Industrial gearboxes are among the most crucial equipment used in almost every
industry. They are primarily used to modify rotational speed received from the input
motor, often reducing rotational speed and thereby increasing torque in many
applications.
This study focuses on the analysis and redesign optimization of the rolling mill gearbox
at Sima Foulad Jahan The gearbox in question is a two-stage gearbox with a ratio of
5.7, designed to handle 1500 kW of power and an input speed of 700 RPM. It features
double helical gears for efficient operation.
This gearbox had specific geometric specifications and recurrently experienced gear or
shaft failures. The aim of this study was to analyze the gearbox using KISSSOFT
software. After conducting the analysis, design errors were identified, and
recommendations for modifications were proposed. Subsequently, the gearbox was
reanalyzed using KISSSOFT software to ensure its optimal performance.
Based on the results obtained, by changing the helix angle of the gears from 12
degrees to 5.14 degrees and adjusting the gear coefficient by 10%, significant
improvements were observed. The fatigue safety factor in tooth bending and contact
stress failure improved by 15% to 20%. The figures below illustrate the specifications
of the studied gearbox and the results obtained from the analysis after the
modifications were made.
Specifications of the gearbox in question
The studied reduction gearbox is a two-stage gearbox used to reduce the rotational
speed of the motor and increase torque for the rolling process in the roughing mill. This
gearbox is considered a high-power unit that requires careful attention to design and
appropriate material selection.To optimize and analyze the design of this gearbox, its
components and elements were analyzed using KISSSOFT software.
The general specifications of this gearbox are as follows:
One of the most critical considerations in selecting an appropriate gearbox is designing
it with the optimal helix angle for the gears. In this study, using KISSSOFT software, a
range of acceptable angles for the helix angle was determined.
The accepted range for the helix angle, based on operational conditions, was found to
be between 7 to 20 degrees. The figures below illustrate the variations of this angle
considering the most important parameters. Based on the observed results and the
selection of the best helix angle, the optimal angle of 5.14 degrees was chosen, and all
subsequent redesign calculations were based on this determination.
Other parameters were specified for the initial gearbox model with their respective
initial values.
Modeling shafts and bearings in KISSSOFT
Modeling of shafts and bearings in KISSSOFT software was conducted to examine the
behavior of gearbox shafts and bearings and compare them with calculated values
based on standards. The image below shows the model of each shaft in the software.
Results extracted from the analysis in KISSSOFT software
Results extracted from the analysis in KISSSOFT software revealed that using a
12-degree helix angle for the gears led to failures indicated as errors or warnings in the
software. Additionally, over the years of operation, the gearbox experienced failures in
specific parts such as gear number 2.
Changing the helix angle did not resolve observed errors in the software, but it did
reduce bending and contact stresses on the gears. However, adjusting the angle
necessitated corresponding changes in the gear correction factor to ensure gear
contact compatibility. These adjustments ultimately resulted in the improvement and
correction of the gear and gearbox assembly.Below are the results obtained from
KISSSOFT software analysis. Due to the importance of shafts 1 and 2, results are
focused solely on these two axes.
Author: Dr. Ebrahim Ebrahimi (Head of Research and Development Unit, Shokri Industrial
Group)
