The engine or compressor vibration analysis determines joint displacement due to unbalanced forces. Joint displacements can be compared versus various allowable deflection specifications and expressed as displacement unity check ratios.
Analysis Type
The analysis label ‘ENGV’ must be entered in columns 7-10 on the DROPT line for engine or compressor vibration analysis. Load Options
Engine unbalanced loading is entered in the form of mechanical unbalanced forces, gas torques or general unbalanced forces. Loading and load options are defined using the ENGVIB, RSPEED and UNBAL lines following the LOAD header line.
Note: Each set of loading requires a separate set of RSPEED and UNBAL lines. Damping Method
Only structural damping input on the SDAMP line is considered for engine vibration analyis.
Note: Because fluid damping is not supported, the FDAMP line should not be used for engine vibration.
Engine Speed Parameters
Engine speed parameters are designated on the ENGVIB line. The beginning speed (the lowest speed) and the ending speed (the highest speed) are specified in columns 7-13 and 14-20, respectively.
The running speed range is defined by the beginning and ending speeds. The program divides the speed range into increments for the purpose of the analysis using either constant increments or varying increments based on modal frequencies. Specify one of the following incrementation methods in columns 21-23:
‘CON’ - Constant incrementation
‘MAH’ - Same as ‘MOD’ except that each harmonic frequency is also included ‘USR’ - Analysis speeds defined by the user using USRSP lines
For constant increments, the speed increment value entered in columns 24-28 is the total number of speed points to analyze. For varying increments on the other hand, the value entered in these columns is used to determine the smallest speed increment allowed between modes.
The following shows the input for a begin speed of 100 rpm and ending speed of 500 rpm using constant speed increments.
User defined running speeds are specified using the USR speed option on the ENGVIB line and USRSP lines immediately following the ENGVIB line. For example, the following input designates analysis running speeds of 120, 150, 200, 300, and 400 rpm.
Nonlinear Interpolation Power
By default, 2.0 is used as the nonlinear interpolation power. Enter the nonlinear interpolation power override to be used for any mechanical unbalanced forces interpolated nonlinearly between running speeds in columns 29-33. If the interpolation power is p, then the interpolation is accomplished as follows:
Calculation Points per Cycle
By default, 10 points are calculated for the highest harmonic determined. This value may be overridden by specifying the maximum number of points calculated for the highest harmonic in columns 37-39 on the ENGVIB line.
Twenty points are calculated for one cycle of the fundamental frequency by default. Enter the minimum number of points to be calculated per cycle in columns 34-36 if this value is to be overridden.
Note: If harmonics are encountered, the number of points calculated per cycle is the maximum of the number of points per fundamental and the number of points calculated per harmonic.
Allowable Option
The deflections determined by the program can be compared to published allowable deflections and expressed as displacement unity check ratios. The allowable option is specified in columns 40-41 as follows:
‘DL’ - D Line Allowable ‘SN’ - SNAME
‘ML’ - US Military Specification ‘VE’ - Maximum Velocity
The following illustrates the input required for displacement unity check ratios to be determined using the D Line allowables.
If the allowable option is ‘VE’ (maximum velocity), the maximum velocity allowed is entered in columns 42-46. Joint Selection
By default, all joints are monitored in the engine vibration analysis. Joints may be optionally selected to be included using the JNTSEL line. For example, the following designates that only joints 101, 103, 105, 107, 109 and 111 are to be monitored in the analysis.
Note: As many JNTSEL lines as required may be used. If JNTSEL lines are specified, only joints specified are monitored during the analysis.
Unbalanced Force Input
Engine unbalanced forces may be expressed in terms of gas torques, mechanical unbalanced forces and/or general unbalanced periodic forces at various running speeds. For a particular running speed, unbalanced forces can be input as separate load conditions where maximum response from each is added and/or may be specified in the same load condition if phase angles between unbalanced forces is known.
Each load condition that unbalanced forces are to be defined is designated by a RSPEED line with the running speed designated in columns 9-15. Gas Torque Loading
Unbalanced forces due to gas torque may be expressed as maximum gas torque at various harmonics or in the form of a total gas torque curve.
When unbalanced forces due to gas torque are known for various harmonics, loading is specified in the form of a periodic loading using UNBAL lines specified immediately after the RSPEED line.
The joint to which the load is applied is designated in columns 8-11. The force type ‘SIN’ is used for load described by a single sine wave (amplitude and phase angle) and must be designated in columns 12-14. The forces acting on the joint are designated in columns 17-58.
Enter the phase angle in columns 59-65, the interpolation type, either ‘LN’ or ‘NL’, in columns 66-67 and the harmonic number in columns 68-69.
The program allows loading to be grouped and considers each load group to act independently. The maximum displacements resulting from each load group are summed together to determine the total displacement. The load group to which this force is assigned is stipulated in columns 70-71.
For example, a gas torque about the global X axis at joint 107 is known for the first 3 harmonics at a running speed of 300 rpm. Since phasing is known, each value is to be assigned to the same load group, load group 1. Linear interpolation is to be used between running speeds.
