Layrub Couplings

[David Morris]

Hi all,

Recent discussions with Moises and his problems with steering arm contact with the top of his crankcase reminded me that the Javelin engine is deliberately set to be lower at the rear, shown in detail by the engineering drawings Keith kindly attached to the various posts on this subject. The engine sits at an angle of about 3.5 degrees to the horizontal. Why?

I think the answer is due to the Jowett use of Layrub couplings. Looking at the data sheet for the couplings, they should not exceed around an angle of around 3 degrees offset from the horizontal. They can go as far as 8 degrees, but only momentarily. There are three Layrub couplings in the Javelin drive train. I suggest that the one behind the gearbox never has to suffer much angular deflection and the same goes for the one by the centre bearing. The propshaft is kept horizontal or at least within the 3 degrees allowed by the Layrub specification.

However, the Layrub coupling just in front of the rear axle pinion has to experience much more than 3 degrees. I believe it routinely runs at around 6 degrees from the horizontal, probably much more on rough ground that the Javelin was designed to cope with overseas.This is not good engineering practice and I suspect Jowett's knew this to be the case. Hence the tilting backwards of the engine, to try and mitigate this angular displacement. I suspect Javelin owners routinely find that this rear Layrub is the one that fails most often?

Probably Jowett's should have used a Hardy Spicer joint here? I am not a Jupiter enthusiast, but don't Jupiters use a Hardy Spicer coupling here?

Jowett's were always keen to shave costs and probably had a supply of Layrub couplings available at a good price? I managed to source some, in bright blue and brand new, from a car show at a knock down price because nobody knew what they were? The ones used on the Bradford are much smaller.

All the best,

David

[Keith Clements]

Somewhere Ì read that Ĺayrubs need an angle and should not be straight through. Of còurse, there is an angle which is too big and one where power loss is minimal. The spec sheet for the Twiflex Layrub says running angle should be 3.5 deg and max 8 deg.

The rear prop has to cope with about 8 inches of suspension travel and thus the length of the rear prop keeps the max angle within limits.
Normal loading of the car should keep the Layrubs at the optimum angle which is possibly 3 deg.

[David Morris]

Hi Keith,

Thanks for your email. As you mention, the manufacturers of our Layrub couplings (Twiflex) give quite comprehensive guidance on-line for our couplings, which from their extensive range appear to be the 'Two-Four' series. They give advice on the 'normal' operating deflection angles, which are up to 3.5 degrees mis-alignment, with momentary mis-alignment of up to 8 degrees.

I suspect that the rear coupling experiences rather more than the 8 degrees at the extreams of the axle movement, leading to premature failure. I am sure Jowett's knew this and just kept their fingers crossed!

All the best,

David

[Forumadmin]

The rear prop is about 120 cm long and the travel is 10 cm in each direction from when car is normally loaded. Using my elementary trigonometry that gives an angle of deflection of 4.7 deg. So well within spec.

[David Morris]

Hi Keith,

Agreed, but 4.7 degrees is outside the running spec of 3.5 degrees and would be in addition to any displacement from horizontal when the car is unloaded.

Let's just agree that a Hardy Spicer here would probably be a better engineering solution? All water under the bridge now.

All the best,

David

[Keith Clements]

The layrub has some advantages over the Hardey Spicer with regard to vibration absorbtion and off axis loads.
Replacement requires a splined telescopic shaft and regular greasing.
The amount of time the joint runs out of spec, that is over 3.5 deg , is minimal, even in rallying conditions.
Layrubs are remarkably reliable compared with CV and Hardy Spicer joints.