Saturday 22 May 2021

The AJS V-Twin Crankshaft: Part 5 - Mainshafts

The nominal dimensions for the mainshaft lengths can be readily calculated from the locations required for the timing-side pinion to the camshaft drive on the timing side and the final drive sprocket on the drive side. The inside diameters for the mainshaft and big-end in the flywheels had already been determined and reamed to 29mm and 1.000’’ respectively.

I had also decided on a 40 taper angle for the final drive sprocket and to not key the sprocket to the shaft. I discussed the question of keyed versus non-keyed final drive sprockets at length with Max Nightingale at Alpha Bearings many years ago and he was of the opinion that non-keyed is a better solution. Why? Well, because the loss of frictional contact surface area by the presence of a keyway on the tapered shaft is not compensated by the shear strength of a key. Velocette clearly came to the same conclusion in the 1920s because Mk1 OHC Velos also do not have a keyway to fix the externally-splined shock-absorber body to the crankshaft.

There are two main decisions that now need to be made before the mainshafts can be designed and manufactured. In no particular order, these are:

1)    Should the mainshafts be surface hardened?

2)    What should be the interference fit between the mainshafts and the flywheels?

I had originally planned to finish the mainshafts to size and then have them nitrided rather than going the more traditional route of using a carbon steel and then case-hardening the surface and grinding to size afterwards; this is why I had chosen EN40B steel. Nitriding is carried out at a much lower temperature of ~ 540 0C rather than the ~800 0C required for hardening carbon steel.

However, there was a flaw in this plan that I was not initially aware of. It turns out that, in addition to the desired diffusion layer of hard nitrides, the nitriding process also produces an unwanted “white layer” or “compound layer”. This white layer is undesirable because it can flake off and, because it is hard, could cause damage elsewhere in the engine and would also result in dimensional changes of the shaft. For anyone that is interested, there is more information here and here.

I contacted a supplier of gas nitriding and they stated that the thickness of the white layer with their process is typically 15 microns. This is 0.0006” in Imperial units and, on a shaft, would equate to a change in diameter of 0.0012”. As I was planning to have a tolerance of 0.0005” for interference fits and even less for the bearing fit on the shaft – a tight sliding fit - I decided not to use nitriding as a surface hardening process.

It is not critical that the crankshaft mainshafts are hardened. Unlike, say, gearbox mainshafts, they are not rotating with respect to another component and so in-service wear is not really an issue although if there is repeated assembly and disassembly then the shaft could wear. It is unlikely that the mainshaft and bearings would be repeatedly assembled and disassembled, however, there is also the final drive engine sprocket on a taper to be considered and for which hardening would be desirable.  

I had, in the meantime, investigated electroless nickel plating as a possible technique for increasing the surface hardness. This is an entirely different technique to electro-deposition of nickel and produces a thin layer of hard nickel by chemical deposition. More details can be found here. The hardness of the deposited layer depends on the composition of the aqueous solution used and, in particular, the phosphorous content - the less phosphorus the higher the hardness. Unlike electroplating, the layer formed from electroless plating does not flake off and has been used in many applications that require wear and impact resistance. Another major advantage of electroless nickel plating is that the deposited layer has a uniform thickness. Finally, heat treatment to increase the hardness of the deposited nickel is also possible.

Rather than send the shaft away to have it plated there are kits on the market that provide the chemicals and instructions to do this in the home workshop. I bought a kit from Delway Technical Services and started some experimentation. In terms of hardness, the deposited layer using this kit is quoted as 600 VPN200. This Vickers harness value translates to a Rockwell value of around 54 HRC. For comparison, the EN40B steel I am using for the shaft is delivered in the “T” condition which has a maximum value of around 32 HRC; my heat-treated O1 tool steel gears for the OHC drive are around 60 HRC. And so electroless nickel plating would increase the surface harness substantially and I would be quite satisfied with a value of the 54 HRC.

There are some additional bits of paraphernalia required in order to start plating – a pan to hold the parts (ideally stainless steel), a couple of litres of purified water, a heater to heat the liquid up to its optimum plating temperature of 90 0C and a thermometer to check the temperature of the plating bath – see picture below.

I first made up a test piece of a similar sized round bar of EN40B to the mainshaft, supported on a mild steel rod, and put that in the plating bath for 1 hour.

The quoted deposition rate is 15 – 25 microns/hour and, as best as I could determine (I can’t measure to the accuracy of microns!) the diameter increased by 0.0015”. This equates to a surface deposition rate of 19 microns/hour. This is useful information because the immersion time can be used to determine the thickness of the layer that is deposited. I don’t have hardness testing at home and have to resort to trying a fine file on the surface for my very approximate “harness tester” but I could tell that the surface was substantially harder than the “as delivered” metal. On this basis, I decided to go the route of using electroless nickel plating for the drive-side mainshaft bearing contact surfaces and taper.

The second main consideration was the interference fit between the mainshafts and the big-end into the flywheels. I spent some time researching this and eventually came up with the nominal values of 0.0035” and 0.0025” for the mainshafts and big-end respectively. How did I arrive at these values? I talked with a couple of people whose opinion I respect – and they each came up with quite different values (!!) and by making calculations of the insertion force that would be required.

For those with a mathematical inclination, there is an analytical solution to the pressure between the contacting shaft and flywheel surfaces. Multiplied by the surface area of contact, this gives the force required to press the shaft into the hole. I have reproduced 4 pages of hand written notes on my calculations for both the mainshaft and big-end.

A couple of points to note: the coefficient of friction is an important parameter and much time was spent in searching available online literature to come up with an appropriate value; the necessary mechanical properties of EN24T (the flywheels) and EN40B (the mainshafts) are very similar and a simplification is then possible in the equations.

The required force to press home the mainshafts and the big-end came out to be around 12 tons and 7 tons respectively. My hydraulic press has a capacity of 20 tons and has a pressure gauge and so it will be interesting to see what pressure is actually required.

Drawings were made for the mainshafts, shown below

and machining was started. The picture below shows both shafts after completing the lathe work. There is excess material on the left side for holding and excess thread on the timing-side mainshaft to allow for a centre.

The surfaces have been machined to 0.008” oversize to allow for grinding. I have used an outside machine shop, Jayess Tools, for grinding as I do not have a cylindrical grinder in my workshop. Prior to grinding, oil transfer holes (one axial and 2 radial) for the oil feed to the big-end were drilled in the drive-side mainshaft and  tapped at the end to allow a high-tensile cap screw to be inserted for blanking off the drilling. An allowance of 0.002'' on the diameter (0.001'' thickness on the surface) was allowed for subsequent plating of the mainshaft.

The final operation was to use the electroless plating bath to put a hard nickel surface on the drive-side mainshaft. I did not want to plate the area in contact with the flywheel for the interference fit as this had already been finished to size and I found that workshop blue-roll (duct tape) can be used to effectively mask an area that doesn’t need plating.

This was then put into the plating bath for just over 1 hour

to deposited the required thickness of nickel

The mainshafts are now finished and ready for assembly into the flywheels.


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