While experimenting with a balanced valve it was
found that steam pressure alone was sufficient to hold a freely floating
'O' ring against the steam chest cover to seal off the required area. (see
drawing)
To assess the durability of a ring used in this manner it had
to be subjected to 'worst possible' conditions, ie high steam pressure
combined with high speed, and the engine needed to be modified in order
to cope with these extremes for an indefinite time.
Since the power the engine developed wasn't important, lap was
increased to give a very short cut-off and keep steam consumption within
the capacity of my boiler, whilst the port width was greatly increased
to permit high speed. Wick feed lubricators were provided to keep
the crankshaft journals and the cross-head happy and the small brass tank
with drip feed was fitted to feed the big-end via a banjo. (The thickened
rim of the disc attatched to the side of the crank web is actually a 'U'
shaped channel. Oil dripped into this is flung to the bottom of the 'U'
from where it is led, via a drilled hole, to the centre of the crank-pin.)
The little end of the con-rod was split and clamped by an adjusting screw
and the pin was case-hardened, an oil well was machined in the eccentric
strap, while balance weights were fixed to the crank webs.
Together with a lighter piston carrying a single cast iron ring
to Prof' Chaddock's design, these mods allowed the engine to run continuously
at 70PSI and 4000RPM developing 1/10HP. The lubricators being kept
topped up with a hypodermic needle through the breather holes in the caps.
It was found during these tests that the 'direct' exhaust had
little effect on performance below 3000RPM and a simple stepped disc fitted
to the standard 'D' valve to carry the ring, with a small bleed passage
between the cavity and the inside of the ring, would have been just as
effective at any normal speed. - see below.
