Gearbox Selection
First of all, gearbox selection is very important to ensure a flawless operation and a long lifetime. To make the right selection, required technical information and operating conditions for the gearbox should be provided during the order process.
The most appropriate gearbox is selected taking into consideration the technical information and operating conditions stated below. The selection can be made either by the technical staff of the company which will be using the gearbox or by our technical staff upon the submission of the necessary information. If there is more than one gearbox model adequate for the job or project, the most economical option is selected.
Gearbox selection is based on the following criteria:
• Mechanical data of the gearbox
• Type and specifications of the driving motor
• Type and specifications of the driven machine
• Operating conditions
• Environmental conditions
• Cooling possibilities
Mechanical Data of the Gearbox
Power Transmitted by the Gearbox: Pre (kw)
The power of the gearbox Pre, is calculated by multiplying the power required for the driven machine, Piş (kw), with the operation safety factor F.
Pre = F . Piş (kw)
Operation safety factor calculation is explained in a separate section and the factor can be taken from the table.
Gearbox Input Speed n1 (rpm)
Gearbox Output Speed n2 (rpm)
Gearboxes are generally used for speed reduction. Higher speed of the motor is reduced to lower speed required for the driven machine. Gearbox input speed is higher than output speed. n1>n2
The shaft of the gearbox connected to the motor is input shaft and the shaft connected to the driven machine is output shaft.
If the gearbox is connected to the motor with a coupling, the input speed is equal to the motor speed.
If the gearbox is connected to the motor via a belt system, the input speed of the gearbox changes according to the ratio of the pulleys.
Reduction Ratio i = n1 : n2
The ratio of the input speed to the output speed is the reduction ratio.
Gearbox Type
The gearbox type suitable for the project can be chosen from our product range shown in our catalogue. There can be more than one type of gearbox suitable for the same job. The price, delivery time, ease of assembly, space available for the gearbox and operating conditions play an important role for the selection process. Project engineers should select the type of gearbox in the project phase.
At low power ranges, motor gears are preferred for economical reasons and easy mounting advantages.
Monuting Positions of Shafts
The input and output shafts of gearboxes can be arranged in different positions. The mounting positions of the shafts are shown in dimension pages for each gearbox type.
For example, the input and output shafts of DA2 model parallel shaft gearbox can be arranged in four different positions. These are respectively; with right input – left output, with left input – right output, with right input – right output, and with left input – left output.
The requested mounting positions for the shafts of gearboxes without motor should be stated during order according to the descriptions on dimension pages.
Special Requests
The gearbox can be equipped with backstop, torque arm, electromagnetic brake or electromagnetic coupling.
The diameters and lengths of input and output shafts of gearbox can be different than given values on dimension pages. In some gearbox types there’s also the possibility to put double input shafts and double output shafts. All special requirements should be stated during order.
Type and Specifications of the Driving Motor
All types of electric motors, turbines, hydraulic motors, diesel and gasoline motors can be used as drive machine.
The electric motor for the motor gear can be supplied either by us or by the customer.
The power of the motor to be connected to the gearbox should be selected greater than the power Piş required for the driven machine. As in gearbox selection, the motor power is calculated by multiplying the required power for the driven machine by a certain safety factor. This safety factor and the safety factor for gearbox selection don't have to be the same. It is not economical to choose the motor power greater than necessary since operating costs might increase due to excessive use of electricity.
According to type of plant and daily working hours, the motor safety factor can be taken between 1.25 and 1.8.
At the time of order, the motor safety factor or the actual power required (Piş) for the driven machine should be stated.
The ideal motor speed to run the gearbox is 1500 and 1000 rpm. It is not recommended to use high speed motors with the gearbox. High speed increases the noise level of the gearbox as well as decreases the lifetime of the bearings and gears. If it is necessary to use a 3000 rpm motor, it is recommended to decrease the gearbox input speed to 1500 or 1000 rpm by placing a V-belt pulley between the motor and the gearbox.
Connecting Motor to Gearbox
Elastic couplings, hydraulic couplings and other special coupling types and V-belt pulleys can be used as connecting elements. Using rigid couplings at gearbox input should be avoided.
