Iron and steel are used in mines in the form of rigid and yielding props, beams and girders, reinforcement in concrete, corrugated sheets and roof/floor bolts. Disused rails of 36 lbs. section or heavier sections are sometimes used. Steel is incombustible and its use greatly reduces fire risk in mines.
Steel props used in the mines are (a) rigid props and (b) yielding props. An ordinary H section steel girder of suitable length with the web cut away and flanges turned over at one or both ends is the common type of a rigid prop. If the prop buckles in use it can be straightened by hydraulic press situated at the pit-bottom. Another type of steel prop consists of a steel pipe containing a well-fitting Umber core through major length and extending 25 to 40 mm beyond the steel pipe at either end. The timber core yields to some extent to the roof pressure and gives indication of roof weight.
Advantages of rigid steel props over timber are:
(1) a row of steel props offers uniform resistance if properly set.
(2) A steel prop has a greater ultimate strength than timber.
(3) Recovery of steel props and frequency of its re-use is greater. If buckled, it can be straightened and re-used but a timber prop once broken cannot be re-used except as lagging.
(4) A steel prop does not deteriorate to the extent timber does; timber decays in a number of ways, particularly by dry rot.
(5) They are incombustible.
Disadvantages of steel joist props are;
(1) Large props are difficult to tighten against the roof and may therefore be knocked out.
(2) A large manpower is required for handling them.
(3) Wooden props give warning of heavy weighting, which steel props do not.
(4) A simple joist prop cannot be used in seams of varying thickness. Cutting of a long prop and turning its flanges cannot be done at the site of erection.
Yielding props
Extensible non rigid props or yielding props are used in this country and are manufactured indigenously. They are of two types.
(1) Friction props: The common example is the FP3 series (Tubular friction prop) manufactured by MAMC, Durgapur.
(2) Hydraulic props: The familiar example is the MAMC — Dowty prop, TU-40 prop of Usha Telehoist Ltd, etc.
Before we describe these props, certain terminology must be explained.
The term setting load is used for that load which, by means of a setting device, is imparted to a yielding prop, purpose being to ensure that the support is firmly in place and can resist lateral pressure due to the passage of the face machinery. In other words, setting load is the reaction offered by the prop to the strata. It is applied to a prop before its yoke is tightened and it is considered essential that it should not be less than 4 te in a seam of moderate thickness (that is; above 1.2 m). The setting load does not affect the load-yield characteristic of the prop.
The yield load is that load on a prop at which the upper member begins to slide; Hydraulic props are specified by the yield load e.g. a 20-tonne prop, a 30-tonne prop, etc.
In the friction type of yielding props the yield load depends upon the force with which clamps or wedges are tightened. But clamping load is not the same thing as yield load.
Load-bearing capacity of a prop is the load at which an axially loaded prop reaches its elastic limit or at which it begins to buckle.
Characteristic curve or load-yield curve is a graph in which yield is plotted along the X-axis and load along the Y-axis and represents the behaviour of a prop under load.
Early bearing props are yielding props which accept the maximum load with a minimum yield. An early bearing prop (one which is capable of building up of maximum load with a minimum of yield, thus reducing to a minimum both convergence and bed separation) should be used for a routine installation on the coal face while a late bearing prop should be used to stiffen the break-offline.
It exerts a resistance which continues to increase with increase .n yield. Hydraulic props are early bearing props.
Friction props rely upon the friction grip between two members, one telescoping into or sliding against the other.
If two bodies are held in contact and one moves with respect to the other, the force P for just moving the body is
P = µ X Q
where µ is the coefficient of friction and Q is the normal force. If the upper member is held by two friction clamps, resistance to sliding, P (or in other words, the bearing capacity) of the prop, is — 2 µ X Q.
P can be increased by increasing µ or Q or both. In a friction prop Q is raised by means of a clamp which is tightened in various ways or by compressing parts of the prop by the use of wedge-shaped upper member or by drag wedges which come into operation as the prop yields. .
When setting the prop, first operation is to extend the prop to the length required in the position for setting and then it is tightened against the roof or bar by a setting wedge or claw attached to the prop for this purpose. The clamp is then tightened up and the setting device removed.
MAMC friction props
The friction prop type FP3 manufactured by M.A.M.C. consists of two seamless steel tubes of which the inner member is made captive to the outer member by means of a spring locking pin which prevents the inner member to come out completely from the outer member beyond the extended length. The clamp unit which is fixed at the top of the outer member provides the necessary friction grip by hammering two locking wedges alternately. The inner member and the clamp unit are provided with special coatings which give the required frictional characteristics of the props and at the same time prevent from corrosion. In the closed position of the props adequate finger clearance has been given to avoid the injury to miner's hand during operation. The important features of these props are as follows:—
(1) These are early bearing props and accept the roof load very quickly.
(2) These have constant load yield characteristics.
(3) The weights of these props compared to their nominal load are less in comparison with other types of props.
Friction props are not much favoured these days though they were adopted on the longwall face with sand stowing using Anderton shearer (for the first time in India) at Chinakuri colliery, and later at Gidi A colliery for the experimental method of extraction by inclined slicing with French collaboration. They were also used at Jeetpur colliery on longwall faces with sand stowing.
Hydraulic props
A hydraulic prop is simply a hydraulic jack. These props have been used at longwall mechanised faces in our country. A hydraulic prop can be set to take immediately three quarters of the maximum load (yield load), but when overloaded it will yield at the designed load after which the resistance is uniform and about 3/4th of the maximum.
