"How are we going to take that stud out?" There's nothing left of the broken stump."
When you are dealing with machine repairs, you will be asked
this question sooner or later. The shipboard environment is especially noted for
its corrosive nature. Seawater and mild steel does not mix very well. The steel
becomes corroded very fast.
In many instances, what is left of the stud is just a narrow piece of steel sliver that crumbles when it is knocked. Threads in screw bolts get eaten away. Hexagonal bolt heads or nuts become rounded with corrosion. It's no easy task removing them. Even penetrating oil sprayed on them will not loosen them.
Sometimes, there is no choice, but to break the bolts. It might be the faster way. Sometimes forcing them to turn loose results in shearing them off.
Ships with apprentice Cadet Engineers, or junior ratings may encounter screw studs breaking due to over tightening. Tightening a bolt or nut is a skill that has to be learnt by experience. Inevitably, the learning experience results in bolt or studs breaking.
If there is still a stump left, then it might be easy to remove. Just a properly sized pipe wrench or a vice-grip plier may do the trick. A bit of penetrating oil and some sharp knocks with a hammer helps. Sometimes, a bit of heating with a gas flame helps to create some expansion and contraction in the threaded joints.
Sometimes, it does not help. The short stump becomes broken. What to do then?
A well-stocked ship may carry a set of screw extractors for this purpose. Basically, it is a set of 3 tapered tools with left-hand threads. The idea is to offer the tool a good grip on the broken stud while at the same time able to turn anticlockwise to remove the latter.
To remove a broken stud, the steps to take are as follows:
- Punch a depression at the center of the broken stud with a center punch.
- Select a drill bit, about half the size of the stud diameter, and together with a drilling machine, drill a hole in the center of the stud.
- Choose a screw extractor tool that can fit into the hole snugly. Turn it anticlockwise. You will find that the tool lodges itself tightly into the hole as you turn.
- You have now created an extra length of stud that can be turned using an ordinary wrench.
Why does it work? Remember the drilling of the hole? There is some clearance space for the stud to contract after the hole is drilled at the center. The heat from the drilling is also useful as a thermal stress agent. The extractor tool is made of hard steel and is able to dig into the softer mild steel studs. The particular design of the left-hand thread of the extractor makes it possible to get a good grip on the broken stud.
A well managed ship have stocks of special tools. CLICK HERE to look for special
tools. A Marine Engineer who signs-on a ship does not know whether there
are such tools on board the ship he is going to. It is wise to keep a set of screw extractors as a personal tool, just in case it's not there. It will save a lot of unnecessary misery when you are faced with broken studs!