The cause is the failure of the hydraulic cylinder. This failure results from the fact that it is placed beneath the elevator and buried in the soil. Electrolytic action causes the steel cylinder to corrode and burst. When this happens the column of oil supporting the elevator is expelled into the surrounding earth and the elevator drops - in more or less free-fall.

 

Catastrophic failure of the buried cylinder has been, in all of the accidents to date, related to the failure of the bulkhead at the bottom of the cylinder. This bulkhead is a circular steel plate - a sort of plug - which is welded inside the cylinder to make it water (or in this case oil) tight. The electrolytic action seems to attack this weld in preference to the other parts of the cylinder. When the weld has been eaten away the bulkhead "pops" out - something like a champagne cork.

 

The first major accident of this type occurred in the late sixties. In the late seventies the various safety codes were revised to require a double bulkhead. This design has the same bulkhead as before but in addition a second bulkhead placed slightly higher up in the cylinder. In this second bulkhead a small hole is drilled so that if the first bulkhead fails the elevator will move down very slowly. This preserves the safety of the installation and at the same time gives a warning that something is wrong.

 

This design seems to deal with the safety problem. The side of the cylinder will still be attacked by corrosion but for whatever reasons there has been no example of a catastrophic side wall failure. The typical failure is characterized by pin holes in the steel wall with consequent slow loss of oil. There is therefore enough warning of trouble and potential hazard to allow corrective action to be taken.

 

More recently, the codes have required that buried cylinders be installed within plastic pipes so as to ensure that the cylinder will not rust.

 

Given the difficulties attendant upon the use of buried cylinders the trend today is towards hydraulic designs with exposed cylinders. In particular, the "holeless" hydraulic and the "roped" hydraulic are becoming more popular. These designs avoid the problem altogether since there is no buried cylinder.