ESPC Bucket Elevators

Let’s focus on our ESPC Bucket Elevators

With this newsletter we’d like to outline the design factors of central bucket chain elevators that are key to providing a low-cost and long life solution for transporting bulk materials

For industries where vertical transport of solid bulk material is necessary, Gambarotta Group supplies bucket elevators according to the physical characteristics of the material to be conveyed, such as particle size, temperature, humidity, stickiness, abrasiveness, etc

Whereever vertical transport is required, bucket elevators play an important role in the lifting of bulk materials.

Even the cement industry has to deal with the issue of raising different types of materials (limestone, pozzolan, chalk, various additives, raw meal, clinker, cement, alternative fuels, ash, and others).

Other than the slow elevators used for lifting large materials (100-350 mm) with gravitional discharge, there are two categories of elevators frequently used in the cement plant – belt bucket elevators and chain bucket elevators:

• Belt elevators: suitable for materials with fine or small grain, and with a temperature of less than 120°C (max 150°C). They allow high lighting capacities (1800 mc/h and more with bukect filling coefficient < 80%) or high lifting heights (wheelbases up to 150 m and more).
• Chain elevators: suitable for materials with medium grain size (up to 80-100 mm) and also for high temperatures. However, these elevators are not suitable for large lifting heights (60-70 m wheelbases are already exceptional). The limitation of their use for large wheelbases is due to the weight of the chains, which grows as the breaking load, that becomes necessary to use, increases

Factors to be considered for chain elevators

Generally speaking, chain drives are characterised by the discrete nature of the pitch of the meshes and the number of teeth “Z” of the wheels (Z real if the wheel is toothed, or fictitious if the wheel is smooth). This makes the chain drive unique and offers advantages and disadvantages. The advantages include, besides the lower load on the shaft bearings, the certainty of a transmission without slippage between the chain and the wheel, if this toothed.

The disadvantages are the vibrations and the resultant noise caused by the impact of the teeth on the wheel, and even more from a phenomenon that we will call “polygonal action” that occurs due to the fact that the chain, wrapped around the wheel, forms a polygon rather than a circle. This leads to a floating movement of the chain.

    In designing a conveying machine that uses mechanical chains (mesh formed by pins and bushes joined by side plates), the designer should consider a fairly long chain pitch (distance between the pivot points) to minimise costs. The chains in fact, often constitute a very important part of the overall cost of the machine. However, this clashes with the polygonal action, which requires the chain pitch to be reduced to more contained values.

    When the drive wheel of a chian drive operates at constant speed, the speed of the chain itself is not constant but is subject to periodic fluctuations. This fluctuation, which is caused by the fact that the chain, when wound on a wheel and wheter it is toothed or not, forms a polygon rather than a circle, and is known as a “polygonal action”; the chain is said to move with a “polygonal speed fluctuation” gene rating dynamic loads to be taken into due consideration.

            Several studies contacted on this phenomenon show that the dynamic load of the chain due to this “polygonal action” is similar to that of a simple forced vibration where its intensity increases with increasing speed up to the value of the critical speed. 

            Higher speeds produce lower dynamic loads It should be noted that the speed in bucket elevators is much lower than the critical speed, however, the masses who have to move the chains due to their own weight, the weight of the buckets and the weight if the transported materials, are considerable. Such masses, subject to frequent speed reversals, and therefore subject to alternate accelerations, generate non-negligible efforts of cyclically variable intensity, generating vibrations and therefore noise.

            The experimental results show that the number of teeth of the wheel and the chain pitch are the main factors that influence the dynamics of the chain, i.e. the polygonal speed fluctuation.

            The fluctuation decreases with the increase in the number of teeth of the wheels and with the reduction of the chain pitch.

            The mechanical chain with the shorter pitch can work more fluidly, and with less vibration, due to the less polygonal speed fluctuation

            From here it is undestandable why the round steel chain ring elevators, which have a very short wheelbase, have a less noisy operation

            Designers of chain elevators therefore need to identify the geometric parameters of chain pitch and the number of teeth of the wheel that determine a fair compromise between the cost-effectiveness of the transmission and transportation that is as fluid as possible

Special features of the central chain elevators type ESPC

In recent decades it has been noted that the type of Central Single-Chain Elevator, following the strong marketing push of some leading manufaturers, has become the most requested chain elevator

Gambarotta is also following this trend at the expense of the historic EVD and EVR elevators, which use the two round steel chains with oval rings.

Among the varios bucket elevators with chains that are in the Gambarotta Gschwendt manufacturing programme, there are also bucket elevators of the ESPC series, classified as fast elevators with centrifugal discharge with a single central chain.

            Gambarotta produces ESPCs with a single row of buckets driven by a single gear wheel.

            They are used for materials with medium grain size, <100 mm, and have a maximum lifting capacity limit of about 600 mc/h. This capability fulfils 80 – 90% of the current market demands.

