J
jim beam
Guest
Kerry Montgomery wrote:
> "jim beam" <[email protected]> wrote in message
> news:[email protected]...
>> [email protected] wrote:
>>> Doug Landau writes:
>>>
>>>>> It's conversion of kinetic energy to heat, not pull or coefficient of
>>>>> friction that lies at the root of this. Aluminum rims dissipate brake
>>>>> heat while soft brake pads generate it.
>>>>> When I read this stuff, I get an indication of how poor engineering
>>>>> was in those days. Sharp was apparently not a mechanical engineer and
>>>>> not skilled in the art of thermodynamics or he would have known what
>>>>> steady state heat rejection was required to descend the 12% grade.
>>>>> It reminds me of the SF Cable Cars that have an even worse emergency
>>>>> brake that doesn't work. As a last resort the third (Red) lever in
>>>>> front of the gripman is used to drive a steel wedge into the cable
>>>>> slot in the belief that because it can be jammed in there tightly, it
>>>>> will dissipate heat and slow the car. Nothing of the kind occurred
>>>>> the few times it was used, but no one noticed because when the cable
>>>>> car crashed into a truck at the bottom of the hill, the lubricating
>>>>> molten steel froze, welding the car to the street.
>>>> Since you mentioned flat spots on train wheels recently the subject
>>>> has shown up on comp.risks a few times. Once because of the advent of
>>>> ABS, which misinterpreted wheelspin on mulch as excessive speed and
>>>> put on the brakes, and once because of the use of disk brakes - which,
>>>> because they don't sweep the circumference of the wheel, don't scrape
>>>> the mulch off them, either.
>>> Mulch (snow, ice, and just water) is on the track and furnishes water
>>> that is the prime lubricant for the rest of the train after the
>>> locomotive has passed. It is up to the driver to assess the lubricity
>>> of the rails.
>>>
>>> Well ABS is better than noting but it is too little too late. RR
>>> wheels do not gradually begin to squeal like rubber tires as they
>>> begin to slip, but cross a traction threshold that causes the wheel to
>>> slip, be that on wet or dry rails. At that moment ABS cuts in but by
>>> that time there is already a skid mark (molten steel) on the wheel
>>> that can be heard when riding in high speed trains on good track (so
>>> the train isn't rumbling already). Subsequent wear does not reduce
>>> the effect which only becomes greater with repeated wheel rotations
>>> that increase the flat spot, as small as it is. For that reason,
>>> braking must be carefully controlled to remain below the critical
>>> threshold so ABS doesn't ever cut in.
>>>
>>> http://tinyurl.com/2z6zua
>>>
>>> I'm not convinced that stopping distances are designed around clean
>>> dry rails and that there is no margin. Wet rails have always made
>>> starting and stopping distances greater. ABS may reduce flat wheel
>>> generation but it cannot prevent it.
>>>
>>> Jobst Brandt
>> flat spots are not exclusive to locked wheels, although a locked wheel
>> will indeed cause one. they can be caused by debris or even defects in
>> the steel "tire" or the way it was fitted. once a flat spot exists, it
>> grows by hammering each time the wheel rotates.
>>
>> also, wheel slippage is not exclusive to braking. any cornering, since
>> the axles are solid, is accompanied by differential slippage, and that can
>> be substantial. on sharp bends, sometimes one of the rails is greased to
>> mitigate [the screeching] noise and abrasion.
>
> The wheels are conic sections, so that at the correct lateral position,
> there is _no_ slippage during cornering when that equilibrium position has
> been reached.
> See:
> http://www.channel4.com/science/microsites/S/scrapheap2002/challenges/RailRoadRacer/Science.html
> and:
> http://www.spikesys.com/Trains/whel_faq.html
> Kerry
>
>
for large radius curves in open country, that's right, but for short
radii, such as in urban areas where land is restricted, equilibrium is
exceeded and wheels squeal. hence the use of greasers.
http://www.xl-lubricants.com/rail_curve.htm
> "jim beam" <[email protected]> wrote in message
> news:[email protected]...
