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Getting a Firmer Grip PUBLIC ACCESS

After a Ski Resort Tragedy, Engineers Design a Safer Connector Between the Cable and Chair on the Lift.

[+] Author Notes

Les M. Okreglak is president and principal engineer for Polx West, an engineering consulting firm in Carson City, Nev.

Mechanical Engineering 122(02), 74-76 (Feb 01, 2000) (3 pages) doi:10.1115/1.2000-FEB-7

This article explains joint engineering efforts of engineers in designing a safer connector between the cable and chair on the lift. Whistler Mountain, along with two other ski areas in the British Columbia/Alberta region, Silver Star and Lake Louise, joined to find a replacement for the grips that would fit the existing chairlift structure and could be implemented quickly. The new grip design eliminates dependence on gravity to secure the grip to the cable on the ski lift. Helical springs exert the entire gripping force and prevent slippage. Linear static stress analyses were performed under two specific conditions: when the jaws were closed and attached to the cable, and when the jaws were open for transition to the loading track, where the force of the springs was the greatest. Built-in visualization tools in the finite element analysis (FEA) software were used to view the stress results with a von Mises display. According to the conclusive results of the testing, the grip was solid. With the tough design criteria, rigorous testing in manufacturing, and scrutiny by a regulatory body, Pol-X West was presented with a situation that ensured a very safe product while using the existing lift equipment and minimizing the resorts’ downtime. The lifts installed with the new grips have since operated successfully, without incident.

The Silver Star Mountain Resort (shown here), Whistler Mountain, and Lake Louise ski areas in the British Co-lumbia/Alberta region of Canada hired Pol-X West to redesign a failed chairlift grip that led to a fatal accident.

Grahic Jump LocationThe Silver Star Mountain Resort (shown here), Whistler Mountain, and Lake Louise ski areas in the British Co-lumbia/Alberta region of Canada hired Pol-X West to redesign a failed chairlift grip that led to a fatal accident.

Most skiers and Snowboarders rate ski resorts by their average powder base and the overall challenge and number of slopes. Few probably even consider the safety of chairlifts at their favorite mountains. Luckily, ski resorts and governing authorities perform regular maintenance and inspection of chairlifts to ensure passenger safety, but in spite of rigorous standards, sometimes accidents can occur.

Les M. Okreglak is president and principal engineer for Pol-X West, an engineering consulting firm in Carson City, Ne v.

On Dec. 23, 1996, Whistler Mountain in British Columbia, Canada, was the site of the worst chairlift accident in the provinces history. Around 3 p.m., while skiers were riding downhill, four chairs detached from the cable and fell 30 feet. Several other chairs shd down the cable and collided with chairs in front. Two passengers died and 10 others were hospitalized.

Investigators from British Columbia’s Ministry of Municipal Affairs, Engineering and Inspection Branch, determined that the accident was caused by the grips that attach the chairs to the transport cable. The grips relied on gravity to maintain contact with the cable.

Shortly thereafter, during a routine inspection of a lift at another ski resort, safety officials from the Ministry of Municipal Affairs discovered cracking in a grip similar to the type of grip that was involved in the Whistler Mountain accident. Safety inspection revealed problems with other detachable lifts, and ski areas throughout North America began replacing them. Twenty-six lifts have been replaced or retrofitted since the accident and several more are under scrutiny by safety officials.

The cracking occurred on a metal insert used to maintain contact with the cable. Such cracking was not found in the grip involved in the Whistler Mountain accident because that design did not include the insert.

The Ministry of Municipal Affairs determined that both the failed grip and the grip with a steel insert were unsafe, and immediately required that ski resorts discontinue use of all lifts that employed the grips.

Whistler Mountain, along with two other ski areas in the British Columbia/Alberta region, Silver Star and Lake Louise, joined to find a replacement for the grips that would fit the existing chairlift structure and could be implemented quickly. The chairlifts had to be operational before the next ski season the following November, or the resorts risked a drop in revenue if they lost part of the busy holiday ski season. To solve the problem, the resorts enlisted the services of Pol-X West Inc., an engineering consulting firm based in Carson City, Nev., to design and test a reliable alternative to the failed grip.

Pol-X West studied both the operation of the existing chairlift systems and the Ministry’s conclusions about the design flaws of the original grips before beginning a redesign.

The purpose of a detachable-style chairlift is to enable passengers to board and dismount the chairs at a comfortable pace while maintaining a constant cable speed, so passengers reach the top of the slope quickly. With this system, the cable stops only for emergencies, such as when a skier falls or misses the chair at the loading or unloading stations.

Each chair is attached to the cable via a detachable grip. The cable travels on a pulley system at a rate of 1,000 feet per minute. As the chair approaches the passenger loading station, the grip releases from the cable and transfers to a track, which slows the chair to let passengers get on or off. The chair assembly then accelerates along the track to the speed of the cable at which time the grip reattaches to the moving cable.

The Ministry of Municipal Affairs identified many factors that could lead to grip failure and developed a set of criteria for the new design to which the engineering firm had to adhere.

In the first of two linear static stress analyses that were performed, the stresses on the components were assessed under normal operation when the grip is attached to the cable.

Grahic Jump LocationIn the first of two linear static stress analyses that were performed, the stresses on the components were assessed under normal operation when the grip is attached to the cable.

The new grip design eliminates dependence on gravity to secure the grip to the cable on the ski lift. Helical springs exert the entire gripping force and prevent slippage.

