Bulleid wheelslip question

Slipping is governed by a number of factors, some are a part of the condition of the locomotive, others are due to design decisions made, then you have rail head conditions, the skill of the crew needs to be considered as well as their familiarity with the type in their charge and the route over which they are working it.

Tyre condition is important, regular turning to maintain the correct profile is viewed as being a necessity. The driven wheelset suffers from micro slip, it takes all the traction forces before the coupled wheels receive drive from the rods. The solution taken was to turn the driving wheel tyres to a slightly larger diameter than the coupled but this is not recognised in the UK. Tyre wear is such that it is not even about the circumference, they do not wear in a regular manner. Areas of wear take place at those areas where there are torque peaks and so the tyre ceases to be of a constant diameter profile, or more simply lack of smooth power delivery causes unwanted tyre wear patterns. This leads to weight unloading at certain rotational speeds. Rather like poor balancing, at certain speeds the wheels loose contact with the rails.

Leaking lubrication lines can contaminate the tyres and ideally a locomotive should be designed with due care being given to the positioning of any oil lines, and particularly any unions, in order to avoid such occurrences. Sadly we are frequently dealing with engines built before this requirement became recognised.

The regulator should offer easy manipulation and predictable flow control. It should also prevent working at less than full opening so though it is highly controllable when needed for short periods it is weighted so that latched up fully open is the only comfortable long term solution to dealing with it when driving. Having just stated 'only' that is not true since some drivers can open the regulator to full, remove the handle and control the locomotive and train throughout a journey by means of adjusting the reverser and making use of the brakes. An insensitive and unpredictable reverser would make this very difficult.

Rail head condition is a factor that has been paid attention to by few designers, rail washers, enhanced sanding in the Chapelon manner (sanding applied in front of the leading bogie really does work and the 231Es were famous for not slipping), though offering advantages also introduce extra costs and complications.

 

So, how do you explain the fact that, in BR days, every Duchess departing north from Crewe station slipped before getting to grips with its train? They probably did the same at the south end, but the vast majority of us spotters were at the north end.

 

I can only offer the anecdotal evidence in one of my previous posts Tom, but my impressions of relative adhesion were accumulated hundreds of journeys on the same stretch of line in the latter days of steam, and over all seasons and rail conditions the standards had better starting adhesion pulling the same load on the same line than the light pacifics. However, once above 20mph there was probably nothing to choose between them and of course the Bulleids superior steaming capability then came into its own at higher speeds. Whether starting adhesion was down to two cylinders not three or whether it was just a combination of valves, motion and frames leading to a smoother application of force I'll never know - but some of the excellently researched posts so far on this thread have improved my understanding of possible reasons.
Just another anecdote a propos of nothing , the down Bournemouth local would often pull in while we waited for our train. Usually three coaches pulled by a humble Brighton Standard 80000 tank. In 1967 with engines being withdrawn every week and a captive audience of watching schoolkids the drivers would often "give it a go" on the regulator pulling away and on the slight downhill out of Hinton Admiral station I saw the fastest acceleration I've ever witnessed anywhere from a steam engine on one of these - it could have been a 4-VEP! No slippage at all with the regulator wide open. The VEPs replaced these fine workhorses that same summer.

 

One thing that I do recall from the last years of steam and in particular the Bulleids, was the amount of oily filth covering everything below the running plate, let's face it , all of that oil used for lubrication was on a total-loss system and it had to end up somewhere, no doubt a fair bit on the tyres with inevitable consequences. Also, by the early sixties, most of the network had diesels in either locomotive or DMU form and I'm sure that we have all witnessed the fallout of oil and fuel from these on the tracks adjacent to platforms, particularly at platform ends, it's no wonder that we saw so much slipping from locomotives - including the diesels and DMUs. Throw in a liberal sprinkling of flange-lubricators around the system for good measure and slipping - even at speed under power - is inevitable.

Cheers,

Alan

 

I'm sure you are right in your observations, but it isn't quite comparing like with like. For example you'd no doubt expect a Standard 5 4-6-0 to slip less than a 4-6-2. So the real question is not "did Bulleids slip more than other engines?" but "did they slip more than other pacifics?" A better comparison might be with a Britannia, for example, rather than a Standard 5 which has lower TE, higher adhesion and no trailing axle compared with a Bulleid. This thread has already shown a lot of examples of Duchesses slipping when starting - hence my querying whether the Bulleids were objectively worse than other similar locos, or just have a worse reputation?

