Wednesday, 20 February 2013

Design and manufacturing of gears

 Gear design is the process of designing a gear.
Designing is done prior to manufacturing and includes calculation of the gear geometry, taking into account gear strength, wear characteristic of the gear teeth, material selection, gear alignment and provision for lubrication of gear.
A gear is a rotating machine part which has cut teeth, that mesh with another toothed part in order to transmit torque.
In a gear drive, the shape of the tooth depends upon the pressure angle .
A 20° pressure angle full-depth involute gear tooth has various advantages over the other pressure angles.

The correct manufacturing of a gear requires a number of prerequisite calculations and design considerations.
The design considerations taken into account before manufacturing of gears are: Strength of the gear in order to avoid failure at staring torques or under dynamic loading during running conditions.
The advantage of split hub is that it reduces the cooling stresses in the gear and facilitates the mounting of the gear on the shaft.
If the lay out of the shaft is known, the diameter of the gear shaft can be calculated. Toothed wheels fixed on the shaft are fitted by interference-for example, press or light press fit.
If the wheel is to be removed from the shaft medium fit are used. The following types of gears are most commonly used in industry for power transmission purposes: Spur gears
A gear having straight teeth along the axis is called a spur gear.
A rack is a straight tooth gear which can be thought of as a segment of spur gear of infinite diameter.
The prime requirement of a gear drive is to transmit power at a particular velocity ratio for certain working condition, such as, operating time, nature of load, etc. The following points must be considered while designing a gear drive: Highest static load acting on the gear tooth due to high starting torque
Wear characteristics of the tooth for increasing its life Besides the above basic requirements, the following aspects are also considered: Lubrication of teeth alignment of gears
Deflection of gear teeth and shaft should be specified tanα where α is the pressure angle. is responsible for transmitting torque and hence the power while the is called the separating force, which always acts towards the centre of the gear.
In the force analysis of a gear drive, an assumption is made that the tangential force remains constant in magnitude as the contact between two teeth moves from top of the tooth to its bottom.
The torque transmitted by with respect to the centre of the gear is Also by using the relation P = ×v, the tangential force responsible for transmitting power can be obtained, where is the power
The continuous change in the point of application of load on the tooth profile and the change in magnitude and direction of the applied load make accurate stress analysis of a gear tooth a complicated problem.
In 1892, Wilfred Lewis published a paper titled,"The investigation of the strength of gear tooth", in which he derived an equation for determining the approximate stress in a gear tooth by treating it as a cantilever beam of uniform strength. The following assumptions are made for the beam strength calculation: The tangential component, , is uniformly distributed across the face width. But practically the distribution is non- uniform. This assumption is valid for small face widths , i.e. ≤12.5m, where m is the module of the gear.

They are traditionally high wear items, thanks to their usual task: converting high-speed, low-torque power from electric motors into the low-speed, high-torque power needed by machinery.
Still, gearboxes remain widespread and popular because, as one of the tried and true rules of thumb in the power-transmission industry says, “Speed is cheap, torque is expensive.” And relying on motors to generate the torque required by many loads is usually more expensive than generating the torque with a motor/gear reducer combination.
It also means the motor doesn’t have to generate as much power, so designs can use smaller, less-expensive motors which don’t take up as much space.
Efficiency also means less heat generation, which prolongs the life of a gear reducer as well as the oil in it.
Hot-running gear reducers have been known to burn distracted or less-than-careful employees. But what type of gearing is most efficient?

Worm gearing is self-locking, which eliminates the need for motor brakes in some applications.
Worm gears usually experience sliding friction, which wastes energy, creates heat, and increases tooth wear, all of which shorten the life of a gearbox. And when worm gears are used in applications in which the motor reverses, backlash grows as the teeth wear over time.

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