The type (radial or axial), magnitude, position and direction of the external forces on the input shaft of the gearbox should be stated.
Necessary Information About the Driven Machine
Type of Driven Machine
It is very important to know the type of driven machine in gearbox selection. There are four main groups:
1. Uniformly operating machines without shock loads
2. Machines with moderate shock loads
3. Machines with heavy shock loads and high moment of inertia
4. Machines with very heavy shock loads and very high moment of inertia.
Required Power P (kw) to Run the Machine at Full Load
The required power Piş and the speed for the machine is calculated by the technical staff. Similar machines can also be taken as reference.
Speed of the Machine niş (rpm)
The speed for the machine is also calculated by the technical staff. Or similar machines can be taken as reference.
Connecting the Gearbox to the Driven Machine
Elastic couplings, rigid couplings, gear coupling and other special coupling types, chain mechanisms, spur gears and bevel gears can be used as connection elements.
External Forces on the Output Shaft of the Gearbox from Coupling System or Driven Machine
Type (radial or axial), magnitude, position and direction of the forces should be stated.
Operating Conditions
The important factors are: average daily operation hours, number of stop-starts within an hour, operation time (as percentage) under load in an hour, whether the driven machine is loaded uniformly or variably and the extent of shock loading (low, medium, high) if the machine is subject to shock loading.
It should be stated whether a flywheel will be put to decrease the effect of variable loads and shock loading. If a flywheel will be added, the mass momentum of the flywheel and other technical specifications should be considered.
Environmental Conditions
The important factors are: lowest and highest average temperatures of the place where the gearbox will operate, whether the gearbox will work in a closed or open area, whether there is excessive dust, moisture, water or a heat source to affect the gearbox at the operation place.
Cooling Possibilities
The important factors are: if a forced cooling system is needed, whether there is tap water, well water or other source of water and the temperature of the water to be used.
Gearbox Selection Stages
After above mentioned criteria are considered, gearbox selection will be done in two stages:
1) Selection of gearbox type
2) Determining the size of the selected gearbox type
Selection of Gearbox Type
The gearbox type suitable for the driven machine is determined during project phase. The technical staff designing the project decides on the type of gearbox according to his past experiences or examining previous applications.
For systems requiring less power, motor gears are the recommended solution. Motor gears are less expensive and are easily mounted. Shaft mounted gearboxes are also preferred due to same reasons.
Determining the Size of the Selected Gearbox
Gearbox size can be determined with the help of the gearbox power tables. For each gearbox type a separate power table is laid out. The table shows the maximum power (kw) a given gearbox size can transmit according to the input speed and transmission ratio. The output torque the gearbox can transmit is also given in a separate table.
The power and torque values given in tables are nominal values. The power and torque needed for the driven machine should be less than the nominal power and torque the gearbox can transmit so that the gearbox can operate at a certain safety.
The power Pre, the gearbox should transmit, is determined by multiplying the power Piş needed for the driven machine by the operation safety factor. The nominal power of the selected gearbox should be equal to or more than the Pre value.
Please find below the method to calculate the operation safety factor.
Operation Safety Factor (F):
Coefficients determining the factor:
1) The coefficient for the driven machine, fiş
2) The coefficient for the driving motor, fm
3) The coefficient for the daily operation time, ft
4) The coefficient for the start-up number, fdk
Operation safety factor F, is attained by multiplying the four coefficients:
F = fiş . fm . ft . fdk
The Driven Machine Coefficient (fiş)
This coefficient depends on the type of driven machine. Driven machines can be classified in four main groups according to their level of shock loading and moment of inertia.
Uniformly operating machines without shock loading: For machines with constant power demand, which operate without shock loading and without sudden moment increase, the coefficient fiş is taken as 1.0.
Machines with medium shock loading: For machines which operate with medium shock loading and where load increases or decreases by at most 50%, the coefficient fiş is taken as 1.5.
Machines with heavy shock loading and high moment of inertia: For machines with high inertia which operate with heavy shock loading and where load increases or decreases by at most 100%, the coefficient fiş is taken as 2.0.
