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Expansion LInks

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Peter Maloney30/07/2017 13:00:11
28 forum posts
12 photos

After several failures at drilling and filing my Pansy expansion links and now several months later, I have finally had another attempt.

Not having a rotary table or a milling spindle I resorted to swinging them by hand on a bar attached to an angle plate on cross slide with cutter in the 4 jaw.

With small cuts and steady movement on the bar I am very pleased with the result. OK they needed a little draw filing and polishing but the method seems to work. I am about to machine the links blocks by the same method with high expectations.

I spent a lot of time thinking and making the set-up but would like to think that I have learned a lot in the process.

Nige30/07/2017 13:46:52
avatar
370 forum posts
65 photos

Some photos would be good if you have any Peter 😀

Peter Maloney30/07/2017 13:58:23
28 forum posts
12 photos

Hi Nige

Will take some photos and try to work out how to post them.

My first attempt at this 'swinging bar ' method burnt out my cutter but for the next 2 all went well when I drilled a series of holes to remove the bulk of the material.

mick H30/07/2017 14:03:02
709 forum posts
21 photos

I have always found expansion links difficult, even with a rotary table so well done. As far as the die blocks are concerned, I have given up on trying to make the traditional shape a good fit throughout the length of the link and now make them cylindrical so that they roll along the link. They can then turned to an exact fit in the expansion link slot and they work extremely well.

Mick

Norman Rogers30/07/2017 14:44:38
17 forum posts
2 photos

I drilled and filed my first set of expansion links ... they're still in the Maisie 30+ years later but due for replacement. I then used the swinging arm technique to great effect but now have the luxury of a rotary table. I found that light cuts with a new cutter was the key and I'm sure you will be well pleased with the outcome.

N

Peter Maloney30/07/2017 15:31:49
28 forum posts
12 photos

Hi Mick

I too have thought about cylindrical die blocks but have been concerned about lack of surface area contact and subsequent increased wear. Do you use steel or bronze for the cylindrical die blocks? I suppose that with the relatively little use the models get, that the blocks will last some time. After all they would be easy to replace hopefully after a few years.

Pete

Peter Maloney30/07/2017 16:12:50
28 forum posts
12 photos

Hi Nige

Have created an album for photos so another success. The 'G clamps' are holding in place simple stops to limit swing travel in both directions. The pivot pin is a 'fitted bolt' and pin hole was drilled using the 4 Jaw and the cross slide traversed to give the require swing radius and locked in place. I also locked the saddle in place for each cut. A little time consuming but done to reduce vibration.

Pete

Andrew Johnston30/07/2017 16:13:32
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5173 forum posts
599 photos
Posted by Peter Maloney on 30/07/2017 15:31:49:

I too have thought about cylindrical die blocks but have been concerned about lack of surface area contact and subsequent increased wear. Do you use steel or bronze for the cylindrical die blocks? I suppose that with the relatively little use the models get, that the blocks will last some time.

Well done, a tricky item to make.

I believe that the supplier of castings for my traction engines uses cylindrical bronze dieblocks for the complete engines he builds for sale. Likewise I'd be concerned about wear on what is essentially a line contact, as the loads can be quite high. In theory the harder materiial can wear first, so using bronze seems the wrong way round.

I've only ever made one set of expansion links, for a fellow builder rather than for my engines, and I cheated by using a CNC mill:

expansion link me.jpg

For scale the material is ½" thick gauge plate. I still did a lot of hand filing to get a good fit though. The outside profile was left rough for the recipient to finish. The inner slot was left undersize from the mill and draw filed to fit, using a Hoffmann roller as a guide. The dieblock was left oversize and likewise finished by draw filing to fir the slot. These are Hoffmann rollers, in this case bought on Ebay at an advantageous price:

hoffmann rollers.jpg

Despite the optical illusion height and diameter are the same.

Andrew

Clive Hartland30/07/2017 17:24:18
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2514 forum posts
40 photos

Surely, if the die blocks were round then they would rotate? Not slide friction wise like square ones.

Clive

Saxalby30/07/2017 17:39:03
159 forum posts
14 photos

I agree with Mick H, and have used round die blocks on several models and found they work extremely well.

