Problems
In practice, the arrangement had a number of disadvantages. Cracks developed in the oil bath casing due to incorrect welding procedure. Condensation caused corrosion, and oil leaked out through inadequate seals, causing wheelslip and fire hazards. Valve timing was highly unpredictable; this has been attributed to chain stretch, although Bulleid claimed that it was allowed for. The chains were replaced as part of a valve and piston examination at around 30,000 to 36,000 miles, by which time they were considerably worn. This wear, in the chain pins and sprockets, could result in an elongation of the chains by as much as 6 inches. Bulleid asserted that a slack of 3 inches would be absorbed by the chain coming under load, with the remaining valve irregularity corrected by adjusting the cut-off.
Chain-stretch is widely considered to be the cause of the gear's inefficiencies, but another cause may have been geometrical due to the sequential proportions of levers, especially the multiplying rockers that transmitted the limited movement of the "miniaturised" valve gear to the union link driving the long travel piston valve. These rockers alone would have made accurate valve timing difficult to achieve, giving rise to stress in the drive mechanism, a symptom of which would be an increase in the likelihood of chain-stretch. The maximum throw of the expansion links was 2.25 inches (5.7 cm). To provide the maximum valve travel of 6 inches (15.2 cm) the rocker arms multiplied the motion of the auxiliary crankshaft in the ratio 8:3. The effects of wear in the pin joints, also multiplied by the same ratio, caused valve openings to vary between cylinders even if the driver maintained the same cut-off.
A further complication was the unpredictable behaviour of the Eastleigh type of steam reverser employed. If this reverser misbehaved under the fluctuating pressures present in the steam chest, the locomotive seemed to have a will of its own. For example, if the reverser dropped into full gear, the slackness in the motion would make the valves overrun their designed full travel. This would result in the locomotive taking off like an unleashed race horse, high coal consumption, throwing the fire out of the chimney and running the risk of a violent high-speed slipping.
These problems combined contributed to the gear's eventual replacement by three separate sets of Walschaerts gear mounted in the conventional manner.
Read more about this topic: Bulleid Chain-driven Valve Gear
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