page_title: South African Class 19C 4-8-2
section_title: Rotary cam poppet valve gear
hierarchical_section_title: South African Class 19C 4-8-2 / Characteristics / Rotary cam poppet valve gear
caption_attribution_description: English: SAR Class 19C (4-8-2) Class 19C after a record test run, during which the locomotive achieved 67mph. A.G. Watson, SAR Chief Mechanical Engineer from 1929 to 1936 standing 6th from left with hat in hand, Dr Eric Manken, 1930's Railway Circle member 7th from left
context_page_description: The South African Railways Class 19C 4-8-2 of 1935 was a steam locomotive.
  In 1935, the South African Railways placed fifty Class 19C steam locomotives with a 4-8-2 Mountain type wheel arrangement in service. It was the first South African locomotive class to use rotary cam poppet valve gear and also the first to be built new with a Watson Standard boiler.
context_section_description: The rotary cam poppet valve gear consists of two admission valves and two exhaust valves per cylinder, operated by a camshaft arranged in a box fixed to each cylinder between the valve chambers. The cam boxes are self-contained and detachable, and replacement cam boxes can be fitted at running sheds within a few hours. The camshafts are driven from the return cranks on the driving wheels through worm wheels and universal propeller shafts, arranged so that the camshafts and the coupled wheels revolve at the same speed.
  There were originally eight cams for forward motion and three for reverse, which gave a range of cut-offs from 15% to 85% in forward gear. The hand reversing screw spindle carried a disk, suitably notched with a locking device to ensure that the cam rollers would be in the centre of the selected cams under working conditions. During drifting, the reversing gear is placed in the neutral position where the exhaust valves are held fully open. These conditions were ideal for drifting and superior to any Walschaerts valve gear in admitting steam to the steam chests, even with cylinders equipped with by-pass valves, snifting valves and drifting valves.
  In service, some trouble was experienced with transmission shaft joints and with worn cams and rollers. If the valves were set with the engine cold, there appeared to be some distortion under working conditions due to the high temperature of superheated steam which caused deterioration of the valve events. Drivers also reported that, when the engines were standing or moving at very slow speed, it was difficult to reverse since the cam rollers were not readily able to climb over the inclined surfaces leading from one cam to another. Trouble was also experienced with steam leaking past the tappet spindles into the cam boxes where it boiled up the oil and caused it to overflow from the cam boxes. This was overcome by serrating the collars of the spindle bushes to allow the steam to escape and prevent entry into the cam boxes.
  The photograph alongside shows a Class 19C loco­motive after a record speed test run during which the engine achieved a speed of 67 miles per hour (108 kilometres per hour). A.G. Watson is standing sixth from left in the group in front of the locomotive, with hat in hand.
  An official test was conducted in 1938 to compare the steam consumption of the poppet valve Class 19C with the piston valve Class 19D. The Class 19D performed slightly better. However, the valve travel of the Class 19D is 7 ¹⁄₂ inches (191 millimetres) compared to a valve travel of about 4 ³⁄₄ inches (121 millimetres) in all previous engines fitted with piston or slide valves. It was concluded that, had the valve travel of the Class 19D been retained at 4 ³⁄₄ inches (121 millimetres), the test would have shown an advantage for the poppet valves.
caption: Class 19C after a record speed test run
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