Would it be easier to pull a train with flat wheels?

[student715]

A train applies steel wheels to a steel track.
Therefore, without any distortion of the wheel or track, the contact patch would be extremely small.

Now, if you slice a chord from the wheel, creating a flat spot contacting the track, the result is a larger contact patch.

With a larger contact patch, more friction will occur between the wheel and track, but the weight will be spread over a larger surface area.

So, here is my question: Will the train be more difficult to pull/slide along the track with standard round wheels or with flats cut on the wheels?

 

[Danger]

Flat cuts will not roll, which is the whole point of wheels. You would be dealing with a purely frictional sliding force as opposed to whatever rolling resistance you would encounter from a decent wheel.
I should mention that railroad wheels and trucks are some of the most efficient mobile devices ever designed. A single man (albeit in good physical shape) can pull a railway flatcar with tonnes of load on it. The acceleration won't scare a Top Fuel Eliminator driver, but any movement is good and it continues to accelerate until a walking pace is reached. After that, it's just yawn city as he walks along with almost no resistance. Given the mass of the parts, I find it hard to believe myself, but it happens on the show circuit. My best advice to find this stuff is to check out the Daily Planet website and go to the archives for a copy of the episode that contains this.

 

[student715]

In both cases the wheels will not be allowed to rotate. ( I forgot to add this in the first post)

The wheels will be fixed; therefore, no rotation will occur.

With the wheels not able to rotate, will the train require more force to move with flat spots contacting the track or the small contact of round wheels?

 

[Crazymechanic]

student715 hello well please don't feel offended but can I ask you a question ? Which scenario and outcomes seem logic to you?

Sliding would put much more friction on the metal to metal contact area , especially under heavy load, dust or rust conditions , the only way i see a sliding "wheel" could work is if it was lubricated by oil constantly the way a bearing is.But then the train wheels would be called bearings now wheels.

A train wheel is and makes the train a extremely efficient way of transport because the wheel touches the rail at a very small area + the resistance is also very small (ofcourse as long as the wheel is rolling) and this is all possible thanks to the properties of metal.

But as to sliding wheels with flat surfaces well I think that is a bad bad idea as it would have a lot more friction + how do you imagine the train would move forward? Electromagnets? well then why the sliding track in the first place. :)

 

[student715]

I am not offended

Let me try this again.

The train was just an analogy to describe an object readers would be familiar with.

Here is the situation:

I designed a very large weldment which rides on 6 steel wheels sitting on a steel track. (much like a train)
I have pneumatic cylinders attached to the weldment that will be pushing a fixed object and applying force against the weldment.
The steel wheels will lock by a brake on the motor used to move the weldment up and down the track.

The engineer i am working under has completed several coefficient calculations using steel on steel and decided the weight of the weldment alone, will be enough to allow the brake to keep the weldment from being pushed my the cylinders.

However, my mind tells me that the contact patch of the steel wheel on a steel track is so small that will be very little traction for the wheels.

Here is where "flat wheels" come into the story. the calculations did not take into effect the surface area of the wheel. Therefore, since a flat wheel has a larger surface area it does not make a difference, the friction would stay the same, so the amount of force required to move the weldment would not change from a flat wheel to a round wheel.

My opinion is the bigger the flat the bigger the contact patch will be. ultimately leading to more friction and less chance for the weldment to move.

Is my opinion correct or am i way off?

 

[Bandersnatch]

Let's see, the dry friction force would be given by:

$F=μF_n$

where μ is the coefficient of friction and F_n is the normal force due to weight.

Contact area doesn't factor in it.

In other words, if you increase the contact area by x, the weight per unit area gets divided by x, so it cancels out.

 

[student715]

Yes! That is exactly what the engineer said.

It just doesn't seem logical. I believe the information, but a car with a larger contact patch is known to have more traction. Racers always want a wider tire apposed to a skinner tire.

That was a great answer, the engineer I am working under is genius, so you're answer, being the same answer as his, proves you may have the genius gene as well. Thank you.

Let's see, the dry friction force would be given by:

$F=μF_n$

where μ is the coefficient of friction and F_n is the normal force due to weight.

Contact area doesn't factor in it.

In other words, if you increase the contact area by x, the weight per unit area gets divided by x, so it cancels out.

 

[Crazymechanic]

It is not an easy answer actually , as if we speak about trains they usually have enough mass so that even the small portion of the wheel that touches the rail in a given time is enough to get friction when braking so that the brakes are actually effective.

There are two things that basically affect the amount of friction something has, the first thing would be weight or you could also say the pressure some object exerts on the pavement or rail and contact area.
Now small pressure and small contact area surely would have less friction than small pressure and large contact area.

 

[Bandersnatch]

you may have the genius gene as well.

Lol, no. I'm pretty sure this was included in the secondary-school curriculum.

