How to size a geared motor?

What is a mechanical gearbox?

A mechanical gearbox, also known as a speed reducer, is a device used in various industrial, agricultural, and other engineering systems to reduce the rotational speed of an electric or hydraulic motor. Its primary role is to decrease the output speed while increasing the output torque. This allows the system's power to be adapted to specific needs.

Mechanical gearboxes are commonly used in various fields such as industry, automotive, robotics, and other applications where it is necessary to reduce the speed of an application while increasing the torque to be transmitted.

They are composed of a set of gears that work together to reduce the rotational speed. Gears can be designed in different ways, including spur gears, helical gears, bevel gears, a bronze worm wheel and a worm screw, etc. By choosing the appropriate gear type and reduction ratio, the rotational speed can be adjusted according to the specific needs of the application.

What is the difference between a gearbox and a geared motor?

The difference between a mechanical gearbox and a geared motor is simple. A mechanical gearbox can be driven by a pulley, a sprocket, a cardan shaft, a flywheel, or any other type of accessory. A geared motor is a mechanical gearbox coupled to an electric motor or a hydraulic motor.

A geared motor can drive all types of industrial applications such as:

• Industry: Geared motors are used in industrial machinery such as conveyors, machine tools, presses, production lines, grinders, pumps, etc.
• Robotics: Geared motors are present in a multitude of robots installed in industry. They enable the entire application to move.
• Agricultural equipment: They are present in agricultural equipment such as potato diggers, spreaders, etc.
• Access control systems: They are found in automatic doors, barriers, and other access control systems.

In summary, geared motors are used in many applications where it is necessary to convert the power of an electric motor into mechanical motion at a specific speed and torque. They play a crucial role in many industries and contribute to the automation and efficiency of mechanical systems.

What are the different types of geared motors?

Many geared motors exist to drive industrial applications. We will present the main types and explain the advantages and disadvantages of the different geared motors present in the industry.

• Worm gear motors: These geared motors use a steel worm screw and a bronze gear (worm wheel) to reduce speed. They are compact, economical, quiet, and can offer a high reduction ratio.
• Helical geared motors: Helical gears have oblique teeth, which reduces noise and improves smooth operation compared to spur gears. These geared motors are suitable for applications requiring quiet power transmission.
• Spur geared motors: These geared motors use spur gears to reduce rotational speed. They are simple, compact, and efficient, suitable for high torque applications.
• Bevel geared motors: These geared motors use bevel gears to achieve speed reduction. They are often used in applications requiring power transmission at right angles.
• Parallel shaft geared motors: These geared motors have spur or helical gears; this type of geared motor is very popular because it is more compact than a cylindrical geared motor or a bevel geared motor.
• Planetary geared motors: These geared motors use a set of planetary gears to provide efficient power transmission with high precision and compactness. They are often used in applications requiring high torque and reduced space.
• Epicyclic geared motors: Also known as cycloidal geared motors, they use a set of epicyclic gears to provide reliable and precise power transmission.

What information is needed to replace a geared motor?

Much information is indicated on its nameplate. We will explain what each piece of information corresponds to.

1: The geared motor type:

The geared motor type consists of letters and numbers; the first letter "R" defines the geared motor type (cylindrical geared, worm geared, bevel geared, parallel shaft, etc.). The number "37" defines the gearbox size. The letters "DRE" define the motor series. The number "90L" corresponds to the motor size and type (compact, long, or short stator), and the number "4" defines the number of poles of the motor driving the geared motor. (2 poles = 3000 rpm, 4 poles = 1500 rpm, 6 poles = 1000 rpm, and 8 poles = 750 rpm)

2: The serial number:

We are using the nameplate of a SEW-USOCOME geared motor as an example, but the serial number is indicated on the nameplate of all geared motor manufacturers. It allows to find the specificities and options of the geared motor.

3: Frequency:

Electrical frequency, also called alternating current frequency, measures the number of complete alternating current cycles per unit of time. It is expressed in hertz (Hz), where one hertz equals one cycle per second.

