Let’s first look at the definition of vibration and what we need to know about vibration.
What is vibration?
Anyone with a basic knowledge of vibration should be clearly understood when it is to be accurately measured and used as an indicator of the mechanical condition of a machine. Technical vibration is defined as the oscillation of an object in relation to its resting position. It can be explained by the simple vibration system shown in the figure below.
If the object is moving, some of the upper and lower limits move back and forth. This mass motion in all its positions and back to the point where it is ready to repeat the motion is defined as a vibration cycle. The time it takes to complete this cycle is the vibration period. The number of these cycles at a given time (for example minutes) is the frequency of the vibration. Frequency is usually expressed in cycles per minute or cycles per second or “hertz”.
Find the cause of vibration
Vibration is always the result of one or more exciting forces, otherwise there will be no vibration. In rotating equipment, excitatory forces may be generated due to the following:
- Rotating part imbalance
- Loose parts
- Defects in the balances
- Bending of shafts or shaft connection sets
- Defects in bearings, gears or belts – air turbulence and so on
These are all defects in rotating equipment, and each effect creates a different type of vibration. The overall level of vibration is the result of all vibrations for different reasons. These are generally the most likely causes of vibration in rotating machines. This type of vibration becomes more alarming if the device rotates at a very high speed.
Advantage of vibration monitoring
The first fundamental question is whether a particular machine should be monitored at all. There are several purposes for vibration monitoring.
- Physical safety of factory personnel and prevention of possible damage to the device and equipment around the device.
- Avoid the cost of performing mechanical repairs both in terms of material (replacement parts) and in terms of time (work).
- To avoid breakdown costs or damage caused by machine breakdowns.
Safety schedule :
The performance of any rotary machine involves high forces and energy levels. When this energy is operated in a controlled manner, the device performs its desired function in the overall process. But when these forces operate out of control, they will suffer severe damage to the point of machine failure. The greatest safety consideration is that the release of energy from the destruction of the machine can endanger the lives of factory personnel. This is especially true for all large and / or high-speed rotary machines, but is especially important for machines that are hazardous in a factory process.
There are usually two types of monitoring.
Intermittent as done by the inspection department.
Continuous monitoring using the Bentley Nevada monitoring system
A car protection system can significantly reduce maintenance costs. Because monitoring systems can reduce machine damage due to malfunctions, the requirements for potential replacement parts can also be minimized. The stock of replacement parts for a well-supervised car does not have to be as expensive as a car without supervision. Reducing maintenance costs can be achieved by saving maintenance personnel time. The monitoring system can show a part of the device in trouble. So that if the need for repair, the problem can be removed quickly. Continuous monitoring also saves factory personnel from having to measure the vibration parameters of machine tools by hand (portable tools) and can make them more accessible for other job tasks.
The ultimate goal of any monitoring system is to improve the normal operating time of the plant. Even the most complex and expensive monitoring system can be paid for in most factories, saving a few hours of factory downtime. By evaluating and processing the data provided by the monitoring system, a given car train can be kept in line as long as the readings are satisfactory, instead of lowering the machine based on periodic inspections, whether required or not. Also, most monitor systems offer two levels of warning, most of which are machine failures that can be detected at first, allowing operation and factory management personnel to process conditions for a regular shutdown instead of losing the line product as a result. Unexpected, adjust. Shutdown can even minimize the effect of increasing failure by reducing load, speed, etc., and therefore keep the device on line for longer before shutting down if production is required.
Vibration measuring tool
The main converter or pickup for measuring vibration is of the three types described below.
- i) Pack your speed
- ii) Shetab Singh
iii) Pickup without connection
Pack your speed
The design of a typical speed pickup consists of a spring-loaded suspension coil surrounded by a permanent magnet attached to a gearbox. When the pickup case is connected to the vibrating machine, the magnet vibrates back and forth from the side of the coil, which remains fixed. The magnetic flux cut by the coil creates a voltage proportional to the speed of vibration in the coil. The schematic of the pickup is shown in the figure below.
