Introduction to Prop-shaft Balancing
You can use your PicoScope with an NVH Kit and an Optical Sensor Kit to balance a prop-shaft without having to remove it from the vehicle. There are two balancing methods, Pinion Range and Hose Clamps.
This article discusses Balancing using Hose Clamps. An overview of balancing is also included in our new NVH Training Course.
Understanding Balancing
If a shaft is unbalanced, it will create an uneven (spinning) force that will act in the direction of the imbalance as the shaft spins. This is a so-called first order vibration - one shake per revolution. If a mass (see the note on the right) is added to the shaft that is equal to and exactly opposite (180° from) the excess mass, the shaft will be balanced and the vibration will disappear. Remember that if two equal masses are added to (or removed from) opposite sides of the shaft, the shaft will remain balanced.
What is an Accelerometer?
An accelerometer measures force. A tiny weight, micro-machined on a silicon chip causes a strain guage to bend when the device accelerates (or decelerates). When the strain guage bends, a tiny voltage is produced which is amplified. A three-axis accelerometer has three weights and / or strain guages and measures forces, simultaneously, in three directions (X, Y and Z). The three outputs can be combined (by software) to determine both the extent and the actual (or 3D) direction of the acceleration, i.e. as a vector.
Accelerometers have many applications such as in intertial navigation systems, 'G' force measurement such as cornering, braking and acceleration forces in vehicles (and fighter aircraft), collision detection in air-bag deployment systems and, of course, to analyse and detect vibrations.
The accelerometer is usually mounted on the differential during testing to measure vibrations induced in it by the shaft. Only one axis is used to detect the vibrations in this application.
See our article on NVH Kits.
What is the Optical Sensor for?
The optical sensor uses a red beam and the device detects when the beam is reflected back. In balancing applications, the optical sensor is used to measure the rotational speed of the shaft, simply by creating an electrical pulse whenever a small reflective sticker passes in front it. This pulse is also used by the software to calculate the position (in degrees) of the shaft at any moment. This information is combined with the accelerometer's output and allows the system to calculate the extent of the imbalance and its direction (angular position on the shaft).
Hose Clamps used as Balancing Weights
Hose clamps (jubilee clips) consist of a strap which wraps around a hose (or prop-shaft) and a screw that pulls the clamp tight. If a hose clamp is tightened around a shaft, the screw assembly will cause an imbalance. If a second hose clamp is added and its screw assembly is 180° (or opposite) the first screw then the imbalance is cancelled.
The mass of the screw (imbalance) needs to be measured carefully (by cutting off the strap on a sample hose clamp and weighing the screw section). The usual (and software default) mass of the screw section of the hose clamps used for prop-shaft balancing is 14 grammes. Excess strap is also removed from the hose clamps (cut off) when tightened to prevent or reduce overlap.
Adjusting unbalanced masses
As previously mentioned, if two equal masses are added to opposite sides of the shaft, they balance and their effect is zero. If two masses (of 14g) are added to the same side of the shaft, the imbalance will be 28g in the direction of the position of the two masses. This means that the mass can be adjusted from 0 to 28g by simply changing the angle between the masses from 180° apart to 0° (together) and the direction of the resultant force is halfway between the two screws.
To balance a shaft, the amount of unbalanced mass must be found and the direction must be determined. That mass must be corrected by exerting an equal force that acts in the opposite direction to the imbalance. This is achieved by moving the hose clamp screws symetrically towards the opposite side of the shaft until the imbalance is cancelled (technically, the sum of all of the centripetal force vectors coincides with the centre of the shaft).
Equipment
- Automotive PicoScope, e.g. 4425
- PC (Laptop) running Windows 7, 8 or 10
- The free PicoDiagnostics Software
- NVH Kit (single or 3 axis accelerometer) to measure vibrations
- Optical Sensor Kit and reflective tape to measure shaft rotation
- Three identical hose clamps (large enough to be wrapped around the shaft)
- Measuring Tape
Connections and Test Setup
Remember to check the tyres for stones and debris that might fly out when the wheels are rotating. When conducting the tests, place the vehicle on stands so that the ride height is correct.
The PicoDiagnostics software and help guides you through the process. In short, you mount the accelerometer to the differential and connect it to channel B of the scope. You attach a piece of reflective tape to the shaft in front of the optical sensor which is connected to channel A.
Hose Clamp Preparation
You measure the mass of the screw section of the first hose clamp by cutting off the strap and weighing it (it is discarded). You tighten the remaining two hose clamps around the shaft and remove any excess strap.
Measuring the shaft
Make a mark on the shaft - zero and use the PICO-TA188 Directional Tape Measure for Propshaft Balancing to measure the circumferance of the shaft and to position the hose clamps during the procedure.
Calibration
Four preliminary tests are performed. For each test, the hose clamps are positioned as instructed by the PicoDIagnostics software and the shaft must be spun up to within 100 RPM of the test speed (e.g. 2500 RPM)
In test 1 (Initial) the screws are positioned at 0° and 180°. In test 2 (Calibration 1), the screws are both at 0°. In test 3 (Calibration 2), both screws are at 120° and in test 4 (Calibration 3) both screws are at 240°.
Results and Fine Tuning
The software calculates where the screws should be to counteract the imbalance and you set the two screws to the position indicated by the software. In test 5, the result of the correction can be seen. Minor adjustments may be required and test 6 can be repeated until you are satisfied, while you make fine adjustments to cancel out any remaining vibrations.
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