Loco Power - Motor or Weight?

[Mark Laidlay]

I've long believed that the haulage capacity of a loco is a result of weight on the driven wheels and wheel material (type of metal or traction tyres), this assumes that the motor is up to spinning the wheels and the model runs smoothly.  Does gearing or motor characteristics really have any affect?

[AndyID]

Gearing affects max and min speeds and the torque (turning force) but adhesion is only a function of weight on the driven wheels and the coefficient of friction between the wheels and the rails. As you correctly say that depends on the materials and any contamination on the surface.

 

The number of driven wheels makes no difference to the adhesion. You can convert an 0-10-0 into an 0-4-0 and it will pull just as well. (I expect someone will object to that last bit  )

 

[Michael Hodgson]

40 minutes ago, AndyID said:

 You can convert an 0-10-0 into an 0-4-0 and it will pull just as well. (I expect someone will object to that last bit  )

 

 

I don't think an 0-4-0 tender engine would make much of a banker on the Lickey.

[AndyID]

18 minutes ago, Michael Hodgson said:

 

I don't think an 0-4-0 tender engine would make much of a banker on the Lickey.

 

Assuming it was the same weight at the 0-10-0 version it would do just as well. The problem is the civil engineering department would never allow that much axle load as it would wreck the track   Fortunately that's not something we have to worry about at the smaller model scales.

 

There is a common misconception that more wheels means more traction but traction is only a function of total weight on the driven wheels and the coefficient of friction between wheels and rail. The number of wheels makes no difference. The real reason for more wheels is the need to distribute the weight over a greater area of track and to keep the axle loads within limits that the bearings can handle.

[Nova Scotian]

1 minute ago, AndyID said:

 

Assuming it was the same weight at the 0-10-0 version it would do just as well. The problem is the civil engineering department would never allow that much axle load as it would wreck the track   Fortunately that's not something we have to worry about at the smaller model scales.

 

There is a common misconception that more wheels means more traction but traction is only a function of total weight on the driven wheels and the coefficient of friction between wheels and rail. The number of wheels makes no difference. The real reason for more wheels is the need to distribute the weight over a greater area of track and to keep the axle loads within limits that the bearings can handle.

Ugh, I'll bite.

 

In exactly controlled track conditions yes... but the 0-4-0 has a much greater risk of losing traction due to any track irregularities - any patch of rail with reduced adhesion, a change in incline etc, especially where it causes a shift of weight distribution.

 

For toy trains.... if the motor is powerful enough to slip the wheels, it's mostly determined by weight on the driven wheels. If the motor is not, then it'll burn out... If there are lots of axles not driven, or the weight is not generally balanced, there can be a loss of traction despite an overall high weight - 0-4-4Ts are notorious for being challenging to get good traction.

 

 

[GWR-fan]

For a rigid chassis then a multi axle model in essence becomes a two driving axle model.  Contact with the rail for the intermediate "driving" axles will degrade traction over track irregularities.  Compensation will transfer load to those driving wheels able to maintain both contact and traction.  Thus,  while both torque, power and traction are essential to consider,  the state of the chassis,  rigid or compensated,  is just as important.  

[AndyID]

6 minutes ago, Nova Scotian said:

Ugh, I'll bite.

 

In exactly controlled track conditions yes... but the 0-4-0 has a much greater risk of losing traction due to any track irregularities - any patch of rail with reduced adhesion, a change in incline etc, especially where it causes a shift of weight distribution.

 

 

Ah ha! But unless some of the weight transfers to non-driven wheels the weight carried on the driven wheels is exactly the same The weight doesn't mysteriously disappear. The wheels that become lightly loaded produce less traction but that means other wheels are more heavily loaded and they produce more traction. It nets out to the same amount of traction.

 

Adding more wheels was not something the designers really wanted to do. More wheels means more complexity and cost but they had to add more wheels as locomotives became ever heavier.

[AndyID]

2 hours ago, GWR-fan said:

For a rigid chassis then a multi axle model in essence becomes a two driving axle model.  Contact with the rail for the intermediate "driving" axles will degrade traction over track irregularities.  Compensation will transfer load to those driving wheels able to maintain both contact and traction.  Thus,  while both torque, power and traction are essential to consider,  the state of the chassis,  rigid or compensated,  is just as important.  

 

Alas, no

 

As some wheels lose contact other wheels have to support more weight and that increases their ability to transmit force without slipping. The ability to transmit force without slip depends only on the total weight supported by the wheels in contact with the rails. The weight is constant so it makes no difference whether it's ten wheels or three wheels in contact with the rails.

