Specifications for 3 1/2″ Gauge

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Specifications for 3 1/2″ Gauge

This topic has 13 replies, 7 voices, and was last updated 14 November 2011 at 20:43 by Sub Mandrel.

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8 November 2011 at 19:54 #1048
Sub Mandrel
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@submandrel
Are there standard specs for model locos?
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8 November 2011 at 19:54 #77502
Sub Mandrel
Participant
@submandrel
Are there standard specifications for 3 1/2″ gauge locos. I’m meaning standard gauge models at 1/16 scale?

So far I’ve worked out that the buffers should be about 4 1/2″ centres and 3 1/2″ above track level. But:

How fast should a model go? A shunter with real life speed of 14 mph should crawl around at 1mph, but will that frustrate other track users? I’m guessing 2.5mph would be reasonable, but is there an accepatble speed to fit track ettiquette?

How thick should a coupling hook be and what size fixing bolt to take the load of towing a human being? Should I provide a fixing point for a safety chain, and if so what form does this take?

Flanges? How thick and how deep. There seems to be a lot of variation, so is there a standard.

Wheel and gear bearings. I’ve used ‘Tich’ hornplates which are hot pressed gunmetal(?) will these be OK with axleboxes machined from cast iron or steel? If I use cast iron for axleboxes, will I get away with plain CI bearing surfaces. Or should I use phosphor bronze bushes or even ball bearings?

Ol’ LBSC said just wobble a reamer about to provide enough play for the axles, but if I make nice fitting bushes or use ball bearings, then the axleboxes will need to have some ‘slop’. How much. And come to think of it, how much ‘suspension travel’ should there be? As for the gear train, should I use ball bearings or are bushes ok here?

Sorry to bombard so many questions in one posting – I now have a creditable chassis, and it’s time to start adding the bits that go round and round!

Neil
8 November 2011 at 20:59 #77513
Tony Martyr
Participant
@tonymartyr14488
“There seems to be a lot of variation”

Indeed there is, when I was building my 3 1/2″ William a went around and measured the rail to rail distance on a few tracks and decided that a bit of tolerance was required on all dimensions except those concerned with diameter and quartering of wheels and effective lengths of coupling rods.

My buffers are at 4 1/4″ centres and 3″ above the table when the loco is sitting on its flanges. The rise and fall has to be sufficient to cope with the track you are running on and must allow for skew across and axle (one rail going up relative to the other) so I prefer phosphor-bronze axle bushes not CI and not ball-bearings. How much running are you going to do is the question.

I will be interested to hear from experts

Tony
8 November 2011 at 21:23 #77514
Clive Hartland
Participant
@clivehartland94829
The horn blocks and main wheel bearings on my Evening Star I made a very good sliding fit until I realized that axles also had to ‘Tilt’ so I went back in and filed a curve in the flanges of the wheel bearing blocks so that it would tilt over.

This then seemed very sloppy but when assembled it worked fine.

I have about three eighths of an inch springing which seems adequate, even if not, there is not much I can do about anyway.

The Horn blocks are stamped brass and my wheel bearings are Alu/bronze that I got off cryogenic valves that were scrapped. Quite tough to machine but I hope will be long wearing as there are 10 of them.

The buffer beams on the tender seemed to me to flex in the middle so I have braced them with a rod from front to back to take the strain. The beams are made from 3mm plate, may be a bit of angle iron behind the beam might be better.

There is reference to a coupling hook a little while back that a member kindly posted for 3.5gauge.

I hope some of this helps

.

Clive
9 November 2011 at 13:16 #77548
AndyP
Participant
@andyp13730
I geared my 3 1/2″ electric for about 5mph which can keep up on the track but the 4 year old complains it isn’t fast enough – the throttle might as well be a switch for her though!

There are wheel standards here as well as lots of other really good stuff.

On working leaf springs I have 3/16″ travel from rest position so around 3/8″ overall and it hasn’t derailed since I added several pound os lead behind the front buffer beam to remove the tendency to pull wheelies.

My coupling hook is 1/8″ thick but the working coupling is a clevis and pin 3/16″ bar and 1/4″ respectively.

Some clubs insist on safety chains, mine doesn’t despite one of the guys having to rugby tackle a runaway 3 1/2″ Brit a few years ago.

Cheers, Andy
9 November 2011 at 21:35 #77577
Sub Mandrel
Participant
@submandrel
That’s incredibly helpful, thanks everyone.

I have been worrying quite a bit about how ball bearings can work OK with moving axles. Looks like PB bushes are the way to go, and be generous with the clearances (suits my style…).

I’ll rivet an extra bit of angle behind the buffer beams – the front already has holes for some ‘dummy rivets’ in the right place.

Andy those pages have really helped me, especially the wheel profile. I’m not sure my shunter will weigh in at 100lbs, perhaps it will reach 20?

I have already decided to pop some lead in the front, as the motor and battery end to end will run from the back of the cab to the front of the engine cover with only a fraction of an inch to spare. This means most of the battery behind the rear axle.

The figures on drawbar pull help me get a perspective. I need at least as much pull as a Tich (10lbs).

