Rivets or Fitted Bolts.. Ex FS thread.

Not the case with the new P2, it seems, where they appear to be employing it in some areas.

 

Indeed......

 

Anybody tried pre cooling fitted bolts (dry ice possibly) so they can be pushed in easily and then pulled up tight with the nut? I know that they will need retightening once they've warmed up

Rob

 

Although freezing is the technique used for fitting cylinder and valve liners I doubt you'd get sufficient contraction from the typical sized fixings used for horns etc.

On the subject of the stretching when loaded that occurs to a fitted bolt causing it to slacken in the reamed hole.
Although I haven't seen a mathematical analysis, I would expect the amount of radial interference when the fixing is unstressed is sufficient to ensure a sensible degree of radial interference remains when fully loaded longitudinally. I.e. there will be some elastic "necking" but insufficient to completely unload the radial interference.

 

If I can make a few observations regarding hot riveting,
1) yes some will be lost off the diameter of the rivet on cooling however as anyone that has ever tried to knock out a rivet once its head has been removed will tell you, it is still a much tighter fit than the shank of a fitted bolt,
2) They also contract on length, meaning that they pull the two halves of the joint tightly together
3) Whenever we do some riveting, our gaffer always reminds us that 50% of the strength of a riveted joint comes from the corrosion between its plates.

 

That's an interesting statement! I always thought that past practice was to coat the surfaces with red lead (not today's red oxide) before riveting and that prevented any corrosion.

 

I guess it depends what it is you're actually riveting together.

In the case of a pressure vessel, once it's riveted together you then need to caulk the seams, however even then the blessed things will still occasionally fizz and whisp in places until a healthy bit of corrosion and scale has built up in the exceedingly small gap between the plates. The list of traditional 'remedies' that can be added to the water to help speed up the process and provide temporary respite whilst 'corrosion' does its trick is legendary.

Of course if the resulting joint simply has a leak, then time won't do anything to heal it - the blowing out of steam/water will just erode the plate and make the matter worse. A good fit of the plates in the first place followed by effective caulking should prevent that though.

 

I think we may be drifting a little off topic, Although there are similarities I feel demands on a fixing on a wagon or a pressure vessel are somewhat different to that for the components of a loco where the forces are continually changing direction with every turn of the wheel.

Years ago a very competent practicing engineer explained to me a fitted bolt or a cold set rivet is also akin to a dowel in holding the components in precise location/alignment. The clearance required to fit a hot rivet allows a degree of "float". The result of this can sometimes be seen when a hot set rivet is removed and a slight joggle may be noticed along the rivets shank although the holes through the items where in perfect alignment when the rivet was fitted.

To me pressure vessels, and boilers in particular, are a bit of a black art. I'm sure boiler-smiths have a pact with the devil; practicing a form of engineering that is a cross between black-smithing and tool-making.

 

Finding joggles on a rivet (after cursing getting the thing out) is an indication that the plates weren't aligned and bolted properly in the first place when the rivet was fitted. It's true that the rivet hole wants to be a rattling good fit before it's inserted - if it isn't then you'll struggle to get the white hot rivet in place - but once it's held up, the first blows of the riveting gun will cause the shank of the rivet to swell and fill the clearance gap. Having ground off the head, removing rivets it is usually infinitely easier to drill and burn away the centre rather than just a brute force attack with hammer and punch.

In the vast majority of cases with road steam vehicles, the boiler itself forms the chassis of the vehicle. The rivets holding the pressure vessel together are also subject to all of the vibration and shocks from rattling down the road, in the case of rollers on steel wheels and no springs. The rivets of the pressure vessel are also absorbing the cylinder forces, seeing as the cylinder is bolted to one end of the barrel and the crankshaft fastened to the sides of the outer firebox at the other end.

As to whether rivets or fitted bolts are better for holding horn blocks on - I really wouldn't like to pass comment, but any suggestion that hot rivets will cool and be slack in their holes has clearly come from someone who hasn't spent much time taking the little blighters out of a boiler. By comparison, my experience of fitted bolts is that they'll come out fairly easily so long as you have room to swing a hammer or wedge in a jack.

 

I think that you pay your money and make your choice. There are advantages and disadvantages to both. One thing that I have never seen, though, is a boiler that has been entirely bolted together, yet there should be no theoretical reason why not. If a fitted bolt is a true interference, there should be no gap through which leakage could occur. Flat tubeplates (as opposed to drumhead ones) should make an ideal bolted joint, whether with fitted bolts or not, and I wonder why you don't get them, especially on smaller boilers. It would make access to the barrel so much easier..

 

Sentinel boilers substantially bolt together. They're not entirely bolted though, I think various sections of the barrel are held together with rivets, but certainly the central firebox section bolts into the outer barrel.

