Weights on the driving wheels of steam locomotives.

Discuss model railway topics and news that do not fit into other sections.
TheObserver
Posts: 66
Joined: Thu Apr 12, 2012 7:48 am
Location: Swansea, South West Wales. UK

Weights on the driving wheels of steam locomotives.

Postby TheObserver » Sat May 14, 2016 8:41 am

Hello, I was brought up on a feast of "How It Works" cutaway diagrams of machinery in the Eagle Comic of the 1950's era.

I have a memory of weights on the inner rims of the driving wheels of locomotives all being 'inline' with each other and on the opposite side of the wheel to the connecting rod, believing that the intention was to counterbalance the weight of the connecting rod.

I was recently looking at this image and noticed that the three counterweights were not in line with each other.

Have I been misinformed about the need for weights to counterbalance the weight of the connecting rod? Could it be there is some other reason that has passed me bye for all these years?
Regards, John, TheObserver,
Swansea, south west Wales, UK.
Our local club website:
http://www.srmg.org.uk

User avatar
Bufferstop
Posts: 12455
Joined: Thu Mar 11, 2010 12:06 pm
Location: Bottom end of N. Warks line

Re: Weights on the driving wheels of steam locomotives.

Postby Bufferstop » Sat May 14, 2016 10:33 am

The weights on the driven axle will be different as they have to counter the thrust of the connecting rod as well as the weight of the crank and boss. Pushing against the crank on the driven axle produces a tangential force which tries to rotate the wheel, (and all the others) and a vertical force which attempts to lift the wheel. The balance weight needs to be in the correct position and heavy enough to minimise the upward force and divert as much as possible into the tangential one. I got that much from "Mechanical Principles 3" and perhaps a bit about swept volume of cylinders the rest involved too much calculus for this electronics and comms man.
Growing old, can't avoid it. Growing up, forget it!
My Layout, My Workbench Blog and My Opinions

User avatar
stuartp
Posts: 1162
Joined: Sat Feb 28, 2009 7:40 pm

Re: Weights on the driving wheels of steam locomotives.

Postby stuartp » Sat May 14, 2016 10:47 am

As well as the coupling rods (visible on the outside) the centre axle also has the cranks and connecting rods (from the pistons) attached to it between the frames. So the forces acting on the centre axle are different to those on the two outer axles, so the weights need to be different/bigger.

You weren't wrong, but it's a lot more complex than just putting them opposite the coupling rod.
Portwilliam - Southwest Scotland in the 1960s, in OO - http://stuart1968.wordpress.com/

TheObserver
Posts: 66
Joined: Thu Apr 12, 2012 7:48 am
Location: Swansea, South West Wales. UK

Re: Weights on the driving wheels of steam locomotives.

Postby TheObserver » Sun May 15, 2016 11:40 am

Thank you both for your explanations.

It's nice to see that my basic understanding of so many years was not actually wrong, although it was well short of a 'full understanding'.

I am reminded of a quotation we will all know of:

    The truth, the whole truth and nothing but the truth.
Thanks again.
Regards, John, TheObserver,
Swansea, south west Wales, UK.
Our local club website:
http://www.srmg.org.uk

RFS
Posts: 890
Joined: Tue Jan 12, 2010 3:09 pm
Location: Derbyshire, UK

Re: Weights on the driving wheels of steam locomotives.

Postby RFS » Sun May 15, 2016 1:37 pm

Robert Smith


Return to “General Model Railway Discussion / News”

Who is online

Users browsing this forum: No registered users and 16 guests