My motor is 1amp 2500rpm no load, and about 2000rpm at 7A when I load it. I work out that a Tich must have less than a couple of kilowatts of heating power, but at 1% efficiency that’s only 20 watts at the drawbar. As an electric will be perhaps 40-50% efficient, even with a three stage gearbox and plain bearings, I should be OK. I have been playing with different gearings and was going to go for a 2.5mph top speed at 2000 rpm, I’ll aim for 4mph instead. I can do this by changing just the motor pinion, so if its underpowered I’ll make a smaller pinion and drop the motor down 1/2″.

Progress so far is the frames, buffer beams and stretchers, including a strange, very tall one that the motor will fix to. My camera batteries have gone flat! I’ll post a picture or two soon.

Neil
9 November 2011 at 22:13 #77583
Brian Dickinson 2
Participant
@briandickinson2
I have just read his book ‘road rail n steam’ or what ever and i find it very fitting to my engineering skills. I only have small time slots to do owt in and it fits me bill quite nicely. Then again i aint a rivet counter if you know what i mean.
10 November 2011 at 00:16 #77588
AndyP
Participant
@andyp13730
Neil

I have added some photos in an album called 3 1/2″ electric.

What you can’t see is that the motor/gearbox assembly is mounted to a stretcher via a paxolin ? sheet that flexes to give suspension movement.

The speed contoller is mounted in the body – sort of in the top of the bonnet and the battery in the cab – hence the wheelies.

My grandchildren love it but since they are both girls I am having to resist painting it pink

Andy
10 November 2011 at 20:06 #77625
Sub Mandrel
Participant
@submandrel
Brian

Shop, shed and road? I must look out for a copy…

Andy, your drive train looks quite modest compared to mine – the motor I’ve got is 4 1/2″ long and 3″ diameter! I’m less worried about not having enough power.

This is my chassis, sorry about the carpet

Neil
10 November 2011 at 21:08 #77631
Brian Dickinson 2
Participant
@briandickinson2
I was never very good remembering things!

I would like to build a low level driving trolley for 5″, are there any plans for them anywhere?

–and i quite like the carpet
13 November 2011 at 20:59 #77801
Sub Mandrel
Participant
@submandrel
Lack of phosphor bronze has put a brake on things. To keep some momentum I just turned up a set of buffer stocks, quite an interesting job as they need finishing both ends and there isn’t a lot to hold on to. In the end I bored out the centre shaped the ends, and then used a parting tool to create along neck 6mm diameter before parting off. Luckily I found some wonderfully free-cutting 3/4″ hex that was in one of the last ‘Whistons Metal packs’ from Proops – parting it was like cutting butter.

Neil
13 November 2011 at 21:55 #77802
Nicholas Farr
Participant
@nicholasfarr14254
Hi Neil, not my colour, but I can’t see any problem with the carpet. Neat work on your chassis BTW.

Regards Nick.
13 November 2011 at 22:35 #77806
61962
Participant
@61962
Neil,

Prototype UK and European railway standards put the buffers around 3ft 6inches above rail level. For 3 1/2″ gauge the buffers and the coupling will be 2 5/8inches above the rail. Buffers are 5ft 8 1/2in apart so 4 1/4″ is about right in 3 1/2.

On raised tracks 3 1/2 inch gauge steam locos are capable of of speeds in excess of 10mph if the railway is suitable and many drivers will lap the average track at 7 or 8 mph routinely. For some reason ground level railways slow them down, perhaps because the ground is much nearer.

On springing: I used to think the softer the springs the better would be the adhesion. Then I watched a pacific with soft springs actually lift its driving wheels when it lost traction. What appeared to be happening was that the running position of the axles was actually lower than the cylinder centre line due to the springs being set to maximise the load on the drivers. When the wheels slipped the mechanics forced the axles to a position nearer the cylinder centre and effectively lifted the drivers off the rail transferring more of the load to the carrying wheels. Since then I have used much stiffer driving springs on all my locos.

As for vertical movement of the driving axles, a full size loco will typically have no more than 2″ of vertical travel available. It has to be very rough railway that needs as much difference in height between the drivers as that. 2″ translates to 1/8″ at 3 1/2″ gauge and if that is not enough then it is the railway that needs looking at not the loco.

Finally, in terms of power, a 3 1/2″ gauge pacific will be capable of achieving a drawbar pull of 10lbs at 10 mph which is about 1/4 brake horse power, or about 180watts. The equivalent electric loco on 24 volts would therefore be running with 7.5 amps plus losses, probably 30amps on the clock when flat out if any of the ones I’ve handled are typical.

Regards

Eddie
14 November 2011 at 20:43 #77838
Sub Mandrel
Participant
@submandrel
Thanks Eddie,

I’m confident that a 12V gel battery should be able to haul me around, if not at 10 mph – it can give a good 7A.

I just converted 10lbs and 10 miles roughly into metric (10 lbf= 45 newtons and 10 miles=16 km), used force x distance moved = work done 720,000 joules, then divided by 3600 to convert into joules per second and got… exactly 200 watts for your steamer!

power in watts ~= 2 x drawbar pull in footpounds x velocity in mph!

If efficiency is about 3%, good by IMLEC standards, that suggests the fuel usage of about 6.6kW so my guess for 2kW for a Tich might not be that far out. I think what you say about springs must apply doubly to a small shunter – with a tiny 5″ wheelbase 1/8″ ought to cope with anything.

Doe anyone have views on using oilite bushes (I’m thinking of using these for the whole power train – 10 needed).

Oilite bush

They work out about £1.30 each for ten including p&p.

Neil
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