Pickup parts include pickup case, coil, damper, mass, spring, magnet. The typical frequency of a pickup is 15 Hz to 2000 Hz / 600 to 60,000 rpm. Measurements outside this range usually require the application of a correction factor. Due to its high output and other design features, it is relatively sensitive to minor cable problems and is not affected by changes in cable length or long cable length (for example, up to 750 feet). When held in hand for rapid periodic reviews, it gives reliable results and can only work with minimal attenuation. The speed pickup is designed to measure the speed of vibration, but when used with a tool that can be integrated, it can also be used to measure vibration displacement. For very low frequency measurements of about 150 rpm, special speed lifting can be used. Such a pickup has a clamp that is attached directly to the coil. In this case, the pickup box is held and the fork is attached to the vibrating surface. This pickup is also useful for measuring the vibration of lightweight structures whose vibrational response is significantly affected by the weight of a typical pickup.
It consists of piezoelectric crystals placed between an accelerometer box and a small mass. The piezoelectric crystal has the property of producing a charge that is proportional to the amount of compression. When the accelerometer is connected to a vibrating machine, the vibration compresses the small mass of the piezoelectric crystal in each cycle. This compression generates an output voltage by the crystals in proportion to the vibration acceleration. The following figure shows a schematic diagram of an accelerometer.
The accelerometer has a frequency range that is usually 120 to 60,000 cpm (2 to 10 khtz). Special units can go higher and lower frequencies. In addition to the wide frequency range, small size, light weight, wide amplitude, impact resistance and ability to withstand high temperatures, the accelerometer is very diverse. However, accelerometers have low output with high impedance. This means that care must be taken to signal the cabling to prevent ground loops, electromagnetic interference, and cable / connector vibration. In addition to measuring acceleration, accelerometers can be used to measure speed and displacement when used with instruments that have the ability to merge and dual merge, respectively. In cases where dual integration is used, there is little sacrifice in the frequency range or the ability to detect low frequency vibration due to instrument noise in the integration process.
This pickup is widely used in industry to measure vibration and displacement. It senses vibration in a different way from the speedometer and accelerometer. A coil of fine wire placed at the end of the pickup (also called a probe) generates a magnetic field when a very high frequency ac (1 MHz) is applied to it. When the pickup coil is attached to a conductive surface such as the shaft of a rotating machine, the magnetic field generates a eddy current in the shaft that acts as an additional electrical resistance in the coil circuit. If the shaft is vibrating, the output output voltage changes according to the shaft vibration amplitude. Therefore, the non-contact type pickup is able to detect both the gap distance between the pickup and the shaft and the range of vibration displacement. Unlike pickup speedometers and accelerometers, which measure “absolute” motion (ie, motion relative to interior space), non-contact pickups measure the “relative” motion between the pickup support structure and the surface adjacent to the pickup coil. Slow (for example, movement between the bearing and the shaft). The figure shows a schematic diagram of this type of pickup.
Non-contact type pickups are mainly designed to measure the relative vibration between the shaft and its bearings. While they have a frequency range of about 0-60000 cpm (0-1 khtz), the shaft vibration shifts at higher frequencies, even for rough-working machines, become so small that the frequency range is actually around 0-60000 cpm. is limited. The resolution of the non-contact pickup range this time is 0.05 to 0.1 million.
The proxy meter is always powered by -18V DC from an external source such as a power supply or monitoring device containing a -18V DC power supply. The proxy meter converts this 18V to an RF signal that is applied to the probe via a 95 ohm coaxial traction cable, as shown in the figure. The probe coil radiates this RF signal to the surrounding area as a magnetic field. If there is no conductive material at a certain distance to intercept the magnetic field, the RF signal is not lost, the RF signal is approximately at the maximum output terminal of the proxy meter. 14.V When a conductive material approaches the tip of the probe, a vortex current is generated on the surface of the material, resulting in the loss of RF signals. As a power loss occurs at the RF signal, the output signal voltage at the proxy meter OUTPUT terminal decreases relative. As the observed conductive surface reaches the tip of the probe, more force is absorbed by eddy currents on the surface of the material. When the probe is very close to the surface of the conductive material, almost all of the radiation emitted by the probe is absorbed by the material. This is reflected as the maximum power loss of the RF signal, which results in a DC output signal at the proxy meter output terminal. The proximeter measures the magnitude of the RF signal and provides a negative DC output voltage signal corresponding to the peak RF signal.