            For higher capacities, Gambarotta uses chain elevators of the ESPLV series. These have buckets connected to the two side chains, driven by two toothed wheels (see Figure 3), and can reach capacity lifting up to 2000 mc/h with buckets width 1600 mm, speed 1.6 m/sec and filling 80%. This is more compact and cost-effective solution for large lifting capacity.

            The ESPC central chain bucket elevator boasts some unique chain and wheel drive characteristics.

The chain

The choice of the chain pitch adopted by the company was made in consideration of the technical issues presented earlier in the article, combined with the company’s extensive experience with the supplies of chain buckets elevators

These considerations and experiences, for a maximum ascent speed of 1.6 m/sec (96 m/min) of the ESPC Gambarotta, led the company to adopt only two types of chain pitch: pitch 1500 mm for smaller machines, and 180 mm for the larger ones.

Gambarotta’s goal was to seek the maximum benefit for its customers: low-cost and long service life. The company chose a simple mechanical chain, which follows the standardisation base of the DIN 8167 regulations. The chain is easily reproducible, and is made of commercially available materiales (plates and round bars). It is also workable on machine tools by many manufacturers, to form the three characteristic elements of the chain; plates, pins and bushes.

            Various sizes of resistant elements are provided to obtain chains with different breaking loads suitable for the chains of particular elevators.

            Other manufacturers use chains with special parts and expensive forged materials, which require the use of appropriate moulds and equipment.     

            The considerable cosst of these chains limits the production to a few models, perhaps only 3 or 4. This sometimes means that elevators have to be equipped with an exaggerated chain, due to their cost.

            Furthermore, three chains, designed and patented, often induce the customer to be “tied” to the bucket elevator fora n expensive spare parts.

            Gambarotta Gschwendt determined some important parameters of the chains, including the thickness of the bush and the hardness of the contact surfaces subject to sliding

• The bush is of the thick type. The hardened peripheral areas (internal and external) are thus separated by a tenacious interposed zone to withstand those dynamic loads, which cause undesired vibrations, due to the polygon action mentioned above.
• The surfaces in contact between the pin and bush and between the bush and wheel teeth are heat treated in order to impact high hardness, and then resist for long wear.

To ensure a long service life, the chain is selected by comparing the real safety factor (ratio between breaking load and working load) with a minimum safety factor calculated with a particularity.

This minimum factor is variable from elevator to elevator as obtained, starting from a common basic value, which is then influenced by the value of wear cyclicity (of wear cycles per unit of time). The cyclical wear is due to the ascent rate and the wheelbase parameters of that precise elevator; directly proportional to the speed and inversely proportional to the wheelbase. In a transmission chain, each link undergoes four wear cycles for each complete devolution: one at the beginning and one at the end of its contact with the upper and lower wheels. The cyclicity value is therefore maximum for low and fast elevators and is minimum for high and less fast elevators.

With this type of selection, and with the geometric dimensions chosen for the chain, the specific contact pressure between pin and bush has low values, generally lower than 30N/mm2, with a consequent increase in operating life

The toothed control wheel

            Gambarotta’s elevators chains are manufactured with control gear, with interchangeable sections. Therefore, ESPC elevators also have a control gear wheel (see Figure 3)

The main reason for this choice is that only the gear wheel can guarantee a slip-free transmission for all operating conditions.

            Although when it is subjected to vibrations due to a “polygonal action”, that generates the fluctuation of the speed of the chain,. The company believes that this fluctiation can cause the chain to slip on the smooth periphery of toothless wheels, causing unwanted wear.

            Again, it should be noted that the phenomenon of polygonal action also occurs on smooh wheels where the real teeth constituted by the points of support of the bushes on the wheel circumference.

            During the operation of an elevator, possible slippage between the smooth wheels and the chain can occur.

            This for example, may be due to the pulsations caused by the dredging of the material or the possible discontinuous flow of material being fed into the base.

            Furthermore, the smooth wheel system requires consistent values of chain tension to create the friction necessary to guarantee the transmission of motion from the wheel to the chain. This causes an unwelcome chain overload and an undesired increase in the load on the drive shaft bearings.

            Some chains designed to work on smooth wheels, in addition to the bushings, also have the edge of the plates that rest, sometimes causing a flexure, which is not suitable for secure resi stance.

            The plates of a chain wound on a gear wheel, on the other hand, are always subjected only to pure traction

            Finally, even the choice of the diameter of the drive wheel has an effect on the wear of the chain since it determines the magnitude of the angle of friction between the pin and bushing which occurs at the beginning and end of the rotation of the chain on the wheel. A higher number of teeth , and therefore a larger diameter wheel, corresponds to a lower rotation angle, followed by a lower intensity of wear.

Here, a compromise is needed to prevent large wheels from deading to a greater frontal dimension of the elevator and an increase in cost for a control unit with greater torque.

 Gambarotta Gschwendt is always committed to producing machines of great strenght, durability and above all maximum reliability