing sag and contact with the track frame.
Track Stabilization and Alignment: It constrains the vertical and lateral movement of the track chain, minimizing whip, bounce, and oscillation during machine travel and slewing, thereby ensuring smooth and stable operation.
Friction Reduction: By supporting the return strand, it significantly reduces frictional drag between the track chain and the track frame, enhancing mechanical efficiency and reducing power consumption from the final drive.
Derailment Prevention: The flanges of the carrier roller interact with the track chain links (or the guide grousers) to keep the track properly centered, preventing lateral derailment, especially during counter-rotation (pivoting) turns.
A properly functioning carrier roller assembly is essential for mitigating abnormal vibration, controlling track slap, and protecting other undercarriage components from premature wear.
2.0 Major Components and Their Detailed Specifications
The assembly is a sealed, pre-lubricated unit designed for high load capacity and extended service life under severe operating conditions.
2.1 Roller Shell (or Housing)
Material & Construction: Fabricated from a high-carbon, heat-treated alloy steel, typically through a forging process. This ensures a superior grain structure for high impact strength and exceptional resistance to abrasive wear.
Design: Features a double-flange configuration. The flanges are integral to the shell, providing a robust guiding surface for the track chain. The contour and height of these flanges are engineered to match the specific track chain link design of the ZX450/ZX470.
Running Surface: The outer diameter (O.D.) is machined to a precise tolerance and undergoes induction hardening. This process creates a deep, consistent case hardness (typically 55-60 HRC) on the surface while retaining a tough, ductile core to withstand rolling contact stresses with the track chain bushings.
Internal Bore: The bore is precision-honed to provide a well-defined interference fit for the bearing cartridge, ensuring optimal heat transfer and structural integrity.
2.2 Sealed Cartridge Bearing Assembly
Hitachi employs a unitized, "maintenance-free" cartridge bearing system, a hallmark of modern, high-performance undercarriages.
Bearing Type: Utilizes a matched pair of Tapered Roller Bearings. This design is selected for its superior ability to handle the combined high radial loads from the track weight and substantial axial (thrust) loads encountered during dynamic turning and side-hill operation.
Bearing Cartridge: The bearings are pre-adjusted to a specified pre-load, pre-lubricated with high-quality grease, and housed within a single, sealed steel sleeve. This cartridge is pressed into the roller shell as one unit, guaranteeing correct internal alignment and eliminating the need for field adjustment.
Lubricant: The cartridge is packed with a high-temperature, high-pressure, lithium-complex extreme pressure (EP) grease, formulated to maintain viscosity and lubricity over the entire projected service interval.
2.3 Advanced Multi-Component Sealing System
The longevity of the roller is directly dependent on the effectiveness of its seals. The ZX450/ZX470 carrier roller features a state-of-the-art, multi-stage sealing system.
Primary Seal: A Spring-Energized Radial Lip Seal serves as the primary fluid barrier. The spring ensures constant lip contact pressure against the seal seat, compensating for minor wear and dynamic runout. The lip material is often a advanced compound like Hydrogenated Nitrile Butadiene Rubber (HNBR) for excellent resistance to heat, oxidation, and grease.
Labyrinth Seal: An external, multi-path "labyrinth" seal is formed between the roller shell and the Flange of the Axle Boss. This complex, tortuous path is filled with a special barrier grease during assembly, effectively blocking the ingress of fine abrasives like silica dust and clay.
Exclusionary Dust Lip: A primary outward-facing lip, often made of a tough, flexible polyurethane, acts as a scraper to remove bulk mud, clay, and debris before they can penetrate the labyrinth.
2.4 Axle Boss (Shaft) and Mounting
Axle Boss: This is the stationary shaft component. It is made from high-strength alloy steel, hardened and ground to provide a perfect, wear-resistant surface for the inner bearing race and the primary seal lip.
Mounting Bracket: The axle boss is an integral part of a Carrier Roller Bracket. This bracket is bolted to the track frame using high-tensile cap screws. This design facilitates straightforward removal and replacement of the entire assembly. The bracket may also incorporate shims or an elongated bolt hole to allow for fine lateral adjustment, ensuring perfect alignment with the track chain.
3.0 Operational Context and Wear Characteristics
3.1 Load and Service Conditions:
Carrier rollers on an excavator of this size (45-47 ton class) are subjected to intense, dynamic loads. The primary stress comes from the significant weight of the heavy-duty track chain itself, compounded by inertial forces from track slap and shock loads from the track's engagement with the sprocket.
3.2 Common Wear Patterns and Failure Modes:
Flange Wear: Measured as a reduction in flange thickness and height. On excavators, the inside flange often wears more due to the high lateral forces during pivoting turns.
Running Surface Wear: A reduction in the roller's outer diameter (O.D.). While not as rapid as on bottom rollers, it should be monitored as part of a complete undercarriage survey.
Bearing Failure: The predominant failure mode. Indications include:
Non-Rotation (Seizure): The roller stops turning, leading to rapid, grinding wear as the track bushing scrapes against a stationary roller.
Excessive End-Play or Radial Play: Worn bearings allow the roller to wobble, causing irregular wear patterns and potential damage to the track links.
Root Cause: Seal failure is almost always the precursor, allowing abrasive contaminants to enter the bearing cavity, leading to lubricant breakdown and accelerated bearing spalling and fatigue.
4.0 Maintenance, Inspection, and Replacement
Preventive Inspection: During daily or weekly walk-arounds, visually inspect for grease leakage (seal failure) and ensure all carrier rollers rotate freely by hand (with the track raised). Listen for rumbling or grinding noises during operation.
Wear Measurement: As part of a scheduled undercarriage inspection, use calipers to measure flange width and roller O.D. Compare these measurements against the manufacturer's specified "wear limits" published in the Hitachi service manual. Proactive replacement before reaching these limits prevents collateral damage.
Replacement Procedure:
Safely elevate the machine and clean the assembly and mounting area thoroughly.
Remove the mounting bolts securing the carrier roller bracket to the track frame.
Using a pry bar or puller, carefully remove the assembly. Avoid damaging the track frame mounting surface.
Clean the track frame mounting boss and inspect for damage or corrosion.
Install the new carrier roller assembly and torque the bolts to the manufacturer's specified value using a calibrated torque wrench.
