rests on the two spring-plank angles, fitting between the vertical legs and having on the outer side a lip which extends down over the ends of the angles and comes flush with the lower bar of the side frame. The spring seat (shown in detail) from which the brake hangers are suspended, is rigidly secured to the side frame by two wrought iron keys driven between it and the sides of the columns. The lower part of this casting, which fits between the spring-plank angles, extends inward from the side frame almost 14 inches, stiffening the connection of the angles to the side frame and keeping the side frames in alignment. The angles are, however, sufficiently flexible to insure freedom from derailment or injurious strains due to undulating track or poor road bed. No bolts are used in the construction of the truck and all keys are cottered.

Q. Can you describe the "Economy" four-wheel engine truck?

A. It has a rigid frame made of two rolled steel side bars to which are secured a central bolster-supporting member and two pedestal transom castings; the four pedestals being combined with a cross transom that supports the air-brake cylinder. The pedestals have renewable pressed-steel shoes held by vertical side flanges with lips turned over top and bottom of the pedestals, without bolts or rivets. The pedestal tie-bars are inside of the journal boxes so as to give room for deep cellars.

Q. What is the maximum displacement of leading trucks under medium weight engines with about 26 feet wheel base?

A. About 311⁄2 inches.

Q. How much lateral displacement is necessary when the wheel base is 30 feet?

A. About 411⁄2 inches, on account of the greater overhang.

Q. How is this attained with three-point hangers?
A. By longer hangers.

Q. What is the result?

A. Increased wheel-flange wear.

Q. What other way can be used to get swing without weakness and flange wear?

A. By resting the bolster on the points of curved rockers in tangent position; so that when the center pin moves laterally the point of contact comes between the curved surfaces of the rockers, and inclined under surfaces of the bolsters.

CHAPTER LX

WHEEL BASE

Q. What name is given to the distance between axle centers?

A. Spread.

Q. What name is given to the total distance between the centers of the front and back wheels?

A. Total wheel base.

Q. What name is given to the distance between front and back driving-wheel centers?

A. Rigid wheel base.

Q. What is the effect, on the resistance to rolling, of lessening the distance between truck axles?

A. Up to a certain point to diminish it.

Q. What is the advantage of placing the driving axles between furnace and smoke box?

A. That the overhanging weight of the furnace in the rear balances that of the cylinders, smoke box, etc., in front, thus distributing the engine weight.

Q. What is the disadvantage of having over nine feet between any two drivers?

A. It makes a coupling rod which is too heavy and too liable to break.

Q. In ordinary ten-wheel engines is the distance greater between the front and the middle pair of driving axles, or between the middle and the rear pairs?

A. Between the middle and the rear.

Q. What is the objection to the six-wheel connected engine with an axle back of the fire-box, as is sometimes built?

A. The overhanging weight of cylinder, smoke box, etc., brings undue weight on the front pair of wheels.

Q. What is one of the principal objects in inclining the cylinders?

A. To get the leading wheels well forward.

Q. What is the advantage of getting the driving wheels well back?

A. To give the greatest weight where it will cause adhesion, and to lessen to some extent the tendency of the connecting rod to cause pitching and rolling.

Q. What is the measure of the wheel base?

A. The distance from the center of the trailing axle to that of the leading axle.

Q. What measures the rigid wheel base of an engine? A. The length between pin centers of the parallel rod; or where there are more than one on each side, the total lengths of such rods on one side.

CHAPTER LXI

WEIGHT DISTRIBUTION*

Q. How much weight is it safe, as far as the rails are concerned, to put on each axle, with rails weighing 30 pounds per yard?

A. About 8,000 pounds.

Q. How much is it safe, for heavy steel rails, to place on each driving axle?

A. About 30,000 pounds.

Q. What enabled the Mogul engine to be possible?

A. The invention of the pony truck (see Fig. 272), which permits the front driving wheels to be placed further forward than on a ten-wheel engine with a fourwheel truck one axle of which is in front and the other back of the cylinders.

Q. What may be said of the tractive power of the Mogul as compared with the ten-wheeler?

A. It has greater hauling power, by reason of having a greater proportion of weight on the drivers.

Q. At what point is an engine's weight supported when it is in working order?

A. At the spring hanger and equalizer fulcrums, transferred by means of spring saddles and equalizers to drivers, lead and trailer wheels, except in case of fourwheel trucks, in which part of the weight is supported by the center casting resting on the truck frame, and transferred to the truck wheels by the springs and equalizers.

Q. What is an equalizer, equalizing beam, or equalizing lever?

A. A beam connected at each end to a driving or truck spring, or to the end of another similar beam, to distrib*See also chapter on "Frames."