Durability in Inserted Valve Seatings
Page 56
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I T does not seem so very long ago that mileages between valve grinding operations were something around the 5,000 mark. Since the Harold Andrews Grinding Co., Ltd., has, however, introduced special Stellite-faced renewable valve seatings the distance likely to be covered between valve overhauls is nearer 100,000 miles.
The renewable valve seats are of a special type, ease of fitting and positive lock being the two outstanding features. Before dealing with the actual construction of the device mention might be made of the extreme hardness of the facing material—Stellite. In its refined state it is impossible to machine it, so that once the seating has been " Stellited " it is necessary to resort to grinding for all subsequent operations.
The drawing reproduced gives an idea of the method of construction. A ring of centrifugally cast iron is first machined to suitable dimensions for fitting to an engine, the recess for the square-sectioned spring-steel band being formed in the cylinder head (or cylinder block, as the case may be) and in the valve seating as well. Now comes one of the trickiest jobs—fusing Stellite on to the portion of the detachable seating which will be in con tact with the valve. This operation is done by an expert who, by dint of perseverence and practice, has acquired the art of fusing the Stellite to the ring at just the right temperature to obtain a good contact yet, at the same time, to avoid blow-holes.
As we have already indicated, it is necessary to grind the seatings to their proper shape and size when they are "Stelfited" so the various diameters are roughed and then finished-ground until finally the seating is ready for fitting to the engine. As the sketch indicates, the seating is first pressed into the block, an interference fit being arranged so that adequate contact for cooling is ensured. Once " home " in the cylinder block the spring-steel band expands until half of it lies in the block and the other half in the valve seating.
An interesting point concerning this locking device might be mentioned. Experiment has shown that in the case of a valve seating which depends for its hold entirely upon its interference fit a pressure of about 500 lb. to 600 lb. will dislodge it, whereas about 10 tons is needed to disturb one of the Harold Andrews Grinding Co.'s fastenings.
As an indication of the care with which the seatings are inserted the final operation may be quoted. Using a mandrel inserted in the valve guide as a mounting for a rotary grinder, the seat is finally trued up—by this means concentricity to within .001 in. can be guaranteed.
Prices naturally vary somewhat according to size and type. Odd samples can be executed with Stellite facings for 16s. 8d. per seating. On the other hand contracts for quantities facilitate production and so prices can accordingly be reduced.