Real World Statics/Dynamics Question 1

[Superman2]

I need to find the force a train exterts on a brake air compressor unit sitting on the flat bed. The train can reach a top( constant) speed of 100 km/h, and it's shunting speed is 5-15 km/hr.(Shunting is the coupling action of the carriages).

Sorry to ask such a basic question, but I have not done any problem solving like this for many years (just changed jobs) and would appreciate some guidance as to how to go about setting up my FBD for this and any equations of motion applied.

Assume mass of train carriage(inc load) is 30,000kg and there are 100 carriages coupled together(at 10 m long, 3.4 m wide, stretching 1 km).

The air coompressor unit is rectangular, with dims 1700(L) X 890(W) X 1060(H) and weighs 710 kgs.
The unit would have its long axis poerpendicular to the carriage centreline.

I need to find the max force exteted onto this compressor unit, and hence select and develop a dampening system & cradle to hold the compressor.

Any help would be very much appreciated. Once again, I apologise for the basicness of this request.

Cheers
Peter

 

[MintJulep]

The largest forces will likely result from shocks imparted on the carriage as it travels along the right of way. Poor track conditions, switches, frogs and the like.

Lacking any directly measured values for your particular application, take a look at IEC 61373 for guidance.

 

[sreid]

In the US, train coupling is called humping. The railrods certainly have information on the forces that can be encountered. There may also be military specs on this as humping tests are required for military equipment.

 

[Barry1961]

I agree with MintJulep that the max force will not be due to normal accel and decell.
I worked for several years by a large rail yard and know that there is a lot of shock produced while hooking up cars. The nite crew was very rough when hooking up cars and would from time to time ram a group of cars hard enough to break the coupling.
There is also a fair shock as the train starts and stops. As I remember the strongest of these was starting soon after stopping while the train was still stretching back out. The combined effect of the rear cars rolling backwards and the front cars being pulled forward would produce a series of very loud booms that could be heard for miles. But I don't think it comes close to the nite crew hooking up cars.
You might be able to get some information about the ratings of the coupling or other purchased components and design to that.

Barry1961

 

[CanEngJohn]

Mark's Mechanical Engineer's Handbook (at least the older edition I have) has a detailed section on this. Worth reading and will probably point you in the right direction.

Total retaring force in pounds force per ton is given as
F = (PLef/W + 20G) where
P = total piston pressure
L = muliplying ratio of the linkage
ef = the product of the coefficient of brake-shoe friction and foundation-gear efficiency
W = loaded weight of vehicle
G = grade, percent

Note an ef estimate of 0.15 is common.

This is what my book reads, mind you it talks about those nifty Deisel Engines which are starting to be manufactured.

 

[Superman2]

OK. Thanks Guys.
I don't have access to Mark's Mechanical Engineer's Handbook , but I do want to expand my library, so I will get it soon.

I can visualise what's going on, and that the shunting action imparts the Max force, but just trying to get the theory sorted and crunching the numbers is the challanging part. Even if I get something approximate, it is good enough to begin formulating a anti-shock package and price it. Any further help wopuld be extremely appreciated.

Regards
Peter

 

[IRstuff]

Your other choice might be to simply design to MIL-STD-810 Method 516 Transport shock requirements.

TTFN

 

[Superman2]

Firstly, Thanks for the help so far.

I have finished my calculations and also spent a considerable amount of time researching on the net.

Basically, I arrive at the figure of 1g loading acc and decc (even in shunting). I am not sure on the degree of reasonableness of this as, I am getting conflicting information. On source mentioned up to 4g dec forces involved during shunting, and another said approx 1g.

Does anyone have any input before I make my spring selection for the shock resistant cradle?

Cheers
Peter

 

[MintJulep]

IEC 61373 suggests testing with shock pulses of 30 m/s/s in the vertical and lateral directions and 50 m/s/s in the longitudinal direction. 30 ms pulse duration for all.