We take a closer look at how vibration and rotation is measured
As we learned in the previous Fundamentals Series post, titled “What is Vibration?”, vibration occurs when spinning objects become unbalanced and a lopsided weight causes a wobble in that rotation.
This video is designed to explore the speed-sensing and vibration-sensing devices that record that wobble and work together to provide the analyzer with data it needs to provide a balance job solution.
First we’ll dig into the subject of vibration sensing devices, known both as accelerometers and also sometimes as velocimeters.
These are devices that you attach to the surface of a vibrating engine. Vibration is picked up by the sensor’s housing and electronic circuits inside the housing convert vibration into voltage. This voltage pulse is then transmitted to the analyzer through connecting cables.
How that works is pretty interesting. It’s all based on Newton’s First Law of Inertia which says: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Using this law of physics, there are two common methods for constructing a vibration sensor: Mechanical and Piezo-electric.
In the mechanical-type sensor, an object or mass, is suspended inside the sensor’s housing. When a force (such as a vibration) is introduced, the mass moves up and down the axis and as it does, it generates a small electrical pulse. As you can image, this tiny bit of mass inside the sensor’s housing is suspended on delicate springs. This explains why you must replace or recalibrate them after they’ve been accidentally dropped.
In the Piezo-electric version of the accelerometer, a tiny crystal made of quartz or tourmaline for example, is installed inside the sensor housing along with a bit of floating mass. When movement is introduced into the sensor, the mass squeezes and releases the crystal. Because the crystal has the magical ability to generate a tiny electric pulse when it’s squeezed, the sensor is able to pass that pulse along to the analyzer.
One thing to understand about these sensors, though, is that they can only measure vibration in one axis. That means the sensor must be mounted perpendicular to the axis of the rotating component. Vibration is like a wave and it moves in a direction. When the sensor is placed perpendicular, the wave strikes the sensor head on and moves the floating mass inside. If the sensor is not mounted correctly the wave hits the sensor’s floating mass on the side, which results in no movement and therefore no pulse.
So now we have sensors that measure the vibration, but the vibration analyzer must also know where the peak of the vibration occurs during one rotation of the component.
For that, we need a device called a tachometer. There are two common versions of this sensor too: magnetic pickup and optical tachometer. Let’s look at both more closely to better understand how they work and why one might be better than the other.
A magnetic pickup is basically a magnet inside a sensor housing and mounted near a spinning object, like the propeller shaft or a spinning disk. When a metal object passes by this magnet, the object pulls on the magnet which sends an electric pulse to the analyzer. Each time this object breaks the magnetic field, the analyzer counts one revolution of the shaft or propeller.
This type of sensor has some advantages and some disadvantages. One of the most obvious advantages is that many twin turbo-prop airplanes already have them installed. We can just tap into that signal and use it to balance the propellers. One of the disadvantages to consider is that the mass must pass by the sensor with very little room to spare. Since there is little room for error, the chances of having a collision at 4,000 RPM are very real.
An optical tachometer, however, is far more forgiving and uses a beam of light reflected off a small piece of reflective tape attached to the rotating part. The beam leaves the sensor from one port, hits the reflective tape and returns to a second port on the sensor to complete the circuit. In actuality, the beam is constant, but since it only returns to the unit when it is reflected, the unit can then use these moments of reflectivity to count rotations. At ACES Systems we offer two types of optical tachometers. The PhotoTach sensor can be mounted up to 18 inches from the point of reflection, while the Lasetach can be mounted up to 30 feet from the spinning surface. To make things even more interesting, though, we’ll just mention that we can even shoot the Lasetach around corners for greater flexibility!
So while we are in the neighborhood, we should also talk more about reflective tape, because not all tape is built the same. We highly recommend that you use 3M 7610. You can buy this tape in smaller 10-foot rolls from our website at: www.AcesSystems.com or in larger rolls from any source you can find on the Internet.
We recommend this tape because it’s just that good. In fact, according to the manufacturer’s specifications and testing, it is 700 to 900 times brighter than a perfectly diffusing white surface (depending on the angle of reflection). The reflective properties are far superior to even the “Slow Moving Farm Machinery” tape sold at your local hardware store. Plus it’s very thin, which keeps weight down and eliminates the possibility of the propeller going out of balance when you remove the larger and heavier Farm Machinery tape. Of course, that doesn’t mean you should ever leave tape on your propeller or fan. We recommend you always remove the tape to prevent any possibility of corrosion.
Check out our Training Library where you’ll find additional training resources, including manuals, application notes, and videos.
Coming up soon in our Fundamental Series…
- Mounts & Cables
- How Vibration Analyzers Work
- Types & Causes of Aviation Vibration
- Engine Vibration analysis
- Special Challenges of Helicopter Vibration
- Balancing Basics
- Using Acoustics to Measure Vibration