GEAR NUMENCLATURE & DEFECTS OF GEARS

Spur Gears are the very common gears for many gear boxes. As the helical gears are the development of the Spur gears, learning spur gears can help almost any type of gears. The design parameters for the gears depending upon the torque which is to be transmitted at a certain required speed. The area of contact will dictate the load capacity of the gears. For designing and application of these gears one should able to know the numnclature of the gears. I have given the details in the below picture.

For measuring the tooth thichness there always some mistakes people used to do. Here in the below picture I have given the method and the various tooth thicknesses acording to the gear modules in the below picture.

DEFECTS OF THE GEARS:

1. Wear is the major defects on the gears. The wear may be for one tooth or few or all the teeth.
2. One or few teeth broken or twisted.
3. Burrs on the bore or the key way.
4. Burrs on the internal splines of the gear bore or the tooth surface.
5. Crack or damage on the rim or the bore of the gear wheel.

LIMITS OF WEAR FOR DISCARDING:

Limits of wear for discarding is one of the important thing which many engineers will try to search here and there for the details. Hence I have given here for easy referrence.

OPEN GEAR TRANSMISSION and the surface speed up to 2 meter / second – The tooth thickness (Sx) reduces due to wear by (0.5m) mm, where m is the module, or about 33% of the original tooth thickness.

CLOSED GEAR TRANSMISSION (GEAR BOXES) for surface speeds over 2 meter / sec. – the tooth thickness (Sx) reduces due to wear by (0.3m) mm or about 20% of the original tooth thickness.

MESHING OF GEARS:

Perfect Meshing gears is one of the prerequisites for an healthy transmission of the power.
The spur and helical gears must be assembled in such a way that the axes of the meshing gears are parallel and the distance between their axes is correctly established.

The correct meshing of the spur and helical gears is decided by applying blue paste on the teeth of the driving gear, giving it several rotations and seeing to the blue marks on the teeth faces of the meshing driven gear.

The defects due to mis alignment which found during blue matching is shown in the below picture.

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GEAR TRANSMISSION & TYPES OF GEARS

Gearing is a very vital form of transmission of mechanical power. Gears are universally adopted for almost all machines. We can say that without Gear boxes there hardly exists Mechanical Equipment. Hence the knowledge on these gear boxes are very essential for the Mechanical Engineers.

Type of Gear Transmissions:

1. Spur Gear,
2.Helical Gear,
3.Herring bone Gear,
4.Bevel Gear,
5.Rack and pinion,
6.Worm and worm wheel.

1. SPUR GEARS:

Spur Gears in any transmission system are employed for changing the speed, the torque and are fixed on shafts either rigidly or with sliding traverse.

2. HELICAL GEARS:

Helical Gears can be used for transmitting motion between parallel shafts as between perpendicular shafts. Helical gears used for transmission at an angle are called SPIRAL GEARS.

Advantages of Helical Gears:
•Noiseless motion even at higher speeds.
•Smooth transmission of small gears with fewer teeth, at large transmission ratio ( up to 15:1)
These gears must be rigidly installed on the shaft. These gears result in an axial force in one direction depending upon the direction of rotation and are used for transmitting small power.

3. HERRING-BONE GEAR:

HERRING-BONE Gears are pairs of Helical Gears. These gears are used for large power transmission.

Advantages of Herring-bone Gears in addition to Helical Gears:
•No Axial Force. It is balanced axially in both direction of rotation.
•Large power transmission can be done at impact loads having frequent starts (inching).
These gears also must be rigidly installed on the shaft.

4. BEVEL GEAR:

Bevel Gear transmission is employed for transmitting motion from one direction to the other at any angle. In general 90° transmission is used mostly.

5. WORM & WORM WHEEL:

Worm and worm wheel gives a large single stage speed reduction in perpendicular direction. Such transmission gives lower efficiency of transmission due to increased frictional losses. For a noiseless motion, worm 1 is usually made of medium carbon / alloy steel and the worm wheel 2 of bronze (or in some cast iron).

6. RACK & PINION:

The rotary motion of pinion 1 (small gear) is transformed into linear motion of the rack 2. For transmitting large power worm & worm rack transmission is employed.

These gears are used separately or with combinations as per the requirement of the transmission.