HOW ROCK BOLTS ARE MADE
Most solid rock bolts are manufactured by producing a deformed bar in a hot rolling mill. These deformed bars have deformations or ribs which run around the circumference of the bar, and they provide an interlock and load transfer mechanism with the resin or grout used to anchor the bolts in a borehole.
Some research has been done on the shape, height and spacing of ribs on a rock bolt to maximise its load transfer characteristics (eg the HPC bolt developed by BHP), but generally speaking, the design of the rib pattern on a rock bolt is principally designed for ease of manufacture.
Specifically, rock bolt manufacturers have to remove the ribs on a bar before they can cold roll a thread on the outside of the bar. Therefore the rock bolt manufacturers like small ribs (eg the “T” bar profile bolt), or widely spaced ribs (eg the J bar bolt) or ribs made from soft steel with no quench and tempered steel (ie no Tempcored bolts). The principle reason the HPC bolt did not catch on was because the manufacturers did not want to push it because of higher cost of manufacture.
There are two main processes to remove ribs on bars, either to bar peel the ribs off the bar which involves a physical peeling operation; or, a swaging operation where the ribs are hammered back into the core of the bar. This latter process is used exclusively by Jennmar in the Australian market and saves material and does increase the strength of the steel slightly. However, there are concerns about swaging very high tensile steel bars, since cold working and embrittlement can be a concern.
The threads are formed by cold rolling where a set of dies are forced into the bar to form a thread, normally called “plunge rolling”. The process of cold rolling threads also makes the threads approximately 10 to 20% stronger than a machined or cut thread, but does cause cold working and some embrittlement in the steel to occur. The weakest part of the bolt is the root diameter of the cold rolled thread, and this is the part of the bolt most likely to be subjected to damage (eg sticking out from the roof and being hit by a machine).
The most common cold rolled thread used in the Australian rock bolt industry is an M24 thread which has a 3mm pitch and provides a good torque/tension relationship. Typically tension generated in a rock bolt from an applied torque are 3 to 7 tonnes with standard nuts and up to 10 tonnes with low friction nuts. The other process to form a thread on a rock bolt, is to actually roll a thread form into the bar in the hot rolling process in the rolling mill (“hot rolled threads”). Basically the ribs are aligned up into a thread form. This considerable advantages in that:
- the thread is already formed into the bar;
- the thread form is not a weakness in the bar;
- the bar has a thread form along its entire length and bolts can then be coupled together; and,
- the threads are almost indestructible and are not damaged by rough handling (unlike cold rolled M24 threads).
These bolts are commonly used in the hard rock mining industry and in civil engineering applications. However, their major disadvantage is that the rib spacing is at least 10mm and the thread pitch is very coarse. Therefore it is difficult to generate high loads in the bolt from an applied torque and there is the danger that the nuts could actually unscrew due to vibration etc (eg vibration from conveyor belts, pipework, etc).
With the current trends of rock bolt manufacturers desperately cutting prices in a vain attempt to gain increased market share from their competitors, it is inevitable that the design of bolts will be to reduce manufactured cost, rather than to increase bolt performance.”
This article has been contributed by Peter Grey