Flexibility and power of Simex cutter heads | Dr. Digger’s Prescriptions

by Marco Garofalo, engineering geologist at Simex Application Dept

Simex cutter head range has two different product lines: on one hand the double-drum cutter heads, on the other chain cutters with continuous cutting.

Simex TF double-drum cutter heads are ideal for trenching, profiling rock and concrete walls, tunneling, quarrying, demolition, dredging, finishing operations and underwater works. They are effective where conventional excavation systems are too weak and percussion systems have little effect, due to material characteristics. Their quiet operation allows them to be put to work near sensitive areas such as residential zones, bridges and infrastructures.

Simex cutter head range Includes 7 models, for excavators from 2,5 to 70 tons, all characterized by the direct drive technology (Simex patent). Hydraulic power is directly transmitted to drums and the shaft is free from loads, thanks to the double support bearings for each drum, granting high torque and high performance. The direct drive piston motor has no lubrication points and needs no regular maintenance.

Moreover, the double security filter system preserves the excavator and the attachment from hydraulic system contamination, while the mechanical seals on drums protect from external agents, even when the attachment is submerged into water or muddy ground.

Finally, the shape of the frame, with a reduced distance between the drums, improves the total working width. The two sides of the frame have a symmetric shape which avoids rubbles and stones to get stuck between the drums and the frame.

Simex TFC cutter heads with continuous cutting are characterized by the patented central chain that allows the whole width of the attachment to be milled without gaps at center or side footprints.

The benefits are many. In finishing and profiling works there is a substantial saving of time along with high performances: while working on the material, thanks to continuous cutting, there is no unmilled portion left between a pass and the other.

In the excavation process, the continuous cutting allows to keep the trench width as narrow as possible since there is no need to move the excavator boom laterally.

Quiet and precise in the work area, their versatility gives high performance in various applications:
* Tunnel profiling, foundations and concrete surfaces
* Milling asphalt concrete and concrete
* Milling plasters (due to the lateral disk there’s a perfect control on removed layers)
* Chipping roots and tree trunks

The TFC range includes 4 models, TFC 50 and TFC 100 for mini-excavators from 1.2 to 4.5 tons, and TFC 400 and TFC 600 for excavators from 6 to 12 tons.

The concept of a cutter head is somehow like a small roadheader with some major differences and yet offering some important advantages. Roadheaders were created for the mining industry some seventy years ago.

Mining industry demanded some specific features such as generous excavation output, self-loading capacity for mucking. These and other technical constraints led to specifically designed machines that over the years improved dramatically in power, weight, drum width and cutting capacity.

There is no doubt that milling in general is a profitable and low impact option for excavating. Surface mining also proves that whenever possible, milling rock is indeed an appealing option. With many units deployed around the world the idea of milling rocks as opposed to drill&blast or hydraulic hammer has developed over the years.

Cutter heads, on the other hand, due their flexibility can also be used in concrete demolition, trenching, wall profiling and many different applications.

However, given the wide variety of materials and power supplied by a standard carrier the bottom-line question remains: what material can be successfully milled?

The short answer would be any material with a strength within a specific range proportioned to the size of the attachment. That certainly includes masonry and concrete that has in general limited strength range compared to rocks, and limitation is rather the type and spacing of reinforcement (up to 20 mm diameter for big size attachments).

The real challenge, however, comes with rocks. With so many rock types in Nature one needs to find a sort of guideline to make an operational decision. Indeed, profitability is the real limit. With big cutter heads some hard rocks are still well cuttable but, as with any machine, output decreases and wear increases.

So, how to decide?

There are many rocks and, geologically speaking, classified by their origin and features; traditionally grouped in three main types, igneous, sedimentary and metamorphic. Unfortunately, that does not say much about their behavior when it comes to excavating them, or in engineering geological terms, their excavatability.

One of the first diagrams on excavatability is by Deere back in the seventies where prediction is made combining fracturing and strength for standard excavation, ripping and for drilling&blasting. No provision was made at that time for hydraulic hammer or milling that came in use later on. A conceptual updated version is presented below.

What the Franklin diagram however pointed out, is the important difference between rock as a sample and rock mass, which is the fundamental concept in rock mechanics. In a loose analogy is somehow the difference between a single brick and a brick wall. Rock mass contains fractures of various origin. At a certain scale the interaction between the single rock units may influence the behavior. As for milling, penetration is essential to excavate but if the rock is heavily fractured, quite intuitively, excavation is easy, as removal is dictated by the single blocks moving past each other. The smaller the blocks the more the mass behaves like an almost loose material. The concept is illustrated by figure below.

Without entering the theoretical approach invoking the specific energy concept (how much energy one needs to excavate a single volume of rock) as that requires a more sophisticated knowledge about rock mechanical parameters, we may focus, on two more readily available aspects. Rock strength and degree of fracturing. Indeed, experience shows that for a given power, excavation output is directly dependent on rock compressive strength (also known as UCS, expressed in MPa), The Simex chart shows production vs rock strength according to the different TF models.

