Ian Plunkett of Iptron Technology, draws attention to the limitations and potential problems of using general purpose winches for control of conveyor tension. Some customers complain that, every one of these, suffers from rope entanglement and rope jamming. This is highly dangerous, difficult to clear, and usually leads technicians to conclude incorrectly, and unfairly, hat the electronic controller has reversed the winch direction even though this is an astronomically impossible occurrence.
Conveyor tension control by means of a take-up winch is potentially
the most versatile and best performing method. For decades, general purpose winches have been widely used for this purpose
but come with major complications.
The following are the reasons:
1) General-purpose winches are intended for manually controlled and
observed, ad hoc, haulage applications, eg, recovering vehicles.
Conveyor tension control is an automatic process which operates ‘unattended’; this is acceptable provided that there are no rope problems.
The contrasting peculiarity of conveyor tensioning winches is that the rope tension varies from zero on the first turn of the winch drum, increasing
with every turn up to the high tension required for conveyor starting. In addition dynamic tension transients generated during conveyor start-up and stopping, cause extreme tension transient stresses on the winch.
These are primarily associated with a take-up system that is too slow.
When the high-tension rope overlays a low-tension rope, the latter is damaged and displaced during the control process. Each time a new rope layer is formed, the inner layers are repeatedly damaged and displaced.
When the rope is paid out manually, and unattended, for maintenance, the
rope reaches the point where the jam occurred and cannot release it.
This results in the rope now being drawn back into the winch in the wrong direction and renders all and any tension control system inoperable and dangerous, particularly because the technician is not watching what is happening. Warning! Never manually operate a take-up winch without observing the winch!
General-purpose winches have the same ‘safe speed’ of +/- 6 metres
per minute. This is too fast for short conveyors and too slow for long conveyors. Fast-winches are difficult to control requiring a wider ‘dead
band’ and the consequent poor control.
Slow winches are the principal cause of extreme tension transients and rough starts. Locked take-up starts cause the highest possible stress during conveyor start-up. The take-up speed of the winch must exceed the conveyor’s peak rate of elongation by at least 10% during acceleration.
Winch manufacturers spool the rope tightly prior to supply to prevent rope-entanglement. However, the rope has to be spooled out for installation and the manufacturer’s initial tensioning is to no avail.
Rope Jamming
1) Winch design
Rope jamming is caused by the combination of varying rope tension and rope-overlaying. Conveyor take-up winches should accommodate the entire
working length of the rope on a single layer on a wide, plain grooved drum. Note that, even ‘lebus screw’ grooving will not prevent rope jamming
when the tension varies widely. A wide drum may require a greater distance to the first rope sheave or a diamond screw rope layer control where space is limited.
2) Detecting a rope jam
A jammed rope is dangerous and often extremely difficult to clear. Every winch used for conveyor tension take-up that cannot accommodate the rope on a single layer should have a safety bar located in front of the winch drum that will trip an emergency stop switch linked to a safety contactor in the event of the rope changing sides following a rope jam.
3) Minimising the possibility of a rope jam
a) Fit safety bar to front of winch.
b) Design take-up winches with wider drums.
c) Do not include unnecessarily long displacement in the take-up system.
d) Keep the length of rope on the winch drum to an absolute minimum.
e) Do not store ‘spare rope’ on the winch drum; keep it behind the tension transducer (load cell).
f) Operate with two falls of rope if possible.
Correct characteristics
a) Take-up winches should be rated in tension only.
b) Manufactured in tension intervals of √2. Final gear should be epicyclic.
d) Helical/bevel gear motor chosen to provide required speed and tension.
e) Operation of winch with starting level at or below 50% of winch rating.
f) Optional integral low-inertia slip-clutch set to slip at 125% of winch rating.
g) Provision for drum-position by proximity switches.
h) High Speed AC brake where possible for small winches.
i) Variable-speed dynamic control for all long, heavy and extendable conveyors.
j) Rope-jam safety-switch linked to a safety contractor.
k) Optional drum width specified for the particular
conveyor.
l) Optional diamond screw rope layer control.
The performance and reliability of any conveyor tension control system, depends on having a take-up winch matched to the conveyor needs under proper electronic control together with mechanical design for minimal friction.
Iptron Technology cc
www.iptrotech.co.za
