In earlier times, the machines had huge cylinders on which the ropes were winded up. This was connected with the disadvantage that - with heavy loads in the pit cages and especially in the inner windings - a great strain was effecting the ropes, because they were winded up very tight. That caused a heavy wear and tear of the rope. That is the reason why a clever man named Koepe had the idea that this effect could be avoided by not winding the rope up, but leading it around a driving wheel and then back to the winding tower and into the shaft.
The principle works in the following way: The rope coming out of the shaft is led around the wheels at the winding tower's top and by this to the winding machine in the corresponding building. Then the rope is driven by the driving wheel and after this it is sent back vertically into the shaft after passing another wheel at the winding tower. So there are two pit cages fixed at each rope - one moving upwards and the other moving downwards the shaft at the same time.
After that one of the pit cages arrived at the onset of a certain level and the other one arrived at the pit bank, there is a strong difference concerning the sharing of the loads: At the first side of the driving wheel just a few metres of rope and the pit cage are pulling and at the other side there are up to 1.5 metres of rope and the pit cage, so that another rope is used for levelling out these differences: It is fixed under the pit cages and connects their floors. In this way the load of both sides is levelled out, so that the machine just has to bring up the power to move what has been put into the pit cages, for example the miners, the mine cars or the coal. The rope connecting the bottom ends of the two pit cages is in most cases a flat one. If you imagine the rope at the top, the both pit cages and the rope connecting them at the bottom, you can nearly see a circle. That is why this kind of hoisting is called "endless rope hoisting".
In given figure.
- Upper head sheave
- Friction pulley
- Skips
- Ropes
- Tail ropes
In Koepe hoisting system, a friction hoist is used that uses the principle of friction to drive the hoisting rope. The driving force of the motor is transmitted to the rope by static friction between the rope and the tread of the Koepe sheave. As long as the frictional force available is greater than the driving force of the motor and greater than the difference in rope tensions between the heavier and lighter loaded ropes at the drive sheave (see figure), the rope will not slip under any condition of operation.
Terms used in the formula:
F₁ = the rope pull or tension in the heavier loaded rope;
F₂ = the rope pull or tension in the lighter loaded rope;
α = the angle of contact (180° - 205°); and
μ = the coefficient of friction between rope and rope tread, which, for design purposes, is taken as equal to either 0.20 for locked-coil or 0.25 for stranded ropes.
A balance or tail rope must be used with Koepe friction hoists to maintain an adequate value of F₂ in order that the rope does not slip.
The Koepe hoists may be classified depending on:
Method of mounting
- Ground-mounted hoists, and
- Tower-mounted hoists.
Number of ropes used
- Single-rope friction hoists, and
- Multi-rope friction hoists.
Method of drive
- Direct-driven D.C. hoists, and
- Geared A.C. hoists.
Single-rope and multi-rope hoists may be used as production hoists for single-level hoisting with two conveyances in balance, or with a conveyance and a counterweight for single or multi-level hoisting. They can also be efficient as service hoists with conveyance and counterweight for single- or multi-level hoisting.
Advantages of Koepe Hoisting
Compared with the drum hoisting system, the Koepe hoisting system has the following advantages:
- The Koepe system is most suitable for heavy-duty hoisting from great depths. With drum hoisting, large drums are required for hoisting large payloads from great depths and multi-layer coiling results in reduced rope life on account of crushing and heavy wear.
- When it is required to hoist from a greater depth with an existing hoisting plant, the drum hoist cannot take the greater length of rope. In such a case, it is possible to replace the drum by a friction sheave and use the existing plant to raise the load from the deeper loading level. If the existing hoisting plant is a friction hoist, it can easily be adapted to hoisting from greater depth by fitting a longer rope and altering the depth indicators and the signalling and controlling devices.
- The Koepe hoist is simpler, more compact, and lighter than the drum hoist. Less costly engine house foundations are required. The carrying capacity and the span of crane required will also be less. The capital cost with Koepe hoisting will, therefore, be lower than with drum hoisting for similar duty, the saving in cost increasing with depth.
- With Koepe hoisting, the inertia of rotating parts is less than with drum hoisting, This is, however, partly offset by the greater inertia of the ropes and conveyances. The smaller inertia and the balance rope lower the peak h.p. demands. For the same duty, the r.m.s. motor power can be as low as two-thirds of that required by an equivalent drum hoist. There is an appreciable saving in the overall consumption of electrical energy.
- With ground-mounted hoists, the head sheaves can be arranged so that the underlay and overlay ropes do not deviate at the head sheaves as well as at the friction sheave rope tread. In this way, the fleet angle becomes zero. With drum hoisting, on the other hand, the ropes deviate at the headgear sheaves and also fleet across the drum.
Example
Find the power transmitted by a belt running over a pulley of 600 mm diameter at 200 r.p.m. The coefficient of friction between the belt and the pulley is 0.25, angle of lap 160° and maximum tension in the belt is 2500 N.
Solution
Given :
d = 600 mm = 0.6 m ;
N = 200 r.p.m. ;
μ = 0.25 ;
θ = 160° = 160 × π / 180 = 2.793 rad ;
T₁ = 2500 N
We know that velocity of the belt,
V = πDN/60 = π(0.6)(200)/60 = 6.284 m/s
Let T₂ = Tension in the slack side of the belt
T₁/T₂ = e^μθ (here ^ means power of)
= e(0.25 x 2.793) = 2.01
T₂ = T₁/2.01 = 2500/2.01 = 1244 N
Power transmitted by belt will be
P = (T₁ – T₂)v = (2500 – 1244) x 6.284 = 7890 W
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