A total gas torque curve may be input as a series of equally spaced points in time using an UNBAL line and LDFACT lines immediately after the RSPEED line. The joint to which the load is applied is designated in columns 8-11. The force option ‘TIM’, designating force input by a series of equally spaced time points, must be designated in columns 12-14. The applied forces acting on the joint are designated in columns 17-58.
Enter the interpolation type, either ‘LN’ or ‘NL’, in columns 66-67 and the highest harmonic number to be used from the Fourier series in columns 72-73. The load group to which this force is assigned is stipulated in columns 70-71.
Note: The harmonic number and phase angle fields must be left blank when inputting load described by equally spaced time points as designated by the ‘TIM’ force option.
The following example shows a total gas torque curve for moment about the global X axis at joint 107 defined at 300 rpm. The curve will be defined at 18 degree
increments (20 points). Each value on the curve is to be assigned to the same load group, load group 1. Linear interpolation is to be used between running speeds and the highest harmonic number to be used is 10.
Mechanical unbalanced forces are specified in the form of a periodic loading using UNBAL lines specified immediately after the RSPEED line.
The joint to which the forces are applied is designated in columns 8-11. The force type ‘SIN’ is used for load described by a single sine wave (amplitude and phase angle) and must be designated in columns 12-14. The forces acting on the joint are designated in columns 17-58.
Enter the phase angle in columns 59-65, the interpolation type, either ‘LN’ or ‘NL’, in columns 66-67 and the harmonic number in columns 68-69.
The program allows loading to be grouped and considers each load group to act independently. The maximum displacements resulting from each load group are summed together to determine the total displacement. The load group to which this force is assigned is stipulated in columns 70-71.
For example, a compressor has primary and secondary mechanical unbalanced forces that create moments about the global Y and Z axes which are phased 90 degrees apart. At 300 rpms, the primary and secondary unbalanced forces create 16000 in-kip and 2600 in-kip moments about the Y axis and 2400 in-kip and 750 in-kip moments about the Z axis respectively, applied at joint 107. Because the unbalanced forces are assumed to vary with the square of the running speed, nonlinear interpolation with a power of 2 is to be used.
General Unbalanced Periodic Forces
General unbalanced forces may be input in the form of time history or periodic loading.
When unbalanced forces are known for various harmonics, loading is specified in the form sine waves of a known amplitude and phase angle using UNBAL lines
specified immediately after the RSPEED line. Unbalanced forces may also be input in the form of a time history with equally spaced time points using an UNBAL line and LDFACT lines.
In either case, the joint to which the load is applied is designated in columns 8-11. The force type, either ‘SIN’ for single sine wave or ‘TIM’ for time history, must be designated in columns 12-14. The forces acting on the joint are designated in columns 17-58.
Single sine wave type loading requires the phase angle in columns 59-65 and the harmonic number in columns 68-69 while the time history type requires only the highest harmonic to be used from the Fourier series in columns 72-73.
The interpolation type, either ‘LN’ or ‘NL’, must be designated in columns 66-67. The load group to which this force is assigned is stipulated in columns 70-71.
Note: The program allows loading to be grouped and considers each load group to act independently. The maximum displacements resulting from each load group are summed together to determine the total displacement.
The following illustrates an unbalanced force along the global Z and about the global X axis at joint 107. The force is known for the first 3 harmonics at a running speed of 300 rpm and each value is to be assigned to the same load group, load group 1. Linear interpolation is to be used between running speeds.
The following example shows a force time history input for moment about the global X axis at joint 107 defined at 300 rpm. The curve will be defined at 18 degree increments (20 points). Each value on the curve is to be assigned to the same load group, load group 1. Linear interpolation is to be used between running speeds and the highest harmonic number to be used is 10.
Output Options
The engine vibration analysis calculates generalized forces and joint displacements for the various conditions defined. Joint displacements may be compared to allowable displacement curves and expressed in terms of a displacement unity check ratio.
Generalized Forces
Generalized force print options are designated in columns 53-54 on the ENGVIB line. Enter ‘PT’ for the standard generalized force print or ‘FL’ for a full print. Joint Results
Joint results for all joints that exceed the allowable displacement at any running speed may be printed by specifying ‘PT’ in columns 57-58 on the ENGVIB line.
Joint displacements may also be plotted by specifying ‘PL’ in columns 55-56. Be default, all joints are plotted when the joint plot feature is instigated. Joints to be plotted may be designated using the JNTPLT line following the ENGVIB line.
For example, the following designates that joint results are to be plotted for joints 101, 103, 105 and 107.
Plot Options
Optional plot options may be designated using the PLTOPT line. Up to three allowable curves may be plotted on the joint displacement plots. Designate the allowable curves to be plotted in columns 11-16 as follows:
‘DL’ - D-Line Allowable ‘SN’ - SNAME
‘ML’ - US Military Specification
Enter ‘GR’ in columns 35-36 if grid lines are to be included on the plots. Plot size and character sizes may also be specified in columns 17-34.