Author Topic: Repairs to camshaft and Crankshaft taper  (Read 7348 times)

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Offline BARRYM

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Repairs to camshaft and Crankshaft taper
« on: 16 Sep 2010 at 22:36 »
Hello All. Thanks to Doug for speedy reply regarding Brake drum plates and footrests on Douglas SW5 And to Roy I have added my location and a few more details.
Next problem to get help on is I have two crankshafts to choose from but both have damage to the flywheel taper and the corresponding taper in the flywheel itself, this having been caused in the past by the flywheel coming loose and taking the woodruff key around a good few times. If my lathe was larger I could bore out the flywheel and re-bush it, so that can be handled by someone with a larger lathe; but what is the best solution for the crankshaft taper I have heard varying reports on the success of welding up with hard steel and regrinding. This in turn leads to the next problem- that being both camshafts I have are badly worn with probably  only one lobe on one cam still near to its original profile, one camshaft is totally worn through the case hardening on two lobes probably through triple valve springs having been fitted at some period. Is there information out there on the details of the SW5 camshaft profiles,lift, e.t.c. and advice on the best method to renovate the camshaft? BARRYM.

Offline Doug

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Re: Repairs to camshaft and Crankshaft taper
« Reply #1 on: 18 Sep 2010 at 00:32 »
Barry,

Both welding up the taper and building it up with electroplate have been tried. From what I heard the weld leaves the surface too soft (the original surface is case hardened) and they are prone to fracture where the weld stops. This make sense as besides the transition in metallurgy, the case hard surface adjacent goes through a rapid heating and quenching (from the mass of the journal) leaving an abrupt transition from dead soft to dead hard. And that creates perfect place for a stress riser to initiate a fatigue crack.

A friend tried hard chrome plating to get around the deficiencies of welding. This lasted two years of intermittent sprinting before it snapped the taper off. Oddly enough the flywheel did not go belting off but stayed with the bike. The shaft broke clean at the top of the taper where the plating stopped. It seems again the likely culprit was a stress riser. Aggravating the situation was the low coefficient of friction of the chrome, which made it difficult to ever get the flywheel taper stuck dead tight. It seemed tight, but probably was able to shuffle slightly.

Depending on how badly torn up the surface is, you may be able to get it to clean up 80-90% by skimming the taper smaller. You will need to grind the taper or turn it with a CBN tipped tool. Some bad tapers and other crankshaft ills can be seen in this post:

https://www.douglasmotorcycles.net/index.php?topic=402.0

Before going to the trouble, have the crank Magnaflux tested for cracks, particularly cracks from the corners of the Woodfruff key seat.

For the flywheel, I would be more inclined today not to sleeve it but build the inner diameter up with weld and re-cut to an undersized taper to compensate for that removed from the crankshaft. The flywheels were not heat treated (though the taper probably would have benefited from case hardening.) You could use hard-face or Stelite filler metal, but probably regular mild steel MIG welding would be good enough and would put the minimum amount of heat into the hub. I have not had to do this myself, because the job was always done in conjunction with machining a new flywheel; which was just made to suit.

Camshafts. Well which one do you have fitted? Douglas used several different timings, the IN.OP.20 and 10.55.63.20 are the most commonly seen. I have a drawing for the IN.OP.20 cam. These were used in the 1934-35 OHV road machines and give a nice tractable engine on a single carb. And of course they probably gave good torque out of the turns on the cinders. I have been told they tend to have too much low-mid range power for sprinting effectively (the bike wants to rear-up), and that better overall results (shorter times) can be had with timings that sacrifice low end for better high end breathing. But for a 3-speed road bike torque is probably more desirable.

Plenty of folk have welded up cams and had them re-profiled. Not sure what they used, and maybe neither did they! Cam life was probably not a major concern. I have heard of several folks in the UK using a material call (I think) Comalloy. Some sort of carbide micro-spheres dispersed in a bronze binder. It does not seem to be availbe in the USA. It is applied as a brazing operation. I can hardly believe it works, but I know several that swear by it. I would not mind finding out what would be best along the likes of a welding alloy, as I have some camshafts that could do with a weld and a regrind. You will also likely find the tappets are knackered as well. Presumably these should be welded up with a slight different alloy to avoid the engineering no-no of rubbing like materials together. If you have access to a medium to large surface grinder, there is a fixture you could build that would allow you to grind your own camshafts. The principle is like the rear swing arm of a motorcycle.

