
Michael Quin Heavener	Search:

Railroad springs

What a difference they make

-----Original Message-----
To: 

I am writing a newsletter for kids at our local train museum (Age 
of Steam, Dallas Texas). I am trying to understand the difference 
between leaf and coil springs and why steam engines use both. I found 
a brief reference to this but wonder if you could further enlighten 
me as to the function of both and if diesel also use these springs.
	Robin

Having crawled under an awful lot of railroad equipment …

… here's what I know about springs:

A pair of NW1600 yard diesels near BNSF superintendent's office at Balmer Yard, Seattle, shows coil springs inside the truck frames. Coil springs dampen up-and-down motion that could damage the locomotive or the rails.

Springs serve multiple purposes

Coil springs are used when up-and-down motion will cause damage to locomotives or rail cars. They are easy to "box up" into enclosed mounting brackets on truck frames. Making coil springs bigger, or adding more of them, provides better jolt retardation. Coil springs (shown under BNSF yard diesel above) protect fragile shipments and passengers from being jounced around inside the cars, but they don't do much to control motion in other directions.

A railroad gone to 'war'

My good friend Jerry and I went over to the local railroad yard to scout photos of springs in action. We eventually plan to do full-motion streaming video on this page showing springs flexing and unflexing real use—but that'll have to wait until the war cools down.

We didn't get far. "I can't give you permission to be in the yard," said Mike, the trainmaster. "We're in a red alert lock-down [because of the September 11 terrorist attacks]. Nobody's allowed in the yard except employees who need to be there." We hadn't thought about it, but he's right. They have to protect their track and trains, as well as property surrounding the rail lines.

No other industry stretches so much of itself over so wide an area in so visible a manner as a railroad. Though BNSF plays peek-a-boo through the mountains, it's a big visible target in the urban areas of the Northwest. Even with safety fences and big "No Trespassing" signs, they still encounter problems with the public.

I warned Jerry in advance that I wasn't going anywhere near the trains without first securing the railroad's permission. I didn't even carry my camera into the office (didn't want to be instantly labeled 'railfan'). We still got frowns from the men at the car repair shop, who sent us down to the superintendent's office (closed for the weekend), and upstairs to the trainmaster.

So Jerry and I contented ourselves taking photos of the locomotives parked next to the crew-change office at Seattle's Balmer Yard. ("I … can't stop you from taking pictures right here," said Trainmaster Mike. "It's a public street.")

With a trio of GE C36s on a northbound freight, two more on a southbound, an aging EMD SD7 partly hidden to the south, and four NW1600/SW1600 switch engines parked right beside the street, there was plenty to photograph. There was also a brand-new Boeing 737 fuselage on a flatcar and two cabooses (one advertising the D.A.R.E. drug-awareness education program).

Leaf springs, on the other hand, don't do much to minimize up-and-down activity. They are used where side-to-side or front-to-back motion must be controlled or restricted. What leaf springs (shown below, under running boards of retired D&RGW K-37 steam locomotive) do is provide better stability and faster speeds on rougher track by continually shifting weight from one end of the spring to the other, allowing wheels to move up and down to compensate (or "equalize") for track conditions.

Springs serve yet another purpose, significant to railroad business managers—they alleviate excessive damage to the tracks. Jolting can (and has) fractured railheads, leading to early track replacement and even derailment. "Side-to-side", or lateral, motion wears down the insides of rails and the outsides of wheels, especially on curves where the flanges act as rolling grinders. Eventually, wheels and rails become so worn and thinned that replacement is the only prevention for accidents.

Steam locomotives hang their spring equipment on the outside, where it is plainly visible and fun to watch. Diesels hide everything away inside the frames and under the carbodies.

Track conditions dictate types of spring used

If you've look down a long stretch of straight track, you'll probably see that it has "hills" and "valleys" corresponding to the rail joints and centers. Rail joints wear down much faster than the rest of the rail and on really poorly-maintain track, you'll also see the rails twist and wobble sideways from overuse. This is not universally true, the major transcontinental railroads have a vested financial stake in maintaining runway-smooth continuous-welded track but that's extremely expensive (on the order of $½ to $1 million per mile).



Normal wheel-rail alignment		Wheel "seeking the curve"



Banked curve alignment		Wheel "climbing the rail"

Take Ballard Terminal, for example, a short line about 2½ miles long, of nearly-abandoned ex-Burlington Northern light industry switching sidings in the Seattle area. The track was allowed to deteriorate under BN ownership and its new owners face prohibitive costs bringing it to American Association of Railroads (AAR) standards. Their (second-hand) diesel is a short, powerful, slow-speed locomotive capable of staying light-footed on the twisted, buckled, and generally miserable rail. Coil springs make this possible.

The famous AT&SF railroad on the other hand, faced different track conditions. Now part of BNSF, Santa Fe operated a fleet of high-speed luxury trains catering to the elite of Hollywood. Their track was impeccable, well-laid, gauged and trued to millimeters, and super-elevated (banked) for train speeds in excess of 90 miles per hour. They didn't need jolt retardation, there weren't any bumps or twists in the jointless welded Santa Fe track. But the banking necessary for high-speed travel through curves obligated them to do major side-to-side compensation. Otherwise, passengers would be thrown from their berths, and meals would slide off tables in the dining cars, as the outside wheels tried to "climb the rail".

