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Success Riding on the Rack

Precision Design, Operating Techniques, Proper Maintenance, Are All Key To Getting The Most Life Out Of A Swing Rack.

By Mark S. Kuhar

It measures 2.8-m (17-ft.) in diameter and weighs more than 20,411 kg (45,000 lb.) It’s the often forgotten, less-visible workhorse of the electric rope shovel. Hundreds of thousands of kilograms are supported by this structure, which disperses the stress from every crowd motion, dipper hoist and loading swing cycle of the upper works.

While massive, the swing rack is a precisely engineered system, critical to the longevity of an electric rope shovel. Swing racks support the entire weight of the upper works as well as absorb the reaction forces when digging material. Though steel is often thought to be rigid and immovable, in the mining industry, even the most robust steel castings, such as a 20.4-tonne (22.5-ton) swing rack, can deform like a spring plate in operation.

The base of a swing rack design consists of two drive assemblies working in unison. Each assembly drives output pinions via a planetary dual-output final drive. They form a single part with multiple critical functions.

“Most designed components serve a single purpose,” commented Mike Haws, technical steward for Caterpillar’s electric rope shovel product. “However, a swing rack acts as both a gear and a bearing, while at the same time supporting the entire weight of the massive upper works. It’s a critical component to the electric rope shovel.”

Swing gearsets bear tremendous loading from the dual output gearboxes, able to totally revolve the full inertia of the 952,000 kg (2.1 million lb.) of upper works in just under 12 seconds from a complete stop. With so much riding on the swing rack, electric rope shovel manufacturers painstakingly perfect the design and gearing precision. Premium quality gear design and manufacturing directly results in a longer system, and, therefore, a lower total cost of ownership. “The Caterpillar design is optimized to protect the gearing against contact, bending and wear failures,” added Haws.

Tight Tolerances
Historically, swing racks were multiple-piece assemblies, welded or bolted together in the field. Some designs included gear teeth as cast, while others were welded on. “Repeated shovel loading and unloading cycles induce fatigue stresses in components,” said Caterpillar Powertrain Design Engineer Timothy Schultz. “Every weld, bolted joint and discrepancy in quality is at the mercy of the engineer’s attention to detail and understanding of the system during the design of the swing rack. That is why it’s critical for a swing rack supplier to understand the loading the swing rack will endure.”

The swing rack design for Cat electric rope shovels has evolved over time. Today, the entire rack is cast in steel prior to welding a hardened steel piece ring to provide a stable base for the revolving frame and flow of stresses through the machine.

“We analyze the molecular structure of the liquid hot metal to ensure quality,” adds Schultz. The change to minimal welding and the steel casting, along with other updates, has led to lowering swing rack system maintenance by up to 40%.

The swing rack roller assembly and center pintle are the only components connecting the upper works to the lower deck of a machine weighing more than 952,000 kg (2.1 million lb.) Therefore, gear alignment for the large swing rack is extremely precise, as the gearsets require tight mesh control of 0.254 mm (0.010 in.) radially and 0.025 mm (0.001 in.) angularly.

“It’s hard to imagine such fine precision is achievable in such a behemoth working machine, but the attention to detail and fine tolerancing on each component related to the swing system is what allows the system to perform correctly for many years,” said Schultz.

Like a fine bearing, the swing rack also serves as the structure support for the removable and segmented bearing race, or rails. The geometry and metallurgy of the rails and rollers are extremely important as the full weight of the upper works bears directly on these components, while they facilitate machine rotation. Understanding how the machine is loaded during operation is essential to developing the proper roller and race designs. If these are poorly designed, pitting and progressive wear will quickly develop, resulting in costly repairs, machine downtime and lost productivity.

Proper Operation – Key to Longevity
As the swing rack system is difficult and expensive to replace, electric rope shovel manufacturers target a design life of 50,000 hours, or 10 years. Replacing a swing rack unexpectedly can cost a mine site millions of dollars in repairs and lost production, as the entire machine must be taken out of service and undecked, which can take weeks to complete.

For this reason, Schultz said that proper operator training and machine operation are critical for the mine to maximize the service life of the swing rack. “Electric rope shovels are built to withstand the demands of moving thousands of tonnes of material every day, over many years, but they are not indestructible,” he said. “Improper operating techniques put more undo stress on the rack, which can damage the component.”

One such ill-advised technique is swinging in the bank. When the operator swings the upper works at full speed, the machine responds by putting as much torque on the motors required to rotate at full speed. If the dipper remains in the bank while the machine begins its swing, the motors are giving full effort and begin to stall.

“Stalling the motor squeezes out every bit of lubricant from between the gear teeth before the metals contact,” warned Haws. “These teeth weld to one another under extreme pressure and are subsequently ripped apart upon the dipper breaking free from the bank. Swinging in the bank tears the metal out of your gearing.” This is the reason shovel manufacturers recommend a specific open gear lubricant (OGL) type to prevent metal-to-metal contact under stalling loads.