 

The firebox and the casing are bolted together. The casing seam is both welded and provided with a riveted butt strap whilst the firebox seam is just welded ( good few years before Bulleid started with his welded boilers!). Various scantlings are riveted on.

 

Question - if using bolts on a boiler barrel is it necessary to radius the head on the inside so it fits flush. If you put the nut on the inside there will be a small tedancy for only the outside edge of the nut to be in contact with the barrel, less of a problem with a small bolt in a large barrel than a big bolt in a small barrel

 

I've not really considered it as Sentinel boilers bolt together on flat flanged surfaces and my earlier musing on why boilers are never bolted was merely pipe-dreaming. However, you could probably put a slight countersink on the hole and a matching tapered seating to the bolt head if you wanted to go down this route.

 

As Steve says, I'm not aware of a conventionally bolted together boiler barrel - The Sentinel design is a vertical boiler with the joints at the top and bottom. You're right about the bolt heads in a curved barrel, this can be clearly seen when you bolt up a barrel/tubeplate prior to riveting. I'd also agree that a countersink would be good to increase the seating area, but the countersink would need to be concentric and square with the bolt hole. Acheiving this on the inside of a barrel isn't easy, if it's not square then the taper on the bolt head will once again only make limited contact. I guess the ideal would be a bolt head with an underside curved in one direction only at a radius to match the inside of the barrel, but I can see some problems with even this.

Overall, for holding a boiler together I think rivets have far too many advantages over bolts. Once you're set up with a couple of helpers they're quick to put in, they tighten further as they cool (you can hear the boiler creaking as it all cools afterwards), their shank swells to fill the rivet hole when fitted, and slight misalignment with the direction of rivet hole is taken up and compensated for as the heads are formed, there's no on-going maintenance as they don't shake loose - there's no checks to keep them tight, and although they're not as quick and easy to remove as a nut & bolt, neither is it particularly difficult. Once you've mastered the skill of burning out threaded stays without damaging the threads in the surrounding platework, burning and donking rivets becomes child's play.

 

In answering this, maybe we should look to the current Vehicle Acceptance criteria currently imposed.
How are Tornado's Hornblock's fastened down? What are the plans for the P2?

The merits of both have been discussed at length already, Personally I would Bolt them, with a close fit. Primarily so they could be replaced more easily, but also to dial out the inherent residual shear stresses placed on the interface by rivets expanding on forming in the hole, which you wont get with the bolting method, residuals being non present due to lack of thermal effects.

Further up the thread someone mentioned FEA. It is my understanding this was a requirement on the P2 to overcome the issues with the crank axle, a notable weak point on the original design.

For anyone unfamiliar with the subject, modern FEA simulation allows you to subject the component to very realistic (and extreme case) loading scenarios with cyclical loading being they key interest here to determine fatigue effects.

Additionally, given the P2 group are using Solidworks, running the FEA simulations is very straight forward once you have the model drawn up. Merely a case of setting the constraints on the component,plugging in the material specs, setting up the magnitude and nature of the loads, setting the meshing requirement (the finite element part, how many iterations/calculations do you want) and running the analysis which then shows you the stress rises of concern (Von Mises etc as desired) and factor of safety. This can be done in SW's or exported to Ansys, another industry standard among a few others. One obtained, add/remove material and modify geometry as required.

Whats really interesting for me personally on this subject, is seeing just how conservative (or not) some of these old designs are in the face of modern engineering practice. Whats going to be really interesting as more of these designs are fed in to the modern software packages, is just how right ( or wrong) some of the existing preservation engineering practice is. An example below.

 

No, Keith; as you tighten the nut, the thread will deform slightly - but if you put any piece of metal under tension, it will elongate along the entire line of tension, so that the whole bolt will lengthen, not just the thread. This extension was actually used as a way of measuring the applied torque on some Triumph engine big end bolts. The big end bolt was measured for length with a micrometer before assembly, and the fitters were instructed to tighten the bolts until they had lengthened by so many thousandths of an inch as compared to their original state.

In the case of those fitted bolts, as the mass of metal is unchanged, but the length is increased, it is physically impossible for it not to reduce in diameter.

The point which strikes me about hot riveting is the enormous pressure which the rivet exerts, on the two components being riveted together, as the rivet cools down. I'd have thought at least a distinct possibility that, as the rivet cools and shrinks, the sections of metal under the rivet heads bulge inwards into the hole, thus tending to fill the annular gap formed by the rivet shrinking in diameter.

I don't think I've ever come across a rivet which didn't need to be punched out of its hole - even when the rivet had sheared at the joint face, and not even in the case of the cold rivets used to fix mower blades onto the type of finger mowers used on Ferguson tractors.