Cars are a different kettle of fish, as the wheels are not rigid and hard.

 

[Danger]

As an uneducated guy who just likes to build things based upon common sense, may I make a suggestion? (Don't bother answering; I'm going to anyhow. )
How about tapping your hydraulic supply off to one more cylinder operating a clamp that gloms onto the rail?
By the bye, what on Earth is a "weldment"?

 

[a1call]

Formulating friction/drag is not that simple. Weight distribution cancelation does not always hold.
As a mental simulation of an extreme case consider:

• A sharp nail is welded (on flat side) on the center of a smooth disk of 1 m diameter which weighs 100000 g
• The disk is placed on soft wood with the nail on top
• ​
  The disk can be pulled/pushed by an average person
• The disk is tuned
• ​
  The weight of the disk forces the nail through the soft wood
• ​
  The assembly can not be pulled/pushed as long as the nail is not bent.

 

[Bandersnatch]

Formulating friction/drag is not that simple. Weight distribution cancelation does not always hold.

While it might be true in general, you'd be hard pressed to find a real-world example closer to the idealised friction than steel wheels on rails, as in OP's contraption.

 

[rcgldr]

In the case of rubber tires, the coefficient of friction decreases as the load increases, known as tire load sensitivity. In the case of steel on steel, the decrease in traction due to a smaller contact area wouldn't be as significant, unless heat become an issue.

 

[OCR]

By the bye, what on Earth is a "weldment"?

http://help.solidworks.com/2012/Eng....html?id=ce2c398f80f640f496375cc697e68208#Pg0

http://help.solidworks.com/2012/Eng...w.htm?id=f8438bade661455e9a591e1f64ca3014#Pg0

http://forum.solidworks.com/message/129347#129347


Basicaly, anything you weld to something.

On a dozer, for example, some of the wear parts are called weldments... you cut the old part off, and weld a new part back on.



OCR

 

[Integral]

Note that train wheels do not normally slide on the rails, they roll. There is a big difference. At the point of contact the surface of the wheel is stationary with respect to the track. The force of friction that needs to be overcome to initiate motion is that of well oiled bearings and that force is less then the force of static friction between track and wheels. Your square wheels can only slide, to get it moving you need to overcome that same force of static friction. The force required to initiate rolling motion is much less then that required to start sliding between the same materials. That's why we have wheels.

 

[rcgldr]

I think what the OP is asking is if the goal is to stop a train in motion, and assuming the choices are to lock up the wheels, or to extend flat metal plates (of the same material as the wheels), on to the track so that the wheels are lifted off the track and all of the weight is on the flat metal plates, will there be a significant difference in stopping distance?

 

[Danger]

Thanks for the links, OCR, but that just confuses me more. Am I to understand that this entire thread is about something that doesn't exist other than in a draughting program? Even if it is a real physical joining of components, how on Earth does that qualify as an object rather than a condition and even more weirdly require it to be on 6 railroad wheels? This whole thing is squirrelly.

 

[student715]

This is an excellent analogy.
Describes what I am trying to accomplish very well.

I think what the OP is asking is if the goal is to stop a train in motion, and assuming the choices are to lock up the wheels, or to extend flat metal plates (of the same material as the wheels), on to the track so that the wheels are lifted off the track and all of the weight is on the flat metal plates, will there be a significant difference in stopping distance?

 

[student715]

Yes, this weldment only exists on my computer.
However, the weldment will be built in a couple weeks.
I am trying to give as much information as possible, but if I uploaded the file my boss would hang me.
I will muck something similar and try to upload.

Sorry to sound squirrel.

Thanks for the links, OCR, but that just confuses me more. Am I to understand that this entire thread is about something that doesn't exist other than in a draughting program? Even if it is a real physical joining of components, how on Earth does that qualify as an object rather than a condition and even more weirdly require it to be on 6 railroad wheels? This whole thing is squirrelly.

 

[student715]

1234

 

[student715]

Thanks for the links, OCR, but that just confuses me more. Am I to understand that this entire thread is about something that doesn't exist other than in a draughting program? Even if it is a real physical joining of components, how on Earth does that qualify as an object rather than a condition and even more weirdly require it to be on 6 railroad wheels? This whole thing is squirrelly.

https://gb5u8w.blu.livefilestore.com/y1p8ferb_PYvJoicKYiAYHZ2nsIPwicvnmyokxvhSachDSK4DQhUeMXTTtgE3gpoJrzkZz7rYxPO8HAqeJznCIpiuJfMKtx5kGP/Capture.JPG?psid=1 https://skydrive.live.com/?cid=5a50d29114bb1240

https://skydrive.live.com/?
cid=5a50d29114bb1240#cid=5A50D29114BB1240&id=5A50D29114BB1240%21107These are a couple links to the screen I uploaded.