 

Compensation will certainly improve electrical contact and help to prevent derailments but it won't improve traction. The only way to do that is to increase the downward force (magnetic attraction or more weight) or alter the coefficient of friction (sand, traction-tires etc.)

 

Traction depends on friction and a gent called Amontons figured all of this out around 300 years ago.

 

His Laws of Friction:

 

The force of friction is directly proportional to the applied load.

The force of friction is independent of the apparent area of contact.

 

As far as I'm aware nobody has been able to disprove his laws.

 

 

[Miss Prism]

On typically undulating track, the dynamic variation in wheel loading on a multi-axle coupled chassis will be less than the dynamic variation in wheel loading on a chassis with fewer axles. Hence multi-axle coupled things are less prone to slipping than singles. Also, the dynamic variation in the wheel loading for a multi-axled coupled chassis will be reduced if the axles are loaded equitably in the static situation.
 

[AndyID]

44 minutes ago, Miss Prism said:

On typically undulating track, the dynamic variation in wheel loading on a multi-axle coupled chassis will be less than the dynamic variation in wheel loading on a chassis with fewer axles. Hence multi-axle coupled things are less prone to slipping than singles. Also, the dynamic variation in the wheel loading for a multi-axled coupled chassis will be reduced if the axles are loaded equitably in the static situation.
 

 

Well put, but it still ignores the underlying science that was established more than 300 years ago 

 

Singles tend to slip because a large percentage of their weight is carried on non-driven wheels, excepting my Caley 123 of course. It has Magnahesion although that doesn't work for toffee on nickel-silver track.

[GWR_Modeller]

What about braking - do the same principles apply? Were bogies and tenders braked and how did this improve stopping? 

[GWR_Modeller]

3 hours ago, AndyID said:

 

Alas, no

 

As some wheels lose contact other wheels have to support more weight and that increases their ability to transmit force without slipping. The ability to transmit force without slip depends only on the total weight supported by the wheels in contact with 

 

Traction depends on friction and a gent called Amontons figured all of this out around 300 years ago.

 

His Laws of Friction:

 

The force of friction is directly proportional to the applied load.

The force of friction is independent of the apparent area of contact.

 

As far as I'm aware nobody has been able to disprove his laws.

 

 

Are these true is a non ideal situation or does the real world get in the way.  Steel track and steel tyres presumably deform very little but in the movies the way to get a good wheel spin in a car chase is to lower the tyre pressure, the total weight of  the car does not change but the area of tyre contact does and it slips more easily, what causes this?

[NIK]

1 hour ago, GWR_Modeller said:

Are these true is a non ideal situation or does the real world get in the way.  Steel track and steel tyres presumably deform very little but in the movies the way to get a good wheel spin in a car chase is to lower the tyre pressure, the total weight of  the car does not change but the area of tyre contact does and it slips more easily, what causes this?

Hi,

 

Just a guess:

 

The car tyre behaves in a non-linear way so when the pressure is lowered the coefficient of friction goes down faster than the surface area of the tyre in contact with the road goes up?.

 

Alternatively the tyre ends up putting most of the force (weight of the car) in the region of the sidewalls so the area most effective in creating grip is reduced?.

 

Regards

 

Nick

[Mark Laidlay]

3 hours ago, GWR_Modeller said:

Are these true is a non ideal situation or does the real world get in the way.  Steel track and steel tyres presumably deform very little but in the movies the way to get a good wheel spin in a car chase is to lower the tyre pressure, the total weight of  the car does not change but the area of tyre contact does and it slips more easily, what causes this?

Changing the subject very much but to increase grip for car tyres the pressure is lowered, raising pressure thus reduces grip.  Maybe if you go to a ridiculously low pressure (<10PSI) then grip will be reduced but even that I have doubts.

[Nova Scotian]

11 hours ago, AndyID said:

 

Ah ha! But unless some of the weight transfers to non-driven wheels the weight carried on the driven wheels is exactly the same The weight doesn't mysteriously disappear. The wheels that become lightly loaded produce less traction but that means other wheels are more heavily loaded and they produce more traction. It nets out to the same amount of traction.

 

Not true - if there is a weight transfer to the rear wheels (in your 0-4-0 example) and it is the rears that have lost traction, then it nets out to less.

[Nearholmer]

15 hours ago, AndyID said:

There is a common misconception that more wheels means more traction but traction is only a function of total weight on the driven wheels and the coefficient of friction between wheels and rail. The number of wheels makes no difference.

