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Accumulators - why?

I viewed a decent RM that is for sale recently, but the owner says the accumulators require charging or possibly a refurbishment. The bus drove and stopped well even when the flag came down as it often did. So if the accumulators are discharged of air, why does the bus on all occaisions stop well, even doing a superb emergency stop?

Why does the brake system on the RM have accumulators, and could the brake system be altered to operate without them?

Re: Accumulators - why?

In theory although there is insufficient room to convert to full air, one COULD alter the system to an air over hydraulic set up, but that would be a retrograde step by introducing a fluid which which is hygroscopic.

Several manufacturers, ERF and IIRC Foden and Daimler to name but a few, tried a full power hydraulic braking system without accumulators and very soon realised that such a system is unsafe. If the engine driven pump stopped turning fast enough then there was no asssistance. A full power hydraulic system requires a source of stored energy, so some tried an accumulator with a big spring inside it. Try Geoffrey Hilditch's writings for chapter and verse.

In regular daily service the accumulators are less troublesome than they are when the vehicle is only in intermittent use.

Since any fool can redirect from Googleand read this site to draw the wrong conclusion you won't be getting an answer from me to the other question.

Re: Accumulators - why?

A lot of years ago, I drove Midland Red D9.s.( Their 'version' of the Routemaster, designed and built for them). The brake pedal had a long travel, and the first application of the brakes gave a lovely assistance, pedal going about half way down, nice, but when the pedal was released and re applied, the pedal went hard, at the top of it's travel, and the only assistance was from the Driver's leg, and often the handbrake was used as well. The hydraulic pressure had been used up, and with no accumulators there was none in store until the bus had been driven along and some more built up by the pump, which I believe was on the rear of the gearbox. I read that the Chief Engineer at Red told Leon Daniels,(who was operating these buses on the Round London Sightseeing Tour), that he could not understand why Drivers were having problems with the brakes in London. I would say that the D9 was a very fine motor, semi automatic gearbox, original fitted with disc front brakes,( later changed to drum), set back front wheels to give a better turning circle, rubber suspension, power assisted steering,(but only with the engine turning fast). The engine intruded into the cab as it was not offset, but a fine successor to the D7, and I am told, well liked by Drivers in the Midlands. They finished service in the Midlands in 1979, I am told without any of the ceremony associated the the London RT's which finished at the same time. I believe only one D1O was built, same layout as FRM1, using the same principles.

My bus number (if any): RT 1702 Society

Re: Accumulators - why?

There is at least one RML with air brakes. You can stop an unloaded bus OK with empty accumulators but you are only getting half the pressure, with passengers the braking performance would be poor.

My bus number (if any): RTL 960, RMC 1458, RM 1585 and several RTs

Re: Accumulators - why?

Not quite right there Brian from what I understand. You ought to get full line pressure circa 1200psi with low or no accumulator pressure. the big problems are as per the Midland Red thread above, plus if the pump fails or the belts break, you then have no brake pressure and no emergency brake. The accumulators act as an air spring to provide hydraulic pressure in these cases.

Service brake pressure is about 600psi, emergency pressure is 1200psi, the last half inch of travel on the pedal. Remember that when you're on annual test.

I've just received a copy of the Lockheed manual on RM brakes which gives a very good description of how they work.

My bus number (if any): RML2532

Re: Accumulators - why?

Thanks to all, great help as always.

So, are we saying that an empty bus is safe to drive without working accumulators if the rest of the braking system is good (inc belts/pump)? I'd have drive the bus home.

Another point comes to mind, if the bus is safe to drive, would it pass a basic MOT and thus be legal? Otherwise surely I'd void my insurance.

Re: Accumulators - why?

If the accumulators are empty then get them charged. If they are empty you will have a warning light in the cab (if the bulb works) and the flag down, the flag says STOP. It means STOP.

A client of ours drove an RML with empty accumulators and rear ended a car, apart from the car it caused significant and costly damage to the front, RM grilles, radiators and fans are not cheap, total was over £800 parts alone.

So to sum up, nobody is saying go anywhere with it with only half the braking system functioning. What you going to do if the quill drive breaks as they do, or the engine cuts out...

My bus number (if any): RTL 960, RMC 1458, RM 1585 and several RTs

Re: Accumulators - why?

You are not going to get anyone to give you the answer you want. No, the bus is not safe to drive home with discharged accumulators and/or with the flag showing STOP. If presented for MOT or at a roadside check in that condition then it would fail and you would be going home on a hook. At the roadside you would be receiving a fixed penalty notice and notice of licence endorsement with the vehicle immobilised. If the flag and warning light were showing STOP with the engine running then it would probably be taken further before the magistrates with charges against you both as driver and owner.

Re: Accumulators - why?

roythebus
Not quite right there Brian from what I understand. You ought to get full line pressure circa 1200psi with low or no accumulator pressure. the big problems are as per the Midland Red thread above, plus if the pump fails or the belts break, you then have no brake pressure and no emergency brake. The accumulators act as an air spring to provide hydraulic pressure in these cases.

Service brake pressure is about 600psi, emergency pressure is 1200psi, the last half inch of travel on the pedal. Remember that when you're on annual test.

I've just received a copy of the Lockheed manual on RM brakes which gives a very good description of how they work.
Yes totally agree about danger of brake pump failure and no accumulators to then enable braking Roy, but I was under the impression that continued use of a bus with empty tubes could or would suck fluid into the accumulators thus reducing braking efficiency. The RML owner in France said the brakes didn't work as normal, when she hit the car I don't know if the flag was down or not, but the accumulators were empty as we went and re-filled them.

My bus number (if any): RTL 960, RMC 1458, RM 1585 and several RTs

Re: Accumulators - why?

I suppose there is a chance that the accumulators will fill with hydraulic fluid if there's no air pressure in there, but the reservoir if fiiled or even showing "empty" on the gauge glass ought to have enough reserve to cope with that eventuality.

I suspect the RML in France had other issues such as badly adjusted brakes which is a common fault on almost every one I've looked at at home and abroad. Seized slack adjusters, seized rollers and brake shoes... If the handbrake is correctly adjusted, that should be enough to stop if the hydraulics fail.

My bus number (if any): RML2532

Re: Accumulators - why?

RML2745 was converted to air brakes in the early 90s. The cost was probably too great to justify any more being done.
https://www.flickr.com/photos/33747326@N05/44085279754/in/photostream/

My bus number (if any): Formerly RML2613