But that is only part of the picture. A more detailed analysis has been proposed by some authors that includes the degree of fracturing, that, as anticipated, represents the second key factor.

The illustration below shows the influence of fracture on production output on road headers. It easy to see that they often diverge especially at lower strength. Extrapolating analysis to different rock conditions and different machines always leads to uncertainty but the conceptual framework holds true. As a rule of thumb, the comfort zone for larger cutter heads is up to 50–70 MPa. Beyond that cutting is well possible but excavation profitability depends largely on the degree of fracturing and rock type. A highly fractured 100 MPa non-abrasive rock may be “easier” to excavate than a massively abrasive 50 MPa rock.

To understand how many variables play a role, one should also have an idea of a third parameter known as brittleness. To simplify, rocks can exhibit a brittle or a ductile behavior. Some rocks in other words can deform under load more than others; that means that if two given rocks of different strength are compared the energy required to break them may be the same if the weaker is also ductile.

Without getting into the engineering explanation, it just serves the purpose of explaining the more we know about the mechanical properties, the more reliable the production estimate can be.

So, how can we obtain strength and degree of fracturing? As for strength UCS (Uniaxial Compressive Strength) that is normally obtained in the lab using a press. That is possibly the most reliable method available. The degree of accuracy required by excavatability predictions does not require necessarily a lab test; say +/- 5–10 MPa is still an acceptable accuracy for the purpose. For small projects even a simple geological hammer can provide a first and fast estimate.

Fracturing is more difficult to obtain without a detailed geotechnical investigation but if some drill logs or a rock face are available the value of RQD can be a useful tool for a preliminary assessment. With these two parameters a production estimate is possible by various algorithms. Simex, for instance, has its own analytical tool that is a mathematical expression tuned on the experience gathered over the years in different conditions.

Last but not least, an estimate for pick wear is also needed. Possibly this item is the one that has the major impact on profitability. There are some tables developed for road headers and they tell us something quite intuitive. The harder the rock the higher the pick wear. But again, this is only part of the picture as we need to check rocks abrasiveness that depends on the mineral composition. One of the most common parameters is CAI (Cherchar Abrasivity Index). Knowing strength and CAI allows estimation of the pick consumption, normally expressed as no. of picks per cubic meter. That tells us that sometimes even medium strength rock can have a significant impact on operation profitability.

Simex TF offer maximum flexibility as they can be easily attached to an excavator of any size given the wide range of sizes available. That means calibrating the operation to the scale of the project, ensuring the perfect balance between excavation output and accuracy. In essence, the right tool for the job.

It is certainly worth mentioning that too often attachments’ power is rated according to maximum theoretical pressure and flow. The two parameters should rather be put in context. In the real world excavators can ensure a certain flow at a given pressure; that means that the operational power out is a combination of pressure and flow and cannot be the theoretical maximum too often indicated on commercial brochures.

So, when estimating excavation output realism is needed; production and hydraulic power are certainly related but in order to obtain realistic figures the theoretical maximum power must be derated. If operating within the correct rock strength, Simex TF can deliver significant output, up to 60 cu.m./hr. But production is only one of the aspects as milling has manyfold benefits. Milling also means accuracy. Consider a tunnel, for instance. Overbreaking is one of the issues when using explosives and scaling is often necessary. Using a TF means excavating and at the same time profiling the tunnel walls. The accuracy can be so high that TF can remove wall lining from building or shotcrete or even concrete lining.

So even if rock is too hard to deploy a cutter head for the main tunnel, they can still prove a valuable tool for profiling. Same applies to trenching where excavation to the exact design profile saves extra volume of excavation. Besides, TF does not disturb rock, no overbreaking; that translates in less stress to the rock mass leading to increased stability of rock faces or walls in application such as quarrying.
Avoiding explosives also means minimizing vibration noise and dust. That is an important factor when working in urban or densely populated areas, as shown in the diagram below.

To sum up, cutter heads are highly flexible tools, less demanding in terms of investment and logistics than a specifically designed equipment; still, they can deliver significant output in excavation and demolition. A careful analysis should be carried out to verify feasibility and profitability; once those are met, they can be applied successfully to various contexts.

For this reason, at Simex, our team of experts, made up of qualified technicians together with application specialists, will be able to guide the user in the most appropriate choice, specifically analyzing the material to be processed. The skills of our consulting service represents a further plus, along with the construction quality, durability of the attachments, a capillary distribution network and a qualified product support service.

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Call #SuretechInfra 1800 120 7873 sales@suretech.co.in www.suretechinfra.com ​

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Originally published at https://doctordigger.chapter24.blog.