A hydraulic prop (Fig. 9.17) basically consists of two oil-filled cylinders, the upper one telescoping into .the lower one. A piston head is fitted to the lower end of the top member and this provides a seal with the inside wall of outer cylinder. The piston (and the top members to which it is fitted) does not slide down, unless the load on it exceeds certain limits. The resistance to downward movement of the upper member is provided by the pressure of the oil in the cylinders and this oil pressure builds up by the operation of a pump at the time of setting up the prop in position. There are two ways of building up the oil pressure in the prop by a pump.
1. 
Closed circuit system.
Closed circuit system.
2. Open circuit system.
In the closed circuit system a built-in pump is provided in the prop itself and forms an integral part of it. The pump is operated by an external detachable handle. In the open circuit system an external pump, serving a number-of props from one central site, is connected to the prop by high pressure hose pipes and operation of the pump builds-up the pressure of oil contained in the cylinders of the prop. Non return valves provided on the prop retain the oil pressure.
The largest manufacturer of hydraulic props in the country, M.A.M.C. Durgapur, manufactures props of both designs, i.e., of the closed circuit system (Example MAMC Duke hydraulic props) and also of the open circuit system (example- Salzgitter/ MAMC hydraulic props).
The MAMC hydraulic prop consists essentially of an inner tube, a pump cylinder with oil, a guard tube, a release valve, a non-return valve, a main piston, a top extension fitting, and a pump and release shaft. The inner tube and pressure cylinder can be extended like a ram by hydraulic pressure. The pump can be actuated by an outside key or handle. A large diameter steel tube connects the lower reservoir with a relief valve capsule. Action of the handle pumps the oil from the inner tube to the outer tube, thus extending the inner lube. After the prop has been so extended up to the roof, further operation of the pump handle provides the initial bearing pressure (or initial setting load) which is 5-8 tonnes.
The pump handle is then withdrawn. In course of time when the roof pressure on the prop gradually increases the inner tube will not slide down till the load is 40 Te, (in the case of4Q-te prop) as the hydraulic fluid is compressed till that load is reached. When the load on the prop exceeds 40 te, a relief valve operates. The relief valve is a capsule permitting adjustment and testing prior to insertion in the prop and it is set to the correct yielding pressure before the prop is assembled in the factory. During the yield, the oil pressure may be from 200 to 500 kgf/cm2. The prop can be withdrawn easily by pulling the release shackle, which actuates a cam and lifts the valve assembly off its seating, allowing a free flow of oil back to the top reservoir. Withdrawal can be effected from a distance by attaching a chain to the relief valve shackle and pulling it. A hydraulic prop can be tightened in a few seconds to any length within a wide range. The pump handle is the only too) required for operation and (he prop can be released and withdrawn in a few seconds. The hydraulic oil used may be water with 10% suitable oil. The oil is DTE light oil, servo-system 311 supplied by Indian Oil Corporation. It has anti-corrosive characteristics.
M.A.M.C. manufactures 40-te props of its own design A precaution which must be borne in mind in connection with hydraulic props is that they must not be left lying on the floor, and the prop must be withdrawn before full closure i.e., before it becomes "solid". On longwall faces having sand stowing, hydraulic props which were not used carefully developed scratches due to sand rubbing on the inner cylinder thereby partially losing the oil-sealing capacity. Such neglect renders the prop ineffective.
Salzgitter/MAMC hydraulic prop.
This is a hydraulic prop of open circuit type, normally used with sliding roof bars (Fig. 9.19). The roof bar itself can be pushed or retracted, when in position, by hydraulic pressure. The Salzgitter/ MAMC hydraulic prop can be extended, according to prop type, by mounting of extension pieces.
Setting of Salzgitter/MAMC hydraulic props is effected by hydraulic pressure, which, produced by a central high-pressure pump, is delivered to the setting gun via. a lead in the supporting system. The setting gun is put on the filling valve of the prop, mechanically fastened by one-hand locking device and operated by one hand.
The pressure medium flows under the piston, pushes out the inner ram tube and sets the prop safely with the setting load pre-set by the hydraulic pressure between cap and bottom. The hydraulic oil used is a 5% oil emulsion, and 95% water; pH, value 5—8 pH.
The prop can take the roof load until the nominal load adjusted in the operating valve has been reached. As soon as the nominal .toad has been reached, the operating valve opens and some pressurised fluid spurts out until a pressure spring closes the operating valve again. This operation occurs whenever the nominal load has been reached. Thus the prop is always protected from over-load of rock pressure.
Large strokes admit large convergence as well. Due to the extension feasibilities according to prop type, the prop lengths can be easily adapted to seam heights underground.
When the prop is to be withdrawn at the goaf edge, the releasing valve can be actuated by means of a releasing key which is extended by means of a rope or chain from the miner's safe position effecting the flow-off of the pressure fluid out of the cylinder space. An installed powerful return spring rapidly retracts the inner ram tube. After this the prop is ready again for another setting process. Releasing keys are available in accordance with service conditions (service heights) in lengths of 150, 280 and 800 mm.
Technical Data (of one of the props); Salzgitter/MAMC prop
Type
Nominal Load (Tonnes)
Material
Min. Yield Strength
Inner Ram Tube (mm)
Outer Ram Tube (mm)
Piston Area
Stroke
Foot plate
Head plate
Length (extended)
(Retracted)
Weight.
Aluminium-alloy prop, S1S40/L
40
Aluminium-alloy
45 kgf/mm2
105 X 13
138 X 14
95 cm2
800 mm
180 mm dia. (Area 250 cm2)
4-prong crown.
2500 mm
1700 mm
62 kgf.
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