>> [email protected] wrote:
>>> Doug Landau writes:
>>>
>>>>> It's conversion of kinetic energy to heat, not pull or coefficient of
>>>>> friction that lies at the root of this. Aluminum rims dissipate brake
>>>>> heat while soft brake pads generate it.
>>>>> When I read this stuff, I get an indication of how poor engineering
>>>>> was in those days. Sharp was apparently not a mechanical engineer and
>>>>> not skilled in the art of thermodynamics or he would have known what
>>>>> steady state heat rejection was required to descend the 12% grade.
>>>>> It reminds me of the SF Cable Cars that have an even worse emergency
>>>>> brake that doesn't work. As a last resort the third (Red) lever in
>>>>> front of the gripman is used to drive a steel wedge into the cable
>>>>> slot in the belief that because it can be jammed in there tightly, it
>>>>> will dissipate heat and slow the car. Nothing of the kind occurred
>>>>> the few times it was used, but no one noticed because when the cable
>>>>> car crashed into a truck at the bottom of the hill, the lubricating
>>>>> molten steel froze, welding the car to the street.
>>>> Since you mentioned flat spots on train wheels recently the subject
>>>> has shown up on comp.risks a few times. Once because of the advent of
>>>> ABS, which misinterpreted wheelspin on mulch as excessive speed and
>>>> put on the brakes, and once because of the use of disk brakes - which,
>>>> because they don't sweep the circumference of the wheel, don't scrape
>>>> the mulch off them, either.
>>> Mulch (snow, ice, and just water) is on the track and furnishes water
>>> that is the prime lubricant for the rest of the train after the
>>> locomotive has passed. It is up to the driver to assess the lubricity
>>> of the rails.
>>>
>>> Well ABS is better than noting but it is too little too late. RR
>>> wheels do not gradually begin to squeal like rubber tires as they
>>> begin to slip, but cross a traction threshold that causes the wheel to
>>> slip, be that on wet or dry rails. At that moment ABS cuts in but by
>>> that time there is already a skid mark (molten steel) on the wheel
>>> that can be heard when riding in high speed trains on good track (so
>>> the train isn't rumbling already). Subsequent wear does not reduce
>>> the effect which only becomes greater with repeated wheel rotations
>>> that increase the flat spot, as small as it is. For that reason,
>>> braking must be carefully controlled to remain below the critical
>>> threshold so ABS doesn't ever cut in.
>>>
>>> http://tinyurl.com/2z6zua
>>>
>>> I'm not convinced that stopping distances are designed around clean
>>> dry rails and that there is no margin. Wet rails have always made
>>> starting and stopping distances greater. ABS may reduce flat wheel
>>> generation but it cannot prevent it.
>>>
>>> Jobst Brandt
>> flat spots are not exclusive to locked wheels, although a locked wheel
>> will indeed cause one. they can be caused by debris or even defects in
>> the steel "tire" or the way it was fitted. once a flat spot exists, it
>> grows by hammering each time the wheel rotates.
>>
>> also, wheel slippage is not exclusive to braking. any cornering, since
>> the axles are solid, is accompanied by differential slippage, and that can
>> be substantial. on sharp bends, sometimes one of the rails is greased to
>> mitigate [the screeching] noise and abrasion.
>
> The wheels are conic sections, so that at the correct lateral position,
> there is _no_ slippage during cornering when that equilibrium position has
> been reached.
> See:
> http://www.channel4.com/science/microsites/S/scrapheap2002/challenges/RailRoadRacer/Science.html
> and:
> http://www.spikesys.com/Trains/whel_faq.html
> Kerry
>
>
for large radius curves in open country, that's right, but for short
radii, such as in urban areas where land is restricted, equilibrium is
exceeded and wheels squeal. hence the use of greasers.
http://www.xl-lubricants.com/rail_curve.htm
