5. Discuss the following topics using participles as well as

Introductory and connective words.

1. Kinds of fasteninge. 2. Their application. 3. Kinds of threads.

Power-transmission equipment

UNIT 7

1. Learn the notions and their definitions.

Mechanism A component of a machine that transmits or changes motion.

Coupling A machine elements used to join one shaft to another, permanent or semipermanent connections.

Clutch A mechanism permitting easy and quick connection and disconnection of two shafts.

Key a machine part composed of one detail insert into another, used to transmit torque between a shaft and a machine part assembled on it.

Cam A rotating or sliding piece of machinery that acts as part of pair to impart or receive motion.

Follower The other part of a cam mechanism, usually a rod and shaft that receives and transmits motion from the cam.

Ratchet A mechanism that works with a pawl. The ratchet is a bar

or wheel with inclined teeth; the pawl is usually a rod that can drop between the

teeth to permit motion in only one direction.

Text 1

1. Read and translate the following text. Define the words –connectors between the title, text, paragraphs and sentences.

Couplings

Couplings are machine elements used to join one shaft to another, which are needed because of specialized operations of the machine – for example, installation or servicing. The coupling of an electric motor driving a wa­ter pump or a gear box may transmit power while preventing an overload, compensate for misalign­ments of machinery, and minimize vibrations. Couplings normally form permanent or semipermanent connections between shafts, while clutches are associated with rapid engagement.

Fig. 14. Gear and dental flexible coupling.

Solid couplings consist of rigid flanges on each shaft, bolted solidly together to form a continuous stiff shaft. Their use is favored when it is desired that the connected machines act as a single unit (for example, the rotor of a motor or generator may be used as the flywheel of a reciprocating engine or compressor) or to reduce over-all length (such as an overhanging blower) or to

eliminate bearings (by coupling components together such as a gear pinion

to a turbine). Shaft misalignment can result between the two shafts, increases bearing loads, and produces a frictional power loss. It is this frictional power that imposes a limitation on the allowable mis­alignment of high-speed

Fig. 15. Solid couplings.

shafts, because the friction heat increases faster with speed than the heat-dissipation ability.

Couplings for high-speed and / or high-power machinery are almost

exclusively of the gear (or dental) type. Torque is transmitted by two coupling halves bolted to­gether, which have at each end an internal gear that meshes with gear teeth machined on the periphery of the shaft flanges. Under running conditions, misalignment, errors in tooth spacing, etc., will cause movement between the mating gear teeth, requiring that lubricating oil or grease be used to prevent damage by fretting.

Disconnectable Couplings. When it is necessary to dis­connect or connect machines while in operation a jaw-type coupling can be used provided it is operated only when the two shafts are running at the same speed. Friction clutches allow the shafts to be engaged while revolving at different speeds, and overrunning clutches will disengage automatically when

the speed of the driven unit exceeds that of the driver. Overrunning clutches

may be of a jaw type or may use oval pins between an inner and an outer raceway which jam when the torque is in one direction and roll out of engagement for reversed torques. These devices can be used to prevent reverse rotation.

Fig. 16. Rubber flexible coupling..

Flexible couplings can be designed to compensate for shaft misalignment or to modify the torsional flexibility of shafts to reduce the effect of torque variations. Torsional flexibility is obtained by using rubber bushings between the bolts and the flanges or, for jaw-type clutches, by use of rubber inserts or rubber bushings between the flange and the shaft. Still other designs use straps to transmit the torque. Couplings that are designed for misalignment use straps, knuckle joints, wobble plates, or some other mechanism that allows a sliding action between meshing parts.

Hydraulic coupling. The hydraulic coupling consists of two open-faced

vaned impellers placed face to face and en­closed in a housing containing oil. The rotation of an impeller connected to the driver creates a vortex. The output impeller, connected to the driven machine, obtains its power by slowing down the vortex. The shafts are coupled by filling the case with oil and are dis­connected by draining.

An electric coupling consists of a metal ring connected to the driver shaft, which surrounds an electromagnet mounted on the output shaft. Shafts are coupled by energizing the magnet and disconnected by demagnetizing the field.

Variable speed is obtained by regulating the current to the electromagnet and slip losses are dissipated with air or water circulation.

Note. open-faced vaned impeller – відкрита гідропередача.