Lesson 13

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Preliminary preparations, Starting the machine for the first time

The anti-corrosive slushing compound, applied on the machined surfaces before packing the machine for shipment, is to be removed with the aid of white spirit or petrol (gasoline). The washed surfaces are dried and then wiped dry.

Special care should be taken in washing the saddle mechanisms, screw for saddle travel, and the index worm wheel, of the work spindle. Remove the guard for access to wash the worm wheel.

All unpainted surfaces are then coated with a thin layer of the same oil used to fill the oil reservoirs. Before starting the machine, storage it in dry premises at least three days to remove all moisture from the insulation of the motor windings and the wiring, which may have been picked up in transportation. Drying increases the re­sistance of the insulation. At least one hour before starting the machine, all oil reservoirs are to be filled with oil, grade "Industrial ЗО", and the machine is to be lubricated.

After becoming acquainted with the purpose of the control levers and devices, it will be necessary to rotate the drive pulley by hand (removing the guard) and to check the operation of all the units in this manner. The machine is then connected to the power supply and is started on an idle run.

Lesson 14

I. Read and memorize the following words and word-combinations:

II. Read and translate the following text: Elementary hydraulic systems

The simplest hydraulic system is perhaps the hydraulic shock absorber which consists of a pump (piston) and resis­tance. Another elementary type of system is the hydraulic weighing machine, which consists of a non-continuous pump element (piston or diaphragm) and a pressure gauge. Other elementary systems consist of a jack, transmission line, and a pump of the non-continuous type without control gear, although valves may still be present for secondary purposes. Systems of this type are used for remote transmission of manual or pedal effort or motion, or for remote transmission of instrument indication.

The simplest best known system of this type is the hydraulic brake.

Bearings

A Bearing is a machine part which supports a moving part and confines its motion. That part of a shaft which rotates in a bearing is called a journal. Bearings in which one rubbing surface slides over another are called plain bearings and may be divided into two classes: Those with a continuous rotary motion and those with an intermittent motion. To the first class belong journal bearings, which carry a load acting at right angles to the shaft axis, and thrust bearings, which take a load acting in the direction of the shaft axis. To the second class belong bearings of parts having a rocking motion, as wrist pins, or a liner reciprocating motion, as crossheads.

Bearings with a continuous rotary motion form the great majority of all bearings.

Bearing Failures.— The failure or need of replacement of a bearing with sliding contact may be due to:

(a) excessive wear of the bearing surfaces;

(b) overheating; or

(c) cracking of the bearing metal.

Excessive Wear.—Wear is caused by metal-to-metal con­tact. It cannot be entirely eliminated but can be reduced by providing a sufficient bearing area and by adequate lubrica­tion.

Overheating, if not stopped in time, may cause either sei­zure of the journal if it runs in a hard-metal bearing or mel­ting of the bearing surface if the bearing is lined with a metal having a low-melting temperature. Overheating is primarily caused by metal-to-metal contact. Lubrication decreases the danger of overheating.

Cracking of the bearing metal is due to heavy shock loads, such as are taking place in the running gear of internal com­bustion engines when the compressive stresses in the bearing metal exceed its endurance limit.