Last century was a time of continuous revolution in underground coal mining technology. At the beginning of the century, traditional pick mining with hand loading was nearly universal. Undercutting machines largely replaced picks by 1930, when mechanical loading began the first great transformation of mining. Other revolutionary developments included the replacement of drilling and blasting by continuous miners during the 50's and 60's and the rapid growth of longwall mining during the 1980's. But, perhaps the most significant development in underground coal mining during the last century was the introduction of roof bolting during the late 1940's and 1950's.
In first half of 20th century, falls of roof were the greatest single safety hazard in underground coal mines claiming thousands of lives annually. In fact, roof falls were responsible for between one-third and one-half of mining fatalities in each decade. But with the introduction of roof bolting technology and its further development over the years reduced the toll taken by roof falls. Bolting was substituted for timbering in underground coal mines in the late 1940's, and it was accepted by the mining industry with greater rapidity than any other mining change since the inception of mechanization. Without a doubt, roof bolting has been the single most important technological development in the field of ground control in the entire history of mining.
Roof bolting is used to prevent mining accidents by securing the roof of the mine and making them self-supportive. From an engineering standpoint, roof bolts are inherently more effective than the wood timbers they replaced. As a support, roof olts are theoretically superior to timbers because timbers offer support after the strata they are supporting have failed; whereas roof bolts reinforce the roof rock, which contributes to its own support.
Roof bolting machine or roof bolter is bolting rig which usually uses pneumatic or hydraulic power to drill deeply into rock strata and firmly secure various kinds of bolts, ranging from resin roof bolts to cable bolts. The roof bolting machines vary significantly in size, from compact handheld devices to heavy-duty machines that move on tracks irrespective of their shape or size. According to their different structure they can be divided into individual type, skid type or machine-mounted and according to the power it can be divided into electric driving, pneumatic driving or hydraulic driving.
A regular handheld roof bolter will usually have an extendable air-operated leg which is used to push the drill into the rock strata. Larger, heavier machines which are often called crawler rigs, up the game by using a two-stage frame and carriage to deal with varying roof heights and extend the drill deeper. Most roof bolters come equipped with a hydraulic timber jack to support the mine roof while the drilling process is in progress.
The larger Roof bolter machines are basically of two types: Single boom and Dual boom. Single boom roof bolters use one setting to install roof bolts. After installation, the machine needs to be repositioned in order to insert the next bolt. The total area of support is commonly 5 feet by 5 feet square meters.
Dual boom roof bolters have a bar or beam type ATRS (automated temporary roof support) system which enables multiple roof bolts to be installed from the same machine. This, however, needs a much larger area of support to ensure non-collapse, with a typical load capacity usually in excess of 15,000 kilograms. The advantages of a dual boom roof bolting machine over a single boom are that firstly, it will take half the time to do the job, blessed as it is with two drill booms instead of one. There's no need for repositioning, so multiple roof bolts can be installed over a larger area in a shorter space of time.
Fortunately, equipment improvements for roof bolting machines have evolved with technology over time. Today's bolting machines are more powerful and maneuverable. Through improvements in hydraulics and controls, the machines now have the ability to drill holes faster than they did only few years ago. In some cases, miners can expect push-button functionality. Technological advances have made roof bolting a completely automated process in order to minimize risk of life. However, handheld devices still remain immensely popular and are considered by many as the safest, most cost effective to undertake secondary support work.
Since the roof bolting machine operates in the underground atmosphere, the main design requisites of the machine are:
1. The machine must be light in weight.
2. The machine must be able to readily move in the most confined areas in the underground mines and must be safe in operation.
3. The machine must be provided with an excellent combination of maneuverability, rotation and thrust for drilling and bolting operations.
4. The machine, while at operation, must have rapid drilling capabilities in order to provide immediate support in the exposed roof areas.
5. The components of machine must be corrosion and impact resistant.
6. The machine must have a simple but compact design with ready accessibility to all components for quick maintenance with minimum downtime.
7. The machine must have low noise emission levels.
The right kind of roof bolting machine not only ensures the safety of all personnel involved in the underground mining operation, it also boosts the mining productivity by providing clear passages for other equipment to operate effectively.