Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(151) Reverse Gear.

As explained under “Cylinders” the travel of the valve bears a definite relation to the piston position so that the ports may be opened and closed at the proper times. It may be shown by a rather complicated diagram that this relation of the valve together with that of the eccentric that drives it is only correct for one direction of rotation. For any other direction of rotation the relation of the valve and piston position must be changed. This may be done in several ways but the most common types are the Stevenson Link and the Wolff slotted yoke.

The Stevenson link motion used on the majority of engines, consists of two independent eccentrics, one being fixed in the relation for forward motion and the other for the reverse direction. The ends of the eccentric rods leading from these eccentrics are connected by a slotted bar or link, in which a block is placed that is connected with the valve rod. The block is free to slide in the slot of the links, that is, it may be moved from one end of the slot to the other. When it is desired to have the engine rotate in a right handed direction, for example, the link is lowered so that the rod from the forward eccentric is brought directly in line with the block so that this eccentric alone acts directly on the valve through the valve stem. When the reverse is desired the link is raised until the rod from the reverse eccentric is brought in line with the block and valve stem, drive being by the reverse eccentric.

When the block is on the link in a position between the two points mentioned, the valve has less travel and it cuts off earlier in the stroke than when driven directly by one eccentric, for the motion at an intermediate point on the link is much different than at the ends of the slots. This fact is taken advantage of in operating engines with the idea of economy in view, and is known commonly as “hooking up” the engine. The best point at which to “hook up” the engine is best determined by experiment, and is equivalent in many respects to the problem of advancing and retarding the spark of a gas engine. We earnestly advise an engineer of a traction engine to take up this subject and determine the best point of cut-off for different loads as he will find that different positions make a considerable difference in his coal bill. Of course the proper way is to determine this point with a steam engine indicator, but as few engineers have such an appliance, the work is generally of the cut and try order. Wear and varying adjustment soon change the points marked on the reverse sector, and for economy’s sake these points should be checked occasionally.

In the Wolff motion, a single eccentric is used for both directions of rotation, in connection with a slotted link. A single eccentric is securely keyed to the crank shaft. The eccentric strap has an extended arm which is pivoted to a block that slides back and forth in a curved guide. The angle at which the guide stands with the horizontal determines the direction of rotation, the angle being changed by the reverse lever. The degree of the angle made by the block also determines the point of cut-off. This is a very efficient and simple valve gear.

Guides and Cross-Head.

The outer end of the piston rod is supported by a sliding block known as the “cross-head” which in turn is supported by the guides. An oscillating rod called the “connecting rod” connects the reciprocating cross-head with the crank pin, this rod is used in the same way as the connecting rod of the gas engine except that it is connected to the cross-head instead of the piston.

Clutch.

The clutch affords a means of connecting and disconnecting the driving wheels and engine shaft. It is usually of the friction type described under “Gas Tractors.” By releasing the clutch the engine is disconnected from the driving gear so that the tractor remains stationary while the engine is driving a load through the belt.

Use of the Exhaust Steam.

The exhaust from the cylinders is used in two ways, first to create a draft for the fire, and second to heat the feed water pumped into the boiler. The draft is increased by exhausting a portion of the steam into a nozzle placed directly under the stack. The friction of the steam on the surrounding air, draws the air with it, forming a partial vacuum over the grate at each puff, and in this way it causes additional air to rush through the fuel and increases the temperature of the combustion. As the load increases the “puffs” increase in intensity due to the greater terminal pressure and the fire is accelerated in proportion. This is a simple but rather expensive way of increasing the draft.

A considerable proportion of the heat in the exhaust steam is saved by using it to heat the feed water supplied to the boiler. Besides the saving in fuel, affected by heating the water from steam that would otherwise be thrown away, the strains on the boiler due to the injection of cold water are greatly decreased as the difference between the temperatures of the boiling water in the boiler and the hot feed water are much less than in the former case.

The feed water heater consists essentially of a series of tubes in a cylindrical shell. The tubes are surrounded on the outside by the feed water, and are filled with the exhaust steam which passes from end to end through the tubes. The hot water is pumped from the heater into the boiler. An efficient feed water heater adds greatly to the steaming capacity of the boiler.