Tuesday 1 June 2021

The AJS V-Twin Crankshaft: Part 6 Assembly and Assessment

Before assembling the crankshaft there were a couple of final machining operations.

The first was to machine the final drive sprocket and, in particular, to ensure that the 40 taper positioned the sprocket in the correct place on the mainshaft. I had previously set up a dummy shaft through the main bearings to determine the axial position of the sprocket to ensure that there was clearance between the chain and the crankcase casting and to position the clutch chainwheel.

The taper was then machined on the actual mainshaft to position the sprocket correctly.

It is much easier to measure and check the machining of the sprocket taper in the lathe with the mainshaft alone rather than when inserted into the flywheel. The change in axial position for a change in the radial cut is determined by 1/(tan(taper angle)) which is 14:1  -  in other words, a 0.001” increase in radial cut will move the sprocket 0.014” along the shaft and so care needs to be taken to avoid taking off too much material and positioning the sprocket too far inboard.

The collection of bits and pieces that is now just about ready for assembly is shown below.

Before starting to assemble the various parts, there was one final check to be made and this was to carefully measure the 3 main dimensions that would determine the crankshaft end-float; these are shown in the sketch below.

Here, A and C are the widths of each of the flywheels between the mainshaft boss and the big-end boss and B is the width of the crankpin between its contact surfaces with the big-end boss. Clearly, A and C should be the same and the sum A + B + C should be equal to the distance between the main bearings minus the desired end float. A few thou were removed from each mainshaft boss to satisfy A = C and to simultaneously provide 0.010” end float.

The mainshafts were then pressed into the flywheels.

The pressure on the hydraulic press was monitored during pressing and rose progressively to a value of around 12 tons - very close to the theoretical calculated value. The mainshaft oil holes were lined up carefully prior to pressing and checked immediately to ensure that there was a flow connection between the oil hole and the big-end drilling. A small amount of lubricant (Loctite bearing fit) was applied prior to pressing. I believe that a certain amount of broaching occurred when pressing in the drive-side mainshaft and that this is the result of the high interference fit; more will be said of this later.

The next step was to press the crankpin into the drive-side flywheel, again ensuring that the oil holes lined up. Again, the maximum pressure required was very similar, around 7 tons, to that calculated.

The connecting rods and bearings were then assembled onto the crankpin

making sure that the connecting rods were the right way round (!!) and the pin and timing-side flywheel were pressed together.

To help with alignment, a silver steel (drill rod in the US) ground 12mm rod was inserted through both flywheels prior to pressing.

 The crankshaft was then immediately set up in the crankshaft measurement jig

to check the runout.

I couldn’t have wished for a better result: both the drive side and timing side mainshafts showed a deviation of maximum 0.001” adjacent to the flywheels

and no greater than 0.002” at the far end of the drive-side mainshaft.

I was extremely happy with this result.

The dodecagonal big-end nuts were then tightened to 150 ft-lbs torque and the runout was rechecked to ensure there was no change.

Some final measurements were made:

Total width between mainshaft bosses = 4.118” – 4.120”

The measured distance between the main bearings is 4.128” and this gives the desired 0.008” - 0.010” end-float.

Total mass = 26 lbs

Apparently the crankshaft in the original 996cc AJS V-Twin weighed 42lbs according to this article so maybe my crankshaft can be considered a mere lightweight…In fairness, the OHC R10 engine, on which the original V-Twin was based, had a stroke of 101 mm and this would have invariably resulted in larger diameter flywheels.

The actual balance factor can be calculated by suspending a mass from the small end of one of the connecting rods so that the rotating crankshaft is in perfect balance, that is, it will rotate without stopping in any prefered position.

To achieve the design balance factor of 59.2% the mass to be suspended from the connecting rod is calculated as follows:

Total Reciprocating Mass = (Mass of Con-Rod Small Ends) + (Mass of Pistons)

= 418 + (373.2 x 2) = 1164.4g

Mass to be added for perfect balance should be

= Reciprocating Mass x Balance Factor – Mass of Connecting Rods Small Ends

= 1164.4 x 0.592 – 418

= 271g

However, the actual mass that was added to achieve perfect balance = 338g

It would therefore seem that an additional 67g needed to be added compared to that required for a 59.2% balance factor.