All wheels do what mechanical engineers call "seeking the curve"—that is, they force themselves outward on the outer rail trying to travel in a straight line (dictated by Newton's three laws of motion and inertia). When they strike the rail, they are forced to turn. Then they bounce into the curve and try to go straight as the process repeats itself. Rail repair/replacement crews report that most of the wear on curves is on the inside of the outer rail, confirmed by rigorous scientific testing in the 1930s and 40s.

Leaf springs retard "seeking the curve"—coil springs provide stability.

Equalization requires leaf springs

Leaf springs are flexible bands of steel three to five feet long and half an inch to an inch thick. They are layered together (from three layers to 20) and bent into curves, and then the layers are clamped at each end so the bends can't relax and let the springs flatten out. Big machines are used to bend the springs and clamp them, so when a spring needs replacement, it's the whole multi-layered unit that is removed.

Leaf springs compensated for uneven track, keeping all the mocomotive's driving wheels firmly on the rails. These were under a D&RGW K-37 at Chama, New Mexico.

If locomotives didn't have leaf springs, their axles would all be a fixed height from the top of the rail. The wheels in the middle wouldn't touch when they passed over the joints, and the ones at front or back wouldn't touch when the middle wheels reached the higher rail centers. Traction and speed would be substantially reduced, there would be a lot of wheel slippage, and the real danger would be derailments. Locomotives would not have any flexibility to compensate for less-than-perfect rail conditions.

Equalization provides that flexibility. Leaf springs fasten to the locomotive frame at each end and to the driver axles in the middle. The locomotive's weight shifts back and forth from one end of the spring to the other, depending on where the driving wheel is positioned on the rail. The axles move up or down, depending on which end of the spring carries the most weight, allowing the wheels to compensate for differences in rail height.

Compensation wasn't a problem on two-coupled (American or 4-4-0 type locomotives with only two driving axles), which merely pivoted their short engine frames diagonally to adjust for rough rail. But as more axles were added to create longer, heavier locomotives, flexibility was lost. Leaf springs and equalization restore that flexibility.

Coil springs play pivotal role

Coil springs are still used extensively. You see them on two- and four-wheeled trucks under tenders and rail cars. These are, by nature, capable of pivoting around the bolster pins that connect them to the "carbody," and they also compensate diagonally across the track on rough stretches. (Bolster pins are three to four inch diameter bars pointing downward from the center frames at each end of rail cars. They drop neatly into holes in truck frame's cross bracing. Trucks are NOT fastened to the rail cars, only gravity keeps them together.)

Union Pacific's fleet of big steam locomotives (the "Big Boy" 4-8-8-4s, 2-6-6-2 Challengers, and 844-class 4-8-4 Northerns) pulled "centipede" tenders riding on leaf spring equalization. The six, eight, or 10 fixed wheels were mounted directly to the tender frames (they also had four-wheel coil-spring trucks at the front to lead the tenders into curves).

Coil springs give this huge diesel electric the flexibility it needs for high speed freight service. The callout shows the exact location of the spring inside the High-Adhesion truck that makes this GE locomotive so efficient. These Hi-Ads also have leaf springs hidden inside the frames under the bolster.

Six-wheeled trucks under big diesel locomotives (as on the 3600-horsepower BNSF road diesel at right) use a combination of spring technologies, although not always visible from easy camera angles. They have coil springs to dampen excess vertical motion and these can usually be seen. The axle ends and bearings are also fitted into guides that slide up and down in the truck frames which are mounted on … tada … leaf spring equalizers running across the trucks (although you generally can't see them because they are inside the frames).

The coil springs under this flatcar protect a Boeing 737's fuselage body from up-and-down track damage. It is the excellent springing which makes railroad shipment of aircraft parts possible.

Leaf springs lift one side and lowering the other depending on track conditions, allowing the electric-motor-powered wheels to adhere to the track at all times. On really heavy equipment, multiple leaf springs may be attached to what mechanical engineers call a "swing hanger," which further counters side-to-side motion by literally swinging the truck frame away from the spring assembly in the opposite direction from the weight of the car as it leans outward on the curve.

The crew of this caboose rides safer with leaf springs protecting them from being thrown side-to-side by sudden lurches in the track.

There is one other kind of spring in modern railroading, the so-called "air bag" suspension used by Amtrak on their Amfleet and Superliner equipment. You can see these air bags above the truck frames—they look like skinny tractor tires laying on their sides. They are inflated by the same pipes which supply air pressure to the train brakes, are actually replacements for coil springs and function the same way. And surprisingly, research on Amtrak's Northeast Corridor in the late 1970s determined these "air-bags" caused more track damage than older, metal coil spring technologies.

Big semi trucks, the kind you see on public highways, use leaf springs attached to the trailer's frame and axles to compensate for rough vertical and lateral conditions, just like railroad locomotive springs and work the same way.

Michael Quin Heavener