Prop Shafts

[pugwash]

i have lots and lots of propshafts at home for some reason- all different.

The two with the most striking differences are from a Stage 1 V8 and a Range rover with a TF727

Ignoring the fact that the lengths are different, the stage 1 prop weighs a ton, is double cardon, has a pretty long slip joing on it, and has an OD of about 3" but seems to be hollow.

The prop from the TF727 has no double cardon and seems to be made from a single piece of 1 1/2" solid bar with what looks like slip joints at either end.

So- which ones stronger. For clearance purposes i would really like to use the TF 727 jobby, but is it man enough for the job? what gives things like prop shafts their strenght? is it the amount of material in the shaft or the outside diameter of the material (ie the wider the prop the stronger even if it has a big hole in the middle!)- will a bar weighing say 1lb and being 12" long be stronger if it is solid or has a chuhking great hole and a large OD?

Am i right in thinknig that double cardon front props reduce prop vibration and that's about it?

[Steve Hiatt]

I have a TF prop on the front of my RR and rear of my indy racer. I've never had a problem with either with a 383 Chevy.

[bishbosh]

There are two issues to look at here, ignoring the strength of the UJ's etc:

1. The torsional strength of the tube or bar and

2. The resistance to local buckling.

A tube is a more efficient use of material than a bar for the same given strength i.e. the tube will weigh less. But, the tube can have a thin wall and still carry the torque loads. What you have to then worry about is a local impact from a rock or similar which could potentially cause the wall of the tube to buckle and the prop destroy itself.

I have had solid and tubular props on the RR. A both were designed for that vehicle they both performed faultlessly as you would expect. I think your problem will be that you will be using an uprated engine and gearboxes so you will need to know the capacity of your prop. Time to get measuring and get someone clevery than me (shouldn't be hard ) to do some maths....

The long slip joint on the Stage 1 prop is for leaf sprung suspension. Not needed (to the same degree) on a coiler with the axle moving in an arc.

[Astro_Al]

Uh Oh!

It's the tube vs bar debate again...

It's related inextricably to mass. If you don't care about mass, bar is always stronger. If you care about mass, tube can be a similar strength for a much better mass.

Of course with propshafts you are also looking at impact strength, since it's bound to be clobbered - the tube version will lose out here against solid bar.

The greater the overall diameter is, the more mass efficient it will be (i.e. the more load it will take for a given mass), but as the walls get thinner, the impact resistance goes down.

Maybe just run the solid one - with Mog axles you can't be too concerned about the weight (on the other hand, since you have more clearance under the prop with portals, maybe you don't need the impact resilience of the solid shaft...).

Ahhh decisions decisions...

Al.

[sorry - posted same time as Bish, hence the repetition]

[FridgeFreezer]

Al - a *lighter* tube will lose out to solid bar strength wise, a reasonably thick walled one will be far stronger in most directions (EG torque) and in its weakest (side impact) will be equal to the bar. There's a reason almost everything round and metal tends to be hollow...

[Astro_Al]

John - I'm not sure you're contradicting anything I said?

Al.

[nas90]

The solid bar front propshafts were used on Classic RR circa 1989 when they had a diesel engine (not 200Tdi), Disco I with cats and NAS90 because it has cats. The solid bar to give clearance on the exhaust. Initially I had problems with wear on the front prop on the NAS but with a strict greasing regime after each off-road outing the prop on the vehicle has lasted 4 years or more.

[Astro_Al]

Hold on, I've just re-read it.

>"There's a reason almost everything round and metal tends to be hollow..."

Yeah, weight and cost. NOT strength.

You need to be more specific in your statements - stuff like 'resonably thick' doesn't mean anything. I guess you are comparing bar and tube of equal mass, right? So you're saying a solid bar of mass x has equal impact resistance to a tube of mass x (both per unit length) no matter what the diameter of the tube is?

