USRA Designs

USRA Mikado Steam Engine






Type D Stoker (page 2)



There are two elevators, one at the left for elevating coal to the left of the firedoor, and one at the right for elevating coal to the right of the firedoor. Each elevator has a casing, a helicoid screw, a screw shaft, and a drive and reverse.

The elevator casings are bolted to the top of the transfer hopper and on the top have bushed bearings for the driving shaft and elevator drive and reverse casing.


The elevator driving shafts extend from the bearing in the bottom and the transfer hopper through the top of the casing and reverse. Keyed and pressed on to the bottom of the shaft is a gear which meshes with the vertical teeth of the rack, from which the shaft receives its power. Each shaft bears an elevator screw, the movement of which is controlled by the elevator drive and reverse on top of the elevator casing.

The elevator drive and reverse mechanism operates on the same principle as the conveyor drive and reverse, although it has two sets of pawls and springs instead of three. The positions of the pawl shifter are the same; drive, or normal position, with the shifter down as far as it will go, and the elevator screw elevating coal; neutral, with the shifter half way out, where it rests on a catch arranged for that purpose when the elevator screw is stationary; and reverse, with the shifter pulled out as far as it will go, in which position it has to be held, the elevator screw reversing and allowing the coal to return to the transfer hopper.

The driving engine of the Duplex Stoker consists of a single cylinder, 11 in. by 17 3/4 in. with piston and reverse head, and is lubricated by a tap on main lubricator. It is secured to the transfer hopper beneath the cab deck and is operated by steam taken from the locomotive turret, reduced in pressure by throttling through a 1/2 in. globe valve. The pressure of the steam used by this engine varies from eight to eighty pounds, according to the work required by the quality and size of the coal, and is indicated by a special steam gage located on the backhead of the locomotive and connected in the line between the control valve and cylinder. In normal operation, the piston has a power stroke only when traveling in toward the center line of the locomotive and the entire stoker mechanism is in normal operation, since on the return stroke of the piston the conveying mechanism is stationary; but when any one, or all, of the three screws—two elevator and one conveyor—are reversed by means of their individual reverse mechanisms, the return stroke of the piston becomes temporarily a power stroke. Hence only a very small percentage of the full boiler pressure is required for the return stroke except when the reversing of any of the screws is necessary.

The operation of the Duplex Stoker and the travel of the coal is as follows:—The shovel sheet is provided with an opening 18 in. wide, extending from the coal gates to the slope sheet of the tank, and covered by slides each measuring about 20 in. in length. After passing through this opening to the trough beneath, the coal is conveyed by the helicoid conveyor screw through the crushing zone—where the coal forced against the crusher plate by the screw is broken to a suitable size—to the transfer hopper where it is divided, equally or unequally, according to the position of the dividing rib between the two elevators. These then elevate the coal and drop it into tubes which are fitted into elbows and extend through holes in the backhead, on each side of the firedoor. Constant steam jets in the elbows blow the coal through the tubes above mentioned, which are fitted with distributors located on the inside of the firebox. These distributors deflect and spread the coal evenly over the fire.

Mikado steam locomotive Duplex Stoker
The elevating screws are driven by gears which mesh with a rack reciprocated by the driving engine and the conveyor screw is driven by a shaft also meshed into this rack, and connected to the conveyor screw through gears.


There is a distributing system, or systems, under the control of the fireman, for spreading as much coal over the grate area as may be required to maintain an even fire and maximum steam pressure. The distribution of coal over the grate area is accomplished by means of a low pressure constant steam jet located in the back and bottom portion of each distributor elbow. Deflecting ribs on the distributors place some of the slack coal in right and left back corners of firebox, thus preventing loss through stack. The fireman can direct more or less coal to each side of the firebox by moving regulating lever to either side and thus changing position of the dividing rib. With the distribution as described, a level, light fire can be carried by the fireman, and perfect combustion can be secured. Firebox temperatures average four or five hundred degrees higher than with hand firing.