CGC Kymon has been working with an operator of OEM shovels to develop a superior roller to support their fleet of RH170/200 face shovels. Having encountered unacceptable failure rates of the OEM hollow cast units, the company pursued a solid roller body, grease lubricated alternative.
Review of roller assembly wear and failure analysis indicated a high failure rate of the two-piece hollow cast and welded construction at the weld joint, with the solid roller body producing better life and reliability. A roller body forged to profile delivered superior and more reliable life and eliminated structural failure where properly lubricated.
The review also indicated a high failure rate of the oil filled and lubricated roller assembly. A greased journal system delivered superior and more reliable life for the roller body journal, end cap, bush and thrust washer assembly. Additionally, it was observed that consistent separation of the case hardened layer from the track wear surface, especially at the edge of the surface reduced the life and consistent load bearing of induction hardened roller bodies. Flame hardened track wear surfaces delivered superior effective case depth.
The newly developed roller assembly product has performed with better service life than the OEM product for the past eight years, with the face shovels totalling in excess of 240,000 operating hours on four machines. The rollers have been consistently able to achieve between 12,000 to 15,000 hours at which time new bearings and seals are fitted.
Forged and Spin Hardened Load Roller Body
The forged roller body of the roller assembly product is the primary reason for its superior performance. Forged to near net shape using AS1444-4140 alloy steel, the forged roller body is then machined, and quench and tempered to tensile strength of 1175MPa, followed by spin hardening on the track wear surface to a hardness of 52/55 HRc to an effective case depth of 43 HRc at 25mm.
Structural grain flow in the roller body is a sought after property in terms of application reliability and performance, particularly when high fatigue strength is mandatory. The open die forge process forms the roller body in one piece with the ultimate shape of the roller body becoming an integral part of the manufacturing process.
Forging has the ability to form the material to the desired shape. The forge process ensures that the grain flow is highest in the area of operational fatigue so that the mechanical properties of tensile strength and impact strength (fatigue resistance) are maximised in the region and direction of maximum loading.
OEM product uses a roller body that has a welded joint two piece hollow casting. OEM cast fabrication's mechanical properties fall well below those achieved from a forged roller. The decrease of structural integrity of a fabrication compared to a one piece forging is the driver behind the operators of RH170/200 face shovels specifying forged one piece rollers for high load applications.
Spin Hardened Track Wear Surface of Roller Body
Spin hardening is the preferred method for hardening load rollers. CGC Kymon designed and built a Spin Hardening Facility specifically for load rollers. The roller spins on a turntable in the horizontal plain and a custom-designed ring burner encircles the part. The coupling distance between the part and the flame head and the number of heating tips is configured to ensure the critical parameters of hardness and depth of hardening.
The precision surface hardening process utilises digital high-speed optical-resolution non-contact infrared thermometers. This is the key to the control and repeatability of the process. Each pyrometer unit is equipped with laser sighting, which allows pinpoint placement for extreme accuracy of surface temperature monitoring. The preheat temperature, austenizing temperature at quenching point and residual temperature are all key elements for controlling the hardness and effective case depth. Each unit has its own digital readout and is tied into a digital chart recorder for traceability. The operator is able to scroll back through the recorder to view the heating cycle at any time. When the surface-hardening process is complete, the heating cycle file is recorded for permanent reference.
Advantages of Spin Hardening vs. Induction Hardening:
The spin hardening process is controlled to produce an effective case depth to 25mm. The change in hardness from surface of 52/62 HRc to 45 HRc at 25mm permits a smooth transition from the surface to the high tensile forged steel core of 1175 MPa. This controlled transition zone supports the hardened surface and provides a key into the forged alloy steel core, ensuring superior impact and torsional strength compared to induction hardened rollers machined from quench and tempered commercial bar stock.
Induction hardening has an effective case depth of approximately 6mm. The abrupt transition from hardened surface of 52/62 HRc to low core tensile strength of 860 MPa of commercial bar stock leads to failure of the surface layer under impact loading. The high residual stresses in the transition zone cause the surface layer to delaminate under high impact loading in extreme service conditions.