 

[Danger]

Student, I regret that I don't know what you just posted. All that shows on my screen is a blue box with a white "?" in it. That is usually an indication of an image or video file that isn't Mac-friendly. (I'm running Safari 5.0.6 through Leopard on a MacBook.)
Anyhow, I totally understand why you have to keep details close to the vest now that you mentioned a boss. I had taken your username literally and assumed that you were a kid working on a private project or a school assignment.
I'll keep thinking about the matter, but for now my hands are kinda tied. If there are things that might be helpful that you can divulge semi-privately, feel free to PM me or send me an e-mail through the PF contact function. I'm not interested in industrial secrets or fortune; I just love designing **** and don't really have many opportunities to do so. (I'm not inspired to just do stuff out of the blue; someone has to introduce a need.)

 

[student715]

Think I got this time!

Student, I regret that I don't know what you just posted. All that shows on my screen is a blue box with a white "?" in it. That is usually an indication of an image or video file that isn't Mac-friendly. (I'm running Safari 5.0.6 through Leopard on a MacBook.)
Anyhow, I totally understand why you have to keep details close to the vest now that you mentioned a boss. I had taken your username literally and assumed that you were a kid working on a private project or a school assignment.
I'll keep thinking about the matter, but for now my hands are kinda tied. If there are things that might be helpful that you can divulge semi-privately, feel free to PM me or send me an e-mail through the PF contact function. I'm not interested in industrial secrets or fortune; I just love designing **** and don't really have many opportunities to do so. (I'm not inspired to just do stuff out of the blue; someone has to introduce a need.)

 

[OCR]

That is usually an indication of an image or video file that isn't Mac-friendly. (I'm running Safari 5.0.6 through Leopard on a MacBook.)

Aboot as useless as flaps on a Cessna, eh? :tongue:

Ok, sorry...




Carry on,

OCR... lol

 

[Crazymechanic]

hey Student , now that you have uploaded a schematic it is much easier to see.
By the way just an idea that I would like to throw.
i guess the machine is being moved on a rail since the wheels.
Now making wheels with flat surfaces maybe is not a bright idea but how about attaching a electromechanical brake to the mainframe? Well for example next to the wheel pair attach a coil , similar ones come in old cassette players and other stuff where an applied dc current pushes out a rod of some sort with some given force that depends on the voltage/amperage applied coil windings diameter etc.Now in the end you can attach a simple brake pad.By the way many trains aleast in the earlier days used brake pads to brake down the main wheels , eliptic brake pads.That were pushed with compressed air to add to the braking power.
So what do you think how about a similar device whenever the frame stops the brake pad pushes out against the rail and considering you don't have tons of weight in this device I would say it wouldn't move anywhere , unless specially pushed ofcourse which is not the case I suppose.

 

[Danger]

Aboot as useless as flaps on a Cessna, eh? :tongue:

:grumpy:

Didn't I kill you in a previous thread? :tongue:

Student, that is a beautiful drawing. I still think that my previous suggestion of a clamp is the most practical one, but I'll think on it some more.
You won't hear from me for a few minutes; I have to get myself out to my doorway because there's a beer delivery coming and it takes me 5 minutes to get there.
By the bye, that thing seems to have an awful hell of a lot of bolts for something that you claim is welded...

edit: I just realized upon rechecking something that I suggested tapping your hydraulics for the clamp mechanism, whereas you stated that you are using pneumatics. My bad.

 

[OCR]

<snip> but now I love you...

(I <snip> detest the NRA and everyone who supports them. <snip>)

Oh my... a love-detest relationship!?

And, now this...

Didn't I kill you in a previous thread?

Lol, instead of having coveted my Hemi Cuda, you should have continued with... :tongue:

I admit that I used to ignore you

Think I got this time!

But student, that looks more like a "boltment"... it is a good picture, however. Can you tell us a bit more about it?




OCR...

 

[Integral]

IMHO, that looks like a simple mechanism, something to be said for simplicity. If you go adding mechanisms for braking you just create more fail points.

Since we know nothing of the load, the accelerations, or speeds involved it is impossible to make a meaningful assessment. If he wants control, then just using the wheels and motor it would be possible to achieve a rate of acceleration which just maintains the static friction between track and wheels and will bring the cart to a stop sooner then if the wheels were locked and a slide was induced.

Seems to me that a slide is very undesirable, you could damage wheels and track, in addition to a lower level of control of the final stopping point.

 

[Danger]

Integral, remember that Student specified earlier that the wheels are locked anyhow. I took this to mean that he wants a "parking brake" rather than a deceleration device. It has to overcome the resistance of whatever his pneumatic cylinders are pushing against, not just stop the thing.
The reason that I suggest anchoring it to the tracks is that it then has the full weight and structural support of the rail system.