Machines with very heavy shock loading and very high moment of inertia: The coefficient fiş is taken as 2.5-3.0 for this group.
The types of machines within these four main groups are given in operation safety factor table.
Driving Motor Coefficient ( fm )
This coefficient is determined according to the type of motor driving the system. There are three main groups:
1.Group: Electric motors (Asynchronous, synchronous, direct current motors), steam turbines, hydraulic motors. The fm for this group is taken as 1.0.
2.Group: Internal combustion, 4 - 6 cylinder engines (gasoline or diesel), water turbines. The fm for this group is taken as 1.25.
3.Group: Internal combustion, 1 - 3 cylinder engines (gasoline or diesel.) The fm for this group is taken as 1.5.
Daily Operation Time Coefficient ( ft )
If the daily operation time is between 3 - 10 hours, the ft coefficient is taken as 1.
If the daily operation time is less than 3 hours, the ft coefficient is taken as 0.8.
If the daily operation time is between 10 - 24 hours, the ft coefficient is taken as 1.25.
Number of Start-ups Coefficient ( fdk )
If the system makes at most five start-ups in an hour, the coefficient fdk is taken as 1.0.
If the system makes more than five start-ups in an hour, the coefficient fdk is taken between 1.25 – 2.0. In this case, special precautions might be needed. Connecting the gearbox to the motor with hydraulic or electro-mechanical coupling will reduce the effect of shocks during stop-starts.
Calculation of Operation Safety Factor
The safety factor (F) for a system operating 24 hours daily with an electric motor, making at most five start-ups in an hour, with moderate shocks:
F = fm . fiş . ft . fdk = 1 . 1.5 . 1.25 . 1 = 1.875.
F can be taken as 1.8 or 2.0.
Operation safety factors (F) are given in the table on page 18. In this table, driven machines are classified into four groups; without shock loading, medium shock loading, with heavy shock loading, and with very heavy shock loading and high moment of inertia. This classification is a guidance based on past experiences. Considering economical and safety factors together, the operation safety factor can be taken lower or higher than the value in the table.
Thermal Power (Pt)
This is the power the gearbox can transmit without heating up. Thermal power of gearbox depends on the size of external surface of the gearbox housing, ambient temperature, operation area (covered place or open air) and the operation time in an hour (FD).
Thermal power values each gearbox size can transmit are given in gearbox power tables. Thermal power values (Pt) given in tables are valid for gearboxes operating in a closed area, with 20 - 30°C ambient temperature and for continuous operation (FD = 100%).
For 40°C ambient temperature, 75% of the value given; for 50°C ambient temperature, 60% of the value given in the table should be taken.
If the gearbox is operating 30 minutes in an hour (FD = 50%), Pt can be increased by 25%. For an operation of 15 minutes in an hour (FD = 25%), Pt can be increased by 50%.
If the gearbox is cooled with air or the gearbox oil is cooled, thermal power can be increased by 10% - 100% depending on the effect of the cooling.
Determination of Gearbox Size
Torque required for the driven machine is determined either by calculation or by past experience. Speed suitable for driven machine is again found either by experience or by trial and error method.
The power required to obtain the required torque for the driven machine at the selected speed is calculated in (kw) as:
Mdiş . niş
Piş = _______________
9550
Provided that torque required for the driven machine stays constant, if the machine will operate at variable speed, power required should be calculated considering the highest speed.
Operation safety factor F can be taken from the table taking into consideration type of driven machine, motor and daily operation time.
The power gearbox should transmit (Pre) is obtained by multiplying power required for driven machine (Piş) and operation safety factor. The Pre value is the key for gearbox selection.
Pre = F . Piş
The power table for selected gearbox type shows the nominal power Pn (kw) a given gearbox size can transmit according to the input and output speed. The gearbox size having a nominal power greater than or equal to the Pre should be selected from the table.
The Pre value calculated should be less than the thermal power (Pt). ( Pre < Pt ).
If Pre is close to or greater than thermal power, gearbox should be forced cooled. If there is no cooling possibility, a greater size should be selected.
Hence, gearbox selection is completed.