If the observed surface is rotating and the gap distance changes rapidly, the amplitude of the RF signal is not a fixed amplitude, but changes in direct proportion to the PP motion of the observed surface according to the figure. This peak-peak motion of the observed surface modulates the RF signal. The proxy meter detects the modulated RF signal as an AC signal that fluctuates around the average constant dc voltage (initial adjustment of the probe slit voltage), as shown in the figure below.
If the shaft vibration is 5 mA, around an initial gap of 50 mA, the average dc voltage remains about 8 volts constant, but the AC voltage of one volt PP, 7.5- t0-8.5 volts, is directly proportional to the shaft vibration in this figure. The process of measuring vibration is radial, whether single-plate or double-plate (XY).
A typical calibration curve for a vortex flow displacement measurement system is shown in the figure below. The curve can be divided into three regions starting from the contact of the probe, the conductive surface and zero dc o / p from the oscillator demodulator.
On most systems, the probe may be out for a short time before the output voltage starts to change. In some cases, as the probe exits, the output voltage rises abruptly and is then transferred to the second or linear region, where each change in distance (gap) creates a corresponding change in DC o / p of the oscillator demodulator. he does.
In the linear range, which may extend from 20 to 80 mL, the current standard requires a 100 mV / mL or 200 mV / mm ratio between the gap and the voltage. Therefore, a 10mV change in the gap should cause a 1V voltage change in the 100mV / mil probe or 2V in the 200mV / mil probe.
For partial deflection, the probe, extension cable, and oscillator demodulator form a tuned resonant circuit. Probes, oscillators, demodulators, and expansion cables must be matched and calibrated to establish and maintain a constant ratio between the gap and the voltage. Most probe manufacturers typically specify the diameter of the probe tip, and the overall length of the extension and integrated cables to be used with each oscillator demodulator.
As the probe further out, the system loses its linear relationship between the output and the gap as the output of the oscillator demodulator approaches its supply voltage. Therefore, whenever the accuracy is desired, the probe must be adjusted to operate within its linear range.
The slope of the curve, the linear range and the dc output of a given gap will vary with the change in target conductivity and permeability. If a calibrated oscillator probe and demodulator is used for 4140 steel without recalibration on materials such as stainless steel or inconel, the curve shifts to the left and produces a higher o / p voltage for a given gap. In addition, the slope of the curve will change according to the change in sensitivity. Due to this change and possible inaccuracies, a non-contact calibrated probe system for one material should not be used without recalibration with another material.
Temperature may also affect the range of a non-contact probe and the DC output in a given slot. However, the displacement is generally within the temperature range experienced in a small bearing housing. High pressures may also affect the sensitivity of a non-contact probe. If the probe is installed in an area of high pressure or oscillation, the response must be tested in a real environment to determine what changes in sensitivity or output will occur.
Like everything else, the maximum linear range with a non-contact displacement measurement system increases with increasing probe tip diameter, and as shown in the figure, it also increases with increasing supply voltage. At a sensitivity of 200 mV / mil, the linear range of a typical non-contact measurement system that observes 4140 steel is approximately different. 60 miles with a tip diameter of 0.190 inches (5 mm) and a power supply of 18 volts dc to 85 miles with a tip diameter of 8 mm and a power supply of 24 volts dc. The proximity converter measurement system has a frequency response of 0 to 10 khtz (600,000 rpm), zero indicates non-rotational or static status. The high frequency (-3db) of 1 Khtz (600,000 rpm) does not limit the system’s ability to respond to a radial vibration rate that is many times faster than the machine.
Ki Phasor is a special application of probe and proxy meter. The probe observes a shaft indicator (or a notch or a bulge) to provide a rotational reference of the shaft speed. The figure below shows the relationship between the probe pickup and the oscilloscope display for both incisions and incisions.