Some Results of VALVE-GEAR RESEARCH As Applied to Diesel Engines 420112
EMPHASIZING first that the entire valve gear from the camshaft to the valve seat must be considered as a whole in designing a poppet-valve engine, Mr. Voorhies proceeds in this paper to discuss the mechanical design of each component of the valve gear. Considerable space in this discussion is devoted to the design, operation, and advantages of hydraulic valve lifters.
A summary of the various points set forth in this paper includes the following recommendations:
Camshaft deflection should be held to not more than 0.002 to 0.003 in. Accelerations should be as high as possible on both the opening and closing sides; the opening side acceleration may range up to three times that of the closing side. Cam followers should not be overstressed by the use of too small a nose radius. Mushroom-type followers may have a cast-iron face when operating on cams of any material. Steel followers can be used successfully only on cast-iron cams provided that the cam is properly designed; the section of the cast-iron cam should be about 30% greater than that of a steel cam for equal rigidity.
The pushrod should be of tubular welded-end construction. The rocker arm should be of the lightest construction that will not deflect excessively under maximum load; a rocker ratio of 1.3 to 1.6 is best, considering all factors. The use of a roller is very good practice in large engines where loads are high. Rocker-arm geometry should be such that the movement of the valve is about one-third above and two-thirds below the centerline.
The valve should have a hard tip, high hot strength, and stainless properties. For relatively high speeds a hollow-drilled valve is desirable for lightness. If this design causes excessive heating, sodium cooling should be used.
Valve stem guides should be extended as high as possible on the valve to permit ample clearance without excessive valve cocking.
The spring should have a safety margin of at least 30% at the point of reversal, and should have the highest possible frequency without overstressing the material.
The valve seat insert should be of ample section, maximum rigidity, and of a material which provides maximum resistance to distortion.