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Post by Nigel Bennett on Jun 27, 2022 12:24:59 GMT
The strength of a roll-pinned assembly can be improved by fitting a smaller roll pin inside a larger one. Not always possible if the roll pins are very small! They do have a bit of "give" and so do not provide a rigid fitting. The orientation of the slot in the roll pin will also determine how "springy" the joint is. There is a version of a roll pin called a Spirol pin, which as its name implies comprises several turns of the spring material in a spiral form when viewed end-on. They are supposed (according to the manufacturer) to be vastly superior in terms of fatigue and ultimate strength. I used to have a nice set of punches with little nibs on the end for fitting roll pins, but Somebody borrowed them and they came back with either flat or bent ends... With taper pins you can often batter them in place and then hide the join completely if you never want to take them apart again; roll pins can't be hidden so easily.
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jma1009
Elder Statesman
Posts: 5,901
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Post by jma1009 on Jun 27, 2022 23:33:22 GMT
My take on all this is that for a built up crank axle, well made and with proper press fits to accommodate Stephensons valve gear eccentrics, you might as well fit a plastic straw instead of a roll pin.
I am not too sure of Steve's press equipment, though I know that John Helps has a press in his workshop not many miles away from Steve.
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Post by doubletop on Jun 28, 2022 5:39:10 GMT
My take on all this is that for a built up crank axle, well made and with proper press fits to accommodate Stephensons valve gear eccentrics, you might as well fit a plastic straw instead of a roll pin. I am not too sure of Steve's press equipment, though I know that John Helps has a press in his workshop not many miles away from Steve. I did intend to go the press fit route for my Dart built up crank, three axle parts, four webs, two crank pins and four eccentrics. I made a test piece of one crank and pressed it together. With thirteen components, I wondered how I was going to get everything together and all in the correct angular alignment. The eccentrics were to be pinned to their respective webs at the correct lead angle, as per Don Ashtons view, "They don't need to be adjustable". As the stack would increase with eight joints in total, each one could introduce an error. I could see how I could get press a pair of webs into alignment but getting each of the two stub axles into overall alignment, escaped me. Then was the "How do I get it apart if I need to try again?" That's why I decided on the good fit, "cotton reel" joints with Loctite 620 and pin it all together. I went for tapper pins as they can be pressed home for a good tight joint, then I peened over the thin end so they couldn't work loose. Pete
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Post by andyhigham on Jun 28, 2022 5:52:45 GMT
In his book on building the Shay, Kozo Hiraoka descibes how to machine a one piece 3 cylinder crankshaft with Stephenson eccentrics.(10 centre lines in total) He breaks it down into relatively simple steps
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Post by springcrocus on Jun 28, 2022 7:29:14 GMT
I am not too sure of Steve's press equipment, though I know that John Helps has a press in his workshop not many miles away from Steve. Julian, I have a home-made bodge-up using old bed-frame material, some 4" x 2" box section and a car jack. It can apply about fifteen tons in normal use. Regards, Steve
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Post by jon38r80 on Jun 30, 2022 18:15:40 GMT
Looks better than many sold for silly money, certainly beefy.
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