Saxalby

Andrew Johnston30/07/2017 18:00:23
avatar
5173 forum posts
599 photos
Posted by Clive Hartland on 30/07/2017 17:24:18:

Surely, if the die blocks were round then they would rotate? Not slide friction wise like square ones.

Not sure about that? Assume Stephenson's valve gear with a mid point suspension. The valve rod can only move linearly. If the expansion link is not set so the dieblock is at the centre it moves in a slight arc. So there is some small amount of sliding of the dieblock. I think that the sliding moment reverses as the expansion link reverses, so a round dieblock may just oscillate rather than continuously rotate. Or it may just slide depending on forces and lubrication.

In fullsize at least, the forces in a valve rod (with slide valves) can be significant, hundreds of pounds, depending upon steam chest pressure and slide valve lubrication.

Andrew

Peter Maloney31/07/2017 15:06:35
28 forum posts
12 photos

Hi all

I am still pondering over steel or bronze die blocks. After all, axles are steel with bronze axle boxes as are crank axles and big end bearings

Pete

Bob Youldon31/07/2017 17:25:40
183 forum posts
20 photos

Hello Peter,

For more years than I can remember, I've used gsuge plate in it's normal state for the links with cast Iron die blocks. The load on the slide valve is high as Andrew has pointed out, but the load on the valve isn't that extreme as the exhaust port, being open to atmosphere must be taken ito consideration in any calculation and often the steam chest pressure will rarely match the boiler pressure due to internal losses and wire drawing. I've yet to meet many who will drive a locomotive with a fully open regulator, controlling the progress of the locomotive on the reverser.

Fear nought, a mild steel link with a bronze die block will probably see you out!

Use cast iron for your axle boxes, with cast iron there will be very little wear. Put an oil way throuth the end of the axles to a cross drilling where the axle box runs and you'll always push any dirt out, almost essential with the trailing set in the area of the ash pan.

Regards,

Bob

duncan webster31/07/2017 17:45:53
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2415 forum posts
39 photos
Posted by Bob Youldon on 31/07/2017 17:25:40:

Hello Peter,

. I've yet to meet many who will drive a locomotive with a fully open regulator, controlling the progress of the locomotive on the reverser.

Regards,

Bob

I had to when the regulator on my loco jammed wide open. It was OK until the lifting link started slipping as well, it was still pulling forward strongly with the lever in reverse. Drain cocks open, full brake application brought it to a stand, then wait for the boiler pressure to drop. This was it's first run, I've fixed it now (I hope)

Peter Maloney01/08/2017 21:16:40
28 forum posts
12 photos

Hello All

Thanks for the lively discussion.

Bob

My links are mild steel. I was going to case harden them. Do you think this is required in the light of your experience?

Bob Youldon02/08/2017 11:03:51
183 forum posts
20 photos

Hello peter,

I've used case hardened mild steel in the past and was very successful; the only reason for using guage plate in it's natural state today as it's easier to obtain than case hardening powder and probably cheaper!

Should you fancy it, Model Engineers Laser ( usual disclamer) can supply the links pre cut to shape in gauge plate should that be of any help.

duncan webster02/08/2017 19:44:21
avatar
2415 forum posts
39 photos

The surface of laser cut parts is like a very fine abrasive stick. If you get expansion links laser cut you'll need to polish up the inner curved surfaces with a diamond file or similar. Ordinary files no good. Water jet cutting should be OK, not sure about wire erosion, perhaps a metalurgist out there can advise.

Several people in out club have had success with PTFE die blocks, the loaded stuff, not the white virgin. I machined mine to size and shape, but I've heard of others just making a rectangle, bludgeoning it into the slot and waiting for a few days, seems it creeps very quickly and takes on the shape of the slot. Seemed a bit brutal to me.

IVATTLMS02/08/2017 20:30:07
22 forum posts

Round die blocks will not revolve they will just slide .

Bob Youldon03/08/2017 11:33:48
183 forum posts
20 photos

Hello Duncan,

I think Model Engineers Laser use a water jet cutting system for their links when using gauge plate.

A good friend had his 4" Foster traction engine link and die block wire eroded in gauge plate, the fit and finish is superb, not a cheap exercise though.

Well done Ivattlms pointing out that round die blocks just slide, old LBSC pointed that out in one of his articles back in the 50's and as someone has already said it would only provide a point loading on the link.

Regards

Bob

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