347. Q.—­What are the dimensions of the end studs of a land engine beam?

A.—­In low pressure engines the diameter of the end studs of the engine beam are usually made 1/9th of the diameter of the cylinder when of cast iron, and 1/10th when of wrought iron, which gives a load with low steam of about 500 lbs. per circular inch of transverse section; but a larger size is preferable, as with large bearings the brasses do not wear so rapidly and the straps are not so likely to be burst by the bearings becoming oval.  These sizes, as also those which immediately follow, suppose the pressure on the piston to be 18 lbs. per circular inch.

348. Q.—­How is the strength of a cast iron gudgeon computed?

A.—­To find the proper size of a cast iron gudgeon adapted to sustain any given weight:—­multiply the weight in lbs. by the intended length of bearing expressed in terms of the diameter; divide the product by 500, and extract the square root of the quotient, which is the diameter in inches.

349. Q.—­What was Mr. Watt’s rule for the strength of gudgeons?

A.—­Supposing the gudgeon to be square, then, to ascertain the thickness, multiply the weight resting on the gudgeon by the distance between the trunnions, and divide the product by 333.  Extract the cube root of the quotient, which is the thickness in inches.

350. Q.—­How do you find the proper strength for the cast iron beam of a land engine?

A.—­If the force acting at the end of an engine beam be taken at 18 lbs. per circular inch of the piston, then the force acting at the middle will be 36 lbs. per circular inch of the piston, and the proper strength of the beam at the centre will be found by the following rule:—­divide the weight in lbs. acting at the centre by 250, and multiply the quotient by the distance between the extreme centres.  To find the depth, the breadth being given:—­divide this product by the breadth in inches, and extract the square root of the quotient, which is the depth.  The depth of a land engine beam at the ends is usually made one third of the depth at the centre (the depth at the centre being equal to the diameter of the cylinder in the case of low pressure engines), while the length is made equal to three times the length of the stroke, and the mean thickness 1/108th of the length—­the width of the edge bead being about three times the thickness of the web.  In many modern engines the force acting at the end of the beam is more than 18 lbs. per circular inch of the piston, but the above rules are still applicable by taking an imaginary cylinder with an area larger in the proportion of the larger pressure.

351. Q.—­What was Mr. Watt’s rule for the main beams of his engines?