Among the various series of bucket
elevators in the Gambarotta Gschwendt manufacturing programme is the EVN
series,which are classified as fast elevators with centrifugal discharge. These
are bucket elevators with a rubber belt whose buckets are connected and fixed
by bolting them to a special rubber belt pulled by friction from the drum of
the control station.
Key elements of type EVN
The bucket’s dimensions (litres), pitch (bucket centre distance) and
lifting speed determine the volumetric capacity or lifting capacity for an
elevator (cubic metres per hour). The shape of the bucket’s side influences the
maximum speed that can be reached in a peripheral unloader with any given upper
drum diameter, to avoid excessive recycling of material that falls towards the
bottom of the elevator. So, the bucket’s parameters – shape and size – upper
drum diameter and lifting speed are carefully selected.
The company has conducted research on the shape and size of the buckets to
maximize the capacity of each elevator.
This was carried out using a special test lift, present in both tests and
inspection areas of the workshop, which allows the variation of lifting speed.
Flow tests were carried out on various models of buckets with different
profiles and sizes using various speeds.
The company’s philosophy regarding speed is to limit its value for reasons
of greater safety of good operation and less wear and tear in order to maximise
EVNs the speed was limited to the maximum value of 1.6 m/sec (96 m/min), while
the increase in the size of the buckets was favoured, thus obtaining
exceptional flow values and a containment of the costs of the buckets for the
The result for this research led to
the identification of a series of Super buckets (buckets “S”) used from the
elevator size of 315 up to the size greater than 1800. The size of an elevator
corresponds to the measure in millimetres of the width of its bucket.
With these buckets “S” the larger EVN
lifts, those with a size equal to and greater than 800 have a useful
coincidence of two numbers. The maximum lifting capacity, at a speed of 1.6
m/sec. and bucket filling of 75-77%, is easy to memorise.
The number that espresses the size
(800,1000,1200, etc) also espresses their maximum flow rate in cubic metres per
In fact, the elevator size 800 has a
lifting capacity of approximately 800 m3/h and the elevator size 1800 mm has a lifting capacity of roughly
EVN elevators with a size equal to
and greater than 1000 are made with two rows of mounted bucksets offset from
each other. In this way, the buckets adapt better to the drums’ camber, the
flow of material to the discharge is more regularised and the pulsating efforts
of the dredging material at the base are reduced.
The belt is undoubtedly a crucial element of this type of bucket elevator,
and that contributes to determining its fundamental reliability.
The belt used in the EVN is supplied
by an International supplier in accordance with the specific characteristics
required by the company’s designers, which entail a multi-layer textile
structure type only for very small elevators. In almost all cases, the belt has
a resistant metal structure (steel cord belt) for two reasons: high load
capacity and limited elasticity. The high load capacity is indispensable for
elevators of large capaciy and those with considerable wheelbase.
High capacity elevators of 1500 m3/h
and more, are required, for example, in:
operations of loading and unloading materials in seaports due to the high cost
of parking ships.
with considerable wheelbase, even over 150m, they are required in cement plants
for feeding modern flour pre-heating systems to be loaded into kilns.
Lower elasticity is useful for severely limiting elongation during its
operating life. Excessive elongation causes expensive shortening of the belt
wih consequent undesired downtime.
The metal structure is
made up of main longitudinal warp cables, designed to support the load due to
be transmitted, and a transverse “weft” , which gives strenght to the anchoring
of the buckets to withstand their concentrated loads. The metal structure, to
which the efforts are entraste, is immersed in several rubber layers calendared
on it during the construction of the belt. The rubber (a mixture of natural
rubber with suitable additives) has the task of encapsulating and protecting
the metal part and of constituting an
element with a high coefficient of friction, very useful for towing by the drum
of the control station.
The strenghts and weaknesses of the rubber belt include:
- the lower
weight of the belt in proportion with its possible high breaking loads allows
the construction of bucket elevators of the maximum possible liftinh heights
(150 m and more)
- the belt’s
insensitivity to certain types of chemical aggression allows to lift materials
from various industrial sectors
- its service
life, rather than its wear, depends in the ageing of the rubber – that is its
natural progressive gardening over time. The value of the hardness of the belt,
detected in the controls made with adequate frequency, it should be used to
decide when to have a spare rubber belt and to schedule its replacement. As a guideline,
it is not recommended to keep a belt with a surface hardness close to 90 Shore
- The main
limitation to the use of the rubber belt is the low resistance of the rubber at
hogh temperatures. Standard belts allow material temperatures of around 100°C
and temperatures up to 150°C can be accepted only for those “heat resistant”
with a particular final covering. The temperature acts as a catalyst to the
natural hardening process of the rubber, reducing the service life of the belt.
- The cover
rubber fears scratching and abrasion by abrasive materials in size that can
damage it, negating its task of protecting the internal metal structure
the above, the company provides its EVN bucket elevators to be used in the
lifting of fine or small particle size materials that have temperatures not
higher than 150°C
Jointing the belt
The standard joint (Figure 2) of the belt is mechanical and made up of three
elements in aluminium alloy: two external specular jows and a “Y” centred body
to be joined by two sets of bolts with special crews self-drilling screws.
The union of the two ends of the
belt is carried out at the customer’s site, after mounting the body of the
elevator and its two drums. The lower drum is positioned in the upper area of
the tension stroke
The joint is made above the upper
dream (Figure 3). By means of special equipment, the external bolted jaws block
the two estreme edges of the belt positioned between them and the central “Y”
Thanks to their conical threaded
tip, the special crews, guided by the mechanical joining holes, pierce the belt
in the right area that allows a result to be able to exploit the entire tension
of the belt running during his exercise.
For elevators with demanding height,
generally above 50-60 m of wheelbase, there is a special joint formed by the
standard joint integrated at the rear by an additional safety clamp consisting
of three pieces which have a corrugated contact surface.
In this last zone the ends of the
belt are stripped of the rubber cover to act with a clamping action on their
Parallel tensioning system with
To avoid the lateral sliding of the belt
it is necessary to find a “correct position” of the axis of the lower tension
drum. Often this position is not perfectly horizontal as it must be that of the
axis of the upper control drum. The position must however be mantained during
the excercise despite the elongation over time of the belt. For this purpose, Gambarotta Gschwendt uses a
special parallel tensioning systemmade up of a pair of pantographs with counterweight.
The latter costantly mantains the desired voltage value over time in
The nuts of the two lateral screws
allow the axis tilt adjustement of the drum voltage to find such “correct
Gambarotta Gschwendt is always
committed to producing machines of great strenght, durability and above all