mike3md Posted November 5, 2015 Report Share Posted November 5, 2015 Dear All, Roger Williams' book "How to improve TR2 - TR4A" mentions that the front main bearing should be slightly undercut to relieve stresses, as has also been suggested on the Forum if modifying an engine. The rear main has this feature machined from original manufacture. Can anybody advise as to why it was not incorporated at the front main when new, and also how it is "better" than ensuring smooth radiussing? Thanks Mike Quote Link to post Share on other sites
RogerH Posted November 5, 2015 Report Share Posted November 5, 2015 Hi Mike, let me start off the avalanche of replies A square cut corner is a high stress raiser so probably best to avoid A radius'd corner would be a big improvement - but the radius has to hit the web at a good tangent. if the radius leaves any amount of acute angle then a stress raiser would be present cracks like acute angles. By undercutting you remove this acute angle and probably get a reflex angle - cracks don't like the pointy end of reflex angles - and so you have a very small stress raiser. Or it could be something else. Roger Quote Link to post Share on other sites
Motorsport Mickey Posted November 5, 2015 Report Share Posted November 5, 2015 (edited) As he often is Roger "is on the money". Having the crank bearing surface undercut slightly (we are only talking minimal amounts) doesn't reduce the crank strength enough to worry about in these large diameters, and the benefit of eliminating or vastly reducing the propensity of cracking or crank breakage by doing it, easily outweighs it. Having an undercut removes the necessity to match the bearing dia into the arc of the rad (they will be done by hand) exactly because if not ground well there may well be a "corner" left with relatively sharp edges (seen these quite often) which may as well have a "tear crank across the perforations" instruction supplied with it ! Also the crank will probably have the ability for at least another crank regrind before the undercut would need renewing. No idea why it was not done new but the cranks were a common old everyday item when supplied and the factory would have 3 kinds of fit if they saw the power output increases (over 100%) we can get out of the engines by careful machining and the occasional replaced quality item to help get there, oh...and running -60 thou big end bearings (deliberately)in race trim, these cranks are tough ! The extra machining process of the undercut will take all of about 5 mins if incorporated into a crank regrind but originally the factory will probably not have seen a necessity for it. Mick Richards Edited November 5, 2015 by Motorsport Mickey Quote Link to post Share on other sites
Peter Cobbold Posted November 5, 2015 Report Share Posted November 5, 2015 Not a lot of load on the front m/b I'd have thought. Almost all the torque goes backwards, apart from that needed to drive the cam and auxilliaries, guessing maybe 10-20hp. So as Mick says the factory may have saved 5 mins' machining per unit.... I suggest legitimately. Peter Quote Link to post Share on other sites
RogerH Posted November 6, 2015 Report Share Posted November 6, 2015 Hi Pete/Mick, another way of looking at it - how many cranks have failed due to the front end falling off. I know of none at all. Roger Quote Link to post Share on other sites
RAHTR4 Posted November 6, 2015 Report Share Posted November 6, 2015 Hi, I do not think that the "front journal" was a matter that crossed the mind of the Technical Team. Triumph were aware in period of the crankshaft failures - however these all occurred at the rear journal - and Triumph reacted by modifying the crankshafts during 1963. From the Register Archives we have :- Issued by the Engineering Dept., Standard-Triumph Engineering Ltd : 2-10-63 Engineering Change Number EC/6747 Description of Change : Introduction of Fillet Radii Rolling Procedure to strengthen Crankshaft and minimise cracking on No. 4 Crankpin and Rear Main Journal. NOTE :- Crankshafts produced with Fillet Rolling Operation to be stamped after Part Number with suffix /R I am not a machine engineer, but the introduction of a Fillet Radii Rolling Procedure into the production cycle of a crankshaft must have costs an amount of money, so why would they expend more money on something that had not been identified as a problem.? Given the date of the above change, approximately 11,750 TR4's were subsequently manufactured followed by the entire TR4A production run of approximately 28,000 units - so that all mounts up to a large number of "upgraded" crankshafts. The savings from "not" modifying the front journal must have been quite substantial. Regards, Richard Quote Link to post Share on other sites
LGFromage Posted November 6, 2015 Report Share Posted November 6, 2015 Hi Pete/Mick, another way of looking at it - how many cranks have failed due to the front end falling off. I know of none at all. Roger Hi Roger, My original 4A crank failed at the front web starting at the fillet radius so that makes at least 1. Since then I have always observed the red line. Tim Quote Link to post Share on other sites
RogerH Posted November 6, 2015 Report Share Posted November 6, 2015 Hi Tim, was that the #1 big-end fillet radius with the web or the main bearing journal to web - where there is no radius. Hi Richard, Description of Change : Introduction of Fillet Radii Rolling Procedure to strengthen Crankshaft and minimise cracking on No. 4 Crankpin and Rear Main Journal. much the same as above - is this the #4 big-end journal to web (known failure area today) or the rear main bearing journal that is undercut with the web - not a fillet !! Roger Quote Link to post Share on other sites
AlanT Posted November 6, 2015 Report Share Posted November 6, 2015 Interesting they talk about "rolling" not "cutting". Quote Link to post Share on other sites
Motorsport Mickey Posted November 6, 2015 Report Share Posted November 6, 2015 Rolling- Displacement of metal into a web corner (it's adding material) whilst still mallable during the crank manufacture, then the radii machined into the web and blending with the bearing surface. Mick Richards Quote Link to post Share on other sites
LGFromage Posted November 6, 2015 Report Share Posted November 6, 2015 Hi Tim, was that the #1 big-end fillet radius with the web or the main bearing journal to web - where there is no radius. Hi Richard, Description of Change : Introduction of Fillet Radii Rolling Procedure to strengthen Crankshaft and minimise cracking on No. 4 Crankpin and Rear Main Journal. much the same as above - is this the #4 big-end journal to web (known failure area today) or the rear main bearing journal that is undercut with the web - not a fillet !! Roger Hi Roger, That would be a failure at the no 1 big end fillet radius with the web for my car. I understand that this is a less common crankshaft failure than that at no 4 position, but it can and does happen and is no less serious. Tim Quote Link to post Share on other sites
mike3md Posted November 7, 2015 Author Report Share Posted November 7, 2015 Thanks to all for the interesting and sensible comments. Clearly worth considering an undercut if the crank is being reground. Mike Quote Link to post Share on other sites
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