 

Yes, but, no, but ...... In practice, a more-wheeled loco can be more sure-footed, less likely to spin its wheels, especially a modern loco with independent control over each axle, because it is less susceptible to very localised problems with adhesion - less force is being put through each wheel-rail interface.

[AndyID]

3 hours ago, Nova Scotian said:

Not true - if there is a weight transfer to the rear wheels (in your 0-4-0 example) and it is the rears that have lost traction, then it nets out to less.

Good Morning!

 

if there is weight transfer the wheels that carry more weight can deliver more traction before they slip.

 

Of course it's different if some of the weight transfers to non-driven carrying wheels. That's why 4-6-0's tend to be a bit more "sure footed" than Pacifics.

[AndyID]

55 minutes ago, Nearholmer said:

 

Yes, but, no, but ...... In practice, a more-wheeled loco can be more sure-footed, less likely to spin its wheels, especially a modern loco with independent control over each axle, because it is less susceptible to very localised problems with adhesion - less force is being put through each wheel-rail interface.

 

Modern locomotives with independent motors are actually more susceptible to localized conditions than wheels that are mechanically coupled. That's why they require sophisticated control systems to prevent slip.

[Nearholmer]

23 minutes ago, AndyID said:

Modern locomotives with independent motors are actually more susceptible to localized conditions

 

I get that, but my point (perhaps not well-explained) was that because the control system detects the situation and adjusts accordingly, they remain overall sure-footed and don't spin their wheels.

 

 

 

 

Edited February 14, 2021 by Nearholmer

[AndyID]

9 hours ago, GWR_Modeller said:

Are these true is a non ideal situation or does the real world get in the way.  Steel track and steel tyres presumably deform very little but in the movies the way to get a good wheel spin in a car chase is to lower the tyre pressure, the total weight of  the car does not change but the area of tyre contact does and it slips more easily, what causes this?

 

I would think that has more to do with the temperature of the tire. Soft tires heat up more due to the increased flexing. The change in temperature affects the coefficient of friction.

 

But I'm guessing

[AndyID]

8 minutes ago, Nearholmer said:

 

I get that, but my point was that because the control system detects the situation and adjusts accordingly, they remain overall sure-footed and don't spin their wheels.

 

 

 

Agreed, but why do some locos have four powered axles while others have six? Or to put it another way did anyone ever feel the need to convert a Bo-Bo into a Co-Co just to improve traction?

[billbedford]

1 hour ago, Nearholmer said:

 

Yes, but, no, but ...... In practice, a more-wheeled loco can be more sure-footed, less likely to spin its wheels,

 

 

If the wheels are connected, and the weights are reasonably evenly distributed, the loco will slip when the lightest loaded axle breaks the friction limits. This is because the torque that that axle was producing before the slip gets transfer to the other axles causing their torque to overcome their friction.  

[Nearholmer]

56 minutes ago, AndyID said:

did anyone ever feel the need to convert a Bo-Bo into a Co-Co just to improve traction?

 

I've never heard of engineering it that way, but rostering is a different matter.

 

The combination of 2 x Class 20, each a 1000hp Bo-Bo, was very popular in Britain for heavy coal trains, chosen partly because that combo worked very well in the poor adhesion conditions typical of a traditional colliery in wet weather. I don't think they had sophisticated controls, just basic a basic wheel slip protection.

 

To cut a complicated story short, and grossly simplify history, it took three or four goes to arrive at a single-unit  Co-Co that could do the job adequately, and by that time the job had largely disappeared.

 

 

 

 

Edited February 14, 2021 by Nearholmer

[Nearholmer]

2 minutes ago, billbedford said:

the loco will slip when the lightest loaded axle breaks the friction limits

 

Depends how localised the poor adhesion is, and how much extra torque is thrown onto the others axles, but, within that set of limits, yes.

[Nova Scotian]

3 hours ago, AndyID said:

Good Morning!

 

if there is weight transfer the wheels that carry more weight can deliver more traction before they slip.

 

Of course it's different if some of the weight transfers to non-driven carrying wheels. That's why 4-6-0's tend to be a bit more "sure footed" than Pacifics.

Again, this is only when you assume the coefficient at the rail is consistent and even. If it is not, the weighted wheel from the wheel transfer can very feasibly be on a section of rail with a lower coefficient - thus the net result is not the same.

 

You are approaching this from a very simplistic view - which is *obviously* aligned with the physics of it, but it is not answering the question of the original poster and is misleading on how the rail/tread interface works on the railway system.