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What is a gearbox?

A speed reducer is an arrangement of gears in an enclosed housing. The reducer contains shafts, bearings and some other parts. The gears are mounted on shafts and the bearings support the shafts. The reducer housing generally contains some form of lubricant (e.g. oil or grease) to lubricate the gears and bearings. Reducers come in a wide range of shapes and sizes to accommodate a broad range of uses.

Reasons for using a gearbox:

• Less expensive to manufacture motors at higher speed rather than lower speed, for the same amount of power.
• Most machines require a lower speed than the motors produced.
• Gears are compact in size for the amount of power they can transmit and are also quite durable. They do not slip like belts for example.
• The correct amount of lubricants can be controlled and at the same time kept clean.
• The housing serves as a safety guard, bearing support and lubricant reservoir.

Functions of a gearbox:

• Most commonly being used to change the speed from the input shaft to the output shaft – the speed can be either reduced or increased.
• When speed is reduced, torque is increased and vise versa.
• It can be used to change the direction of a shaft, e.g. worm gear for right angle application.
• It may be used to change the direction of shaft rotation, from clockwise to counter-clockwise.

Coefficient of Friction

The coefficient of friction, often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one.

The static coefficient of friction is also called the starting coefficient of friction, and has the symbol µo.

The sliding coefficient of friction is also called the kinetic coefficient of friction, and has the symbol µ.

Although the dynamic friction coefficient for rolling bearings varies with the type of bearings, load, lubrication, speed, and other factors; for normal operating conditions, the approximate friction coefficients for various bearing types are listed in the table below.

Mechanical Efficiency

Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement. Efficiency is measured as a ratio of the measured performance to the performance of an ideal machine:

Torque

Torque, moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object. Mathematically, torque is defined as the cross product of the lever-arm distance and force, which tends to produce rotation.

Torque = Force X Radius

Power

In physics, Power (symbol: P) is defined as the amount of energy consumed per unit time. In the MKS system, the unit of power is the joule per second (J/s), known as the watt (in honor of James Watt, the eighteenth-century developer of the steam engine).

Power = Force X Velocity

3 types of loads:

Type 1 – CONSTANT TORQUE LOADS

Constant torque loads require the same amount of torque at low speeds as at high speeds. Torque remains constant throughout the speed range, and the horsepower increases and decreases in direct proportion to the speed. Constant torque loads include most positive displacement and reciprocating pumps and compressors as well as traction drives and conveyors. With constant torque loads, the torque is not a function of speed. As speed is changed, the load torque will remain fairly constant and the horsepower will change linearly with the speed.

For example, if the speed increases by 50%, then the power required to drive the operation will increase 50% while the torque remains constant.

Type 2 – CONSTANT POWER LOADS

Constant horsepower loads require high torque at low speeds and low torque at high speeds, which means constant horsepower at any speed. Constant horsepower loads include grinders, winding machines and lathes. For constant horsepower loads, the torque loading is a function of speed up to 100% operating speed. As the speed of the operation is decreased, the torque increases so that the horsepower required remains essentially constant.

Type 3 – VARIABLE TORQUE LOADS

Variable torque loads require much lower torque at low speeds than at high speeds. The torque required varies as the square of the speed and the horsepower required varies as the cube of the speed. Variable torque loads include most centrifugal and axial pumps, fans and blowers and many mixers and agitators.

As the speed is decreased, the torque will decrease by the square of the speed decrease and the horsepower required decreases by the cube of the speed decrease.

As an example, when the speed of a variable torque load is reduced by 50% or one half, the torque required to drive the load is reduced to one-quarter or 25%. The horsepower is reduced to the speed cubed, which is 1/8, or 12.5% of that required to drive the load at full speed.

Service Factors

Service factor FS is a coefficient that enables to take into consideration real operating conditions of the gearbox. In order to make a correct selection, the service factor of the gearbox must be higher than the one required for the application.

Table below gives indications for the right selection having determined the classification of the uniformity of operation.

A) Uniform operation

Light screw conveyors, fans, assembly belts, light conveyor belts, small agitators, elevators, cleaning machines, filling machines, testing machines and belt conveyors.

B) Moderate shocks, non-uniform operation

Decoilers, feed drives for wood processing machines, hoists, balancing machines, tapping units, heavy conveyor, belts, winches, sliding doors, stall dunging machines, packaging machines, cement mixers, crane traveling mechanisms, mills, bending machines and gear pumps.

C) Heavy shocks, extreme non-uniform operation

Stirrers and mixers, shears, presses, centrifuges, rolling stands, heavy winches and lifts, grinding mills, stone crushers, bucket elevators, punching machines, hammer mills, eccentric presses, folding machines, roller tables, tumbling barrels, choppers, shredders, vibrators.

Correction factor fs have to be applied for Gear Boxes operating with more than 10 starts per hour.

Mass Moments Inertia of Bodies

Mass moments inertia (J) of bodies in Kgm²

Technical characteristic of a three-phase AC motor:

Synchronous speed of a three-phase AC motor:

Star and Delta Connections

IEC60034-7 Mounting Arrangement (IM) Code

The following table shows the mounting arrangement (IM) Code with reference to IEC60034-7 Standard.
Code II (Code I)

Remarks:-

It is important to nominate the “IM” code to ensure that drain holes are in the correct position and bearing arrangement is checked for suitability if the “IM” code differs from standard.