Brake Force and Stopping Distance

John Fitton

Brake Force and Stopping Distance

Postby John Fitton » Fri Mar 17, 2017 4:50 pm

I think the railway fraternity has a general obsession with tractive effort and horsepower, but a relative disinterest or lack of knowledge (me!) in brake force. So, given that all vehicles nowadays have brakes, and we see the typical braking force a significant fraction of vehicle weight, why do trains take so long to stop?

John

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Tim V
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Re: Brake Force and Stopping Distance

Postby Tim V » Fri Mar 17, 2017 5:08 pm

Inertia.
Tim V

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Fri Mar 17, 2017 5:41 pm

Because, like acceleration, it depends on the friction between steel wheels and steel rails, which has a relatively low coefficient in normal traffic. Too much power and you either get wheelspin or a non-rotating, sliding, wheel, meaning the train will take longer to stop. And, as Tim says, 300 plus tons of train has a lot of inertia.
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Noel

John Fitton

Re: Brake Force and Stopping Distance

Postby John Fitton » Fri Mar 17, 2017 7:23 pm

Well, let me clarify what I am looking for.

If a net force of 1 tonne is applied to 100 tonnes then we will have an acceleration of 1/00th g. 1 g is 0 to 60 mph (changing units for a moment) in 2.7 seconds, so 1/100th g will take 2700 seconds. A force of 1 tonne is pretty small (2,204 lbs in old money), but a loco producing a T/E of 10 tonnes (22,040 lbs) will get to 60 in about 27 seconds. (ignoring horsepower capability of course, and taking a constant T/E.

So in braking terms, when we see brake forces of 50 tonnes for a large locomotive, why do we not see extremely short braking distances?

I would have to assume that the braking force is an achievable number, so any comments about steel rail etc are overridden by practical measurements.

John

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Fri Mar 17, 2017 8:34 pm

John Fitton wrote:If a net force of 1 tonne is applied to 100 tonnes then we will have an acceleration of 1/00th g


Force [in Newtons] = mass [in Kg] x acceleration [in metres per second^2] where one Newton is the force needed to accelerate one kilogram of mass at the rate of one metre per second squared in the direction of the applied force. Force, therefore, is not measured in units of mass, so the sentence quoted [ignoring the typo] is not correctly stated.

John Fitton wrote: 1 g is 0 to 60 mph (changing units for a moment) in 2.7 seconds


:?:
Last edited by Noel on Fri Mar 17, 2017 8:49 pm, edited 1 time in total.
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Noel

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Fri Mar 17, 2017 8:45 pm

John Fitton wrote: a loco producing a T/E of 10 tonnes (22,040 lbs) will get to 60 in about 27 seconds


This is meaningless without a starting speed and the weight of the loco. Assuming starting from zero, I think this is more than somewhat questionable, even without a train attached, for any loco [no, I haven't tried to do the maths]. It also ignores some non-negligible factors such as gradients and the effective coefficient of friction for the condition of the rail surface.
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Noel

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Paul Townsend
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Re: Brake Force and Stopping Distance

Postby Paul Townsend » Fri Mar 17, 2017 9:54 pm

John Fitton wrote:I think the railway fraternity has a general obsession with tractive effort and horsepower, but a relative disinterest or lack of knowledge (me!) in brake force. So, given that all vehicles nowadays have brakes, and we see the typical braking force a significant fraction of vehicle weight, why do trains take so long to stop?

John

Brunel was asked about the brakes on one of his early locos and replied " tolerably useless"
Who needs to analyse that!

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Re: Brake Force and Stopping Distance

Postby kelly » Sat Mar 18, 2017 12:22 am

As stated, inertia, but also the fact the set up is deliberately favourable to lack of friction. The various electric locos/units can be prone to slipping on wet rails due to the power to adhesion amounts. Then there are the other factors like leaves, which can make the braking much much worse.
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Re: Brake Force and Stopping Distance

Postby williambarter » Sat Mar 18, 2017 8:14 am

Reading back, 50 tonnes brake force is about right for a Class 37, but if the driver were to apply the brake in full all at once I'm sure the result would be derailment of the following wagons. In fact I've seen it happen, smashing 15 vanfits (it was my fault, as I had just said to the chargeman "I think that's the best night we've had this week").

For suburban / metro passenger trains, the limit on braking rate is more to do with passenger comfort, whilst for high speed operations the limit tends to come from the ability to dissipate heat from the brake pads, where there is a trade-off between air flow over the pads, and low aerodynamic resistance.

Hence there is a desire in really high speed operations to stick to use of the regenerative brake if at all possible in normal use, which gives lower braking rates than friction braking.

William

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Re: Brake Force and Stopping Distance

Postby Alan Turner » Sat Mar 18, 2017 4:02 pm

John Fitton wrote: why do trains take so long to stop?

John


as trains weigh a lot then the brakes have to dissipate a lot of kinetic energy. Train brakes are not that efficient at doing that so it takes a long time to stop.

regards

Alan

John Fitton

Re: Brake Force and Stopping Distance

Postby John Fitton » Sat Mar 18, 2017 6:42 pm

Noel wrote:
John Fitton wrote:If a net force of 1 tonne is applied to 100 tonnes then we will have an acceleration of 1/00th g


Force [in Newtons] = mass [in Kg] x acceleration [in metres per second^2] where one Newton is the force needed to accelerate one kilogram of mass at the rate of one metre per second squared in the direction of the applied force. Force, therefore, is not measured in units of mass, so the sentence quoted [ignoring the typo] is not correctly stated.

