Билет 7 bearings with rolling contact

In a bearing with rolling contact the shaft is supported on rollers or balls. A bearing of this type has the theoretical advantage of reduced friction, but to be practical it must fulfil the following conditions:

a) unavoidable sliding should be reduced to a minimum:

b) the rolling elements must be properly guided in their motion;

c) all rolling elements should be of exactly the same size;

d) the rolling elements and their guides or raceways must be extremely hard and be very smoothly polished;

e) the pressure should be approximately normal to the surface of contact;

f) in order to avoid distortion the rolling elements must not be overloaded.

Properly designed and well manufactured bearings with rolling contact have the following advantages over bearings with sliding contact:

a) they will maintain a comparatively accurate alignment over long periods of time;

b) they can carry heavy momentary overloads without failure or seizure;

c) their power loss caused by friction is very small;

d) they are particularly adapted to very low speeds since their friction coefficient is independent of speed;

e) they have a very low starting friction;

f) their lubrication is simple and requires but little attention.

Билет 8 toothed spur gearing

Toothed gears may be considered as friction gears in which the contact surfaces are provided with grooves and projections which prevent slip and insure positive means of transmitting rotation. Toothed gears are used when a constant speed ratio is desired and the distance between the shafts is relatively small. Toothed spur gears are used to connect parallel shafts and are suitable for transmission up to the largest power.

Spur gearing may be classified as external, internal, rack and pinion. If the tooth elements are parallel to the shaft axis, the gears are termed straight-tooth spur gears; if the elements are helices, the gears are called helical gears. If the words spur gears are used without a modifying adjective, the more commonly used straight-tooth gears are signified.

Requirements.

a) The teeth must have a profile which insures a constant velocity ratio.

b) The relative motion of one tooth upon the other should be more of a rolling than of a sliding nature.

c) The arc of engagement should be so long that at all times more than one pair of teeth is in mesh. In practice this requirement is not always fulfilled.

d) The tooth profile should approach a cantilever beam of uniform strength.

Билет 9 worm gearing

Worm gearing is a type of screw gearing used for transmitting power between non-intersecting shafts which are at right angles to each other. By this means higher speed reductions may be obtained in a minimum space. There are two classes of worm gearing in common use, each of which has its advantages: one is a straight or cylindrical worm and the other a worm with a hollow shape similar to that of an hour glass. Due to its nature worm gearing is used mostly as a speed reducer, the worm being the driving member.

The threads of a straight or cylindrical worm have an axial pitch that is constant for all points between the top and the root of the threads. The gear teeth are of the involute form. There exist two methods of cutting the worm-gear teeth. By the first method, used for ordinary worm gearing, the cutting hob has a constant diameter and is fed radially to the proper depth into the gear blank, both hob and blank being rotated in the required relation to each other. The teeth produced by this method are not theoretically correct but are sufficiently accurate for single-thread worm gears with a great number of teeth.

When a higher efficiency and better service are desired, the teeth are cut with a tapered hob fed into the gear blank longitudinally at right angles to the axis of the blank.