Spur and helical gears. A gear having tooth elements that are straight and pa¬rallel to its axis is known as a spur gear. A spur pair can be used to connect parallel shafts only. Parallel shafts, however, can also be connected by gears of another type, and a spur gear can be mated with a gear of a different type.8420
To prevent jamming as a result of thermal expansion, to aid lubrication, and to compensate for unavoidable inaccuracies in manufacture, all power-transmitting gears must have backlash. This means that on the pitch circles of a mating pair, the space width on the pinion must be slightly greater than the tooth thickness on the gear, and vice versa. On instrument gears, backlash can be eliminated by using a gear split down its middle, one half being rot table relative to the other  A spring forces the split gear teeth to occupy the full width of the pinion space.
Helical gears have certain advantages; for example, when connecting parallel shafts they have a higher load carrying capacity than spur gears with the same tooth numbers and cut with the same cutter. Because of the overlapping action of the teeth, they are smoother in action and can operate at higher pitch-line velocities than spur gears. The pitch-line velocity is the velocity of the pitch circle. Since the teeth are inclined to the axis of rotation, helical gears create an axial thrust. If used singly, this thrust must be absorbed in the shaft bearings. The thrust problem can be over¬come by cutting two sets of opposed helical teeth on the same blank. Depending on the method of manufacture, the gear may be of the continuous-tooth herringbone variety or a double-helical gear with a space between the two halves to permit the cutting tool to run out. Double-helical gears are well suited for the efficient transmis¬sion of power at high speeds.
Helical gears can also be used to connect nonparallel, non-intersecting shafts at any angle to one another. Ninety degrees is the commonest angle at which such gears are used.
Worm and bevel gears.   In order to achieve line contact and improve the load-carrying capacity of the crossed axis helical gears, the gear can be made to curve partially around the pinion, in somewhat the same way that a nut envelops a screw. The result would be a cylindrical worm and gear.
Worm gears provide the simplest means of obtaining large ratios in a single pair. They are usually less efficient than parallel-shaft gears, however, because of an additional sliding movement along the teeth. Because of their similarity, the efficien¬cy of a worm and gear depends on the same factors as the efficiency of a screw. Single-thread worms of large diameter have small lead angles and low efficiencies. Multiple-thread worms have larger lead angles and higher efficiencies.
For transmitting rotary motion and torque around corners, bevel gears are com¬monly used. The connected shafts, whose axes would intersect if extended, are usually but not necessarily at right angles to one another.
When adapted for shafts that do not intersect, spiral bevel gears are called hypoid gears. The pitch surfaces of these gears are not rolling cones, and the ratio of their mean diameters is not equal to the speed ratio. Consequently, the pinion may have few teeth and be made as large as necessary to carry the load.
The profiles of the teeth on bevel gears are not involutes; they are of such a shape that the tools for cutting the teeth are easier to make and maintain than involutes cutting tools. Since bevel gears come in pairs, as long as- they are conjugate to one another they need not be conjugate to other gears with different tooth numbers.
Rolling Guides and Bearings
Rolling guides
Rolling linear guides and guide ways are widely used in practice, alongside plain linear guides. The following advantages ate obtained when compared with plain guides: light running forces due to rolling friction, no stick-slip, trouble-free installa¬tion and immediate availability due to standardization of the rolling elements. 机械结构英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_6770.html