IIQ

MODERN STEAM PRACTICE.

ment to provide a separate valve to work expansively, allowing theslide-valve to move always in full gear: thus the valve faces areworn evenly. Some makers have introduced a cylinder having apiston and rod so connected to the link that the reverse movementis actuated by steam pressure; and where marine engines, suchas those in the Royal Navy , require an expeditious means of hand-ling, this plan has a decided advantage in being able to reversethe engine, or manoeuvre the screw propeller quickly, when the shipis in action.

There are a variety of arrangements for link motions applied tothe slide-valves of marine engines, but the arc of the link is describedthe same way in all cases, no matter whether the link is slotted out,or simply solid; when the valve and adjuncts are at half stroke, fromthe centre of the crank shaft to the slide-valve block, or centre ofthe pin on which the link vibrates, is the length of the radius that

describes the arc of the link. Someexamples of valve-gear have two valverods, with eccentric rods on the returnprinciple, one of the valve rods beingplaced above and the other below themain shaft of the engine. The rods areguided on the condenser with a longsliding bar, having snugs forged onto take the rods. The lifting lever is

Fig. 58.—Link Motion and Starting Gear for Marine Engines.

placed on the top of the condenser, having a rod passing down-wards, taking the reversing link at the centre and on the arc line;the lever shaft carrying a toothed sector, working into a worm-wheelplaced vertically, having the starting handle or wheel horizontal.In this arrangement the reversing lever makes a half revolution,consequently this modeof suspending the link is not well calculated