Brake Compounding

[NeilF]

I have just read a short article which mentions 'brake compounding' relating to freight wagons. Can anybody explain this in simple terms? I am interested to know:
1. What exactly this means and how it occurs?
2. What, if any risk, is associated with it?
3. Is it an issue only on freight wagons/vehicles or can it be an issue with passenger stock/vehicles?

Thanks in advance.

 

[twpsaesneg]

Could you post a link to the article? The only time I've heard of this phrase being used is when applying a handbrake on a vehicle which already has the service brake applied. The risk being that once the air pressure leaks off you may need to be Hercules to then release the handbrake.

If this is what it is in relation to then I would only expect it to apply to vehicles with a handbrake, which knocks out most passenger stock straightaway.

 

[edwin_m]

It is discussed in a recent RAIB report. https://assets.publishing.service.gov.uk/media/665836338f90ef31c23ebc01/R042024_240516_Kineton.pdf

Brake compounding is unadvisable for a number of reasons,
including:
i. As the assistance from the air brake system subsides (either due to the
brakes being released or as the air leaks away), the brake block force does
not reduce to the levels associated with a non-compounded application. This
means that the resulting brake block force is being held by the handbrake
cable alone, forcing it to carry more load than expected.
ii. Because the handbrake is being applied in addition to the air brake, shunters
are unable to satisfy themselves that the brake block is being applied to the
wheel by the handbrake, because the brake block has already been applied
by the air brake. This means that a shunter is entirely reliant on the handbrake
indicator which, in the case of these wagons, only shows that the handbrake
control wheel has been turned (paragraph 85).
iii. In the event of a failure of the handbrake system, the air brake will hold the
wagon in place until the air leaks away. This will mask the failure, which may
only manifest itself hours later, possibly when there is then no opportunity for
intervention by staff.
Analysis

 

[Lemmy282]

As it says in the report the shunters applied the handbrake while the vehicle was stationary on the air brake. If the brake cables were in good condition then when the air leaked off the force to release the handbrake would be immense, in this instance it seems they were not, and the handbrake was not effective so the vehicle rolled away.
I drive heritage trams, and on airbrake trams we are taught to release all the air from the braking system while applying the handbrake, if you apply handbrake over airbrake it is almost impossible to release the handbrake when the air had leaked off.

 

[NeilF]

Thank you for the replies posted. My question followed on from the RAIB report (link kindly posted by 'edwin_m' above).

The part of the report that puzzled me was the insinuation that the process for securing vehicles was wrong, given that the method in the instructions to the Shunters working at the location was to apply hand brakes whilst the air brake was still applied. The report states that this is contrary to the industry Rule Book. I have looked this up and it doesn't seem to be the case (although the version of the module concerned that is referenced in the report has been superseded).

If a rake of vehicles was being secured, then releasing the air brake before applying the hand brakes would not be a problem. This could be done one vehicle at a time. Whilst the air brake was being released on a single vehicle, it would be held in place by the others attached to it. Also, if one of the hand brakes had failed on one vehicle in the stabled rake, this would not matter. The risk of vehicles running away is negligible.

But this was a single vehicle stabled on its own. The Shunter cannot be sure whether or not the hand brake had actually applied, regardless of the method used to apply it. I would be interested to know if anybody has experience of this, or if anybody has a view. Would it be standard practice to tell the Shunter to scotch a single vehicle and not rely on the hand brake?

My question about passenger stock has been answered above. Besides, I know that most modern stock is fitted with spring applied parking brakes. These will gradually apply as the air brake leaks off, so presumably this prevents the problem arising.

Overall, my view (having read the report) is that the actions of the Shunter were of little relevance. Given that the report acknowledges that the Shunter cannot be sure that the hand brake has worked, I am not sure what more could be expected. Surely, the primary issue was the failure of the hand brake and the failures in the maintenance regime that should have prevented the failure?

Thanks again for the replies.

 

[edwin_m]

Thank you for the replies posted. My question followed on from the RAIB report (link kindly posted by 'edwin_m' above).

The part of the report that puzzled me was the insinuation that the process for securing vehicles was wrong, given that the method in the instructions to the Shunters working at the location was to apply hand brakes whilst the air brake was still applied. The report states that this is contrary to the industry Rule Book. I have looked this up and it doesn't seem to be the case (although the version of the module concerned that is referenced in the report has been superseded).

If a rake of vehicles was being secured, then releasing the air brake before applying the hand brakes would not be a problem. This could be done one vehicle at a time. Whilst the air brake was being released on a single vehicle, it would be held in place by the others attached to it. Also, if one of the hand brakes had failed on one vehicle in the stabled rake, this would not matter. The risk of vehicles running away is negligible.

But this was a single vehicle stabled on its own. The Shunter cannot be sure whether or not the hand brake had actually applied, regardless of the method used to apply it. I would be interested to know if anybody has experience of this, or if anybody has a view. Would it be standard practice to tell the Shunter to scotch a single vehicle and not rely on the hand brake?

My question about passenger stock has been answered above. Besides, I know that most modern stock is fitted with spring applied parking brakes. These will gradually apply as the air brake leaks off, so presumably this prevents the problem arising.

Overall, my view (having read the report) is that the actions of the Shunter were of little relevance. Given that the report acknowledges that the Shunter cannot be sure that the hand brake has worked, I am not sure what more could be expected. Surely, the primary issue was the failure of the hand brake and the failures in the maintenance regime that should have prevented the failure?

Thanks again for the replies.

Isn't the answer here that one or more wagons must be secured on hand brakes or chocks before the locomotive is shut down or uncoupled? Otherwise you are relying on the air in the reservoirs to hold the train, when this air can leak away in time, as well as risking compounding the brakes.

 

[66701GBRF]

Correct method for applying a handbrake on single wagon is…
Vent air from pipe (keep coupled to loco)
Vent air from wagon distributor
Apply handbrake brake fully
Physically check blocks have applied
Uncouple loco from wagon

The shunter didn’t vent the air correctly and physically check blocks which is where their error was.

 

[NeilF]

The above makes sense. I hadn't considered the requirement to visibly check the blocks. Presumably, the process is the same on a rake of vehicles? Do this one vehicle at a time until the requisite number of wagons have been secured by handbrakes?

A couple of minor points following on from this: Would this method be feasible with disc braked vehicles (visibly checking) and has the whole process got to be followed when running a loco round?

 

[edwin_m]

The above makes sense. I hadn't considered the requirement to visibly check the blocks. Presumably, the process is the same on a rake of vehicles? Do this one vehicle at a time until the requisite number of wagons have been secured by handbrakes?

A couple of minor points following on from this: Would this method be feasible with disc braked vehicles (visibly checking) and has the whole process got to be followed when running a loco round?

Unless on a very severe gradient, the handbrake on the loco would hold the whole train until enough wagon handbrakes had been applied to allow uncoupling.