Free-fall test tower ensures cage safety for Inco's employees.

Free-fall test tower ensures cage safety for Inco's employees

At a repair shop tucked away at Inco's Creighton Mine complex workers and engineers have built a state-of-the-art testing facility to ensure the safety of miners while they travel to and from their underground work stations.

In 1989 Inco Ltd. installed a new, 55-foot free-fall test tower at its conveyance repair shop to replace an existing smaller test tower at another location.

Regulations contained in the Mining Act require that all mine cages used to transport people underground be equipped with safety catches. The safety catches or "dogs," as they are known within the industry, are designed to engage and stop a run-away mine cage in the event that the cable holding it is released or severed.

The dogs are designed to stop the cage in about nine feet after the cage is released.

The safety mechanisms must be tested on every cage prior to service and after any repairs are conducted. Drop tests are conducted on Inco's cages every three months. The cages are removed for a complete overhaul and further testing after every five years of service. The company conducts approximately five tests at the tower annually.

A free-fall test, which engages the safety catches during the cage's maximum rate of descent, can be performed in either the shaft headframe or in a free-fall test tower, as is done at Inco.

The 55-foot high tower was constructed by Inco because testing in a shaft is too risky, says Largo Albert, a senior hoist specialist at Inco.

Free fall tests performed in a mine shaft require extra safety precautions to minimize the potential damage to the mine hoist cables, steelwork, the cage and the shaft itself. Any accident occurring while a test is being performed in the shaft could result in lengthy downtime and a considerable loss of production.

Inco has actually been conducting free-fall safety tests on its cages away from the shaft since 1940.

Prior to 1989 the tests were conducted in the hoist room at the Creighton Number Five shaft. The company constructed the new tower to improve the test procedures and to allow workers to repair the cages in an area identical to a shaft environment.

The new tower is designed to accommodate mine cages weighing up to 60,000 pounds each (plus payload).

Test results are recorded and supplied to the Ministry of Northern Development and Mines.

At a test conducted at the tower in April specialists, including Albert and Richard McIvor, simulated a free-fall of one of the largest mine cages operated at Inco.

A 35-ton crane released its hold on the 44-person cage and dropped it nine feet before the dogs began slowing the conveyance. The safety mechanisms performed flawlessly, stopping the cage within the required distance and without damaging the timber shaft guides.

DEAD TIME

The time between the release of the cage and when the dogs engage is known to the specialists as "dead time" or reaction time.

The distance the cage travels within this dead time is measured by the tester to determine how well the safety mechanisms are functioning.

The typical reaction distance for a nine-foot free-fall is approximately 18 inches.

A monitor records the deceleration rate of the cage and the rate of change of deceleration during the test.

SHAFT GUIDES

Tests are conducted to determine the strength, grain, and moisture content of woods used for shaft guides, including B.C. fir, karriwood and MacMillan-Bloedel Parallam wood. Parallam wood is composed of bonded strands of B.C. fir.

According to past studies conducted at Inco, the moisture content and grain structure of wooden shaft guides will dramatically affect the outcome of a free-fall test.

A report prepared by the two Inco specialists states that because "wood absorbs water and has retained water in the plant cells, it is very difficult to control the moisture content."

At a series of experiments conducted at the Haileybury School of Mines in the early 1980s, researchers determined that the rate of deceleration was decreased by a factor of one gravity for each 45-per-cent increase in the moisture content of the wood.

However, the tests conducted at Inco since the construction of the new tower draw a different conclusion.

"The dry guides failed by splitting and offered much less resistance than the green (fresh) ones," Inco's report indicated.

Inco officials have decided not to discount the findings of the Haileybury study team, but rather to continue conducting experiments to establish with certainty the relationship between wood resistance factors and moisture content.

PHOTO : Inco employees watch as a mine cage ascends to the top of a 55-foot free-fall test tower.

PHOTO : Inco hoist specialist Keith Jones monitors a decelerometer. The device is used to measure and record the deceleration rate of a cage during a free-fall simulation.