This means that the flywheel has been “underbalanced” compared to the design value.

The actual balance factor can easily be calculated to be 56%, which is 3% less than the target value.

Does this matter? Not really. From the discussion in a previous blog, V-Twin balance factors can be anywhere in the range 50 – 60% and so 56% should be quite OK. Why is the value different from that calculated? I have added that to one of the points under Lessons Learned.

Lessons Learned

Designing and making the crankshaft has probably been the largest and most challenging sub-project in the overall V-Twin project and has taken many weeks in design and in the workshop. There were no diasters during the manufacture and no aspect really went badly and I have ended up with a crankshaft that is strong and fulfills the accuracy and balance requirements.

Nevertheless, it is useful to reflect on what has been learned – what went well and things that I would change if were making another crankshaft. I believe it is the Spanish philosopher George Santayana that is credited with the aphorism “Those who cannot remember the past are condemned to repeat it,” …variants of which are also attributed to Edmund Burke and Winston Churchill.

Things that went well…

1)  The use of the Harley Davidson EVO connecting rod and big-end assembly was a good choice. This was bought for a remarkably good price, it is of excellent quality and fits the bill perfectly.

 2)  Design of the flywheels, in particular the balance calculations, worked out well and with a good engineering solution. If I was starting this project again I would automate this process either using a spreadsheet or, better still, use CAD rather than making laborious repetitive calculations as quite a few design iterations were necessary.

 3)  The introduction of a 12mm hole in both flywheels diametrically opposite the crankpin proved invaluable in the accuracy to which the second flywheel and crankpin could be pressed together using a piece of ground silver steel as a guide dowel. No changes or adjustments were needed to the alignment after pressing.

 4)  The use of electroless nickel plating on the drive-side mainshaft worked well. It is straightforward to use, the deposition rate can be checked easily with a test piece and is sufficiently slow that a known thickness can be deposited in a given time period and it provides a thin hard protective layer.

 5)  In the absence of accurate internal grinding, the use of parallel reamers to finish the mainshaft and big-end holes worked well. The only disadvantage is the cost; the 2 reamers that I used were purchased especially for this project and cost 230 GBP for the pair; quite a lot of money to make a couple of holes.  

 Things that I would change….

1)   The interference fit of 0.003’’ – 0.0035’’ for the mainshafts in the flywheels is too great and, as mentioned previously, has probably resulted in some broaching on the drive-side. I would reduce the fit to 0.002’’ -  0.0025” in future.

 2) The initial decision to use EN40B and nitriding as a hardening process was taken on the basis that the shafts could be machined-to-size prior to a relatively low temperature heat treatment process. Unfortunately this did not account for the white layer formed during the nitriding process, which I became aware of later, and which would need to have been removed with a consequential change to critical dimensions. If I were making these shafts again, I would use EN24T as it has similar mechanical properties and is readily available and to apply electroless nickel plating.

 3) The balance factor turned out to be 3% lower than the design value. Having rechecked my calculations I am almost certain that the discrepancy stems from not accounting accurately for the radii in the annular segment – shown below

This reinforces the desirability of automating the balance calculations where all details are accounted for properly.  

In summary, I am pleased with the crankshaft and, although there is one final check to make, namely the piston height relative to the top of the cylinder, I am confident that it will perform well in the engine.

....and finally.... In one of my previous blogs on the crankshaft I included a picture of the "as received" crankpin with a taper for entry into the flywheels and a reground crankpin with a parallel fit. As they were both in the same picture the curious might have asked "why do you have 2 crankpins?". Well, when I was checking the rod assembly prior to pressing in the crankpin I found that one roller from the bearing on the forked rod was missing. I searched everywhere for the roller but couldn't find it ....the workshop fairies had hidden it somewhere.

As it is possible to buy standard sized rollers from bearing supply companies I checked the size to replace the missing roller but it is a non-standard size and so this was not an option. There was no alternative but to order a second complete connecting-rod and big-end assembly ...just for one roller!

A day or so later I noticed what I thought was a shiny piece of swarf of the floor. No, the workshop fairies, who obviously have a sense of humour, had returned my single roller. And so I now have a complete spare rod/bearing assembly for a Harley Davidson EVO engine. I had never planned to build a second engine but I'm certainly acquiring enough parts for one.

No comments:

Post a Comment