What caused the cracks in Hiitachi’s Class 800 trains…
…AND HOW WILL THE MANUFACTURER GO ABOUT fiXING THE problem? PHIiIP HAIdH recalls the problems that befell HITACHI’S NEW flEETS 12 MONTHS AGO, AND EXAMINES THE OFfiCE OF RAIL AND ROAD’S REPORT INTO THE ISSUE
CAST your mind back to school science lessons.
At some stage, you probably hung weights from a coiled spring. The more weight you added, the further the spring stretched. If you took weights off, the spring became shorter.
That’s deflection. It changes according to the load applied to the spring (until the spring has too many weights and stretches beyond the point of no return).
You might have noticed the weights bouncing around on the end of the spring after you added them. This is oscillation, but eventually the movement would stop and you could measure how far the spring had deflected.
Controlling this bouncing is a damper’s job (a motor mechanic would call this a shock absorber). I don’t recall them being introduced into my school science lessons, but that’s bythe-by.
The upshot of all of this is that you can control the movement of one thing relative to another by connecting them with a combination of springs and dampers. So, a train’s wheels and axles move in relation to their bogie according to the springs and dampers between them. They’re quite easy to see under most trains and they’re called the primary suspension. (Steam locomotives and earlier coach and locomotive bogies used leaf springs which provide their own damping.)
There’s another set of springs and dampers between the bogie and the body (sometimes with an airbag). This is the secondary suspension. Both primary and secondary suspension work to give passengers a smooth ride. They also work to reduce the impact that trains have on the track.
I could add that there’s a third suspension system - and that’s in your seat. Modern seats (dare I say ironing boards?) don’t come with coiled springs, but traditional seats have them.
By changing the characteristics of springs and dampers, so you can change the ride you feel. With a hard ride, you feel every imperfection in the track. A soft ride insulates you.
Such is the world of vehicle dynamics. It might be easy to understand what springs and dampers do in isolation, but the subject quickly becomes very complicated with real trains and tracks.
Changing one aspect of a train’s suspension can have unwanted consequences elsewhere. This happened 20 years ago, when rail operators phased out older stock in favour of new trains.
The new trains came with stiffer suspension, particularly in the way bogies rotated (steered) into curves. Faster trains generally benefit from stiffer suspension, but this higher bogie rotational stiffness showed itself in increased gauge corner cracking (GCC) in rails.
And it was GCC that led to the fatal accident at Hatfield in October 2000, when a cracked rail shattered under a GNER express. There followed huge disruption as network owner Railtrack discovered GCC blighting line after line.
Such complex interactions between trains and tracks played a part in the sudden withdrawal of Hitachi’s fleets of ‘800’-type trains in May 2021. Inspections revealed cracks in the weld joining yaw damper brackets to train bodies. These dampers help control bogie rotational stiffness, but Hitachi’s suspension design had the bracket taking loads as the carbodies rolled side-to-side in relation to the bogies.
They were fatigue cracks that come from material (in this case the metal of the weld) being repeatedly loaded and unloaded. It’s more than likely that this loading/unloading comes from a combination of vehicle and track dynamics. But - and it’s a big but - this is not to say the track was faulty or deficient.
In its report into the cracks, the Office of Rail and Road said: “There is no definitive reason identified for the in-service fatigue loads to exceed those defined in the standard. Potential factors that have been suggested include wheel wear and track specification.
“Hitachi’s testing has identified a difference in loads arising from new and worn wheel profiles. No work has yet been undertaken to evaluate whether the characteristics of track on the routes over which Class 80x rolling stock operate differ from those assumed by the bodyshell fatigue standard. No grounds have been presented for asserting that the condition of the track on the routes was anything other than compliant with the applicable standards and therefore conformed to its specification.”
ORR now calls on the rail industry to
examine whether the standards that Hitachi’s designers followed take into account the loads that UK trains encounter. However, it adds that other trains designed to the same standards have not had the same problems.
This means it’s possible that the problem is Hitachi’s. ORR’s report notes: “The examination of the cracks suggested that the extent of the fusion of the weld metal to the vehicle body structure was small. This meant that the loads were being transmitted through a relatively small area. This is likely to be significant as a cause of cracking.”
Elsewhere in the report, ORR records that Hitachi’s investigation had concluded that the design around this joint made it hard to weld. Pictures of Hitachi’s revised design show a much beefier bracket. This makes it likely that the old design wasn’t up to the job - by either its inherent strength or the way it had to be welded to the carbody.
So, the company is now creating a repair programme to work its way through 1,247 vehicles, taking the best part of four weeks for a nine-car train. When I spoke to Hitachi, it was very open that it will foot the bill, not taxpayers or farepayers. But it wouldn’t say how much this bill might be, no matter how hard I pushed.
The repair programme will also deal with the other cracks that Hitachi found in spring 2021. They were found in the lifting points under each carbody. By coincidence, these lifting points sit close to the yaw damper brackets, but that’s where the similarities end.
The lifting points have no loads applied to them in normal service. They are only used when jacks lift vehicles in depots. Yet they cracked under a mechanism called stress corrosion cracking (SCC), which needs three things: a susceptible material, a corrosive atmosphere, and stress.
Hitachi provided the first by using a 7000-series grade of aluminium. It’s stronger than other grades of aluminium, so can result in lighter structures.
The corrosive atmosphere isn’t anything as bad as London’s smogs from yesteryear, but comes from the chlorides which ORR says are endemic to Britain. They come from the road salts we spread every winter, and from the country’s general atmosphere (which ORR describes as “reasonably benign”) as a small island with seas all around us.
Finally, the stress in the lifting plates came not from their use, but from the way Hitachi welded them to the rest of the bolster assembly under each carbody. This welding left residual stress. Hitachi put the bolster through stressrelieving processes, but not the lifting plates.
This brought together all three elements needed for SCC, hence the cracks that Hitachi found. Although Hitachi initially feared the lifting plates might fall off and hit people or infrastructure, its investigation discovered that even plates with cracks were very hard to remove. Six welds hold them in place but even with as many as four fully cracked, the plates would not detach.
So, Hitachi proposes to fit bolts where needed as a secondary way of holding the plates in place.
Hitachi used 7000-series aluminium in 17 areas of its Class 800 trains, and found three areas that needed rework to address SCC including the lifting plates. The other areas are coupler support plate and the yaw damper on driving vehicles.
As part of its fatigue crack repairs, Hitachi will weld over areas of machined extrusions to cover any exposed grain boundaries. It is these boundaries, at a microscopic level, that allow stress corrosion cracks to start developing in corrosive atmospheres. Covering them with weld metal protects them (a bit like painting steel to protect it from rusting).
Whether the cracks came from bad luck or poor judgement on Hitachi’s part, the mass withdrawal of ‘800s’ shows that Hitachi’s inspection procedures worked well. Staff spotted the cracks and acted. Hitachi and the train operators were, I suppose, lucky that COVID-19 had cut passenger numbers, but that initial fleet withdrawal was still disruptive.
Subsequently, the operators could cope with having fewer trains because they were running fewer services. As passenger numbers now recover, Hitachi will need all its skills to keep sufficient trains in service while repairing others.
Taken as a whole, the cracks are embarrassing for Hitachi and expensive. But they weren’t unsafe.
“Whether the cracks came from bad luck or poor judgement on Hitachi’s part, the mass withdrawal of ‘800s’ shows that Hitachi’s inspection procedures worked well. Staff spotted the cracks and acted.”
R