Another common improper operating occurrence that overstresses the swing rack and increases wear on the rollers and bearing rails is stalling a dipper. Stalls occur when the operator crowds too far into the material, and the machine can no longer hoist the dipper, even under full hoist power. This results in overloading the dipper and using the hoist motor to plow through material that is incapable of being removed by the machine.

“The loads experienced by the swing rack may increase by up to 15% when the dipper is stalled,” advised Schultz. “This may not seem like much, but it is significant, considering the fatigue life of steel and the fact that one of the rollers under the upper works may already be supporting more than 226,000 kg (500,000 lb.) during a normal digging load.”

To keep the electric rope shovel in the bank and minimize improper techniques like these, manufacturers have introduced machine operating software upgrades to assist operators. Caterpillar developed its Operator Assist – Enhanced Motion Control system for the Cat 7495 Series shovels. The software both simplifies machine operation and protects it from inadvertent misuse. Through reducing the occurrence of stalling the dipper and swinging in the bank – as well as boom jack prevention, crowd impact prevention and crowd over-speed prevention – it can also help enhance production.

Experience Matters
With machines working in virtually every climate and a wide spectrum of material mined, electric rope shovel manufacturers encounter local conditions that pose challenges to component longevity but also an opportunity to improve component design.

“As the OEM, we are held to high standards to meet longevity expectations from our customers,” said Haws. “When we encounter a unique issue, we consider the entire system and how it operates as a unit to apply a fix.”

He offers one encounter specifically involving swing rack design updates to improve component reliability and durability. The issue originated from a mine with operations in the Atacama Desert in Chile. Known as the driest nonpolar place in the world, the average annual rainfall is approximately 15 mm (0.6 in.) with some locations receiving less than 3 mm (0.12 in.) per year.

Mining operations produced exceptionally fine dust during the loading and unloading cycle. “Many of the particles measure under 10 microns, and it is not uncommon for the shovel and loading area to be engulfed with this dust,” explained Bob Hamann, engineering technical steward for Caterpillar’s electric rope shovels. “Since these particles are prone to float or blow around when disturbed, the dust accumulates to machine components that are not filtered, sealed or pressurized.”

Among other components, dust accumulated in the swing rack, and the fine particles increased abrasive wear to the gearing. Caterpillar and Cat dealer service technicians worked with the mine to analyze the issues found in the swing output pinions and swing rack assembly.

One possible fix could have been to simply increase the pinion and/or rack harness. “This would have only resulted in a minor improvement, and we were looking to add strength without adding unnecessary bulk,” recalled Haws.

Instead, the design team took a different approach. The fix focused on the output pinion on each gearcase. “We doubled the quantity of output pinions to develop our single input, dual output gearcase design,” continued Haws. “The system could still be powered by the same 377-kW (505-hp) electric motor, but this design significantly reduced the loading on each pinion to deliver an improved rack-and-pinion life of nearly 400%. Of course, other factors could influence life beyond the pinions.”

Along with experience, another advantage of this upgrade being made at the OEM level versus a component supplier is reach. “Since we made this swing rack design standard on our electric rope shovels, the success achieved in Chile now benefits the global mining community,” said Haws.

Back to Basics
How an electric rope shovel is operated and maintained tells the final story of swing rack longevity. The basics of proper maintenance, adhering to service schedules, using specified lubricants and replacing components with OEM parts designed to work as part of the entire machine’s system significantly contribute to increasing the time a machine spends in the bank.

One simple way to maintain the swing rack is through machine operating technique in the pit. Electric rope shovels rotate, on average, within a 120-degree arc for loading and unloading, so the same pinion teeth interact with the same gear teeth repeatedly. Schultz recommends frequent machine rotation to even swing rack wear. “Rotating the machine after each shift allows the lubricant to be dispersed to the entire swing rack and pairs new pinion/gear tooth combinations, which results in more gradual and even wear,” he said.

Harsh operating conditions – subzero temperatures, extreme heat, unintended misuse and significant quantities of dirt/fine particles – combine to play a role in the service life of a swing gear. This makes lubrication and using the right type critical. Using a recommended open gear lube, such as Caterpillar OGL specifically designed to prevent metal-to-metal contact under stalling loads, provides a tenacious film that adheres to the gear teeth in extreme conditions. OGL also includes high pressure additives formulated specifically for extreme loads and stalling conditions to protect against gear scuffing and wear.

With the demanding operating conditions swing racks face, it is normal for material to flow from the edges as the machine bears the full inertia of the system on the gear. Proper maintenance practices include trimming this metal flow from the tips and ends of the teeth. Any shavings must be removed from the teeth prior to putting the machine back into service.

Schultz offers one final observation regarding the swing rack. Often operators interpret the robust nature of the machine to mean the swing rack gearing will take anything and everything. “Swing gear systems are highly precise and are sensitive to poor maintenance practices. Inspections, lubrication and responsible operating techniques will make a difference in extending the life of the swing rack. The gear that is cared for will be there to work, both today and tomorrow.”