Within a relatively short period after servicing, the rotary phosphor bronze valve disc tended to seize. This made it difficult for
the driver of a Metro coach to operate.
Partial skimming of the surface was not acceptable as this created air leakage. The application of various lubricants did not work either.
It was suggested that a wear liner made from low friction, low wear Vesconite Hilube be fitted to the under surface of the rotary valve disc.
A prototype was made for test purposes, 134 mm (5.3")in diameter by 3 mm (0.12") thick. To fit the Vesconite Hilube, 3 mm (0.12") was skimmed off the working face of the phosphor bronze. Two shear pins together with cyanocrylate adhesive, Loctite 480, were used to locate the disk
Under the railways authorities supervision a well planned test was executed and recorded in which the equivalent of a three year period of operation was simulated over an actual test time of 176 hours.
The test proved that the use of the Hilube wear liner made possible the free operation of the valve at all times. There was no increase in rotational friction and no loss of vacuum.
The maximum wear measured after the test on the 3 mm (0.12") thick Vesconite Hilube disk was 0.2 mm (0.008").
||175 hrs 42 mins
The conclusion of the test was that the problem was completely solved using Vesconite Hilube. In addition, the modification was highly effective as it greatly increased the servicing intervals.