Weighing a steam loco

I read this recently on the MHR's website (news from Ropley shed) and it raised some questions that I'm sure the experts here can answer!

"For those wondering how we weigh a steam locomotive, wagon or carriage (stuff to large to fit on the bathroom scales!), this is a brief outline of how it's done. The engine is moved onto the pit road (level) where a small hydraulic jack is placed between the rail and the wheel. The jack then raises the wheel clear of the rail. Once clear, the amount of pressure required to lift the engine is measured and this then translates into weight."

1. Where is the jack placed? Surely if the wheel is resting on the rail there's nowhere for the jack to go.

I can understand the next bit, where the jack is lifting the wheel, pushing against the downward pressure of the spring. But,

2. How can you accurately measure the downward force of the spring just by continuing to lift with the jack? Surely that figure will change depending on the amount of upward force that is being exerted by the jack?

3. Is the jack then really 'lifting the engine' at that point, or just continuing to lift the wheel against the force of its spring? I doubt whether there would be any measurable upward movement of the loco itself.

Just curious, that's all!

John.

 

The jack goes under the flange and only lifts it just clear of the rail. On the Festiniog they use a steel rule to check that it is clear.

It's not 100% accurate for the reasons you've already suggested but then the weight of a steam loco will change in service anyway.

Tim

 

I think I can answer part 1, because I saw it being done with Bittern last summer. The jack has a pair of wedge-shaped jaws that are offered up either side of the chord of the wheel in contact with the rail. As the jaws are forced towards each other, the wheel is raised. HTH.

 

Some railways have Kelbus weighing equipment - does this work in more or less the same way. I believe the SVR have gone high tech and have a piece of equipment that can measure as the engine passes over it.

I've heard of the hydraulic jack method before, but would have thought it would be a bit limited by the sensitivity of the pressure gauge used- if there's 8 tons on an axle that's 17920lbs on whatever the area of the ram is and so needs a pressure gauge with a range such that even small errors in reading it make quite a difference in the final calculation.

 

Weighing a steam loco isn't an exacting thing and, if you have a calibrated hydraulic jack with a pressure gauge, it can be done. You are not looking at small differences. As has been said, you raise the wheel a fraction off the rail and note the pressure. Ideally, you want a separate jack for each wheel so you can weigh the whole lot at one go but it is generally accurate enough to do each wheel separately with the same jack. If you do it twice, you'll generally get different results due to the hysteresis of the leaf springs but, at the end of the day, all you are really interested in is that there is a sensible weight distribution across the loco with each spring carrying something like its design load. It's going to vary with the amount of coal/water in the boiler/tender in any case, when it's in service, not to mention the cross level of the track. Proprietary weighing equipment is available - the NYMR have a set - but the principle is the same. You can also strain gauge a dedicated length of track and measure loads this way, even on the move, if required. There are weighing equipment suppliers who will do this; it's not new technology and well proven. When did you last see a railway wagon being individually and statically weighed on a weighbridge?

 

It is also possible to have a pressure transducer as well as a mechanical gauge to read the pressure being exerted by a hydraulic jack. These electronic devices (often using strain-gauge techniques) can measure the pressure with much greater accuracy than mechanical gauges.

 

I think item 1 has been answered. You can lift a wheel by as many methods as I've had hot dinners and I'm always hungry!!

Item 2 - As has been mentioned, you only lift the wheel just clear of the rail. If you continue lifting more weight will be taken by the jack thereby giving a false reading.

Item 3 - All you are doing is lifting the wheel against the force of it's spring and there doesn't need to be any upward movement of the loco itself as all of it's weight is borne by the springs and transmitted through the wheel to the railhead. You are trying to measure the amount of weight taken by the spring you are lifting. Once all the wheels have been measured you can then adjust the weight borne by each spring until they are even.

If you know the area of the jack's ram and measure the pressure required to just lift the wheel then you can work out the upward force required to do the lifting and therefore the downward force/weight of the loco on that particular wheel.

Hope this helps.

 

So could I weigh my Ty-Fighter like that??!!!

 

Ah, but young Luke e should know that in space e don't weigh anything a'tall :lol:

 

Thank you to all the learned gentlemen who have answered my 'weighty' questions so fully. I didn't really expect such comprehensive explanations... and all within a few hours of posting the original question! Top marks for NP!

Steve, I haven't seen any railway vehicles being weighed. I was just curious, having read a short news piece on the MHR website. I'm only an armchair enthusiast but I like to know how stuff works.

Having witnessed the birth and development of the heritage railway industry in my own lifetime, I'm amazed at the skills and equipment that are at work on today's railways.

John.

 

Of course the more modern methods involve using load cells, which I believe are slightly more accurate.

Regards,

Dan

 

A year or so ago, I had an X turn (maintenance turn) at the Bluebell which involved changing all the loco springs on the C class. When we were done, weights were taken on each wheel in turn; the spring hangers were then adjusted until the variation on any axle, left and right, was within a certain tolerance, which I think was 1/4 ton. So you can see it's not an exact science, we certainly weren't interested to the nearest ounce! As I recall, it was done with a kind of hydraulic jack that had two small rollers that could be positioned on the rail either side of the wheel; the jack could then be pumped up, moving the two rollers towards each other until the wheel lifted, and the weight could be read.

Tom

 

See photos of the "portable" weigh jacks here:

http://www.erlestokemanorfund.co.uk/

in newsletter no 53.

The previous posters have given most of the info except that:

1. weighing must be done on level track
2.readings are taken both jacking up (when a bespoke feeler gauge will just pass between the tread of the tyre and the rail) & on carefully realeasing the weight going down (when the feeler is just trapped between the wheel & rail)
By averaging these readings stiction of the axleboxes in the horns is eliminated.
3. you cant weigh more than one wheel at once as any raising of a wheel has an effect on all the other springs
4. this is not an exact science and probably only accurate to within 10% - if you weigh a loco & then run it up & down before reweighing it the results will differ at each wheel even though the total for the loco remains the same.

The SVR also has a device built into the track at Plat 1 Bridgnorth which can check axle weights of vehices passing over it whilst moving at a dead slow speed. This provides a means of checking for locos/coaches.

 

This is quite interesting, I wondered how the 'big railway' weighed things. I knew that they did, but not how.

For smaller items, I supposed some form of weighbridge - as I have seen a loaded carriage on a public weighbridge a few years ago.
As well as a number of 15" and narrow gauge items hanging off a crane.....
The RHDR loco "John Southland" had a short flight when it left Gateshead in 1990, but I have forgotten exactly what the crane said the weight was (and I've lost my photographs....)

StoneRoad

 

Er - when we are talking about weighing we are not so much interested in the total weight - we are interested in the load on each wheel to check that it is as the designer intended.

 

You don't need to weigh a Ty-Fighter you just feel the (down) force