TVT - Reducing Wagon Rolling Resistance.

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TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Thu Jun 20, 2019 3:12 am

This is the result of an investigation I've been working on recently to reduce the rolling resistance of trains, especially long goods trains.

Because wheelsets usually have both wheels firmly fixed to the axle, the rolling resistance and hence the load on the loco, is higher on curves than on straight track. Due to the outer rail on a curve being a little longer than the inner one, one wheel on an axle has to slip a little and the extra friction means it takes more drawbar pull from the loco to keep the train moving on the curve. The smaller the curve radius, the greater the drag. As an example, my bogie wagons have a rolling resistance of 17g on flat straight track and 28g on my sharpest curve of 7'6".

Some manufacturers sell ball bearing wheelsets to counter this effect, but they are expensive. For some reason they also fit ball bearings to both wheels, although only one is needed to give the differential effect.

I make my own rolling stock wheelsets and had been considering fitting a miniature ball bearing to one wheel on each axle, as miniature bearings are easily available and reasonably cheap. Using them takes a bit of precision machining though and I've been looking for a simpler solution.

The photo shows the result of these deliberations. It also shows the result of living near the sea and leaving steel parts lying around unpainted in the workshop. :roll:

Free wheel.jpg
Free wheel.jpg (78.92 KiB) Viewed 5478 times

The difference in rotational speed between the inner and outer wheel on a curve is very small and a ball bearing is overkill. So, instead of fitting a ball bearing in the wheel as originally planned, the wheel seat on one end of the axle was reduced slightly in dia. to provide a plain bearing for that wheel. The free rolling wheel is retained in place on the axle by a little brass collar, which is a press fit and is visible on the r/h wheel set in the photo.

With four of these wheel sets fitted to a bogie wagon, the extra rolling resistance on curves was eliminated. The rolling resistance is now 17g on both straight and curved track, even on the LGB R3 curve I use for checking clearances. Which confirmed my theory that two bearings per axle was not necessary for free running.

Total cost to convert a bogie wagon (4 wheel sets) is 15 cents worth of brass to make the collars, less than 10 minutes lathe time and about 30 minutes disassembling, reassembling and fitting the wheel sets to the wagon.

I know, this is not much use if you don't have a lathe, but it gives some insight into why ball bearing wheelsets are popular in the US. That said, I could buy a lathe and have change left over, for what it would cost me to convert my wagon fleet using commercial wheelsets....... :shock:

Regards,
Graeme

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by philipy » Thu Jun 20, 2019 5:49 am

Thanks Graeme.
Thats interesting to know and not something that had ever crossed my mind before. Food for thought, although being without a lathe I think I'll have to live with a problem I never knew I had before! :roll:
Philip

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by gregh » Thu Jun 20, 2019 5:55 am

Thanks for the information about testing your great idea. It is quite timely for me as I am testing trains on 1' radius curves. I might try your idea on some of my plastic wheelsets.

I'm also interested in your train resistance measurements as I try to do the same. Could you tell me the mass of your wagon please?

I tend to use % as measurements for rolling resistance, curve resistance so I can just add them to the %grade and get the total train resistance. So if I know the mass of your wagon, I can divide it into the 17g you got, and get a %. My values for rolling resistance on straight are around 0.5% for steel wheels. It will be interesting to see if your measurements are in the same ball-park.

Also is your 7'-6" a radius or diameter?
Sorry if this is a bit off your topic of free-wheeling.
Greg from downunder.
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Re: TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Thu Jun 20, 2019 2:29 pm

gregh wrote: Thu Jun 20, 2019 5:55 am
I'm also interested in your train resistance measurements as I try to do the same. Could you tell me the mass of your wagon please?

Also is your 7'-6" a radius or diameter?
I measure rolling resistance of rolling stock and drawbar pull of locos using a little digital balance, basically the electronic equivalent of a spring balance. I used the inclined plane method for both measurements in HO scale, but that needs too much space in large scale.

The bogie open I was experimenting with weighs approx. 550g and the wheelsets are fitted in LGB bogies. Using your method the rolling resistance of a wheelset is 0.8%, or 3% for the wagon I think?

I define track curves by their radius.

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Graeme

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by -steves- » Thu Jun 20, 2019 3:27 pm

Great idea, when I get the time I will try this on some of my Accucraft rolling stock as so e of those struggle on my tight 32mm track, it will turn them into useable rolling stock on 32mm instead of converting them back to 45mm, awesome :thumbup:
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Re: TVT - Reducing Wagon Rolling Resistance.

