Bogie Rotational tests
The resistance to rotation of a bogie is very important to the safe running of trains.
The bogie should be able to rotate without being too stiff or too slack. High rotational stiffness can lead to excessive flange wear when negotiating curved track or 'crabbing' on straight track and too little resistance can lead to dynamic bogie hunting on straight track. An ideal stiffness should be able to control hunting and allow curving without flange contact.
Rotating the bogie within certain limits on new trains can also reveal areas where cables/pipework can foul and where airbag suspensions can be stretched too far.
Bogie rotational resistance tests were always undertaken at two speeds 0.2 deg/sec and 1.0 deg/sec. The rationale was that the lower rate (0.2 deg/sec) was more applicable to how a vehicle might perform when running on the main line at speed (large radius curves). This part of the test was mainly used to give some measure of the stability that might be provided by the rotational resistance. That is why the original limits were set – too low and it goes unstable – too high and you get excessive flange wear and increased derailment risk.
Many of the derailments which occurred in the past (and it’s probably still the same today) were in yards with relatively tight radius curves. Therefore if a unit is running at moderate speed in a yard the rotational rate will be higher. The rotational resistance will increase on vehicles with yaw dampers and interconnected elements (air bags) which can add a considerable margin to the rotational resistance. A rule of thumb was that the worst case would be approaching 1 deg/sec for UK infrastructure (min Curve radius @ 3 chains) and that is why it was adopted in the UK.
The resulting figure from these tests is known as the 'X-Factor' (BR thought of it first!) and common value for passenger stock is 0.05.
The design X factor of a bogie is almost invariably a compromise. For a bogie to steer optimally in tighter curves, (generally less than 600 m radius), a low X factor is required. However if the X factor is too low the bogie will tend to hunt at speed on straight track. As a result, the design X factor is carefully considered and a material is selected for use on a bogie’s secondary yaw friction surfaces which provides the appropriate balance between the desired curving behaviour and ride stability at normal operating speeds on straight track.
Before the amalgamation of the regional testing services at Derby and the building of the RTC with its new workshop and its specialised facilities, rotational stiffness testing of vehicle bogies was undertaken at Darlington on the small turntable just to the south of the station. This turntable is still in existence but not rail connected anymore.
Click on the pictures for a bigger image - All photos are from the author's collection except where credited.
|EE Type 4 D344 stands with one bogie on the turntable whilst engineers conduct a rotational resistance test on the leading bogie by pulling the table around, and measuring the force taken to move the bogie. Note the test car marshalled behind the locomotive which would have housed the instrumentation.|
|Engineers check the instruments attached to the bogie centre pivot which would measure the rotational stiffness. Since the 1970's there has been a special turntable rig within the RTC workshop for this purpose.|
|Another view of the tests shows one of the engineers on the phone to the test car whilst another takes a well-earned breather|
|A rather grainy picture of the
bogie rotation rig in the EDU c1972 with a Lowliner wagon
|89001 undergoes a bogie
rotational test during its acceptance testing
|59003 undergoes a bogie
rotational test in the EDU prior to being shipped to Europe in 1994
|The Plasser UFM160 high speed infrastructure measuring machine is pictured in the workshop at Derby undergoing bogie rotational tests in summer 2003|
Have a look here for a recent RAIB report into a freight train derailment which happened as a result of the rotational resistance of a bogie being too high.
Similarly this report into the derailment of an XPT set in Australia in 2001 points to high bogie rotational resistance due to poor maintenance of the side bearers between the car body and the bogie
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