Confusing part on brass steam?

When I look at the bottom of a high end brass steam engine, I often see some sort of square or triangular (can’t tell which) part mounted on the axle behind the geared axle. Does anyone know what this part is for?

might be a cam for triggering sound effects switch. a photo would help.

grizlump

I think that griz has got it right: sounds like it’s a cam for activating sound synchronised to the loco’s speed. It can be on any non-geared axle, and its shape is usually square, although I’m guessing it would be triangular for a three-cylinder loco. [swg]

Wayne

You are both 100% correct!!! Have a “cookie”!!! They were also “round” and painted with an unpainted section for the sound contact, I have had this on a very few Tenshodo locos from the factory. They work quite well although the only sounds were the chuff-chuff sounds, but you have to remember this was on engines built in the late 70’s-early 80’s so sound technology was nothing like it is today.

Mark

I had a feeling they could be related to sound. Thanks guys![:D]

For those who don’t quite know what I was asking about, here are a few examples:

OMI Big Boy (round one)
PSC 4-8-4 (square one)

And it looks like it’s still being done on newer models, like this BLI “Brass Hybrid” 4-6-4:
http://www.broadway-limited.com/images/view.aspx?productId=875 (picture 7)

Something like that would be more reliable than purely electronic timing, so it does make good sense.

I have something like that on my Hornby B12. It’s for the chuffing noise. Its fitted to the tender so that the wheelslip is not noticed in the sound.