-Doug


Offline eddie

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Re: Repairs to camshaft and Crankshaft taper
« Reply #2 on: 18 Sep 2010 at 06:35 »
Barry,
          I go along with what Doug has just said - only get the crank welded as a last resort. My 600 sprint engine had a damaged taper on the crank - both scored and bits out of it where the flywheel had fretted and welded itself to the crank. I set the crank up in the lathe and reground the taper to get rid of the scoring but leaving the deeper pits - then made a new flywheel hub to suit (we use smaller dia flywheels on our sprinters - evidently, the tendency to weld the flywheel to the crank has something to do with resonant frequency of the flywheel and crank - change the diameter of the crank and it doesn't happen!).
       When refitting the flywheel, machine the tapers as accurately as possible and then carefully lap the two together - a taper must fit perfectly - there is no such thing as it almost fits - it either does or doesn't. To this end, I prefer to leave the key a little slack - to make sure the taper is not held off by the key. The taper should do the driving - the key only locates it.
         Regarding the building up of camshafts: when I built my sprinter, following advice from Henry Body, I built up a camshaft with mig weld (just plain mild steel) and ground it to the correct profile but about 20 thou undersize. I then coated the lobes with 'Stellite No 1' (applied with oxy-acetylene) and reground to the correct size. This coped with 3 seasons sprinting before wearing through the Stellite coating. This cam ran with standard Douglas followers. I have since produced my own camshafts from EN36 steel which I now run with followers tipped with Stellite. As Doug says, it is important to used dissimilar metals when they are rubbing together under load. I have heard of Colmonoy being used under similar conditions - from what I am lead to believe, it is similar to Stellite but less expensive.
       By the way, I built my own rig to regrind camshafts. It is just an 'H' shaped frame that carries the new camshaft in bearings at the top and the pattern cam at the bottom. This, then rocks about it's centre, with the cams being rotated by light chain driven slowly by a windscreen wiper motor. The pattern cam runs against a roller the same dia as the grinding wheel used to profile the new cam. Just the simple rocking motion faithfully reproduces a copy of the pattern part. With light pressure applied by a spring, this rig can be left to get on with the grinding whilst you attend to something else!
      If Doug's drawing isn't suitable for your purpose, I can probably find a reasonable standard cam for you to copy ( I say standard, but there are several different cams used in the OHV motors - some had different timing and some had 6 splines instead of 8).
               Good luck with the rebuilding,
                                 Regards,
                                      Eddie.

Offline eddie

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Re: Repairs to camshaft and Crankshaft taper
« Reply #3 on: 18 Sep 2010 at 11:17 »
Sorry Barry,
                    Just noticed a couple of mistakes in the previous posting. It should have read "Change the diameter of the flywheel" - not crank!
        Also, the last line should read "6 splines instead of 8" - dont know how the smiley face got in there!

                          Eddie.

Offline BARRYM

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Re: Repairs to camshaft and Crankshaft taper
« Reply #4 on: 18 Sep 2010 at 21:28 »
Many ,many thanks to Doug and Eddie for their concise replies that have given me a wealth of valuable information and will steer me away from welding or even chrome plate building the crank taper as it seems a bad road to travel indeed. After looking through the excellent photos of the new made crank assembly from you Doug I feel a bit more capable of taking my two crankshafts apart and in view of the difficulty of repairing the worst one of the two I just may be able to get one decent crank set up out of the two; as one of them compared to the photos in your reply is not as bad as I had decided. The camshafts I have are both 10-50-63-20 with eight splines Eddie, so if you have a good one of this configuration, I may, when I get to this point in the restoration get back with you. The Douglas will only be used for enjoyment on the road and I plan to run on the original beaded edge 28" wheels as I want that original lean low look that these SW5's had. At 72 years old I don't plan on any fireworks!
I bought the Douglas in 1960 along with a Hartley tuned circa 1925 500cc Ariel, plus another one as spares for 10  which does seem a bargain now! I took it apart almost thirty years ago and am having quite a puzzle remembering how things go - so there will certainly be more postings. Barry

Offline Doug

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Re: Repairs to camshaft and Crankshaft taper
« Reply #5 on: 19 Sep 2010 at 02:34 »
Barry,

I have to point out a misconception. The crankshaft in the post I referred you to is not newly made but a cleaned up and refurbished original. The connecting rods are new manufacture. 

-Doug

 

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