So lets take a 1" solid steel bar, for example. Without changing the mass/unit length, we increase the bars diameter to say 4" outside (obviously it becomes a tube, since the mass is conserved). I can't be bothered to work out the volume but I guess the tube would have a wall thickness of, say, 2 or 3mm? Now you're saying that if I give you a hammer, you stand as much chance of denting the solid steel 1" bar as this new thin walled tube? Nope. If you're still not convinced, work out what the wall thickness would be if it was expanded to a 12" diameter tube, then whack it with your imaginary hammer.

Regarding torsion, the maximum shear stress seen in a tube under torsional load will be higher than in a solid bar of the same outside diameter under the same load. As you increase the size of the central hole, the max shear stress increases (as the outer material takes more and more of the load). So the solid will always be strongest for a given extrenal diameter, but losses in strength are minimal as you take material out of the centre, so it can be mass efficient to remove material (with minimal strength implications), but it's always detrimental in this case.

If you constrain the MASS but play with the max diameter then you can end up with a tube which will take more tosional load before failure than a solid bar.

It just depends where the goal posts are... (I'm not sure you disagree, it's hard to tell what you mean from your message).

Al.

[FridgeFreezer]

I wasn't disagreeing, I was clarifying.

It's swings and roundabouts as you say, but with only a little bit of thought to material specs tube usually wins over bar. Thinks like half-shafts are obviously cheaper to make from a solid bar.

Your example takes things to the extreme, you would end up with a tube made of tin foil and obviously that's not gonna last in any sort of load. I used the word "reasonable" to indicate a modicum of sense was required on the part of the reader rather than getting into debating specific examples with maths and big words. You can prove anything with maths

The strength, in every form of loading, of a prop will depend on the material thickness, the ID (if is is hollow) and the OD. There is a happy medium to be reached fairly easily if you get your calculator out. The fact I remember this from basic mechanics at HNC suggests it's fairly elementary engineering knowledge and that arguing about it is going to result in someone proving you wrong sooner or later.

Personally, I would look at the practical examples and note that the majority of vehicle props are made from tube, except where clearance is a problem.

Does this mean you're gonna build your frame from solid bar rather than tube, after all you might dent it otherwise

[jericho]

Am i right in thinknig that double cardon front props reduce prop vibration and that's about it?

Props with a double cardan joint are used where the angles at either end of the prop are not equal;that is - where the axis of the drive flange and the axis of the diff flange are not parallel.

But a prop with three ujs should only be used where the angle at the single jointed end is close to zero (e.g. where the nose of the diff points at the drive flange) ,or else the two ends of the prop will be trying to rotate at different speeds,causing vibration.

If the angle at each end of a prop are not equal,and neither is close to zero,then a prop with two double joints could be used - though it would be better to change the transmission geometry.

Sorry to waffle on.

[mwncidur]

tf prop is skinny so that it clears the bellhousing/adapter on full bump.

[simonr]

Tube has one other big advantage - Its straightforward to weld two sections together without compromising the strength too badly.

It is of course similarly possible to weld bar, but as the thickness increases, it's more & more difficult to get a good joint.

If you, for example needed a custom prop shaft - it's dead easy to cut a long (say 110 rear) one and loose a bit in the middle and zip it back together.

Both my props started life as 110 rears.

Si

[Bush65]

I wasn't disagreeing, I was clarifying.

It's swings and roundabouts as you say, but with only a little bit of thought to material specs tube usually wins over bar. Thinks like half-shafts are obviously cheaper to make from a solid bar.

Your example takes things to the extreme, you would end up with a tube made of tin foil and obviously that's not gonna last in any sort of load. I used the word "reasonable" to indicate a modicum of sense was required on the part of the reader rather than getting into debating specific examples with maths and big words. You can prove anything with maths

The strength, in every form of loading, of a prop will depend on the material thickness, the ID (if is is hollow) and the OD. There is a happy medium to be reached fairly easily if you get your calculator out. The fact I remember this from basic mechanics at HNC suggests it's fairly elementary engineering knowledge and that arguing about it is going to result in someone proving you wrong sooner or later.

