Have you ever found yourself holding your breath while watching a crane position large steel beams atop a skyscraper construction project? You're not alone. Though catastrophic accidents are typically caused by wind and operator errors, faulty components can also cause disaster. As with any industrial process involving moving machinery, even when part failures don't cause accidents, they can lead to lost productivity and particularly expensive downtime. For this reason, heavy hoisting equipment — such as cranes and hoists used in construction, mining, and material handling — requires extra-durable components.
Here we explore what a few manufacturers are doing to guarantee safety and reliability in challenging hoisting applications.
Double-acting bearing seals fit the bill
Marine environments often call for rugged and robust components that can withstand corrosive seawater and the extreme forces of the open ocean. One company accustomed to meeting these maritime demands is Dutch firm Heerema Marine Contractors (HMC), a leading supplier to the offshore oil and gas industry. Its largest construction vessel — the Thialf — is 661 ft long by 290 ft wide. Working together, the vessel's dual cranes lift objects weighing up to 14,200 tons, more than any other vessel of its type. The Thialf also features a dynamic positioning system that holds it stationary without anchoring as it installs subsea structures in deep waters.
Since the giant semi-submersible barge made its debut in 1986, the Thialf has installed many of the deepwater oil and gas production platforms found in the North Sea and around the globe. As the energy industry continues to develop oil and gas fields in water much too deep for bottom-based platforms, the Thialf is used to install equipment such as subsea production facilities or moorings (directly onto the seabed) for floating platforms. Lifting and hoisting operations now take place in water depths well beyond 6,500 ft.
Underwater operations mean that the hook assemblies, the working ends of the cranes, are exposed to seawater. Measuring almost 20 ft tall, each four-prong hook assembly rotates on a massive roller bearing packed with lubricating grease. Seals from Trelleborg Sealing Solutions, Fort Wayne, Ind., are used to protect the bearing from seawater corrosion, while allowing smooth rotation of the hook shaft.
“Previous seals allowed seawater to seep in when the hook assemblies were used underwater,” explains Jurgen de Jong, HMC's crane superintendent.
In 1999, HMC and Trelleborg worked together to develop a better solution. Armed with specifications provided by HMC, Trelleborg's engineers proposed using a double-acting seal to prevent fluid passing in either direction, to stop seawater going into the bearing or grease from the bearing leaking into the ocean. The team selected the Turcon Roto Glyd Ring, which is widely used in other industries. However, an unusually large size was required — a seal with a diameter of nearly 6 ft. Over the next 10 years, the Thialf continued to perform lifts, and the bearings in the hook assemblies provided trouble-free operation. When the vessel returned to Rotterdam for maintenance in 2010, seals were evaluated for wear.
Trelleborg was called in to inspect and replace the seals as part of preventive maintenance, a job requiring three days for each seal. The results were surprising: The inspection team had never seen seals in such good condition after 10 years in a harsh working environment. Very little deterioration was evident and the bearing and internals of the block were in superior condition, because the seals did their job. Only one question remains for the seal team: How much longer might the original seals have lasted?
For more information, visit tss.trelleborg.com/us.
Brawny brakes suit gold mine hoists
With the price of gold reaching record highs in recent months, it's no wonder that prospecting for gold is making a comeback. While individuals may hope to strike it rich by panning for gold, sophisticated mining operations involving heavy equipment are much more likely to find success. One of these large-scale projects is a gold mine in northern Canada projected to process 7,000 tons of throughput per day, producing 600,000 ounces of gold per year over a 15-year lifecycle.
Among the tools used to extract gold from the earth are heavy-duty mine hoists. Beyond substantial lifting capacity, these specialized hoists also require failsafe braking systems. After evaluating several options, one manufacturer of mine hoists recently specified VMS3/SPS brakes from Twiflex Limited, a division of Altra Industrial Motion, Quincy, Mass. The order called for 10 VMS3/SPS calipers for installation on a 20-ft-diameter, double-drum, single-clutch mine hoist with two brake discs; one disc is positioned on the fixed drum, with the other on the clutched drum. The fixed drum requires six calipers, while the clutched drum requires four. Each brake produces 240 kN braking force at a 2.5-mm airgap, with more than two million cycles of fatigue life available at this rating.
The mine-ready design of the VMS3/SPS features a fortified housing that integrates additional springs for improved braking force. Totally sealed to provide superior corrosion and dust protection, the brake design also offers tamper-proof pad and airgap adjustment, external pad retraction, onsite torque adjustment, and a “Park-Off” feature that enables fast seal changes without special tools. For convenience, seals can be changed from the rear without removing the brake from its mounting.
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The VMS3/SPS also integrates small pistons for quicker reaction times, combined with an improved drainage system and reduced retraction pressure of 137 bar at this rating. Braking force can be easily changed (up to 275 kN at 3 mm airgap) by adding or removing shims located behind the rear cover. Sensors also can be supplied to work in conjunction with a PLC to signal whether the brake is on or off, and to alert operators about pad wear. For more information, visit twiflex.com.
