Week 10: Design of the Rail Cross Section
Text (14 pages):
- Ch. 3 Track Structure (32-45)
- Ch. 17 subgrade geometric design (293-298)
- Ch. 20 drainage (366-392)
- Ch. 21 ballast (393-418)
- Ch. 22 cross ties (436-468)
- Ch. 25 fastenings (562-591)
Objective: To gain knowledge of cross section components, understand their relative contribution to cross section design, be able to apply the concepts by designing a functional cross section
- Click here to see two typical rail cross sections (Fig. 13-4, Wright and Ashford)
- The function of the rail structure is to transfer all loads without settling during all weather conditions
- the measure of support strength is the _____________________, or, the force required (in lbs. per inch) to depress the rail one inch. Typical values are 6000 - 12,000 lbs. per inch
- Rail Guage = Wheel Guage + 3/4 inch = 4' 8 1/2"
- Must be kept in tolerance
- Click here to read a report on Guage restraint (not from Text). A new "split axle" can reduce derailments. See figure 7 for distribution of widening. Look at the dimensions of wheels and rails - how much widening will cause derailment?
- Impacts required superelevation
- To examine forces acting during curves, see Figure 12-7 from Wright and Ashford
- To calculate balanced superelevation, use E = ________________________ (feet, mph) or E = 11.9V2/R (mm, kph) where E = Equilibrium elevation, V = speed, and R = radius of curve
- See Wright and Ashford, Table 12-6, which provides metric values for E, given speed and radius R
- Use a max E of 8 inches (200 mm) (desirable 6 to 7 inches (150 to 170 mm))
- to determine overturning speed, use V = 2.24 R (mph and feet; assumes center of gravity = 7' above outer rail)
- distribute loads (depth 6 - 30 inches); loads are reduced from 100 psi just under the tie to less than 50 psi at the top of the subgrade
- reducing the load allows the subgrade to hold up and not settle (load less than endurance limit)
- anchor ties ("interlocking" is required, so crushed stone is preferred to river gravel)
- provide drainage (use open graded materials, 1.5 inch diameter.) - problem with too many fines will be ____________________________________
- control weeds and dust
- Ballast quality standards:
- Wear resistance
- Cleanliness (can be improved by use of geofabrics)
- Frost Resistance
- Unit Weight (e.g., slags)
- Ballast will usually last for ________ Million gross tons (25,000 trains at 10,000 tons each)
- spread loads
- maintain guage
- prevent longitudinal and lateral movement
- deflect to cushion impact
- making small vertical adjustments
Frogs, switches and turnouts
- 39' long (some double length, now)
- jointed or continuous weld (advantages of welded include decreased maintenance, higher speeds, smoother ride)
- up to __________ of continuously welded rail can be moved on special "rail trains" that "bend easily around curves"
- 115 - 140 lbs/yd.
- welded track must be laid warm, so it will always be under tension (compression causes sun kinks - see safety lecture)
- turnouts have only 2 moving parts (switch points), but they can freeze up!
- the frog is that part of the turnout that allows the flange to pass over the crossing track
- higher frog numbers are used in ___________ turnouts and facilitate higher speed (up to 24, 50 mph+)
- Click here to visit the Q.C., Inc. Home page - Click the Unitrac link to see a catalog of rail cross sections which contains photos and descriptions
- Click here to visit the Atlantic Track & Turnout Co. where you can see Rail, Trackwork, Crane Rail Accessories, and Manufacturing photos, descriptions and a catalog of rail section materials. It also has a program that calculates frog numbers!
Sketch a functional cross section, labeling all of its parts. Include plan, profile and cross-section views.
- for jointed rail, alrternate bolt heads inside and out to avoid joint failure if a car is derailed and cutting inside bolts
- steel tie plates as long as 18" are used to prevent damage of concentrated loads on ties
- rail anchors (spring clips) are used to reduce "creep" - _________________ movement of rail
- purpose of spikes is to prevent lateral movement, not keep the rail from lifting out