In the context of power supply, frequency is an important aspect because it determines how many times the current direction changes each second. In most electrical systems around the world, the standard frequency is 50 hertz or 60 hertz. Countries and regions generally adopt one of these two frequencies.

• 50 Hertz: Used in most European countries, Asia, Africa, and other regions of the world.
• 60 Hertz: Used in North America, South America, Japan, and parts of Asia.

4: The rotational speed at 50Hz:

The first speed corresponds to the motor's rotational speed (at the gearbox input), and the second speed corresponds to the geared motor's output speed. The output speed corresponds to the input speed, divided by the reduction ratio.

5: The electric motor supply voltage:

The geared motor's voltage can be 230V single-phase or three-phase. The three-phase voltage can be 230V or 400V.

6: The electric motor power:

The electric motor's power is an important characteristic. It is expressed in Kw or Hp.

7: The absorbed current under load at 50Hz:

Some geared motor manufacturers indicate the absorbed current under load at 50Hz and 60Hz. It is different because the voltage is not the same at 50 and 60Hz.

8: The Cos Phi:

Cos phi, also called power factor (or cosine phi), is an important parameter in electrical systems. It measures the efficiency of converting electrical energy into useful energy, especially in systems using alternating current (AC). Cos phi is a value between 0 and 1, where 1 represents an ideal power factor.

Cos phi is defined as the cosine of the phase angle between voltage and current in a circuit. A power factor close to 1 indicates that voltage and current are in phase, meaning they simultaneously reach their maximum and minimum values. A lower cos phi, close to 0, indicates a greater phase shift between voltage and current.

A low power factor can lead to inefficient use of electrical energy in a system. This is often due to the presence of inductive loads, such as electric motors, transformers, etc. These loads induce a phase shift between voltage and current, thereby reducing the power factor.

9: The electric motor efficiency:

It allows calculating the absorbed power and thus the current consumed by the geared motor. The higher the efficiency, the lower the electricity consumption. Efficiency is between 0 and 1.

13, 14, 15 and 16: Power, current, power factor and efficiency at 60Hz:

The data indicated in 13, 14, 15 and 16 indicate power, current, power factor and efficiency at a frequency of 60Hz.

10: The frequency at 60Hz:

This frequency allows to find the characteristics of the geared motor at 60Hz. It allows the user to know the input speed and output speed at a usage frequency of 60Hz.

12: The supply voltage at 60Hz:

The supply voltage is not the same at 50Hz and 60Hz. The voltage at 60Hz will always be higher. The voltage 254-277V indicates that the motor must be coupled in delta if the mains voltage is between these values, and the voltage 440-480V indicates that the motor must be coupled in star if the mains voltage is between 440 and 480V.

18: The service factor:

This data is calculated by dividing the power of an electric motor installed on the geared motor by the thermal power of the reducer.

The service factor must be determined during the application design to ensure the geared motor's lifespan.

19: The reduction ratio:

It is determined based on the number of gear trains and the number of teeth of the geared motor's pinions. For example, the geared motor has two reduction stages. The first stage has a 14-tooth and 57-tooth pinion, and the second stage has a 12-tooth and 67-tooth pinion.

The reduction ratio of the first stage is calculated by dividing the number of teeth of the large pinion by the number of teeth of the small pinion (57/14 = 4.07), and the second reduction train is also calculated with the same formula (67/12 = 5.58). Assuming the input speed is 1450 rpm. The output speed of the geared motor will be calculated using the following formula. Divide the input speed by the first reduction ratio (1450/4.07 = 356.26) and re-divide this speed by the second reduction ratio of the geared motor (356.26/5.58 = 63.84). The output speed of the geared motor is therefore 63.84 rpm.

20: The output torque:

The output torque of the geared motor is calculated by multiplying the motor torque by the reduction ratio and the reducer's efficiency.

21: The mounting position of the geared motor on the application:

This characteristic is important for adjusting the reducer's oil quantity. It will be different if the geared motor is installed horizontally or vertically.

22: The oil type and quantity:

This information allows the maintenance department to know if the geared motor uses mineral, synthetic, or food-grade oil and to know the quantity to put in the reducer in case of maintenance.

23: The weight of the geared motor.