type of gears

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Spur gears

These are the most common form of drive. They are cylindrical and have the teeth cut straight and parallel to the axis of rotation. The tooth form can be one of several, but there is no axial thrust component on the bearings as the teeth are straight. The efficiency of the spur gear can be as high as 98% and capable of practical speed ratios of 10:1 although 6-7:1 is more common.
The main disadvantage of spur gears lies in the fact that they tend to be noisy at over 1000ft/min. If made to exceptionally fine limits of accuracy, plain spur gears can be used at far higher speeds in turbine drives.

Single Helical gears

Helical gears are produced by cutting the teeth at an angle to the gear axis and the teeth follow a spiral path thus making for gradual tooth engagement and load distribution. Efficiency is as great as for spur gears. Ratios of 10:1 are possible with increased load over spur gears. A degree of axial thrust is produced which must be catered for in the bearing design. Angular contact or tapered roller bearings are employed. On larger designs where plain bearings are fitted a thrust block arrangement must be fitted.
Single helical gears can be used at speeds up to 4000 ft/min

Double Helical gears

Commonly specified where the axial thrust from a single helical design would be too large or where there plain bearings are used. To balance the side thrust the teeth are formed on each gear in helices of identical angle but opposite hand. For cast commercial gears the teeth are sometimes of the uninterrupted type, cut by the planing process. For hobbed gears a 3 in wide gap is left for the hob clearance. Single reductions of 10:1 with double reductions of 75:1 and triple reductions of 350:1 are used Pitch line velocities from 4000 to 20000ft/min are possible depending on the accuracy of manufacture.

Bevel gears

Bevel gears are used in situations where it is desired to transmit motion between two shafts whose axis intersect. The most common type is that in which the teeth are radial to the point of intersection of the shaft axes or apex and these are known as straight bevel gears. The tooth action is similar to that of spur gears, being in line contact parallel to the pitch line. There is no longitudinal sliding between the teeth, but there is an end thrust developed under tooth load which acts away from the apex, thus tending to separate the gears. Thrust bearings must therefore be provided. The maximum gears ratio is 4:1. The maximum speed at pitchline is 1000ft/min.

Spiral bevel gears

The spirally cut gears like the helical gear in its relationship to the spur gear, can withstand higher speeds than the straight cut bevel and is quieter in action. Unlike the straight cut bevel gears which can be shaped or precision forged the spiral bevel gears must be made on a special machine ( made by Gleason Co). Pitchline velocities of 4000ft/min maximum can be handled.

Hypoid gears

Similar in appearance to the spiral bevel gear it is distinguished by having the pinion axis offset to the wheel axis. They are mainly used in the automotive back axle drives where they provide smooth tooth engagement at the high speeds combined with high load carrying capacity.

Spiral or crossed axis gears

These gears are identical in
every way to helical gears, the only difference is that they are used to transmit power between shafts that are not parallel. Mating gears must have the same base pitch but their helix angled may vary. The contact made by the pitch cylinders of spiral gears is point contact only and there fore spiral gears are suitable for light duties only.

Zero Bevel gears

These gears have teeth that are curved in the same general direction as straight teeth. They are spiral gears of zero spiral angle.

Skew Bevel gears

In this form the pinion shaft is offset in relation to the wheel. The pinion may have straight teeth or it may have skew teeth similar to a helically cut bevel gear. The object is to obtain more gradual tooth engagement than with a straight tooth bevel. An additional advantage is that it sometimes makes possible the provision of bearings at both ends of the pinion shaft. Skew bevels are seldom used as they are difficult to set up.

Internal gears

The meshing condition of this sort of gear are said to be better than those of the external gears for the reason that the contact area is between a concave and a convex surface, while also making better conditions for lubrication.
Other advantages include shaft direction is the same for input and output, Greater load capacity is possible, increased safety as the teeth are guarded. Disadvantages include difficulty in supporting the shaft, range of gear cutting processes is reduced and tooth interference is a common problem.

Worm reducer gears

conditions for worm gears include the following;
1. shafts at right angles
2. large speed reductions in smallest compass
3. smallest number of gears

A worm drive comprises a cylindrical worm having helical teeth or threads, similar to a helical gear, meshing with wheel with a concave face. The tooth contact is a line one and heavy loads can be handled. Efficiencies claimed for worm gears are 97% and above. Ratios of 1000's to one is possible with double worm drives and it is the most popular form of industrial drive.

Spiroid gears

In addition to the above bevel gears there is a special type which has right angle non intersecting gears having tapered pinions with threads of constant axial lead meshing with face type gears. This is used mainly in the automotive