Back Axle Blues

BACK AXLE BLUES Part 2

Mike Cartney

Titanic engineering

The design of rear axle fitted to pre-war Riley cars is of the type known to the black- fingernailed as “three-quarter floating”. This strange description, whose origins are obscure, nevertheless sounds reasonably appropriate, being suggestive of something which is about to sink without trace. It is a term which is unfortunately no help at all in describing the way the axle actually works.

Don’t throw a wobbly (wheel, that is)

In this type of axle, the wheel hubs run on single row ballraces mounted on the ends of the axle casing. Single row ballraces are very good at resisting radial forces, which in this case arise from the weight of the car, moderately good at resisting axial forces and poor at resisting eccentric forces, both of which arise from cornering. In order to resist these eccentric forces (which tend to cause the wheel to tilt over relative to the axle) the half- shafts are made a tight fit in the hubs and take these loads in bending. The half-shafts are therefore subject not only to torsional stresses due to the transmission of power, but bending stresses due to cornering forces. The bearings carry only vertical loads from the weight of the car and the axial component of cornering forces.

Trouble down at ‘t mill (the one at Durbar Avenue, Coventry)

The problems arise with that innocent sounding requirement to make the half shaft a tight fit in the hub. You wouldn’t believe this is particularly difficult, but it is, at least the way Rileys did it. With the benefit of 75 years of hindsight, it is obvious that the easy way to get a tight fit is to use a taper and keyway and pull it up using a big nut, and many manufacturers went down this road with satisfactory results. Rileys, on the other hand, made life difficult for themselves by forming splines on half shaft and hub, presumably in order to transmit the driving torque (we’ll come back to that next month), and force fitting them together. Now if you want to fit a hub onto a shaft so that it is good and tight without using a taper, you need what is called an interference fit, that is, the hole in the hub is smaller than the shaft diameter. When the hub is pressed or shrunk onto the shaft it is forced to expand and so grips the shaft. When the shaft and hub are splined, as in this case, usual practice is to have an interference fit only on the outer diameter of the shaft, the other surfaces being very small clearance fits. This requires the half-shaft and hub splines to be machined to very small tolerances, and it is likely that selective assembly was used. Selective assembly is, as the name suggests, a process of manually matching shafts and hubs so that an acceptable degree of fit, or tightness, is achieved. It is required because in any machining process there will always be some variation, however small, from the designed dimension, and the combination of these errors can adversely affect the fit. (The evidence suggesting selective assembly is the fact that Rileys only sold half- shafts and hubs as assemblies, not separately. They wouldn’t sell you a half-shaft on its own because the likelihood of it fitting your hub satisfactorily could not be guaranteed). Selective assembly is very time consuming and expensive in a manufacturing environment and the degree of success is somewhat variable, depending on the skill and enthusiasm of the people building the axles. The results are apparent in service. Some hubs have remained tight on the shafts to this day, others worked loose quite quickly.

Pin number

As a side issue, if you examine a hub and half-shaft assembly, you will see that there is a pin locking the hub and shaft together. The function of this pin is a mystery. The hub is held on the axle by the bearing and cannot possibly come off unless the bearing collapses. The shaft cannot escape because it is inserted in the hub from the axle side. If the bearing collapses, the hub will come off, complete with shaft, pin or no pin. Answers on a postcard to somebody else, please.

Slopping out

To revert, the mode of failure seems to be that if a hub has a poor grip on the outside diameter of the shaft, driving and over-run forces cause the hub to rotate backwards and forwards on the shaft until the splines make contact in each direction. This causes wear on the cylindrical faces of the shaft and hub and hammering of the faces of the splines. This gradually increasing lateral and angular movement pumps oil through the splines onto the outside of the hub, from whence it gets all over the wheel centres and the outside of the brake drums. The shafts are a bit harder than the hubs, and in the well lubricated conditions pertaining, the principal wear seems to take place on the driving faces of the hub splines, eventual failure being loss of drive. However, in reality long before this point is reached some MOT man will have spotted the play in the road wheel, assumed that it is about to drop off and failed the car. The MOT man may have a point. Single-row ballraces are very accommodating of out of line running, but if the hub gets very sloppy, the bearing will wear rapidly and eventually fail, with the results mentioned above.

Desperate measures

Over the years many Riley owners have faced this problem, and come up with various solutions. Perhaps the most ingenious if not the most successful was the hub dismantled by an SRE member which had lengths of piano wire inserted in the splines to take up the clearance. The solution adopted does tend to depend on how bad things have become. Many years ago, I successfully dealt with the problem on my Monaco by having the shafts built-up by metal spraying, followed by careful hand-fitting, but the degree of movement that had developed in these hubs was still very small. I have also seen a variant of this method, where a run of weld was deposited along the outside leading edge of each spline on the half-shaft and hand filed back to fit. There are various methods of “shrinking” the hubs, and I have seen examples with evidence of peening and hammering, presumably whilst red-hot. This approach can be successful if combined with new half-shafts, but again, is only applicable to hubs with a very small degree of wear.

Hold tight

How tight is tight? This metaphysical sounding question probably has a range of answers, but my own conclusion, which does not necessarily agree with what Rileys did, and may differ from other peoples’ views, is that you need at least one thousandth of an inch interference fit on the outside diameter of the splines. This should translate into a minimum of about one ton peak force on the press when driving the shaft into the hub. Anything less than this is unlikely to remain tight for very long. This means that you won’t be able to do this job in the home workshop, but it does give the hand-fettlers and shrinkers something to aim at. This degree of fit also applies if replacing a broken half- shaft in an existing hub. If you cannot get a ton on the press, it isn’t tight enough.

A practical upper limit for the interference fit would be .002” or 2 tons on the press which stays comfortably inside the limiting tensile stress in the hub assuming it is mild steel or something not much better.

Overcome by fatigue?

When things have been allowed to get a bit worse, it is tempting to consider welding as a solution. The obvious objection to this is that it could become almost impossible to separate hub and shaft in the future, and a broken half-shaft will result in the whole assembly being thrown away. If the hub is badly worn, you may take the view that it is scrap anyway, so nothing is lost. However, in this case, it will be necessary to weld at both the inside and outside faces of the hub in order to distribute the load and I would be very cautious about welding at the inside face of the hub. Half-shafts in Riley axles are very highly stressed anyway, and this is the point where the bending stresses in the half- shaft due to cornering are at their maximum. Welding is likely to introduce unquantifiable residual stresses and metallurgical changes in the half-shaft at this critical point which may increase the already-present tendency to fatigue failure. I am aware that this solution has been adopted on at least one car, so far without problems, but time will no doubt tell.

One situation where welding at the outside end of the hub can be successful is where the hub and half-shaft have an interference fit but less than the one thou minimum. Lay a generous bead of weld at the visible interface between shaft and hub. No adverse fatigue problems should arise as the stresses in the half-shaft are much lower at this point. The weld can be ground away to remove the shaft, should this become necessary in the future.

If all else fails.

I understand that at the time of writing new Rudge Whitworth type rear hubs can be obtained, but not the earlier bolt-on type. If using new hubs, it is an extremely good idea to buy them complete with new half-shafts fitted by the supplier, not only because the shafts are unworn and therefore more likely to be a tight fit, but also they are “zero hours” in respect of fatigue. The one-ton press force requirement also applies, and my advice is to quiz the supplier on this point.

If your hubs are worn, and you can’t get new ones (or better second-hand ones), what do you do? You wait for next month’s gripping episode, that’s what!