Friction gearing

GEARING

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WHY WE CALL IT A «DIESEL» ENGINE

A diesel engine is one form of internal-combustion engine, the latter being an engine from which work is obtained by the combustion of fuel within the engine cylinders themselves. A diesel engine is that type of internal-combustion engine, which injects fuel oil in a finely divided state into a cylinder within which air has been compressed to a high pressure and temperature. The temperature of the air is high enough to ignite the particles of the injected fuel; no other means are used for ignition. Because of this method of ignition, diesel engines are sometimes called compression-ignition engines. This sets them apart from other internal-combustion engines called spark-ignition engines. These latter engines use gasoline or gas as fuel, and the mixture of fuel and air is ignited by an electric spark.

Why, then, do we call this compression-ignition engine by the name «diesel»? Simply because a man whose name was Rudolf Diesel originated in Germany and obtained patents in 1892 on a high-compression, self-ignition engine originally intended to burn powdered coal.

Distinguishing Features of Diesel Engines. How To Recognize a Diesel Engine. Whenever you see an engine you should be able to tell quickly whether or not it is a diesel. Many diesels, at first glance, look like gasoline engines, but it's easy to distinguish them if you keep in mind three essential differences in the way they work. These are: (1) the diesel ignites its fuel solely by the heat of compression, and therefore has no external ignition system; (2) the diesel draws in air alone when it fills its cylinders, and therefore uses no carburetor; (3) the diesel injects its fuel into the cylinder in the form of a spray at high pressure, and therefore uses a fuel-injection pump.

There's another important difference between diesel and gasoline engines - the diesel is built heavier because it must withstand higher pressures, but you can hardly see this from the outside.

Some vertical steam engines of late design look remarkably like diesels, which is not surprising because the designers of these steam engines deliberately copied some of the mechanical features of diesel construction. But the large steam supply pipe gives them away, as does the absence of a fuel-injection pump.

General Considerations. Friction gears depend for their driving action upon the friction of the driving wheel, or driver, against its mate, or follower. The friction surface of a driver should be of a comparatively soft material, such as wood, fiber, leather, paper, or rubber, while that of the follower is usually made of cast iron. This arrangement insures the maintenance of the correct shape of the friction surfaces; whereas if the follower is of the softer material its surface might be injured and eventually ruined when starting under load or when an excessive load has brought it to a standstill

Friction gears are used for light and medium powers in machinery, which is frequently started and stopped, also where provision for a change of speed of the driven shaft or a reversed motion is necessary. Their advantages are flexibility and noiselessness.

The disadvantages of friction gears are the thrust on the bearings and slippage, resulting in a comparatively low efficiency. However, by using metal-to-metal contact surfaces and ball or roller bearings, these objections are partially eliminated in some recent designs.

The driving capacity of friction gears is a function of the coeffi­cient of friction between the surfaces in contact and of the pressure, which holds them in contact. This pressure is limited by the ability of the softer surface to endure it without injury

Дата добавления: 2014-10-17