One other thought: the Bulleid pacifics had a reputation for riding well at speed. But I wonder (warning: uninformed speculation alert...) if that came at the cost of adhesion when starting, i.e. different suspension arrangements in the trailing truck that helped the riding qualities also resulted in more weight than normal transferring to the back?

Tom

 

Great Thread.

Any one ever design a system to unweight the trailing truck on starting ?

 

I believe that the Chinese QJs had a mechanism to de-weight both the pony and trailing truck to increase adhesion.

Cheers,

Alan

 

Didn't stop them slipping though.

 

A personal observation on that very comparison: Both types are fitted with steam chest pressure gauges, with a Britannia on dry rail it is possible to open the regulator until 120psi is showing in the steam chest before the loco is likely to slip on pulling away, with an original Bulleid the figure is 90psi under the same conditions.

 

Sounds like a valid anecdote there Mr Brit, it seems like with all that we have read, heard and seen regarding the original Bulleids, Mr Riddles Standard class 7 locomotives were an advance in some significant areas.

Cheers,

Alan

 

You're right - it was Derek Cross who told this story in one of his erudite introductions to the Locomotives Illustrated series - from memory, it happened at Kilmarnock one morning when attempting to restart a heavy overnight sleeper on the G&SW route. The driver couldn't control the slip for many minutes, by which time the railhead had been worn down to the web.

David

 

A mate had that happen to him behind a Class 87 leaving Glasgow Central so the problem goes beyond the steam age. It was scalding hot coffee so he was less than impressed.

 

I have read that was indeed the case.

 

OK, let's put our 3 cyl loco at a position where one cylinder is at front dead centre (i.e., the con rod is directly in line with the piston rod. If you apply steam to the piston, it is on a position that it cannot apply any turning moment. The second cylinder crank is leading this by 120 degrees so the piston is at 75% of stroke - not quite true because of angularity but I'm being theoretical. Most locos have a maximum cut-off of about 75% so, at this point the valve is just closed to steam and no steam is admitted to this cylinder. That leaves the third cylinder, which is a further 120 degrees in advance as the only cylinder that is applying torque at the crank axle at the moment the regulator is first opened and this is not at the point of maximum torque (mid stroke.)
The above is all theoretical and a lot more needs to be taken into account, including admission of lead steam and, as I said, angularity has a significant effect. Valve gears are also far from perfect. A Black 5 for instance, has, in full forward gear, an effective cut-off of 76% on the front port and 72.5% on the back port; different again in back gear. In theory, anyway. Wear in motion joints will also reduce this.
I believe Gresley originally limited cut-off on his three cylinder locos to 65%, which made them very poor starters; something later rectified, at least in forward gear. (The reverser indicator on 60007 only goes to 65% in reverse.)
In terms of opening the regulator at starting, I invariably open the regulator and, as soon as my senses tell me something is happening, shut it again. Although I've driven both I can't really comment on the differences between an A4 and a Bulleid regulator. Every loco is different; the regulators of 60007 & 60009 are like chalk and cheese (from my limited experience of the latter.)

 

As a boy I can remember seeing Lapford slip to a stand moving out of Bournemouth Central onto the shed. It was very slightly up hill and eventually she limped onto the shed! I was always lead to believe that this was an inherent problem with Pacific locos in that it was due to the front and trailing bogies lifting the driving wheels fractionally off the rail that caused the problem. That along with the oil bath?

Dave

 

Thank you for that clear explanation, by which I am now convinced. I think my argument about evenness of torque is valid between low speed and high speed, but NOT for starting from rest when, as you explain, one piston can be at one end of its travel, so unable to exert any useful torque on the crank, and another one in such a position that its associated inlet port is closed, leaving only the third one to start (or not) moving the loco and train.

 

Have a look at this, about six minutes in. 34102 sliding INTO Waterloo in reverse, although I believe there is a downhill gradient at this point. However, it seems the crew were going slightly too fast for the rail conditions, as there has been a full brake application. They probably didn't even notice it had happened until the wheel flats started!



You might not be able to directly access it from here, but just click on the link in the video box.

 

What's so unusual about that? Locos the length and breadth of the country pick up their wheels and slide, seen it umpteen times and most recently at the Foxfield.
Shows also a "surefooted" Standard 4-6-0 having a right old slip and slide leaving Waterloo.