Selection of the reducer
Reducer selection method
In order to carry out a correct reducer selection, the following main data must be known:
 Power absorbed by the driven machine, corresponding to the power at the reducer low speed shaft PL [kW];
 Reduction ratio
 Input shaft rotation speed [rpm].
Referring to the reduction ratio and the application, the type of reducer (parallel or right angle axis) and the number of reduction stages (1, 2, 3 or 4) is selected; therefore the mechanical efficiency comes from the table.
Basing upon the application and the operation of the reducer, the required service factor can be determined, which measures the safety margin intended for the project, representing the ratio between the maximum power transmissible by the reducer and the power really applied in operating conditions.
Once the service factor FS is known, the minimum necessary nominal power of the reducer can be calculated:
In the end, from the table of the nominal powers of the selected reducer type (C1, C2, C3, C4, K1, K2, K3 or K4), basing upon the reduction ratio and the input speed, the reducer with nominal power at least equal or higher than the here above calculated power can be selected.
Then, the need for forced feed lubrication shall be checked (symbol "•" on the power tables).
The natural thermal rating (corrected if necessary with the relevant coefficients of tables 3, 4 and 5 ) shall be lower than the power absorbed by the reducer during operation; if this is not the case, auxiliary cooling media shall be adopted (fan, cooling coil, external oil coolers...) or, if these are not acceptable, select a larger size so as to satisfy the thermal design requirements with the reducer natural cooling capability.
Then, it is necessary to check that the momentary overloads to which the reducer may be subjected (starting torque or shocks) are not higher than 2.5 times the torque corresponding to the nominal power of the selected reducer.
Please note that, in order to avoid unnecessary overloads at startups, it is a good design practice to select a driving motor not excessively oversized with respect to the power which is really required by the driven machine.
In any case, when the reducer is subjected to frequent startups, a reduction factor of the nominal power of the reducer itself must be considered, deduced from table 2 as a function of the type of application and of the number of starts per hour.
Reducer selection example
Application description:
parallel shaft speed reducer for a fan, driven by an electric motor with continuous operation for 24 hours a day.
Design data:
absorbed power at the low speed shaft:
 450 kW
 input speed: 1500 rpm
 output speed: 165 rpm
 ambient temperature: 30°C

cooling water temperature: 30°C
Calculation of the theoretical reduction ratio and reducer type selection:
the theoretical gear ratio is:
Minimum required nominal power calculation:
the minimum nominal power needed may be calculated as a function of the power absorbed at the low speed shaft, of the reducer efficiency and of the required service factor:
PN=1.5x(450/0.98)=688.8 kW.
Reducer size selection:
from the C2 type reducers power table, for the nominal ratio 9, the suitable size is selected as 320; in fact, the reducer type C2320 has a nominal power of 722 kW at 1500 rpm, greater than the 688.8 kW required.
Hence, the actual service factor will be 722/450=1.6.
Thermal rating check:
the values of 213 kW, 389 kW and 625 kW come from the thermal ratings table of the C2 type reducers, respectively for the natural thermal rating, for the application of a cooling fan and for the application of a cooling coil.
These three values are modified by the coefficients depending upon the ambient temperature (table 3), the duration of operation (table 4) and the cooling water temperature (table 5), obtaining the following values:
Thermal rating with natural cooling:
213 x 0.84 x 1 = 178.9 kW
Thermal rating with fan:
389 x 0.85 x 1 = 330.6 kW
Thermal rating with cooling coil:
[(625213) x 0.91+213 x 0.84] x 1= 553.8 kW
Therefore, the adoption of a cooling coil is necessary, since a mechanical power of 450 kW is continuously applied to the reducer.
Additional considerations:
the exact value of the gear ratio is 9.000, while the required value is 9.09; the oil splash lubrication is acceptable; the type of construction shall be selected (see figure).