 

[256bits]

IMHO, that looks like a simple mechanism, something to be said for simplicity. If you go adding mechanisms for braking you just create more fail points.

Since we know nothing of the load, the accelerations, or speeds involved it is impossible to make a meaningful assessment. If he wants control, then just using the wheels and motor it would be possible to achieve a rate of acceleration which just maintains the static friction between track and wheels and will bring the cart to a stop sooner then if the wheels were locked and a slide was induced.

Seems to me that a slide is very undesirable, you could damage wheels and track, in addition to a lower level of control of the final stopping point.

I am with you on that.
Added complexity will not ensure 100% funtionality, quite the opposite.

If the wheels are locked, and the static friction force between the locked wheels and rails is enough to overcome the force produced by the cyllinder, then the design is complete.

 

[Crazymechanic]

Well considering the device on the wheels doesn't weigh that much I would say if when stopped a cylinder is pushing against something horizontally it would probably make the wheels slip because with a little weight solid smooth metal against another solid smooth metal has very little friction - well that's the whole idea behind train being a very efficient transport.

By the way a simple dc coil operating a single rod that pushes something in or out doesn't count to my mind as a huge complexity b the way.
But forget the brake pad maybe you can combine Danger's idea with the coil for say the coil pushes in or out a rod that anchors the whole device after any given intervals that you make on the rails.
I can't imagine how would you make this anchor without a electrical coil or device considering that this whole machine I guess will run pretty automated or controlled from a computer like a CNC machine I guess.

 

[student715]

Danger.. You're killin me!
That was a prototype in the first picture. The entire machine will be a weldment after all the bugs are worked out.

Here is another image.
I added a mock weight for reference and the cylinder will produce approximately 20,000lbs of force against a stationary object.

:grumpy:

Didn't I kill you in a previous thread? :tongue:

Student, that is a beautiful drawing. I still think that my previous suggestion of a clamp is the most practical one, but I'll think on it some more.
You won't hear from me for a few minutes; I have to get myself out to my doorway because there's a beer delivery coming and it takes me 5 minutes to get there.
By the bye, that thing seems to have an awful hell of a lot of bolts for something that you claim is welded...

edit: I just realized upon rechecking something that I suggested tapping your hydraulics for the clamp mechanism, whereas you stated that you are using pneumatics. My bad.

 

[256bits]

Something doesn't add up here with the figures you give, for your light rail system.
5000 pound weight of the 'weldment' , and a 20,000 pushing force for the 'pneumatics'??

You stated earlier:
"The engineer i am working under has completed several coefficient calculations using steel on steel and decided the weight of the weldment alone, will be enough to allow the brake to keep the weldment from being pushed my the cylinders"

With an assumption of an approxiamate coeficient of friction for dirty or somewhat greasy ( ie not clean ) steel on steel of 0.1, your static friction force to hold the car in place in only 500 pounds, with no safety facture.
Or your car needs to weigh more than 200,000 pounds.

Something is not so genius as you say, or your forces are incorrect.

20,000 pounds supplied by pneumatics.
What bore and pressure are you using?

 

[student715]

There is more then 1 pneumatic cylinder.
I removed the top of the assembly including all other cylinders from this image.
The total force produced from the cylinders is going to be about 20,000lbs.
The tracks and wheels will be covered to eliminate any chance of oil.

The answer i am searching for is to the question of friction.. I think?

When finding the coefficient of steel ( he used.8) this calculation does not take into account the surface area. I am baffled by this because the logically the bigger surface area the more traction.

I am not an engineer, but a car can stop faster with wider tires. Same rubber and same road and same weight, just different surface area..

Something doesn't add up here with the figures you give, for your light rail system.
5000 pound weight of the 'weldment' , and a 20,000 pushing force for the 'pneumatics'??

You stated earlier:
"The engineer i am working under has completed several coefficient calculations using steel on steel and decided the weight of the weldment alone, will be enough to allow the brake to keep the weldment from being pushed my the cylinders"

With an assumption of an approxiamate coeficient of friction for dirty or somewhat greasy ( ie not clean ) steel on steel of 0.1, your static friction force to hold the car in place in only 500 pounds, with no safety facture.
Or your car needs to weigh more than 200,000 pounds.

Something is not so genius as you say, or your forces are incorrect.

20,000 pounds supplied by pneumatics.
What bore and pressure are you using?

 

[Danger]

I am not an engineer, but a car can stop faster with wider tires. Same rubber and same road and same weight, just different surface area..

There's a balance to be struck in that situation, though. The larger the contact patch, the less weight each cm² of rubber experiences and this lowers the frictional force. That's why we switch to skinny tires in winter and is the principle behind snowshoes.
I still think that gravity alone will not hold your doodad in place, though.