Post by gregh » Fri Jun 21, 2019 5:42 am

GTB wrote: Thu Jun 20, 2019 2:29 pm I measure rolling resistance of rolling stock and drawbar pull of locos using a little digital balance, basically the electronic equivalent of a spring balance. I used the inclined plane method for both measurements in HO scale, but that needs too much space in large scale.
The bogie open I was experimenting with weighs approx. 550g and the wheelsets are fitted in LGB bogies. Using your method the rolling resistance of a wheelset is 0.8%, or 3% for the wagon I think?
I define track curves by their radius.
Thanks Graeme. I'm surprised that the rolling resistance is 3% for steel wheels. I measure my plastic wheeled wagons at around that value. (Don't have any steel wheeled stock to check!) I measure rolling resistance by finding the grade they will just roll down. Something is wrong somewhere - I keep wondering if I've forgotten the gravity constant and am confusing Newtons with kg.

On the weekend I'll try and get around to doing your loose wheel method on a plastic-wheeled wagon, measuring before and after curve/grade they can roll down.
Greg from downunder.
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Re: TVT - Reducing Wagon Rolling Resistance.

Post by gregh » Sat Jun 22, 2019 4:10 am

I did try making a 'free-wheeling' axle for a 4-wheel wagon today.
The only 'set' curves I have are some kid's plastic track of 2' radius. The wagon has a wheelbase of 100mm, and has plastic wheels on steel axle running in styrene 'bearings'. And I haven't oiled them for years. Anyhow I tested before I started and the wagon would just run down a 9% grade.
With the free-wheeling wheels fitted there was no difference. So I guess maybe on really sharp radii, the curve resistance is more due to the flanges rubbing on the rails, than the inner wheels slipping.

Anyhow, I like your idea and look forward to hearing if you do a whole train of wagons and can get a good reduction in curve forces.

Back on the rolling resistance forces. I tested my 4-wheel wagons with plastic wheels as above and they just run down a 3.2% grade on straight track (The '.2' really shouldn't be taken as a measure of my accuracy!). It was the same on brass and plastic rails. I did find a Bachmann bogie passenger car with steel wheels and it ran down a 1.8% grade. Even that was much higher than I expected - maybe I should do some oiling?

[I have been doing lots of testing of curve and gradient forces in conjunction with my planning for a small indoor layout, and will try and get the results all together in a separate Topic.]
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Re: TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Sat Jun 22, 2019 4:28 pm

I did some testing last night which bears on Greg's results.

Contrary to popular opinion in the hobby, there is little difference between the rolling resistance of wagons with metal and plastic wheels if they are kept clean. I use steel wheels because they stay cleaner than plastic ones and don't wear as fast.

Most of my wagons have steel wheels, with steel axles running in either brass or plastic bearings. The rest have plastic wheels with steel axles running in brass bearings. While the measured wagon rolling resistance in gram force (gf) varies, it is always about 3% of the wagon mass, independent of the wheel material.

For example on straight and level track, where the rolling resistance reading includes bearing friction -

- a 550g bogie wagon with steel wheels and plastic bearings has a rolling resistance of 17gf, or 3%.
- a 370g bogie wagon with plastic wheels and brass bearings has a rolling resistance of 12gf, or 3.2%.

Repeating these on an inclined straight track -

- the same 550g bogie wagon starts to roll on a 1:35 grade. Rolling resistance 16gf, or 2.9%.
- the same 370g bogie wagon starts to roll on a 1:30 grade. Rolling resistance 12gf, or 3.2%

Removing a wheelset and letting it roll down an inclined straight track, so the bearing friction is not included -

- a 63g wheelset with steel wheels starts to roll on a 1:120 grade. Rolling resistance 0.5gf, or 0.8%
- an 11g wheelset with plastic wheels starts to roll on a 1:80 grade. Rolling resistance 0.1gf, or 1.2%

The plastic wheelset in this last test was noticeably dirtier than the ones on the bogie wagon and that will probably account for why the rolling resistance is a little higher than the steel wheelset.

Not too much significance should be placed on small differences in these results, due to the measurement errors in the methods. For example a rolling resistance of 16gf is somewhere between 15gf and 17gf and a grade of 1:35 is somewhere between 1:32 and 1:38.

Greg, you are right about bearings and lubrication. What also falls out of these numbers is that something like 75% of the rolling resistance of a wagon is due to friction in the bearings and that's with test wagons that were recently serviced.

The moral I get from these results is that to reduce train resistance, use light wagons and keep the bearings lubricated. Also if using plastic wheels, clean the wheel treads regularly.

Now that I think about it, I learnt all that empirically 50+ years ago, running Tri-ang trains on the lounge room carpet. This just puts numbers on it all. :roll:

Greg,

You probably won't see much improvement from fitting a free wheel on the axles of your wagons on the inside layout, as the trains are only six axles long. You'll get more bang for the buck by keeping them light, cleaning the wheels and lubricating the axles now and again, as I suspect the steep grades will have more effect on what you are doing than the sharp curves.

