Bolts or studs

I've part made A A Raymans high speed marine engine at 2/3rds from the drawing. I've noticed and on other steam engines that studs and nuts are used on the cylinder and valve chest,Is this for a authentic look or some other reason .All the engines I've made I have used bolts, as for the studs are they usually cut down from bolts or is studding for 2mm any good as I brought some a few years ago and it is rubbish, or are they generally made on the lathe.

I'm sitting at home lethargic after my operation, sorry if this is a dumb question.

Bob

It's for looks as studs would have been used on the full size though as this is not based on a full size engine you can do what you like, if it were being used to power a model boat then cap heads would be more practical if it was performance and ease of maintainace you were after.

I usually thread my own rod leaving a plain middle section unless whatever is being retained is quite thin in which case I cheat and thread it all the way

Edited By JasonB on 06/10/2020 10:22:39

Studs are preferable in my opinion to reduce the risk of wear and other damage to critical threads when carrying out maintenance. Colin

Thanks for the replies.

That is a great looking traction engine you made Colin is it a working model.

Bob

+1 for studs, and nuts,

Apart from the "look", if a thread wears or strips, the stud or nut can be replaced.

With bolts or setscrews it is the thread in the casting that would be damaged.

Howard

I understood there are three(ish) practical reasons in real life:

No wear on the stud/casting threads, and easy replaceability of the stud.

No risk of bottoming out the bolt in the threaded hole.

Consistent nut torque, owing to the unvariable thread length engaged between stud and nut (40% being thread friction)

In traditional marine work it was easier to lock a nut on a stud, by split-pin or lock nut, than to lock a bolt which needed tab washers or wire-locking of the whole ring of bolts.

In more modern marine diesel work it is the need to have equal torque setting that is important and that is easier with nuts on studs.

Tony

Tony Martyr: I have seen this argument before, but I'm not sure it makes a lot of sense. Of course, you can lock a nut on the drilled stud, and of course, a bolt can come unscrewed. But what stops the stud, with its attached locked nut, twirling round and unscrewing?

And I'm not sure about the 'consistent torque' argument of J Halfpenny - the thread engagement is going to be the same either way, surely?

But then, look at a modern engine - big ends etc with no locking wire, no shakeproof nonsense, no tab washers. Just properly engineered and correctly torqued.

funny old world - Tim

A nut could shake loose on a stud as it has only a small friction area additionally to the thread.

With a stud, there is not only the friction of the nut against the flange or cylinder-head, but probably a lot of extra friction on the stud surfaces in the through-hole and gasket, often a slightly longer thread than in the nut, and the stud is often wound down to the thread run-out.

Nigel 2: The flat contact area of a nut can readily be increased by using a flanged nut, especially if the flange is undercut slightly so that the outer edge bears hard, increasing the friction torque.
Winding a stud down to the run-out is not always a sound idea, as it tends to raise a burr around the stud, preventing the adjacent face from sitting firm, and reducing the even squash of the gasket. You can avoid the problem by counterboring the stud-hole (or countersinking) but this reduces the advantages you list for studs. Or by relying on the stud bottoming in the hole, which requires careful drilling, a bottoming tap, and rigorous cleaning of swarf, both new and in service.

In reality, the problems solved and created by the various options are interlinked, and there is no 'one size fits all' answer.

Cheers, Tim

PS has anyone a reason why accidental double posting happens?

Tim

Bob , thanks for the comment about my traction engine model; It is a working on compressed air model and has been shown at the now discontinued Brighton model world exhibitions several times during construction. It still has some pipework to be done but I have tested the boiler to twice working pressure successfully. Colin

Very good point Howard.

I always thought the whole point of using studs was that they act as a precise locating dowel and should be either a reamed fit or very close to it to prevent lateral movement. Whilst we probably tend to think of circular cylinder covers in relation to model engineering that usually have machined locating spigots there are many instances of rectangular covers with shaft bearings in them where this precision is necessary.

Additionally the plain portion should always be slightly larger in diameter than the threads to avoid thread damage when removing and replacing parts as has been mentioned in previous replies. The same argument can be used in the bolts versus set screws debate with the plain portion of the bolt acting as a locating dowel if all has been made correctly.

We should also distinguish between standard bolts and fitted bolts, the latter being both dowels and very tight fits with round heads for driving into place so that there is no need to use 2 spanners.

When making studs two thread gauges are used, the end that is fixed is made slightly larger than the nut end so that it fits better into the component and locks, this allows removal of the nut without the stud coming out.

Colin I think you are almost correct, however, and I am surprised that nobody has mentioned it, studs should ideally have two different thread pitches. The coarse one going into the casting or other major component. the end that accepts the nut should be a fine pitch thread e.g. BSW thread into a casting and BSF for the nut. This allows the stud to be secured into, say a casting and tightened down. When the nut is fitted this arrangement allows the fine thread to become the one that applies tightening torque and clamping pressure. So far so good, however when it becomes necessary to dismantle the assembly the differing thread piches will seldom operate together, which allows the nut to be removed without loosening the stud in a casting. Anybody who has ever dismantled a British motorbike will know that in 99% of cases undoing the cylinder head nuts does not loosen the studs.