Peter Gl
Member
Why do wheel flanges screech at low speeds?
Pete
Pete
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Screech!
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LexyBoy
Established Member
Because of the wail...
Zoidberg
Established Member
They don't screech because of travelling at low speeds. They screech when traversing a tight radius when the flange binds on the rail. Traversing a tight radius necessitates travelling at low speed.
Peter Gl
Member
Okey doke, that makes sense, thanks Zoidberg.
Pete
Pete
LexyBoy
Established Member
And the longer the wheelbase, the worse the screech because there's more force on the flange against the rail. The joy of Pacers...
It is sometimes counteracted by grease applicators (I'm sure they have a proper name?). Trouble is too little and in residential areas you get neighbour complaints due to the noise (it's would be horrible waking to that every morning at 5.00am!), too much and it can cause wheel slip/slide even in dry conditions.
They cranked it up on the Holywell curve near Shoreditch High St recently but I've now had to change my braking point due to slide on the down approach into the station.
They cranked it up on the Holywell curve near Shoreditch High St recently but I've now had to change my braking point due to slide on the down approach into the station.
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Sir_Clagalot
Member
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Added to that, the axles are fixed, so one wheel effectively gets 'dragged' round. What would probably get rid of if not reduce an awful lot would be to fit differentials to every axle. These allow the outer wheel to turn faster and therefore cover the greater distance at the same overall speed as the inner wheel.
Would also reduce tyre/flange wear and rail wear too.
Would also reduce tyre/flange wear and rail wear too.
What would happen to all the freight stock that only has disc brakes on one wheel of each axle, the diff would effectively reduce your brake force by 50 percent.
I can see your reasoning but it would cost a fortune to impliment and also increase maintenance especially as each braked wheel would be able to lock up easier, it only takes 1 wheel on each axle to have grip now to keep them both spinning.
asylumxl
Established Member
Plus as AndyPJG said, it would counteract the coning. The speed difference between the two wheels is what causes the axles to follow curved track in the first place.
O L Leigh
Established Member
The profile of rail wheels (referred to by others here as "coning") IS the differential for rail vehicles. As a rail axle travels around a curve the outer wheel climbs up the rail while the inner wheel climbs down. The profile means that the circumference of the outer wheel where it is in contact with the rail is greater than the circumference of the inner wheel where it is in contact with the rail, thereby inducing the axle to naturally follow a curved path. The wheel flange itself almost never makes contact with the rail except under the most extreme curvature of the line.
My own thinking is that the screech you hear is not necessarily anything to do with the wheel flanges at all but to do with vibrations set up in the rails themselves as the train passes over them. Yes line curvature has something to do with it together with any sudden changes in direction, which is why you often hear a short squeak as trains negotiate complex pointwork, but I don't necessarily believe that it has anything to do with the wheel flange hitting the rail. If it did, the wear patterns on the wheels themselves would be quite different due to the extent to which the flange would be in contact with the rail. Because the wheel diameter is at it's greatest at the flange and not at the tread, the flange travels faster than the tread. Therefore, if it was in contact with the rail it would wear faster and cause more wear to the sides of the rails.
Flanges do make contact with the rails, I'm just not sure that it is always the cause of the screech you sometime hear on tightly curved lines.
Oh, and the screech is speed dependent. The slower the train is going the lower the pitch. The screech heard in stations is loudest at around 15mph. In yards and depots where the speed is limited to a 5mph maximum the sound is more like a rumble or groan but on high speed lines it is more like a hiss or whistle.
O L Leigh
My own thinking is that the screech you hear is not necessarily anything to do with the wheel flanges at all but to do with vibrations set up in the rails themselves as the train passes over them. Yes line curvature has something to do with it together with any sudden changes in direction, which is why you often hear a short squeak as trains negotiate complex pointwork, but I don't necessarily believe that it has anything to do with the wheel flange hitting the rail. If it did, the wear patterns on the wheels themselves would be quite different due to the extent to which the flange would be in contact with the rail. Because the wheel diameter is at it's greatest at the flange and not at the tread, the flange travels faster than the tread. Therefore, if it was in contact with the rail it would wear faster and cause more wear to the sides of the rails.
