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The friction surfaces on clutches should have a high coefficient of friction and be able to conduct the heat away rapidly. These properties are difficult to obtain in a single material and for this reason; one of each pair of mating surfaces is usually metallic, while the other is either leather, cork, or an asbestos-based facing riveted to a metal plate. Some friction clutches are run dry, while others operate in oil. Dry clutches have a higher coefficient of friction than wet clutches, but the oil helps to carry off the heat.
A half-section of a multiple-plate disk clutch in which input member 2 is keyed to the driving shaft 1 and output member 3 is keyed to the driven shaft 4. The friction plate’s b has external gear teeth or splines that mate with teeth on the inside of member 2, while friction plate’s c has internal teeth that mate with external teeth on member 3. Plate’s b can slide axially in 2, while plate’s c can slide axially on 3. The clutch is engaged by .moving the spool to the left, which, by a wedging action, rotates the lever about the pivot and creates a force that squeezes the plates together.
Magnetic clutches.
Magnetic-particle clutches have an annular (ring-shaped) gap between the driv¬ing and driven members that is filled with powdered iron and graphite. When a magnetic field is induced across the gap by a direct-current control coil, the iron particles form chains across the gap and transmit a torque that depends on the strength of the field. Controlled by varying the current, the load can be engaged smoothly and there is no slippage when the clutch is transmitting the torque for which it was designed.
Ball screw and nuts
Precision ball screw assembly are mainly used in various kinds of precision NC machine-tools, machining center, auto drafting machines, measuring devices and pre¬cision instruments where exactly positioning and continuous controls are required.
Conventional ball screw assembly are utilized extensively in save labour and sensitive gearing, such as adjustments for hospital doors, beds and chairs, starters, lifting jacks, opening and closing of valves/ metallurgical machinery, civil-architectural machining, automic reactors, submarines, satellite, trackers and steering gears for automobiles and tractors, etc.
The development of NC machine tools (see Volume 3) demanded among other things the provision of feed and guiding systems free from stick-slip and backlash. This required the availability of low-friction driving and guiding elements. Rolling guide ways and recirculating-ball screw systems are the outcome of efforts in that direction. A recirculating-ball screw system may be regarded as a rolling guide on a shaft, like all other screw-nut systems, it serves to transform rotary motion into linear motion, or vice versa. The main areas of application are for feed mechanisms and in measuring machines.
The principal positive features of a recirculating ball screw are:
a. very high mechanical efficiency due to rolling friction (up to 95%);
b. no stick-slip;
c. when correctly designed almost no wear and hence a very long life;
d. able to be pre-loaded (no play);
e. adequate stiffness.
The only disadvantage which must be mentioned is their low damping property.
As the balls rotate in the guiding grooves of the screw and nut, they are sub¬jected to a tangential or circumferential movement. Hence it is necessary to pro¬vide for the return of the balls into the system. In the upper dia¬gram a screw with a ball-return track at the end of each thread pitch. The advantage of this design lies in its small physical size. The unfavorable angles of the ball inlet and exit bring about the disadvantage of uneven rolling effects.
In the lower part of the diagram a ball screw is shown on which the ball return is affected over the full length of the nut by means of a return tube. By suitably shaping the tube, the balls leave and return to the load-bearing part of the nut tangentially, resulting in an even and shock-free running condition and permitting high rotational speeds to be used. An important disadvantage of this design is the ease with which the return tube may be damaged; this would hinder the ball motion and thus lead to further damage of the screw-nut assembly.