Gears

Gears are Power transmission elements. It is the Gears that decides the torque, speed and direction of rotation of all the driven machine elements. Broadly speaking, Gear types may be grouped into five major categories. They are Spur, Helical, Bevel, Hypoid, and Worm. A lot of intricacies are there in the different types of gears. Actually The choice of gear type is not a very easy process. It is dependent on a number of considerations. Factors that go into it are physical space and shaft arrangement, gear ratio, load, accuracy and quality level.

Following are the major Gear Types

By a rule of thumb it is wise to choose the simplest gear type which helps in accomplishing the objectives. For example Spur gears are an excellent choice as they are easy to make, accurate and also cost wise cheaper. But this comes with the rider can they do the job. Taking another example of Helical Gears, they show superior features when it comes to load capacity. Other advantages include total avoidance of undercutting that usually happens in small tooth number pinions. By contrast, crossed Helical Gears can be used in skew shaft drive provided the loads are small. Actually the point is each of the gears have their own set of respective advantages disadvantages and limitations. It is only the users that should try to use them judiciously, taking into account, the strong point of gears.





The following table gives a good comparison between the different gear types:

Table Showing Different types of Gears

Crossed Helical	Precision Rating	Features	Applications	Comments about Precision
Spur	Very good	Parallel Shafting, high Speed and loads, High efficiency	Can be applied to all type of Gear trains with wide range of velocity ratios.	Tooth elements are simple that offers maximum precision, recommended for all kinds of gear meshes except in cases requiring high speeds and loads and special features.
Helical	Good	Parallel Shafting, very high Speed and loads, efficiency lesser than Spur mesh.	"Applicable mostly to high speeds and loads, can also be used wherever spurs are applied."	"Quality is at per with spurs except for helix angle complication, recommended for all high-speed and load meshes, Axial thrust component must be taken care of."
Crossed Helical	Poor	"Shows skewed shafting point contact, high sliding, low speeds andlight load."	Velocity ratio is relatively low, can be applied to light loads and low speeds only.	Better to be avoided for precision meshes, limited precision and capacity because of point contact, lubrication is must, can be used as a good substitute for Bevel Gears.
Internal Spur	Fairly good	"Parallel shafts, High speeds and loads."	Internal drives needing high speeds and loads; low sliding; excellent for high capacity; can be applied in Planetary Gears for producing large reduction ratios; high stress loading.	Not to be used for precision meshes; used only when internal feature is necessary.
Bevel	Fair	"Intersecting shafts, offering high speeds and loads."	"Ideal for 1:1 and high velocity ratios, right angle meshes."	Preferred for right-angle drive; low ratios; limited precision; for best performance location should be around one of the less critical meshes of Gear train.
Worm mesh	Fairly good	"Right angle skew shafts; high velocity ratio; high speed and loads; designs are non reversible."	High loads and velocity ratio, angular meshes.	Possible to be made to high precision, suffers from limited precision; preferred choice for a combination of right angle drive and high velocity ratio.
"Special gears- Face, Spiroid, Helicon, Beveloid."	Between Poor to fair.	Skew and intersecting shafts; speed and loads are modest.	Only special cases.	Better avoided on precision meshes; Non conjugate action is significant; fabrication requires special devices, limited inspection.