You are also right about flange binding on sharp curves being another issue. Are you planning to gauge widen the curves?

My issues are different I think, as I have generous curves and insignificant grades, but like running long trains.

Regards,
Graeme

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Sat Jun 22, 2019 4:44 pm

Oily Rag wrote: Sat Jun 22, 2019 10:27 am However as you are probably already aware there has been a series of articles in recent issues of "Australian Model Engineer" concerning bogie twist on larger gauge stock.
No, I wasn't aware of the articles. I have copies of AME from issue 1 to issue 200, but had been losing interest in it since the editorial change. We finally parted ways when they stopped distribution through newsagencies.

I've got a vague memory of seeing free wheels with ball bearings being used on riding trucks on one of the local live steam tracks some years ago. There may even be an old article on the subject in a back issue of AME.

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by gregh » Sun Jun 23, 2019 4:01 am

Fantastic info Graeme.
I'm glad you confirm that steel wheels are not all that different to plastic for rolling resistance. I have always believed that changing plastic for steel would not give a great effect, especially if you have any sort of grades and curves.
Your rule of thumb that most of the rolling resistance is in the bearings is interesting - something to be tucked away in the memory bank. Testing the bogie wheelset on its own to reduce bearing friction was a brilliant idea.

Re my planning for an indoor layout with 1' rad curves. I have done a test with a semi-circle on a 6% grade and it works. But using 'lighter' wagons as you suggest could be a problem as they tend to fall over on the tight curve. More in another topic some day.
I must go and oil the wagon bearings and see if I can measure a difference.
And yes, I did widen the gauge to 46mm on the 1' rad curve.
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Re: TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Sun Jun 23, 2019 5:18 pm

Oily Rag wrote: Sun Jun 23, 2019 11:05 am Another point perhaps to consider is that we tend to look at plain bearings (line contact in truth) usually about 3mm diameter. The smaller scales, "00", "H0" and "N" seem to be all point bearings. With the very tight radii envisaged here, just another thought. I have no idea how they would run outdoors however.
About half the weight of my large scale wagons is in the steel wheelsets. I've never needed to add extra weight as I do in HO.........

All my HO rolling stock has pinpoint brass axles running in acetal bearings, which I found roll better than brass bearings. The bearing is basically frictionless if the end float is minimised and a bogie wagon will roll on the same grade as a wheelset, ie. 1:200. So the rolling resistance is 0.5% compared to 3% for my large scale models. They don't need lubrication either......

There's no significant difference between the friction in cylindrical plastic bearings and cylindrical brass bearings, so the pinpoint is the secret ingredient in the HO results.

I've never considered using pinpoints in large scale. Turning axles isn't a big problem, but machining acetal bearings (or brass) would be a pain. The acetal ones I use in HO are injection moulded by a friend for the HO kits he sells, but I doubt I could talk him into making them in large scale.The only likely problem I can think of from using them in large scale outside is that the greater mass of large scale stock might mean distortion of the bearing and a higher wear rate.

Regards,
Graeme

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Re: TVT - Reducing Wagon Rolling Resistance.

Post by GTB » Mon Jun 24, 2019 4:04 pm

Oily Rag wrote: Sun Jun 23, 2019 8:22 pm Just looking around academically I have found a series of "chamfer" tools with 60deg included angle, apparently (I stand to be corrected as I know little of the smaller scales) the angle used by many manufacturers of small scale stock for their pinpoint bearings and axles.
The milling cutter in the link would be unsuitable for pinpoint bearings as it has a flat end and like most 4 flute cutters won't drill a hole. There are others online that have a point and cost a lot less, so might be suitable.

Common usage in HO and the other small scales is a 60 deg min cone for the bearing and a 50 deg min cone for the axle, which provides clearance so that only the very ends of the cones are in contact. If you use 60 deg for both the bearing and the axle the wagon will have the rolling properties of a brick.

I've been using the relevant NMRA Recommended Practices RP-24.1 and RP-24.3 in HO for 40+ years, but they don't cover any scale larger than S Scale. https://www.nmra.org/index-nmra-standar ... -practices

UK small scale practice seems to be the usual dogs breakfast, but this article gives some details of how they should be set up. http://www.clag.org.uk/bearing-interface.html

I don't know if 60 deg is some sort of magic number found by experiment or just copied from horological practice. It might be possible to use a 90 deg spotting drill for the bearing and turn an 80 deg cone on the axle for the same result.

I do wonder though if the mass of large scale models might gve too high a bearing loading for this style of bearing.......

Regards,
Graeme

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