VIBRATION FOR COMPONENTS ON ENDOTHERMIC ENGINES
When designing engine-mounted components or when vibration tests have to be performed, the fundamental focus is the study of the number of vibration orders.
Typically the accelerometers will be mounted on the engine block or cylinder head to obtain excitation inputs. Accelerations are measured during the passage from the engine to idle at full load conditions with the throttle fully open.
The analysis is performed to study and extract the dominant motor orders. If there is a resonant frequency in the structure, the evidence will be seen in the results.
The test that will be performed to simulate this condition will be the sine sweep which will take into account the frequency and amplitude levels measured directly on the motor.
Optimum design and frequencies…
The objective of an optimal design, with respect to engine vibration, is to avoid any structural resonance within the engine’s normal operating regime.
The fundamental frequency range during the motor’s operating range can be determined as shown in Equation 1.
However, the dominant motor orders are more focused as vibration levels are generally highest among all motor harmonic orders. The dominant order frequency range can be obtained and may depend on the engine type as shown in Equation 2 (assuming four-stroke engines).
The I4 four-stroke engine runs from 600rpm to 6500rpm and has dominant order of the 2nd order frequency range from 20Hz to 220Hz. In addition to the resonant frequency itself, the amplification factor (Q) and damping are also important factors. As shown in Figure 1 for the case of an inline 4 motor, the resonant frequency of the component at 182Hz is fully second order. (stretch up to 220Hz). Figure 1 An example of the frequency response of an engine component
Figure 1 An example of the frequency response of an engine component Figure 2 Example Test RSTD
The Plastlab Laboratory with extremely qualified and competent personnel in the vibration sector performs and provides consultancy for vibration tests.
Resonant Frequencies…and the RSTD Test
Engine mounted components should ideally be designed with the first resonant frequency above the engine operating range.
However in practice there are components whose resonant frequencies cannot be improved easily. Often in these cases it is decided to operate trying to reduce the damping using silent block rubbers.
Once the component has been optimized in terms of damping, a test can be conducted at the resonance frequency for a number of cycles equal to the fatigue limit of the material with which the component was made.
The function to perform this type of test is called RSTD. An RSTD test involves performing a sine sweep to look for resonances of a structure, then dwelling on those resonances to investigate how the structure behaves. The sine sweep setup has all the features of sine wave vibration control whereby the user can specify the start and end frequencies, sweep speed and frequency resolution of the spectrum, i.e. points per sweep.
The duration of the test can be specified in cycles or in time duration. The parameter that allows the shaker to totalize the number of set cycles is the maximum set frequency change. Typically during the test, if the resonant frequency decreases, the machine stops when it reaches the maximum set threshold and this is normally an indication of component failure. If the frequency remains constant and in any case within the set range, the component will reach the number of cycles according to the settings, generally the one corresponding to the fatigue life limit.
For more information visit our site about laboratory tests!
A special thank you goes to Noris Vivarelli, a great expert in the vibration and fluid-dynamic sector, as well as Plastlab Laboratory Manager, who raises awareness on this issue so as to provide us with new content for these #labpills!