RSS Feeds
The Case of the Molten Menace
Kenneth Russell, Professor Emeritus, MIT -- Design News, November 5, 2007
There are no minor accidents in foundries. Any mishap involving molten metal is serious. In the present case, a young man was charged with operating a so-called “bull ladle” that holds about a ton of molten iron. This ladle receives its charge from the melting crucible and travels around on a system of overhead tracks to supply smaller ladles, which in turn supply the molds. The bull ladle is raised or lowered by a hoist in which a wire rope is threaded around a number of sheaves, or pulleys.
The Scene of the Crime
One day a young worker had filled the bull ladle and was replacing the top. Suddenly, the full ladle went into free-fall. The ladle fell about 3 ft which would only have caused a few heart palpitations had the floor been flat. Unfortunately, the ladle dropped partly into a slag pit and tipped, releasing a wave of molten iron. The operator took to his heels but ran full tilt into an electric truck. The molten metal overtook his legs and the intense heat caused his clothing to catch fire. The accident caused third-degree burns over more than half his body. The poor devil also broke an arm in his collision with the truck.
The operator rehabbed fairly well, but refused to take another job working around hot metal. Naturally, he sued. I was retained as consulting metallurgist by the distributor of the hoist.
The Investigation
The foundry hoped to be found not at fault so as to escape the expense of workman's compensation. The injured worker also wanted the foundry off the hook to escape the workman's comp limitations on financial recovery. There were thus two sets of fingers pointing at the manufacturer and distributor of the hoist.
The cause of the accident was obvious. The upper sheave or pulley had come free of the load block, releasing the ladle. The 1-inch diameter steel sheave pin had ground almost all the way through and finally sheared the rest of the way, thereby releasing the sheave and full ladle.
The cast iron sheave was fitted with an oil-impregnated porous bronze bushing that turned against the steel sheave pin. The gritty, dusty environment in the foundry would, of course, cause wear and the sheave assembly was supposed to be inspected weekly. Company records showed the bushings were replaced about every nine months. The hoist was one of several in the foundry that had been used without incident for 20 years.
The Smoking Gun
The subject sheave wound up fused into a puddle of iron on the foundry floor. There was no sign of a bushing. There were signs of excessive wear on the inner diameter of the sheave where the bushing should have been. It was clear the sheave had been operated a long time with a worn-out bushing or possibly no bushing at all. The foundry claimed to have inspected the sheave a week before the accident.
The claim was made that the porous bronze was not suitable for a hoist operating in the hot, dusty foundry environment. Other experts in the case proposed the use of roller or ball bearings.
I have a lot of experience operating farm machinery in a hot, gritty environment similar to that in the foundry. The weekly inspection and nine-month bushing lifetime made sense to me. I did not like the idea of roller or ball bearings that would have to be lubricated probably twice a shift. Lubrication would have required someone climbing a ladder and operating a grease gun in the miserable conditions above the various ladles and crucibles. It was not a job I would care for. In addition, such lubrication causes grease to be extruded from the bearings. These dollops of grease would tend to drip into the bull ladle with possibly nasty results.
The problem in the foundry was lack of what the American military calls PM, Preventive Maintenance. The sheave and bushing had clearly not been inspected for a very long time. My experience in dry land farming helped me estimate that the hoist had been operated for months with an absent or badly worn bushing. Obviously, the specified inspections had not been made. The foundry was clearly remiss.
I defended my findings in a lengthy and hostile deposition. My client settled this case before trial on what I believe were favorable terms.
| Author Information |
| Ken Russell (kenruss@mit.edu) is professor emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files. |
Talkback
-
Ok and how does the molton menace get cleaned up/removed from the floor?
- 2007-11-17 18:44:21
DN's Resource Center Get Free Information, Made Easy
-
2008 Adhesives Sourcebook - Loctite Products for Design, Assembly, Manufacturing and Maintenance
Henkel Corporation | Design Guide
Request Item -
Solutions for the Appliance Industry
Henkel Corporation | Design Guide
Request Item