Personally, I would look at the practical examples and note that the majority of vehicle props are made from tube, except where clearance is a problem.

Does this mean you're gonna build your frame from solid bar rather than tube, after all you might dent it otherwise

I found your earlier post above, anything but clear. It was extremely ambiguous.

In general terms for components manufactured from steel, when determining strength, we usually relate calculated stresses in the component, to the tensile and yeild (or 2% offset stress for the stronger steels) properties published for the material used (as found by a standardised test in tensile testing machine).

Now, how the component is loaded in use, is often much different to how the material was loaded in the tensile testing machine. However, the various failure modes, for different types of loads are well known and procedures established to relate the failure modes to tensile test material properties.

These cover such cases as buckling, brittle failure, shear stress (to tensile stress), 2 dimensional and 3 dimensional stress (to the 1 dimensional stress in the tensile testing machine).

Torsion creates shear stress (not tensile stress) in the material. The shear stress is not uniform throughout the material, but is zero at the centre and increases as the radius increases, for circular parts. For non circular parts, it is much more complicated.

For this reason, increasing the diameter, will increase the strength. The torsional strength is proportional to the diameter cubed - so doubling the diameter results in eight times the strength.

So a 2" shaft can carry 8 x the torque that a 1" shaft can carry. A hollow shaft with outside dia of 2" and inside diameter of 1" will be 7 times (8 - 1 = 7) stronger than the 1" diameter solid shaft.

Now the area of a circle and therfore the weight of circular shafts, is proportional to the diameter squared - so doubling the diameter results in four times the weight.

So it is reasonably easy to see that a tube can be made stronger and lighter than a solid shaft, but the outside diameter of tube must be larger than the solid shaft.

With very thin walled tubes in torsion, the failure mode may be buckling, before the ultimate torsional shear stress is reached.

For a thicker wall tube in torsion, impacts on the tube may lead to buckling failure.

[FridgeFreezer]

I found your earlier post above, anything but clear. It was extremely ambiguous.

That's 'cos I know nuffink, just that Al needed arguing with

Good to have some real tech with numbers and everything, you confirmed what I was blundering approximately towards

[Petergg]

Dragging up the past, so what prop shaft is the strongest? as I need a new rear one made up/cutdown

Peter

[JST]

Jim - try propshaft clinic or....d4x4 they make their own now as well.

[nickm]

The solid bar front propshafts were used on Classic RR circa 1989 when they had a diesel engine (not 200Tdi), Disco I with cats and NAS90 because it has cats. The solid bar to give clearance on the exhaust. Initially I had problems with wear on the front prop on the NAS but with a strict greasing regime after each off-road outing the prop on the vehicle has lasted 4 years or more.

wasnt the solid bar mainly used on v8 auto's ?

[pugwash]

Solid bar was for the TF727 3 speed autos- ZF4 box uses a hollow tube.

[bishbosh]

Not true puggy - V8 auto with cats has a solid bar. Without cats is a tube in my experience.

[pugwash]

that's useful to know Mr B, as my lump has to run cats for emissions

[Petergg]

Is the 130 prop the strongest L/R make as std?

I am after a std L/R prop which I can cut down.

Peter

[FridgeFreezer]

What aspect of the prop's strength are you worried about? I'd have thought the UJ's would be the weak link in most cases. Mine are from the Propshaft Clinic on Jez's recommendation and seem very nicely made - the fact that Jez's have survived two Ladoga's now with seemingly no actual wear was a selling point

Jim - do you need cats or do you just have to pass emissions, cos the 109 (when tuned correctly) passed the cat emissions level at MOT despite only requiring a visual smoke test the tester was quite surprised to be unable to find a cat in the system

[Petergg]

Hi FF

I am after overall strength and reliability from a std prop, need to be 8" longer than a std 90 rear.

I was in the thinking of maybe a 130 prop cut down, but not sure if they are stronger than a 90/110.

Peter