Flanges do make contact with the rails, I'm just not sure that it is always the cause of the screech you sometime hear on tightly curved lines.
Oh, and the screech is speed dependent. The slower the train is going the lower the pitch. The screech heard in stations is loudest at around 15mph. In yards and depots where the speed is limited to a 5mph maximum the sound is more like a rumble or groan but on high speed lines it is more like a hiss or whistle.
O L Leigh
edwin_m
Veteran Member
The easy way to check whether flange contact is occurring is to look whether the shiny bit is just on top of the rail but also on the sides.
Older stock had bogies that would turn relativey easily, giving rise to "hunting" where the bogle effectively zigzagged along the track with (in the extreme case) the flange bouncing off the rail each side. Flange contact for a prolonged period happened only on curves tight enough that the conicity wasn't enough to account for the difference in distance travelled on the two rails.
Anything built in the last few decades has dampers (shock absorbers) which resist the rotation of the bogies. If these are working properly they will eliminate hunting but flange contact may take place on curves where previously the coning of the wheel was enough to steer the bogie. A 158 for example "sings" on quite gentle curves, though it's not enough to qualify as a screech, and I think this indicates is a certain amount of flange contact.
The "variable bush" fitted to 450s attempts to get round this by damping out the high-frequency movements of hunting while allowing the lower-frequency movements associated with going into and out of curves.
Older stock had bogies that would turn relativey easily, giving rise to "hunting" where the bogle effectively zigzagged along the track with (in the extreme case) the flange bouncing off the rail each side. Flange contact for a prolonged period happened only on curves tight enough that the conicity wasn't enough to account for the difference in distance travelled on the two rails.
Anything built in the last few decades has dampers (shock absorbers) which resist the rotation of the bogies. If these are working properly they will eliminate hunting but flange contact may take place on curves where previously the coning of the wheel was enough to steer the bogie. A 158 for example "sings" on quite gentle curves, though it's not enough to qualify as a screech, and I think this indicates is a certain amount of flange contact.
The "variable bush" fitted to 450s attempts to get round this by damping out the high-frequency movements of hunting while allowing the lower-frequency movements associated with going into and out of curves.
Sir_Clagalot
Member
- Joined
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A differential means you can then brake each wheel individually as in a car/hgv/etc, rather than the axle, that way if one wheel locks up the other one the other side of the diff can still be used for brake force...
Would need WSP probes each side of course
Would need WSP probes each side of course
Zoidberg
Established Member
If only I knew what are WSP probes
Wheel Sensor Probe probes? No, too many "probes".
Wheel Sensor Position probes? Maybe.
Go on, enlighten me. Please.
Zoidberg
Established Member
If only I knew what are WSP probes
Wheel Sensor Probe probes? No, too many "probes".
Wheel Sensor Position probes? Maybe.
Go on, enlighten me. Please.
WSP is Wheel Slide Protection. Each axle is fitted with a rotational speed sensor, often in the form of a toothed wheel passing a magnetic probe. The speed of each axle is monitored and compared with the other axles on that vehicle.Should a differential of more than around 7 per cent occur, such as when the wheels pick up on braking, and slide, the brake cylinders on that axle's wheels will be released until the speed differential is removed, ie when the wheels are turning at the same speed as the others on that vehicle.
Zoidberg
Established Member
SemaphoreSam
Member
Mr. Leigh explained the mechanism very well for laymen like me...here's another explanation from my side of the pond...also mentions railway equivalent to a differential around curves, from my fav physicist. Sam
https://www.youtube.com/watch?v=WAwDvbIfkos
https://www.youtube.com/watch?v=WAwDvbIfkos
edwin_m
Veteran Member
Wheel Slide Protection. Effectively an ABS for trains! The probes sense the rotation of each wheel and if one stops suddenly it reduces the brake on that wheel to prevent it sliding and getting a wheelflat. In theory at least, it's not 100%.
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