QMT Features: March 2007
Stop whining
There is not a single passenger car or sports utility vehicle manufactured anywhere in the world whose front and rear axles cannot be checked automatically on a standardised test rig from Geo Kingsbury.

A new, standardised test rig for measuring axle noise, vibration and harshness (NVH) has been introduced by British manufacturer, Geo Kingsbury.  Capital cost of the equipment is said to be significantly lower and lead-time from date of order to delivery and commissioning will be cut by a third.

There is not a single passenger car or sports utility vehicle manufactured anywhere in the world whose front and rear axles cannot be checked automatically on this rig before it leaves the production line.  Axles for some light vans will also fit, although NVH is not so important on commercial vehicles.

NVH testing measures acoustic emissions from an axle under drive and coast conditions, accurately predicting its quality when fitted in a vehicle.  Such objective testing is becoming essential in the automotive industry, as customers’ ever-increasing expectations of their vehicles makes it unwise to rely on subjective quality analysis.  This is certainly the case in the luxury car sector, as drivers will not tolerate whine from the drive train of an expensive vehicle.  OEMs are acutely aware of this and, to minimise expensive recalls that would dent their profitability, place stringent demands on their axle suppliers.

The standard Geo Kingsbury test rigs are the first to be controlled by a full CNC system – the Sinumeric 840D from Siemens.  The AXT 1800-LS (long stroke) has the largest footprint and accommodates any size of light duty axle, including front and rear beam axle assemblies with or without drive shafts (tubes fitted), beam axle centre sections, front and rear independents (side gear or flange output coupling), and outputs coupling with drive shaft hubs or side gear splines.

The short stroke version, AXT 1800-SS, is intended for complete front and rear beam axle assemblies with drive shafts, and the output coupling with drive shaft hubs only.  Model AXT 750 tests front and rear independents (side gear or flange output coupling, and beam centre sections without tubes fitted; while the smallest test rig, the AXT 500, is restricted to rear independents with output drive flanges.  As the size of the test rig decreases, so does cost and area of factory floor occupied. Geo Kingsbury believes its new standardised range of rigs to be unique in terms of their universal applicability to any size and type of car axle.The driveline assemblies are standard in all rigs and are fitted in advance to the stock machines.  The input is up to 6,000 rpm / 250 Nm torque, while the outputs can be controlled to 2,400 rpm / 350 Nm.  Within these parameters, a large range of input and output motors can be specified by the customer to suit its requirements, using a common motor frame size.

Market research has shown that the typical displacement of pinion gear to axle centreline is ± 40 mm, so Geo Kingsbury has incorporated ± 60 mm adjustment in the input lift assembly.  Control of the offset is by servomotor via a ballscrew under instruction by the Siemens 840D, so adjustment can be done on the fly, allowing the loading sequence to be fine-tuned in two CNC axes.  This is crucial for accuracy and repeatability of testing.

The axle is clamped in the same way as it will be mounted in the vehicle so that the geometry is not affected during testing.  In the case of a rear axle, the input shaft that simulates the drive from the transmission is engaged and so also are the two output shafts that will, when the axle is in the car, be connected to the back wheels.

Test scenarios can include varying the speed at constant torque, varying the torque from positive to negative at constant speed, and combining the two.  In this way, the gear set is monitored for NVH under load in both drive and coast directions and at the optimum load conditions.  The speed of the two output shafts is normally synchronised so that only straight line driving is simulated; the differential gears are not usually included in the tests, as noise is more of an issue when cruising rather than when cornering, although differential output speed tests can be accommodated to measure differential ‘growl’.

Reactive coupling

Worldwide patents are pending on a new system invented by Geo Kingsbury for the consistent coupling of the driveline to the axle under test.  This factor most influences test rig repeatability, ie the variability of measurements obtained when repeatedly measuring NVH values for the same axle.

The difficulty with coupling an axle in a production test rig is that the action must be effected quickly to minimise cycle time, yet the linkage has to be rigid to avoid play and unwanted vibration during the tests.  Repeatability is strongly influenced by how well the coupling is connected.  Positive pressure by the input adaptor assembly on the axle companion flange would solve the problem, but this has to be avoided, as any axial force would influence the test results.

So clearance is always left between the face of the input adaptor and the axle flange.  However, the exact position of the latter is uncertain due to the variability of tolerance build-up during manufacture. Until now, it was necessary to take the worst-case scenario, ie the longest axle flange, and program the test rig slides to position the adaptor as close as possible, without touching it. The problem was that axles with shorter flanges ended up further away from the adaptor, causing a larger engagement gap that made the test results less accurate.

To overcome this difficulty, Geo Kingsbury conducted a series of tests at the same oil temperature to find out what the clearance should be ideally.  The test cycle involved acceleration, coasting and deceleration so that both faces of the meshing gears were tested and the dynamic torque measured, which translates into a decibel figure representing the noise generated.

The company then devised a method for positioning the input adaptor accurately to that distance from the flange of any axle as it comes off the assembly line, ie the machine reacts to the real position of the flange and not to the predetermined slide position of the machine.  This is the crux of the Geo Kingsbury reactive coupling patent.

Further tests showed the system to be remarkably effective. With reactive coupling turned on, repeatability of measurements was improved by 50 percent on the drive side and by 70 percent   on coast. l

email: mtools@gkholdings.com

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