Automated Test Stand Ensures Axle Quality
Drivers of the latest Land Rover Discovery and Range Rover models can be confident that the axle and differential assembly, known as the final drive unit (FDU), is as quiet and smooth as in any luxury vehicle in the world. It is because the first-tier supplier, Dana Traction Technologies Europe, has installed a robotically-loaded test stand at its Birmingham factory to inspect rigorously each FDU for noise, vibration and harshness (NVH) as it comes off the production line, rejecting any that do not meet the car makers’ stringent standards.
Designed and built by Burke Porter Machinery in Gosport, Hampshire, the production test stand operates around the clock, five days a week and frequently at weekends. It is one of the most advanced ever built, employing a 6-axis articulated-arm robot from Motoman to load and unload the FDUs, of which there are currently four types weighing up to 65 kg each, in a floor-to-floor time of one minute. Operators are thus spared having to handle 50 tonnes of material a day, avoiding strain injury and other potential health and safety problems. During changeover from one assembly to another, which takes place up to five times a day, the robot automatically changes its own grippers to handle the next FDU model and also exchanges the heavy, bespoke fixtures which clamp them in the rig.
Four types of FDU are tested. Two are rear axles that fit both the Discovery 3 and the recently launched Range Rover Sport, one axle having an open differential and the other a locking differential to enable terrain torque management. The other two FDUs are the front axle for the Discovery 3 and the rear axle for the current Range Rover. Production of the latter, codenamed L322, is currently 450 per week, ramping up to 650, while the remaining axles for the so-called T5 programme are produced at a rate of 1,000 in 24-hours.
According to Dana’s project manager, John Taylor, full NVH testing of vehicle transmission components is being specified more and more by automotive OEMs in response to higher levels of expectation from drivers, as the process does not rely on subjective analysis by a fallible operator. What sets the Dana test stand apart is the use in production of a new system whereby the vibrations of the input and output shafts are measured as the pinion gear engages with the crown wheel, allowing the quality of gear meshing to be determined very accurately. This allows of the level of refinement of the gear teeth when in contact to be determined, leading to an accurate prediction of how quiet the FDU will be in operation in the vehicle.
As it arrives by conveyor at the test stand, the axle is recognised by an electronic ‘handshake’ with the control of the Brazilian-built Muri production line. Provided that the correct fixtures are in place, the robot automatically loads the FDU onto one side of the test stand’s rotary table and waits for it to index to give access to the tested FDU. Provided it has passed, it is picked up and placed onto the conveyor to complete its journey to the dock audit section, from where it is delivered to the Solihull car factory.
Meanwhile, the new FDU is automatically clamped in the Burke Porter Machinery stand at in-vehicle mounting points so that the geometry is not affected during testing. In the case of a rear axle, the differential input shaft that simulates the drive from the engine is engaged and so also are the two output shafts that will, when the FDU is in the car, be connected to the back wheels. At the same time, the differential is filled automatically with temperature-controlled oil, the type and amount of which varies according to FDU variant. Double metering ensure that the correct amount of oil enters the unit, as the axle will be shipped in this condition and it is important that the differential is neither overfilled nor underfilled. (Front axle testing is simpler as there is no differential.)
As the oil fill cycle finishes the shafts start to rotate, taking the assembly through a sequence of tests at different speeds. Dana configures the tests themselves to suit the vehicle model and to accommodate future types of axle that may be produced. Typically, the input ramps up through various speeds to 6,000 rpm, representing about 100 mph on the road after 4 to 1 reduction.
Test scenarios 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 heavy load in both directions and also when the gears are only lightly meshing. The speed of the two output shafts is synchronised so that only straight line driving is simulated; the differential gears are not included in the tests as noise is more of an issue when cruising rather than when cornering.
System control software writing and programming of the test stand were carried out in-house at the Gosport factory. Land Rover specified Anthony Best Dynamics’ software for analysing the output signals from the transducers. Axles are rated for quality from 1 to 10 under both drive and coast conditions and only those achieving 6 or more in both categories pass.
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 transmissions, it has also manufactured test cells for steering components, cylinder heads, hydraulic valves, alternators, starter motors, clutches and both electric and hydraulic motors. With the exception of wear items, the company provides free service and parts for its test stands during the first two years and guarantees 94 per cent uptime.
The supplier was chosen for the Dana project in Birmingham following the successful installation of a similar Burke Porter Machinery stand at the American parent company’s Orangeburg factory in South Carolina, which produces FDUs for the BMW X5 built at the nearby Spartanburg plant. More recently, a test stand has been delivered to Dana in Austria for axle testing. This machine integrates the output torque transducer measurements to the NVH analysis, so instead of just the mean torque being available, high-speed data capture and analysis allows peak-to-trough measurements to be made from the torque fluctuations. The quality of gear meshing is therefore indicated more precisely.