4 ms·
> I find this hard to accept. As did I at first, but it's true. There are several papers that show the math behind this (including one I co-authored called "Pe
by Anechoic 15y ago
> I find this hard to accept.
As did I at first, but it's true. There are several papers that show the math behind this (including one I co-authored called "Performance of ballast mats on passenger railroads: Measurement vs. projections") - unfortunately they are all behind paywalls.
> Just because there's a spring doesn't mean the weight isn't there, bearing down on the wheels, deforming the wheel and the track.
At rest, sure. Once it's in motion, the static load is insignificant (by several orders of magnitude) compared to the dynamic forces, and the dynamic forces are driven by the unsprung mass (and other factors such as the primary suspension frequency).
>Stand next to a train as it goes by, you can feel the ground move.
You're referring to ground-borne vibration (one of my areas of expertise). Again, that is a function of the dynamic forces, not the static forces. The FTA Transit Noise & Vibration Impact Assessment guidance manual (disclosure: I used to work for the firm that wrote the manual and I have a contract with FTA to teach cources based on material in the manual) touches upon this a little bit when it mentions that heavy rail and light rail trains have similar vibration levels (p 7-8, section 7.3, note that "heavy" and "light" in this context refers to train schedule not vehicle weight, but generally speaking heavy rail vehicles tend to be heavier than light rail vehicles) but again, all the good stuff is behind paywalls.
>All that bending/unbending of the track and bed contributes to fatigue damage, and fatigue damage goes up as the cube of the load.
That's a highway stat. Trains are different than roadway vehicles in that there is only one spring separating the vehicle weight from the unsprung mass. In trains, there are two suspension systems, the primary suspension with isolates the wheel/axle from the truck and the secondary suspension that isolates the vehicle body from the truck (and the wheels). When you look at the driving force behind ground vibration, the body weight has little correlation with the forces going into the ground (okay, there is a weak correlation in that heavier trains tend to have heftier axle sets, but the differences here between, say, the Acela and the TGV are minor).
- WalterBright 15y agoI appreciate the detailed reply. I am talking about the dynamic load. A moving train generates a standing wave that is a function of the train's weight and its speed. I don't see how adding a spring changes that. This standing wave bends the track and the bed, resulting in fatigue damage. I know about the standing wave because in the early days of trains, there were problems because the standing wave propagated at speeds the locomotives were able to reach, and this resulted in mysterious destruction of the tracks and a derailment. (The solution was to stiffen the tracks so the wave propagated faster than the locomotive.) Feeling the ground move down as the wheel trucks pass is dynamic, yes, but it is not vibration, as its "frequency" would be the rate at which the wheels passed by, and its amplitude would be proportional to the static weight. Furthermore, there will be more localized bending as the wheels and track must deform so the contact area is enlarged to support the entire weight. (Assuming the same elasticity of the track steel.) This local deformation is going to produce cracking and wear of the surface of the rails.
- Anechoic 15y ago>A moving train generates a standing wave that is a function of the train's weight and its speed. No, it's really not. It's a function of the unsprung mass, the roughness of the wheel, the roughness of the rails and other factors (fastener type, faster spacing, etc). Train weight never enters in to the equation (literally), only the combined weights of the train components south of the primary suspension. >Feeling the ground move down as the wheel trucks pass is dynamic, yes, but it is not vibration ??? That is exactly what ground-borne vibration is! >its "frequency" would be the rate at which the wheels passed by, Among other factors (wavelength of roughness, spacing of supports, etc), yes, and you can see that in train vibration spectra. You can also see the primary suspension resonance and other components. I'll refer you to the report "Vibration Characteristics of the Transrapid TR08 Maglev System" - (yes, I was a coauthor) were we discussed these issues in the section about the TR08 force density. http://www.fra.dot.gov/downloads/rrdev/fra0206.pdf http://www.fra.dot.gov/downloads/rrdev/fra0206.pdf section 3.3. It's not a perfect analog with steel-wheeled trains but a lot of the same principles apply in terms of the different factors that go into the input force. You mentioned "the early days of trains" - what era are you talking about? Specifically are you talking about the days before trains had suspensions, or had very stiff suspensions (which was common as late as the 1950's-1970's in transit vehicles until it's effect on vibration was well known - this was a huge issue with BART)? In that case I could very will see that vibration was proportional to gross weight since a stiff suspension is basically a short circuit to ground. That generally isn't true any more, the only exception being some types of freight cars. edit: a quick Google search provided a paper that supports my thesis: http://library.witpress.com/pages/PaperInfo.asp?PaperID=19575 http://library.witpress.com/pages/PaperInfo.asp?PaperID=1957...
- WalterBright 15y agoI'd really like an explanation of how, if you push down on the track at a point, you are not bending the track down at that point, sprung or not. Move that point along the track, and you create a standing wave, sprung or not. The amplitude of that wave will be proportional to the total weight.
- 15y ago