RomaxDesigner Questions and Answers This document is intended to answer many frequently asked questions about the RomaxDesigner software and the calculations within it. The document is split up into four sections, dealing with the four main topics of discussion about RomaxDesigner: - Conventions - Shafts - Bearings - Gears The information mainly relates to the Core Modeller, with a few topics relating to additional modules, such as Gear Optimisation and Gear Micro-Geometry. Table of Contents Section 1: RomaxDesigner Conventions 4-3 10. What are the Sign Conventions for Shaft Static Analysis? 11. What are the Sign Conventions for Gear Micro-Geometry? 12. What are the Sign Conventions for Misalignment? 13. What are the Right and Left Sides of a Node? 14. How does RomaxDesigner Calculate Duty Cycle Damage?
Section 2: Shafts Questions and Answers Guide 4-13 20. How are the Gear Loads Applied to the Shaft?
10 What are the Sign Conventions for Shaft Static Analysis? In RomaxDesigner the shaft static analysis results contain a large number of graphs relating to the Analysis. Understanding these graphs is dependent on the user understanding the sign conventions, as laid out below. 10.1 Fundamentals of the RomaxDesigner Sign Conventions There are three fundamental rules to follow in the application of RomaxDesigner's sign convention: 1. The co-ordinate system is a right hand set of axes 2. All rotations obey the right hand screw rule 3. A plane can be defined by a single, perpendicular vector or two non-parallel lines running through a common point.
These are all universally accepted conventions, but are explained and related to RomaxDesigner below. 10.2 Definition of a Right Hand Set of Axes A right hand set of axes is one such that, if you look out from the origin, in a positive X, Y and Z direction, you will see the X, Y and Z axes appear in order in a clockwise direction. 10.3 Definition of the Right Hand Screw Rule There are a number of different ways to define the right hand screw rule, but they lead to the same thing. Two definitions are given below. 10.3.1 Definition 1 The right hand screw rule states that, if you grip your right hand around the axis with your thumb pointing in the positive direction along the axis, your fingers will point in the direction of positive rotation. 10.3.2 Definition 2 The right hand screw rule states that positive rotation about the X-axis is defined by moving from the Y-axis to the Z-axis.
The rule continues for Y and Z axes in the order XYZXYZ etc. 10.4 Definition of a Plane As was stated above, a plane can be defined by a single, perpendicular vector or two non-parallel lines running through a common point. Thus the same plane can have two definitions. The Y and Z-axes are clearly not parallel and meet at a common point (the origin). Hence they define a plane, which might be referred to as the "YZ Plane". However, both axes are perpendicular to the X-axis, hence the same plane can be defined as being perpendicular to the X-axis. 10.5 Implementation in RomaxDesigner 10.5.1 Shaft Deflections, Slopes and Rotations In the Shaft Design window the following co-ordinate system is used. Consider the following shaft deflection, and especially the shaft slope at the position shown: When defined in terms of dy/dz, the shaft slope is clearly positive. In other words, the slope of the shaft in the YZ plane is positive. However, when viewed in the plane perpendicular to the x axis, the rotation (or slope) of the shaft about the X axis is negative. In RomaxDesigner, shaft slopes are given as slopes about the perpendicular axis. When assessing the results make sure you interpret the shaft deflection appropriately. 10.5.2 Other Measures The following is a summary of how each of the other measures are defined in the RomaxDesigner shaft static analysis results sheet. Linear displacements are not included since it is assumed that these are self-explanatory. Most of this is fairly self explanatory except the moment diagrams. Note that the moment diagram in the XZ Plane is the integral of XZ Shear Force Diagram plus applied moments about the Y axis. This is because the plane defined by the X and Z axes is the same as that perpendicular to the Y axis. 12.1 Positive Misalignment Separation between the active flanks occurs towards the increasing Z direction (top end) of the face width of the pinion local Z axis 12.2 Negative Misalignment Separation between the active flanks occurs towards the decreasing Z direction (bottom end) of the face width of the pinion local Z axis 13 What are the Right and Left Sides of a Node? The basis of the distinction between the right and left of a node is that, where there is a change in section of the shaft, a node is formed. There are two values of the diameter, so given a constant torque being transmitted across a given section of the shaft, there is a different torsional stress. This is referred to as "to the right of the node" or "to the left of the node". The other occasion when this occurs is where there is a power load or a gear load that applies torque to the shaft, often across a section where the diameter is constant. Since the torque is applied as a point load, and a node is created at that point, the transmitted torques to the left and right of the node will be different and so will the torsional stresses. 14.1 About this Document The RomaxDesigner quotes Duty Cycle results as a percentage damage for bearings and gears. This document describes the theoretical background to Duty Cycle damage and how it is applied to components within RomaxDesigner. 14.2 Duty Cycle Definition RomaxDesigner allows the input of "lumped" loading information in the form of a Duty Cycle definition. The Duty Cycle consists of one or more load cases, representing a level of constant amplitude torque or power applied to the system and duration of the loading. 