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Super Modulator
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Counterweights are the "half moon" part of a driver, the filled in part.

Eccentrics are the small "arms" or "cranks" that power the valve gear.

Regards, Greg
 

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An "Eccentric" is something that is "off center" (an apt description of some old people!) so both the Counterweights and what is named as the Eccentric are "eccentrics".

But the Counterweights are specifically off center to counterballance the weight of the side and main rods on the locomotive (Side rods connect from one wheel to the next and the Main rod connects from the power piston, at the Crosshead to one of the drive wheels).

The part named the Eccentric depends on the type of valve gear that is on the engine.

Stephenson's valve gear (and some others) has a round wheel or pulley shaped part with an off-center hole (hence the name "Eccentric") that makes it wobble about the axis of rotation (the axle).

Walschearts valve gear (and some others) use an arm attached to the main pin of the drive wheel (where the main rod connects) so that the other end of the arm moves in a similar manner to the Stephensons wheel on the axle.

The idea of both is to produce a linear motion (pushing and pulling the valve) from a rotational motion (the wheels turning)...

The Eccentric Rod, which is attached to that outer end of the Eccentric arm (or rides on the Stephenson's Eccentric wheel on the axle) is what communicates that linear motion to the valve gear to push and pull the valve rod to make the valve open and close the steam and exhaust ports on the power cylinder at the correct times.

The drawing below names several of these parts on a Walshearts valve gear.
 

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Not to confuse you even further John, bit it bears mentioning. The Bachmann Mallet is an inside frame locomotive, so what Greg and Semper said regarding it is true. However, on outside frame locomotives, like the K-27 for example, the counterweights are not "the "half moon" part of a driver, the filled in part." The drivers on such a loco are inside the frame, but the counterweights are on the outside connected to the axle ends. I only bring this up here because discussions of Bachmann's previous loco, the K-27, revolved around "loose counterweights."
 

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True, the counterweights can be cast as part of the wheel or a solid metal blocks bolted to the spokes, some are weights fitted into a box bolted to the spokes, or as in the case of some outside frame locomotives they are completely separate parts mounted on the axle just like the wheel is, but farther toward the end of the axle.

What I never understood about the "Loose counterweights" issue was "how" they were loose? Too big of a hole to fit the axle? no setscrew or any method to make them rigid? What was wrong? What was the "fix"?
 

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Posted By tacfoley on 01/18/2009 5:13 PM
BTW - Jules T. Anatole Mallet (23 May 1837 – 10 October 1919) was a SWISS mechanical engineer, NOT a French one.

tac
www.ovgrs.org


OH DEAR! Now I've done gone and insulted BOTH the Swiss and the French!
 

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Thanks,

Now I know the difference between a counterweight and an eccentric.

I will go back and look at the pictures and try to understand what is happening.

John
 
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