John Fitton wrote: 1 g is 0 to 60 mph (changing units for a moment) in 2.7 seconds


:?:

Substitute tonnes force for tonnes and the statement is correct. There is no need to switch to normalized units of force (poundals in imperial, Newtons in SI) as most people don't understand what they mean.

John Fitton

Re: Brake Force and Stopping Distance

Postby John Fitton » Sat Mar 18, 2017 6:45 pm

Noel wrote:
John Fitton wrote: a loco producing a T/E of 10 tonnes (22,040 lbs) will get to 60 in about 27 seconds


This is meaningless without a starting speed and the weight of the loco. Assuming starting from zero, I think this is more than somewhat questionable, even without a train attached, for any loco [no, I haven't tried to do the maths]. It also ignores some non-negligible factors such as gradients and the effective coefficient of friction for the condition of the rail surface.

My earlier paragraph indicated this was a zero to 60 acceleration time, and the mass of the loco was 100 tonnes.

It is generally well known (or so I thought) that a 0 to 60 time of 2.7 seconds is very close to a 1 g acceleration.

John Fitton

Re: Brake Force and Stopping Distance

Postby John Fitton » Sat Mar 18, 2017 6:46 pm

williambarter wrote:Reading back, 50 tonnes brake force is about right for a Class 37, but if the driver were to apply the brake in full all at once I'm sure the result would be derailment of the following wagons. In fact I've seen it happen, smashing 15 vanfits (it was my fault, as I had just said to the chargeman "I think that's the best night we've had this week").

For suburban / metro passenger trains, the limit on braking rate is more to do with passenger comfort, whilst for high speed operations the limit tends to come from the ability to dissipate heat from the brake pads, where there is a trade-off between air flow over the pads, and low aerodynamic resistance.

Hence there is a desire in really high speed operations to stick to use of the regenerative brake if at all possible in normal use, which gives lower braking rates than friction braking.

William

I think this is the sensible answer William, Thank you.

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Sat Mar 18, 2017 7:33 pm

John Fitton wrote:Substitute tonnes force for tonnes and the statement is correct.


No, it is not. Force, as I stated earlier, is calculated from the mass it acts on times the resulting acceleration of the mass. The acceleration is not optional.
Regards
Noel

John Fitton

Re: Brake Force and Stopping Distance

Postby John Fitton » Sat Mar 18, 2017 8:40 pm

Noel wrote:
John Fitton wrote:Substitute tonnes force for tonnes and the statement is correct.


No, it is not. Force, as I stated earlier, is calculated from the mass it acts on times the resulting acceleration of the mass. The acceleration is not optional.

2nd year physics Noel, force is mass times acceleration. So a ten tonnes force on a 100 tonnes mass produces an acceleration of.....

(fill in the blank)

William produced the correct answer by the way...

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Sat Mar 18, 2017 10:35 pm

If you honestly think that force can be measured in units of mass, this link may amuse you...

http://forums.uktrainsim.com/viewtopic.php?f=245&t=111716
Regards
Noel

Alan Turner
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Re: Brake Force and Stopping Distance

Postby Alan Turner » Sun Mar 19, 2017 9:19 am

Unfortunately units of Mass and units of weight are banded about by people as if they are the same thing. For the general populous that works most of the time for what they do (buying apples or what ever). However in the world of engineering/physics they are different.

However force has nothing to do with braking and to talk about a "Braking Force" is meaning less. Braking is to do with the dissipation of kinetic energy. Most often by generating heat but occasionally , now, by generating electricity.

regards

Alan

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Noel
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Re: Brake Force and Stopping Distance

Postby Noel » Sun Mar 19, 2017 10:14 am

Having now had more time to look at this, I offer you a BR document from the Barrowmore website:

http://www.barrowmoremrg.co.uk/BRBDocuments/BR30054_6_Issue.pdf

Section G gives the brake force figures for BR locomotives, in tons. Section C1/6 states that "the brake force of fitted vehicles should be assessed as one-half of the tare, rounded down to the next whole number of tons." Section B5(b) states how the train brake force is to be arrived at, which is by addition of the brake forces of individual wagons and the loco. However the individual figures are arrived at, it seems clear that they are a simplified method of enabling train and yard staff of checking whether or not the train has adequate means of stopping, since it would not be reasonable to expect them to carry out complex mathematical calculations when making up a train and calculating the speed at which it can be allowed to run. Equally clearly, the figures are not 'force' as defined in physics, and cannot be used in such calculations.

On the calculations, there is a whole book:

http://shop.bsigroup.com/ProductDetail/?pid=000000000030250647
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Noel

Alan Turner
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Re: Brake Force and Stopping Distance

Postby Alan Turner » Sun Mar 19, 2017 1:08 pm

Of course the ability to dissipate the train's kinetic energy is limited by the friction that exists between the steel wheel and the steel rail. It is pointless providing brakes that can dissipate more energy than can be dissipated from the rail/wheel interface because if the brakes attempt to, the brakes will stop working and transfer the energy dissipation to the rail/wheel interface.

What is interesting is that if you follow the maths through the stopping distance of a fully braked train is independent of its mass. Only the initial velocity and the coefficient of steel/steel friction governs the stopping distance.


regards

Alan


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