Grahic Jump LocationThe new grip design eliminates dependence on gravity to secure the grip to the cable on the ski lift. Helical springs exert the entire gripping force and prevent slippage.

The grip involved in the Whistler Mountain accident relied heavily on gravity to grasp the cable without slippage. Investigators discovered that the catastrophic failure of the gravity-assisted grips occurred when the lift stopped suddenly, causing the cable to bounce while creating a sudden impact load on the grips. The bouncing movement disrupted the gravitational force that contributed to the steadfastness of the grips on the cable.

In addition, the second flawed design featured a steel insert that experienced cyclical loading when speeds of the chair and cable differed at the instant that the grip reattached to the cable. This cyclical loading caused cracks to develop at the sharp corner of the insert. The cracking increased due to impact loading that was created when the cable made sudden stops to accommodate passengers who experienced difficulty boarding the lift at the loading station.

The impact load placed additional strain on the weakened metal inserts of the grips, which, in combination with a disruption in the gravitational force, could result in grip failure as well.

The firm was challenged to design a grip that would eliminate the dependence on gravity to secure it to the cable, thereby removing an additional source of slippage resistance. In addition, the use of the metal insert that was prone to strain had to be eliminated.

Engineers first created a basic two dimensional drawing using AutoCAD from AutoDesk of San Rafael, Calif., to create a basic plan of its design that could later be used in manufacturing. They designed the clamp as one unit that included two symmetrical portions, including one mobile jaw and one static jaw that rely on a pair of parallel helical springs to exert the clamping force. The clamp was designed to surround 270 degrees of the 17/8-inch-diameter wire cable, so that it could pass the pulleys at both ends of the cable system and release easily from the cable at the loading and unloading platforms.

The firm then created 3-D models for the fixed and mobile jaws, using SolidWorks from SolidWorks Corp. of Concord, Mass. The models were transferred via IGES files to Pittsburgh-based Algor’s finite element analysis software for linear stress analysis. Its automatic meshing tools were used to create a surface mesh. Then, the mesh around small holes in the fixed jaw model was enhanced using Algor’s Merlin Meshing Technology. Next, engineers used Algor’s Hexagen, an automatic solid mesh engine, to create hybrid meshes of both brick and tetrahedral solid elements.

The fixed and mobile portions of the grip were analyzed independently regarding the compressive force of the springs. In addition, each portion was evaluated concerning the load of the cable against the point where it meets the grip. Since the cold temperatures of the mountains would affect the performance of the metal components, specifically A148 casting steel, material properties were factored at -50°F. Boundary conditions were applied at the points where the jaws attached to cable and where the top portion of the mobile jaw would join with the hanger portion of the chair.

Linear static stress analyses were performed under two specific conditions: when the jaws were closed and attached to the cable, and when the jaws were open for transition to the loading track, where the force of the springs was the greatest. Built-in visualization tools in the FEA software were used to view the stress results with a von Mises display.

The analysis confirmed that the design was well within the allowable limit of one-third of the material yield point. A prototype was then built based on the analysis.

Pol-X West built the prototype using A148 casting steel and subjected it to the required physical tests for strain, fatigue, and slippage. The strain gauge recordings were performed in a field test that included an instrumented grip that was assembled on the original chairlift mechanism.

The fatigue analyses consisted of in-plant trials at Ba-com Donaldson, a third-party engineering consulting firm based in Vancouver, B.C.

The grip was attached to a cycling machine that opened and closed it 500,000 times. The second trial included attaching the grip on an assembled hanger and chair with a load equivalent to four passengers. This test put the grip through 5 million endurance cycles. This final test, which checked the grip’s slippage force, was performed in the plant with the unit attached to a 1⅞-inch-diameter wire cable installed on a test stand. The trial tested the grip in normal operation, as well as under conditions simulating the grip’s performance if it lost the use of one or more springs. In addition, the grip was tested for use with cables ranging from 6 percent smaller to 10 percent larger than the standard l⅞-inch-diameter.

Chairs attached to the cable (above) travel at a rate of 1,000 feet per minute. As the chairs approach the loading station, a detachable grip releases from the cable and transfers to a track (inset), which slows down to let passengers get off.

Grahic Jump LocationChairs attached to the cable (above) travel at a rate of 1,000 feet per minute. As the chairs approach the loading station, a detachable grip releases from the cable and transfers to a track (inset), which slows down to let passengers get off.

The grip performed as expected in all the tests and complied with the Ministry of Municipal Affairs’ guidelines. The engineers were able to confidently design a single prototype that tested well, which enabled them to move on quickly to the manufacturing and implementation phases of the project.

The scrutiny of the product did not end with the final testing of the prototype. Since the component was critical to the safety of the entire lift, the manufacturing process was also held to rigid standards imposed by the Ministry of Municipal Affairs. Each of the 1,000 manufactured grips was X-rayed for material flaws while still in the casting molds. Bacom Donaldson, which specializes in metallurgy, inspected every finished product to ensure part integrity.

According to the final results of the testing, the grip was solid. With the tough design criteria, rigorous testing in manufacturing, and scrutiny by a regulatory body, Pol-X West was presented with a situation that ensured a very safe product while using the existing lift equipment and minimizing the resorts’ downtime. The lifts installed with the new grips have since operated successfully, without incident.

Copyright © 2000 by ASME
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