Specialized inspections help prevent catastrophes
It's no surprise that undetected defects in a hoist's load bearing or safety-related components can lead to catastrophic failure. Most of today's compliance and preventive-maintenance inspections rely on visual observations and measurements alone. However, certain defects and deficiencies are not visible to the naked eye, while others may occur within components that are not normally taken apart for inspection.
One company that has developed in-depth inspection methods for cranes and their various parts is Konecranes Inc., Springfield, Ohio. The company's “Critical Components Assessment” uses non-destructive testing (NDT) and other advanced technologies to uncover defects and deficiencies in critical parts. At a minimum, Konecranes recommends a thorough assessment in the following situations: Older equipment with more than 10 years of service, process duty applications, equipment being used beyond the original design duty, machinery that may have been overloaded, or whenever it is recommended by the OEM.
To get an idea of what such an assessment covers with regard to crane life, following is a summary of various aspects of Konecranes' inspection process:
Wire rope assessment — A reliable method to ensure wire ropes are in safe working condition. Thorough rope inspection (what Koncranes calls “RopeQ”) is recommended in applications with a high safety risk, applications with long wire ropes, or where certain rope constructions have a known risk of inner core damage.
Gear case assessment — While cranes themselves are subjected to routine safety inspections, internal gear case components are often ignored. Konecranes recommends — and offers — an in-depth and detailed visual inspection and, where appropriate, NDT evaluation of the internal components of every crane gear case as part of routine maintenance.
Mechanical load brake assessment — A load brake that is out of adjustment operates improperly and may provide a false sense of security. Mechanical load brakes operate through friction, thus generating heat and contaminants in the hoisting gear case. Due to friction material wear, brakes require periodic inspection and adjustment in accordance with OEM instructions. A service life analysis can estimate the remaining life of critical components, such as the mechanical load brake, to avoid unsafe conditions and sudden failures.
Hoist coupling assessment — Failure of a hoist coupling may lead to a load drop scenario and subsequent reliance on secondary braking, such as regenerative braking or mechanical load brakes. However, in this case, regenerative braking isn't available because the motor normally providing retarding torque is no longer mechanically connected. If the hoist gear case is equipped with a mechanical load brake, it must be properly adjusted to control the load's descent speed. A thorough alignment, assembly, and lubrication inspection can help ensure proper coupling life.
Hook assessment — A crane's bottom block assembly is made of many critical components for which proper function is crucial to safe operation. These components are subjected to loads and stresses during each hoisting cycle. NDT techniques such as dye penetrant, magnetic particle and magnetic rubber analysis, visual examination, and measurements are valuable during a hook inspection. Critical components should be examined, including the hook shank, where most fatigue failures originate.
Thermal imaging assessment — Abnormal temperatures and heat patterns are a warning sign of potential problems: They can point to poor electrical connections, worn contacts, worn insulation, overloaded motors due to electrical imbalance or failing bearings, malfunctioning cooling equipment, or equipment being used beyond its intended duty. A thermal imaging camera used on a crane's electrical or mechanical components can uncover maintenance issues before a sudden failure or safety hazard occurs.
Motor assessment — Motor failure can lead to unexpected and costly downtime. Motor insulation assessment tests the motor insulation value by attaching test leads to the motor without requiring motor disassembly. Vibration analysis ensures that mechanical components, such as bearings, are in working order and are not on the verge of failure.
For more information, visitkonecranes.com.
Need a lift?
In many plant floor environments, overhead hoisting isn't practical for moving products or equipment from one level to another. Instead, lifting mechanisms are more appropriate. Why? While it's relatively easy to move heavy or bulky goods and materials across a floor with a pallet jack, cart, or forklift, it's a bit more challenging to move them up to an elevated level. That's why some companies are turning to vertical reciprocating conveyors (VRCs) to efficiently transfer items between levels.
VRCs operate much like an elevator in that a platform raises and lowers material — though not people. Depending on the load, some VRCs use a single-cylinder hydraulic system that prevents twisting during operation, while eliminating all moving hoses, cables, and chains. The advantage is that this design has fewer moving components to enable easier installation, reduced maintenance, and longer service life than other designs.
To minimize hazards, VRCs are equipped with myriad safety gates and features. Safety interlocks prevent the VRC from moving unless the gates are fully closed, and the gates open only when the carriage reaches a designated level. In addition, mechanical stops ensure positive leveling with the upper deck, redundant overload protection prevents the carriage from lifting if loaded to more than 120% of its rated capacity, and velocity fuses safely control descent.
Suitable applications for VRCs include distribution centers, manufacturing and assembly plants, pharmaceutical operations, and other material handling, commercial, and consumer industries. For more information, visit wildeck.com.
Trelleborg Sealing Solutions
(607) 733-7121 (U.S. distributor)