Locomotive balance weights

Any aspect related to the prototype stock.
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Graham Tipple
Posts: 101
Joined: Tue Dec 27, 2016 8:16 pm

Locomotive balance weights

Post by Graham Tipple » Sun Jun 21, 2020 8:46 am

Hi Folks, Were there any rules or protocols as to where the balance weights were in relation to the crank on loco driving wheels or were they a bit like those on the car (just where a particular wheel needed them)? Were they there to help the wheel turn at a particular point in the cylinder cycle or to stop the wheels bouncing around (as in a car)? Sorry for such ignorance in a man of my age!
Keep safe, all. Graham

jimwatt2mm
Posts: 655
Joined: Fri Nov 16, 2012 5:36 pm

Re: Locomotive balance weights

Post by jimwatt2mm » Sun Jun 21, 2020 1:34 pm

Hi Graham. The balance weights were there to balance the weight of the crank and coupling rods on coupled wheels and of the eccentrics, valve gear, big ends etc on driven wheels, whether the valve gear was outside or inside. I would imagine that all this could be done by calculation at the design stage. Therefore they were opposite the crank on the former and generally larger and in a different position, depending on how the valve gear and connecting rods were arranged, on the latter. I'm not an engineer, but I assume it was to reduce vibration and wear on the bearings as well as to probably even out the downward forces on the rails. Others with more knowledge than I of these things will probably correct and expand on this.

Jim

Dave John
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Joined: Wed Aug 15, 2012 7:42 am

Re: Locomotive balance weights

Post by Dave John » Sun Jun 21, 2020 2:43 pm

They hardly ever get shown on drawings.

Generally I have found that the only way to size them is to find a photo and do a bit of guesstimation.

Graham Tipple
Posts: 101
Joined: Tue Dec 27, 2016 8:16 pm

Re: Locomotive balance weights

Post by Graham Tipple » Mon Jun 22, 2020 5:27 pm

Thanks, Jim and Dave. I had not realised the difference between the wheel connected to the cylinders and the others. Thanks for that tip. The photos make much more sense now. Keep safe, Graham

DonaldPeddie
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Joined: Sat Sep 22, 2012 2:40 pm

Re: Locomotive balance weights

Post by DonaldPeddie » Mon Jul 27, 2020 10:16 pm

The balancing of locomotives is a complex mathematical subject - if you want to understand the technical detail, one of the best publications is 'The Balancing of Engines' by Prof WE Dalby FRS, first published by Edward Arnold in London in 1901, with subsequent updates in 1906 and 1909. The book is very mathematical, but covers all aspects of balancing for all types of engines (4, 6, 8 etc coupled), for all numbers of cylinders (2.3 and 4), all crank settings (90, 120, 135 etc degrees), inside and outside cylinders, divided drive, cylinder inclination and so forth. Without the balancing of reciprocating parts (pistons, crosshead, connecting rods and other components) severe draw-bar oscillation in the direction of travel between steam engine and train results. Balance weights are added at precise positions on driving wheels (there are a number of ways to do this, and each of the 'big four' had their own schemes) to alleviate this in-line oscillation, the result being 'hammer blow' on the track. The balancing is a compromise between oscillation reduction and increasing hammer blow. You might be interested to know that using a 'Black 5', running at the equivalent of around 100mph on heavily greased rail (the engine was securely anchored to buffer stops), the LMS made accurate measurements. Due to the balance weights (50% of reciprocating masses balanced), each 'Black 5' driving wheel left the rails by over 1.5 inches and came back to the track with a force in excess of 25 tons. Driving wheel speed in these circumstances was in excess 8 revs per second, the 1.5 inch wheel lift and the 25 tons hammer blow occurring at this frequency for each wheel-set, there being phase displacement between the wheel-set hammer blows and phasing side-to-side on each axle. As one might expect, the test was of short duration and observers may have made their measurements remotely from a bunker.

If you need a book to send you to sleep at night, I would recommend the report of the '1928 Bridge Stress Committee', by HMSO,which looked at the impact of steam engine hammer blow on railway bridges. Hammer blow results are given for many locos including all the main CR 4-6-0 and 4-4-0 Classes. The main authors of this research material included Prof Sir Alfred Ewing FRS, Prof Sir Charles Inglis FRS and of course Prof Dalby. Gresley used Dalby to advise in detail on the balancing of his earliest pacifics.

Happy reading

Donald

Graham Tipple
Posts: 101
Joined: Tue Dec 27, 2016 8:16 pm

Re: Locomotive balance weights

Post by Graham Tipple » Tue Jul 28, 2020 5:58 pm

Thanks for that detailed answer, Donald. I am full of admiration for those who understand arcane topics and try to pass on hard-won data in a readable (?) form. In my own career as an academic, I have written books and papers on equally arcane topics, mainly to do with housing policy in African countries. I could send you to sleep with an explanation on how the dead are one of the main actors in the housing process in Ghana.

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