14.2.1 What is a Load Case? Each load case of the Duty Cycle consists of powerflow information as well as the load and duration. This specifies the route that the power takes through the transmission. In real life this is defined by selecting which clutches or synchronisers are engaged. A similar definition is used in RomaxDesigner. It is expected that each speed will have at least one load case associated with it. However, it is possible to define multiple load cases for each condition (e.g. 1st speed, 100% torque; 1st speed, 80% torque etc.). This allows comprehensive Duty Cycle data to be defined. 14.2.2 How do the Powerflow Analysis and the Static Analysis Differ? For each load case, the user defines the following data at the gearbox level: - Input (or output) power (or torque) and speed - Duration of the load case - Route that the power takes through the gearbox (the powerflow) This defines the boundary conditions for the gearbox. In order to derive the applied loads on all the components arising from this definition, the "Powerflow Analysis" needs to be run for that load case. Running the static analysis runs the shaft/bearing static analysis, which calculates the reaction forces and system deflections. 14.2.3 How do "Run All Powerflows" and "Duty Cycle" Analysis' Differ? When editing the Duty Cycle, the user has the option of selecting "Run All Powerflows" and "Duty Cycle". As described above, "Run All Powerflows" calculates the applied loads on the components arising from the load case definition; "Duty Cycle" calculates the reaction forces and system deflections. This is done for all load cases, and the total Duty Cycle damage calculated. 14.3 Duty Cycle Damage 14.3.1 The Linear Damage Rule The method of damage summation used in RomaxDesigner is the Palmgren-Miner "Linear Damage Rule", more commonly known as "Miner"s Rule". Miner"s Rule is a constant-amplitude, stress-life method used to sum the damage sustained by a component across a Duty Cycle. The rule states that the partial damage at any load case amplitude is linearly proportional to the ratio of the duration of that load case.  (Where C = 1)
This partial damage is shown as a percentage (or fraction) of life used up by a load case within a duty cycle. 14.3.2 Calculating Percentage Damage Take an example components that is loaded in two load cases: | Load Case 1 | Load Case 2 | Calculated Life | 50 hrs | 500 hrs | Required Life | 10 hrs | 150 hrs | Percentage Damage | 100 * 10 / 50 = 20% | 100 * 150 / 500 = 30% | Overall Duty Cycle Reuslts | Combined Percentage Damage = 50% |
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14.3.3 Interpreting Duty Cycle Damage The component life for the Duty Cycle as a whole is obtained by the Miner-Palmgren rule, which sums up fatigue damage to the bearing due to individual load cycles for each gear. Speed | Calculated Life (Hrs) | Required Life for Load Case (Hrs) | % Damage | 1 | 83.3 | 5 | 6 | 2 | 100 | 10 | 10 | 3 | 333.3 | 50 | 15 | 4 | 416.6 | 125 | 30 | 5 | 230 | 57.5 | 25 | R | 125 | 2.5 | 2 |
| Total Damage for Duty Cycle | 88% |
Figure 2 L10 Life and Miner-Palmgren Rule The total damage value of 88% represents the likely total percentage damage to the component arising from the duty cycle. 100% predicts that the component would be likely to fail. 50% predicts the component would be able to last two complete duty cycles. 14.4 What Duty Cycle Results are Available in RomaxDesigner? Duty Cycle results are available for any component where RomaxDesigner calculates a life calculation. It is necessary to highlight which methods are available and for which components. 14.4.1 Bearings RomaxDesigner has two methods for calculating the life of a bearing: - ISO Life - standard ISO life rating, using radial load and dynamic capacity. - Adjusted (ISO) Predicted Life - ISO life including adjustments for misalignment and radial internal clearance. So there are two versions of the Total Duty Cycle Percentage Damage: - ISO Percentage Damage - percentage damage using the ISO Life calculation. - Adjusted (ISO Percentage) Damage - percentage damage using the Adjusted Life calculation. These results are summarised in a Bearing Duty Cycle Report, which adds the following for each bearing: - ISO Pass/Fail - "Fail" is given if the ISO percentage damage is greater than 100%. - Adjusted Pass/Fail - "Fail" is given if the Adjusted percentage damage is greater than 100%. 14.4.2 Gears RomaxDesigner provides the following methods for calculating gear life: - ISO 6336 (method B) - AGMA 2001 For each load case, these rating methods calculate the following parameters: - Percentage Damage in Contact for the pinion - Percentage Damage in Bending for the pinion - Percentage Damage in Contact for the wheel - Percentage Damage in Bending for the wheel For a Duty Cycle, the results are combined across the load cases using Miner's rule to give: - Total Percentage Damage in Contact for the pinion - Total Percentage Damage in Bending for the pinion - Total Percentage Damage in Contact for the wheel - Total Percentage Damage in Bending for the wheel. Overall Pass/Fail - The highest of the four percentages is taken. A gear set is said to fail if any of the total percentage damages is greater than 100%. Shafts Questions and Answers Guide This guide is intended to help users with common issues related to Shafts and other related components. There are separate guides on the subject of Bearings and Gears. The sections covered are: Since the RomaxDesigner Shaft model is a Finite Element model the distributed loads are applied as a series of point loads. The Shaft/Bearing model creates a number of nodes at the appropriate positions and applies the correct forces. The number of points across which the loads should be distributed can be set by the user (see the "General Tips" section on Modelling and Analysis). The diagram below shows a simple shaft with the load distributed across 5 nodes. The load distribution can be seen. Note that the load has not been distributed across the entire face width of the gear. The software has split the face width of the gear into five equal strips and the appropriate load has been applied at the centre of the strip. | 
