Tuesday 23 February 2021

The Clutch: Part 1

A complete clutch came with the project bike and can be seen attached to the gearbox in the “as received” picture below.


My experience of clutches fitted to early AJSs is that they are marginal in their ability to handle the power of the engines. On both the AJS K7 and AJcette bikes that I had built previously I had found that the clutch slipped under power at high engine revs. These clutches were new in every respect and identical copies of the originals but the number of plates fitted (one behind the chainwheel and one in front) is, in my view, insufficient for a powerful OHC engine. Not wishing to make new clutches for a second time for these bikes I solved the problem by increasing the spring pressure and although both clutches will now take full power without slipping the increased pressure results in a heavier clutch.

The original Sturmey Archer clutch fitted to this gearbox has another couple of plates compared to the lightweight singles however it is still a pretty puny clutch and I decided to design and make a new one that would take the power from this V-Twin engine.

As I have mentioned in previous blogs, I often design and make parts “on-the-fly” in the workshop but, here, I needed to at least sketch the bare bones of the complete clutch as there are a few design constraints that need to be accounted for. In particular, I planned to use an off-the-shelf bearing between the clutch centre and the chainwheel rather than the lose rollers of the original and the chainwheel sprocket needs to end up in the correct axial location on the gearbox mainshaft to align with the (not yet made) engine sprocket and the (also not yet made) crankshaft.

Although some details changed slightly during machining, this is essentially the design that was adopted.

I decided to proceed in the following way:

1)    Select a suitable bearing. I choose an NSK 6812ZZ 60mm x 78mm x 10mm which is good quality metal shielded deep groove ball bearing.

 2)    Use a triplex sprocket with an adequate OD between the rows of teeth that will accept later Norton clutch plates. Interestingly, the friction plates on a Norton Dominator clutch from the 1950s are the same as those on a 1930s Sturmey Archer clutch ….Norton didn’t change much after they bought the design rights to Sturmey Archer gearboxes. Machine off 2 rows of teeth, bore the centre and mill slots for the clutch plates. Machine the other end to accept the bearing and make a flat plate cover to protect the clutch innards from road dirt.

 3)    Machine a clutch centre to fit the existing gearbox mainshaft taper, spark erode the keyway, mill slots for the plain clutch plates, thread ¼” BSF for the 6 clutch spring studs and machine the diameter on the inner end to support the bearing.

 4)    Make new longer studs to support the springs and spring holders.

 5)    Bond friction material onto the chainwheel inner surface and on both sides of new friction plates.

The first step was to order the various materials for the main parts, shown below.

The material for the triplex sprocket and the large round piece of steel from which the clutch centre would be made are both EN8 which is a good medium strength steel and quite adequate for these components.

In the end, I didn’t use the upper plates shown in the picture but ordered a set of 3 Surflex plates which are of better quality.

The first step in making the chainwheel/clutch basket is to machine off the hub of the triplex sprocket. I have made a number of clutches using this method in the past and found that the hub is actually welded onto the 3-row sprocket, ie what appears to be a single, large piece of metal is in fact 2 pieces welded together. This is shown clearly in the picture below with the sprocket held in the lathe chuck on one set of teeth and sufficient material has been machined away such that the hub has detached from the sprocket – it literally fell off!

It’s useful to be aware of this to avoid some nasty machining accident!

I have found in the past that the centre hole/hub/sprocket are not usually concentric by quite a significant amount and the only foolproof way of setting this up for machining is to check by using a dial gauge, carefully, on the teeth. It is also much easier if the number of teeth is divisible by 3 as the sprocket can then be mounted in the 3-jaw chuck; the alternative is to use a 4-jaw check. If this is not done, then the sprocket will end up being mounted and machined eccentrically and it will subsequently be impossible to set up the chain tension properly.

After machining off 2 rows of teeth and boring the inside the main body of the clutch is finished

and, after boring the other end for the bearing


 it is set up on the rotary indexing table on the milling machine to machine the slots


The picture below show the finished clutch housing together with the original.


The completed chainwheel, together with the new friction plates, was then sent off to have friction disks bonded onto the surfaces, specifically, a 2mm thick disk on the chainwheel and 2x 1.5mm thick on each friction plate. I have used the company Saftek in Cleckheaton for many years for all friction material and always been very happy with their work. 


Finally, the friction plates and the plain plates were inserted into the new chainwheel to check that all the dimensions had been calculated (and made!) correctly …which they had.


The second part is to make the inner core of the clutch; this is a bit more complicated than the chainwheel.

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