ALGOR, Inc
ALGOR, Inc
 
ALGOR, IncALGOR, Inc Click Here For Printer Friendly VersionALGOR, Inc
ALGOR, IncALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc ALGOR, Inc

SKI RESORT TRAGEDY PROMPTS CHAIRLIFT INVESTIGATION

An engineering consulting firm uses Algor FEA software to design a safer chairlift grip before busy ski season.


The Silver Star Mountain Resort, shown here, Whistler Mountain and Lake Louise ski areas in the British Columbia/Alberta region of Canada employed Pol-X West, Inc., Carson City, Nevada, to redesign a failed chairlift grip that led to a fatal accident. Les Okreglak, president and chief engineer of Pol-X West, used FEA software from ALGOR, Inc. to reduce the design cycle of the new grip to less than six months. Photograph Courtesy of Silver Star Mountain Resort.

Most skiers and snowboarders rate ski resorts by their average powder base and the overall challenge and number of slopes. Few likely 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. In spite of rigorous standards, sometimes accidents can occur.

On December 23, 1996, Whistler Mountain in British Columbia, Canada was the site of the worst chairlift accident in the province’s history. Around 3 p.m., while skiers were riding downhill, four chairs detached from the cable and fell 30 feet. Several other chairs slid 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 involved in the Whistler Mountain accident. This 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 required that ski resorts discontinue use of all lifts that employed the grips.


The failed grip component was part of a detachable-style chairlift that enables passengers to board and unboard the chairs at a comfortable pace while maintaining a constant cable speed. 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 decelerates to allow passengers to unload before accelerating again for reattachment to the cable. Photographs Courtesy of Silver Star Mountain Resort.

Whistler Mountain, and two other ski areas in the B.C./Alberta region, Silver Star and Lake Louise, joined together to find a replacement for the flawed 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 loss in revenue that could extend through the busy Holiday ski season. They enlisted the services of Pol-X West, Inc., an engineering consulting firm based in Carson City, Nevada who used Finite Element Analysis (FEA) software from ALGOR, Inc. to design and test a reliable alternative to the failed grip.

"With only six months to design, test, manufacture and implement the replacement grips, we relied heavily on ALGOR’s FEA software to develop and analyze the new part design," said Les M. Okreglak, P.E., president and principal engineer for Pol-X West. "ALGOR software helped us to ensure that the extensive physical testing required by the B.C. Ministry would be limited to just one prototype."

Okreglak studied both the operation of the existing chairlift systems and the Ministry’s conclusions about the design flaws of the original grips before beginning his redesign.

The purpose of a detachable-style chairlift is to enable passengers to board and unboard the chairs at a comfortable pace while maintaining a constant cable speed so passengers quickly reach the top of the slope. 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. According to Okreglak, 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 decelerates the chair to allow passengers to load or unload. 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 new grip design eliminates dependence on gravity to secure the grip to the cable, which contributed to the failure of the original design, according to the British Columbia Ministry of Municipal Affairs. A pair of helical springs exerts the entire gripping force and prevents slippage. Photograph Courtesy of Silver Star Mountain Resort.

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 Okreglak had to adhere.

The grip involved on 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 when 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 created when the cable made sudden stops to accommodate passengers who had difficulty boarding the lift at the loading station, according to Okreglak.

"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," Okreglak said.

Okreglak and his firm were challenged to design a grip that would eliminate the dependence on gravity to secure the grip to the cable thereby removing an additional source of grip slippage resistance. In addition, he needed to eliminate use of the metal insert that was prone to strain. The firm first created a basic 2-D drawing using AutoCAD to create a basic plan of their 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 270of the 1-7/8" 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. 


Okreglak used SolidWorks to create 3-D solid models of the fixed and mobile jaws of the new grip design and then transferred the files via IGES file format to ALGOR for analysis.

The firm then created 3-D models for the fixed and mobile jaws using SolidWorks, which then transferred via IGES files to ALGOR Finite Element Analysis software for linear stress analysis. Okreglak used ALGOR’s automatic meshing tools to create a surface mesh, then enhanced the mesh around small holes in the fixed jaw model using Merlin Meshing Technology. He then used ALGOR’s Hexagen, an automatic solid mesh engine, to create hybrid meshes of both brick and tetrahedral solid elements.

Okreglak analyzed the fixed and mobile portions of the grip 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, Okreglak factored material properties at -50F conditions. 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.


In the first of two ALGOR linear static stress analyses, Okreglak analyzed the stresses on the components under normal operation when the grip is attached to the cable. Okreglak accounted for the effect -50 F temperatures would have on the performance of casting steel used for the jaws in the setup of the analyses.

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. Okreglak used ALGOR’s built-in visualization tools to view the stress results using a von Mises display.

"Our analysis confirmed that the design was well within the allowable limit of 1/3 of the material yield point. We were then able to build a prototype based on ALGOR’s analysis," said Okreglak.


Okerglak’s second stress analysis studied the jaws when they were open for transition to the loading track, where the force of the springs was the greatest. Stresses for both results were within the allowable range of one-third the material yield point.

Pol-X West built the prototype using A148 casting steel and subjected it to the required physical tests regarding 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 strain gauge analysis matched closely to ALGOR’s FEA results," Okreglak said. "This physical test made me feel confident in the performance of the software and the design."

The fatigue analyses consisted of in-plant trials at Bacom Donaldson, a third-party engineering consulting firm based in Vancouver, British Columbia.

The grip was attached to a cycling machine that opened and closed the grip 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 5,000,000 endurance cycles. This final test, which checked the grip’s slippage force, was performed in the plant with the unit attached to a 1-7/8" 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 cable that is 6% smaller to 10% larger than the standard 1-7/8" diameter.


Stringent physical prototype testing was mandated by the Ministry of Municipal Affairs. Here the grip’s plucking force is tested in an in-plant trial.

"The grip performed as expected in all the tests and complied with the Ministry of Municipal Affairs’ guidelines. With the use of ALGOR’s FEA software, we were able to confidently design a single prototype that tested well, which enabled us to move quickly to the manufacturing and implementation phases of the project," Okreglak said.

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, who specializes in metallurgy, inspected every finished product to ensure part integrity.

"This grip is solid. With the tough design criteria, rigorous testing in manufacturing and scrutiny by a regulatory body, we were presented a situation that ensured a very safe product while utilizing the existing lift equipment and minimizing the resorts’ downtime." Okreglak concluded. "The lifts installed with the new grips have since operated successfully, without incident."



Copyright 2011 Autodesk, Inc. All rights reserved.   Privacy Policy Legal Notices & Trademarks Report Piracy