Standard Test Rig Suits All Light Axles
A new, standardised test rig for measuring axle noise, vibration and harshness (NVH)
has been introduced by Burke Porter Machinery. The capital cost of the equipment
is significantly lower and lead-time from date of order to delivery and
commissioning will be cut by a third. The world launch was at the Automotive
Testing Expo in 2006. It is the largest of four models in the company's new
standard test rig range and will be demonstrated under power testing a beam axle
with tubes and an independent rear from American Axle. 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-higher 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 Burke Porter Machinery 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
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. Burke Porter Machinery believes the new standardised range of
rigs to be unique in terms of their universal applicability to any size and type
of car axle.
Now all rigs are built around four sizes of AXT based assemblies that are constructed
from standard modules. This modular approach will also allow the reduction
of price and production times. 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 Burke Porter Machinery 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 improves test accuracy on a new patented
system that consistently couples the driveline to the axle under test.
This is the factor that 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 adapter 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 adapter 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 adapter as close as possible,
without touching it. The problem was that axles with shorter flanges ended up
further away from the adapter, causing a larger engagement gap that made the
test results less accurate.
To overcome this difficulty, Burke Porter 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. Having identified the optimum
engagement clearance, the company devised a method for positioning the input
adapter 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
Burke Porter Machinery reactive coupling patent.
Further tests showed the system to be remarkably effective. With reactive
coupling turned on, repeatability of measurements was improved by 50 per cent on
the drive side and by 70 per cent on coast.
Other steps taken to improve the repeatability of results have centred on the
design of the machine structure and of the driveline. Anthony Best Dynamics partnered with
Burke Porter Machinery to create a unique system to model mathematically the
amplitude and frequency of resonation around the structure during axle testing
under various simulated operating conditions. Such dynamic mapping is able to
suggest design modifications that minimise resonance at operating speeds where
it is most likely to affect the accuracy of the NVH measurements.
Form many years, the company has been co-operating with ABD for several years and is now on its
fifth generation of test stand, every design iteration having been an
improvement on its predecessor. Each time, the predicted distribution of
resonance and the expected positions of dead zones around the structure were
correlated against measurements subsequently made on the machine itself,
enabling the design to be progressively refined. The company has also worked in conjunction
with ABD to develop a system using on-shaft torque sensors and torsional
accelerometers to measure accurately the dynamic torque signal, or ripple, to
within 0.01 Nm, giving an accurate measurement of gear mesh noise.
The company is often asked to supply production test rigs in a very short
time frame so that the axle manufacturer can correlate test results with those
from the audit rig in their R&D department, or indeed the vehicle itself,
which is essential before the axles can be brought to market. The new, standard
test rig design will greatly assist in meeting these tight deadlines.
Burke Porter Machinery is one of the world's leading producers of automotive test stands,
having custom-designed, built and installed more than 1,000 since 1960 for most
of the major names in car and truck production. Although the company specialises
in test equipment for automotive power train assemblies, it has also
manufactured test cells for steering components, cylinder heads, hydraulic
valves, alternators, starter motors, clutches and both electric and hydraulic
A standard CNC axle test rig manufactured by Burke Porter Machinery
An output driveline with motor, flexible coupling, flywheel, slipping clutch
and intermediate bearing block.
With reactive coupling deactivated, the gap between the axle and input
shaft is 0.5 mm. The dynamic output torque (Y axis) plotted against static
input torque for the same axle tested 6 times shows a 1.8 dB variation in the
results leading to uncertainty as to how noisy the gears are.
With reactive coupling activated, the gap between the axle and input
shaft is maintained at 0.1 mm and the spread of results are much smaller at .9
dB allowing gear noise to be predicted much more accurately.