Tuesday 2 March 2021

The Clutch: Part 2

The second part of the clutch to be made is the clutch centre. This was made from a piece of EN8 round bar and the first machining operation was to make the taper for the gearbox mainshaft and the diameter for the bearing inner race. Narrow angle tapers are notoriously difficult to machine because the male and female tapers will not bind together if there is even the slightest difference between the male and female taper angles. The nominal taper angle, which was  4.50  here, can be determined by measuring the shaft and then set on the top slide on the lathe for machining but a deviation of even 0.10 will prevent proper contact. The only way that I have found to machine narrow angle tapers successfully in what is essentially a reverse engineering operation is to use engineers blue on the shaft/hole contact surface to determine if the angle needs increasing or reducing and to then make very small adjustments during the machining operation and try again until a good tight fit is achieved.

The axial location of the clutch centre on the shaft is obviously important as this is one of the determining dimensions for the chain alignment and because continual attempts at machining the taper would change that dimension the sequence of machining here is: 1) get the taper angle correct; 2) face off the end to get the correct axial location of the clutch centre on the shaft; 3) machine the diameter for the bearing inner race (0.001” interference fit).

The subsequent machining operations are relative straightforward, namely:

1)    Bore the inside diameters

 2)    Drill and tap the 6 holes for the spring holder retaining studs

 3)    Machine the grooves for the plane plates


and, finally, a trip to my spark eroder to make the keyway.

The comparison between the original clutch parts – chainwheel and centre is apparent.

New, longer, studs for the springs were made in EN24T

The spring-holding pressure plate is now at a much greater distance from the “mushroom” that is inserted into the end of the gearbox mainshaft and operates the clutch.

There are 2 possible solutions for this: 1) make a new mushroom that has a significantly longer shaft than the original or 2) modify the spring holder so that the original mushroom can make contact with an extension in the centre of the spring holder. I chose the latter because the shaft diameter of the mushroom is only ¼” and having over 1” of unsupported shaft pressing against the spring holder would be a poor design.

The original pressure plate, shown below in the lathe,

was bored and threaded to accept an insert that was silver soldered to the centre

A new threaded centre was made from silver steel and hardened and tempered at the contact end

And this completed the machining for the clutch centre adjuster/extender

All of the parts have now been made for the clutch and it was time to assemble it for the first time. The bearing was first pressed into the chainwheel and then the centre was inserted and pressed into place.

The change in appearance is a result of chemically blacking the centre. The backplate was now put on and the two main clutch components are now fixed together via the bearing.

After checking that they rotated freely and without interference, the assembly could be put onto the gearbox mainshaft and the plates and springs etc. inserted. There will certainly be some adjustments needed to spring pressure in due course and a locknut put on the centre adjuster but the chainwheel is, encouragingly, in exactly the right place for alignment with the crankshaft sprocket. I also noted that the pressure plate pulled off very evenly when the clutch was actuated so I am hoping that this will result in a very effective and relatively light clutch.

Last, but by no means least, a puller was made so that the clutch centre could be released from its taper on the gearbox mainshaft.



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