Re: Tony Thompson & Scale Weights

Mike Brock <brockm@...>

Marcelo Lordeiro writes:

The correct springs allows you to raise one driving wheel without lifting the locomotive.
Doing that you garantee that all drivers have the same adesion tp the track and you will pull twice as much cars.
I kinda hate to do this but I think I'll respond here on the STMFC. Actually, while the subject has a closer association to steam era frt cars than several recent posts, this subject should be discussed on the Steam Loco group...


Anyhow, many people missunderstand the principle of adhesion so I think I'll comment on it. A locomotive driver...even works for...yuk...diesels...has two primary forces acting on it to move the locomotive. The first is the force from the rod attached and the second is the "normal" force [ weight ] acting downward. The Factor of Adhesion [ FA ] is defined as weight on drivers/tractive force. Hence, a UP Big Boy had 540,000 lbs of weight acting on the 8 drivers and about 135,000 lbs of tractive effort [ at start ]. The ratio is 540,000/135,000 = 4.0. This has long been established as the weight/tractive force relationship required to given traction without slipping when steel wheel encounters steel rail...dry & clean. Guess what? It is...surprise...based on the coefficient of static friction for steel on steel [ the actual figure is given as a range of 0.15 to 0.3 equating to FA's of 3.3 to 6.6 but 4.0 has been determined in practice to be the required number ]. To see the effects of these forces on a single driver, simply divide the whole mess by 16. Thus, 540,000/16 and 135,000/16 gives 33.75/8.4375 = 4.0. Hmmm. Lets convert our 4-8-8-4 to a 4-6-6-4, leaving the weight and tractive force the same. OK...we now have 540,000/135,000 still equal to 4. Hmmm. Now lets make our engine a 4-2-2-4. We still have 540,000/135,000= 4.0. IOW, adhesion has nothing to do with the number of drivers making contact with the rail. We DO have a different problem of course. Axle loading went from 67750 lbs to 270,000 lbs and the wheels broke under the weight in addition to snapping the rails. The point of all this is that a driver could lift off the my Big Boy does on a superelevated curve...but adhesion is not effected at all. The weight simply is supported by those wheels still on the rails and force remains the same. Now...on a real locomotive, springing is VERY important for a number of reasons. First is axle loading. Second is dynamic balance. All kinds of other problems occur if axle loading exceeds that designed and an out of balance engine could wreck havoc with itself.

Mike Brock

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