Performance of Flail Hammers

by Frédéric Guerne [ Digger Foundation ]

The following article discusses the strengths and weaknesses of flail hammer use in the demining field. The author leans on his field experience with the Digger Foundation to analyze types and usage of these tools. In doing so, he aims to give advice on the best methods for using flails to achieve the best results.

Examples from the field show that when carrying out ground-preparation work with demining machines, the parameters that influence the users’ operational choices can be based on ill-suited criteria. A number of factors need to be considered before employing flails in the field, some of which are described here. This article does not detail a full scientific study, but instead shares some practical experience from the field. The article provides information regarding the strengths and limitations of different flail-hammer designs, as well as advice on the best way to use these tools. Our conclusions about flail hammers come from using the Digger D-2 during demining operations in Sudan, and from tests carried out by the Digger Foundation to improve the performance of the D-2 in Sudan.


(Click the image to enlarge)
Influence of the hammer shape on digging efficiency.

The article also stresses the point that demonstrations of machines in short runs or lanes cannot be expected to highlight all the key parameters involved, in particular the life span of wearing parts such as hammers. These factors must be analyzed in detail through field tests. Digger’s field experience can hopefully provide insight and guidance for others to apply to their own contexts.

Flail Hammers

Two of the most important parameters of flail hammers, their shape and material composition, heavily influence the efficiency, lifespan and price of hammers. Identifying the best possible hammer, i.e., the design that provides the best results depends on the projected use and leads to the necessity of finding the best balance between these aspects.

Influence of the Hammer Shape on Digging Efficiency

We define efficiency as the ability of a flail hammer to penetrate soil under specific conditions to a given depth using the least possible flail power. The more efficient a hammer is, the lower the fuel costs. These considerations assume that the desired digging depth is the depth setting of the flail. It is not acceptable to improve efficiency simply by decreasing depth.


Sudanese soil before being flailed.
All photos courtesy of the author

To the best of Digger staff knowledge, and from what has been observed by staff in the field, the most widely used hammer shape is the mushroom-shaped hammer. This hammer design provides good digging efficiency—the sharper the cutting edge is, the more efficient the hammer will be.

Using the Digger D-2 electronic load-sensing control,1 the efficiencies of different hammer shapes and designs were compared by simply measuring the time spent working a lane of given length in a constant type of soil with each flail. The results, reported in Table 1 (above), show, for example, that when using the square-shaped hammer, it took almost twice as long to process the same area compared to using the sharp, mushroom-shaped hammer.

On the other hand, Digger’s experience also shows that the sharper a hammer head is, the shorter life span it will have. Indeed, as soon as the sharpness reduces, the digging efficiency is lost and the hammer loses its advantage.

Life Span

The Digger Foundation’s experience in Sudan has greatly influenced the design of Digger hammers with a 3-mm tungsten coating on the bottom surface. During Digger operations in Sudan at first, the traditional thick, mushroom-shaped hammers were used. In the conditions we met, however, especially in North Sudan, such a shape proved not to be useful. The wear generated by the specific soil conditions existing in Sudan (compacted sand) reduced the hammers’ lifetime to less than three hours for the improved version with a 3-mm tungsten coating and to half an hour for the standard versions (150 HB steel).2 As previously mentioned, the main problem with the “mushroom” hammer is that the tool is not useful once the cutting part of the mushroom head wears out.


Sudanese soil after being flailed.

The use of the costly tungsten coating (about 12 euro or US$173 for the coating of one hammer when producing a minimum of 200 at one time) provided the best compromise between hardness and shock resistance. Though this type of coating increased the lifetime by a factor of four to six compared to the uncoated steel (150 HB), it was still not acceptable (i.e., less than three hours’ operating time before being worn out). Six different tungsten coating systems were then tested, from thin (0.1 mm) to thick coating (2–3 mm). This testing was done using ion projection and flame deposition.

One of the downsides of using a hard tungsten coating was the associated cost. Eventually, the cost/life span ratio was deemed uneconomical and Digger then moved to using square-shaped hammers, which have a longer life span. With this hammer type, using 150 HB steel, the advantage is that the hammers can wear more than 70 mm before having to be replaced.

This new solution and design increased the life span by a factor of two in comparison to the expensive, tungsten-coated “mushroom” version. The hammers could now be used for six to seven hours. However, researchers deemed a life span of six hours was still not sufficient and sought other solutions to further increase the life span of the hammers.

The steel hammer (150 HB) was replaced by one that can be tempered (about 380 HB). This process had the advantage of being significantly less expensive than tungsten coating. With this improvement, the life span of the hammers was increased by a factor of two to three, compared to the non-tempered (150 HB) square-shaped hammers. In North Sudan, a lifetime of around 14–18 hours was finally reached.


Almost new thick “mushroom” hammer.

Digging Efficiency

The remaining question was how to combine a long life span with a sufficient cutting and digging efficiency. The answer was to develop a hammer with a new shape—one with a sharper cutting edge than square hammers that could also be used for a long time without needing to be replaced. These new hammers are made of the same standard steel, square-shaped bar we used for the square hammers, but are cut at an angle and have the chain attachment off-center.

The center of gravity of the hammer head will automatically be positioned in line with the chains by the centrifugal force of the turning flail, ensuring that the hammer position is at the desired angle when hitting the ground and that its cutting edge is optimally placed. The wear of the hammers, due to their position, is minimal while allowing for maximum cutting. These hammers are also tempered to provide the same advantages we had with the square-shaped ones.

Cost Aspects


Uncoated hammers after two hours' flailing.

As indicated above, in order to reach a good efficiency/life span ratio, technological solutions (steel quality, tempering process, etc.) have to be engaged, which can increase the price of the hammers. According to Digger’s experience, it is more effective to have high-quality hammers with an appropriate design than to use low-cost hammers that will be worn out in a few hours. The latter option requires operators to stop the machine frequently to replace damaged hammers. Having to change hammers also represents a logistical problem in terms of transportation and storage of spare parts. Such delays, in turn, further increase the overall running cost. Trying to save money with low-quality hammers can turn out to be quite costly.

Flail Rotating Speed

Another important point with regard to the hammer design is the rotating speed of the flail. The faster a flail rotates, the greater the wear of the hammers. Decreasing the rotating speed of the flail, however, can lead to very dangerous results concerning the digging profile, leaving skip zones, or areas where the hammers will not dig into the ground. It is unsafe and counterproductive to reduce the rotating speed of the flail to try to reduce the wear. This will undoubtedly reduce the quality of the work, and it is not recommended.

Conclusion

Flail-hammer choice represents a critical aspect of mechanical-demining operations with flails. The design and quality of the hammers need to be considered carefully since these factors will have cost and operational implications for the project as a whole in the long run. Regarding the comparison between life span and digging efficiency, Table 2 (above) summarizes the characteristics of the standard hammers Digger developed and corresponding lifetimes based on Digger’s operational experience in Sudan.

Biography

Frédéric Guerne worked for 10 years in Swiss industry as a development engineer. Since 1996, he has worked with mine-detecting sensors as a Team Leader at the Demining Technology Center project at Ecole Polytechnique Fédérale de Lausanne, Switzerland. In 1998 he founded a nonprofit organization, the Digger Foundation, which manufactures demining-assistance machines.


Endnotes

  1. This is a system that ensures the maximum available power from the engine is "sent" to the digging tool (flail or tiller) in order to ensure the highest possible efficiency in terms of m2/h. That means using this system, if a tool has a better digging efficiency than another, the forward speed of the vehicle will automatically be higher than for a less efficient one.
  2. BH refers to the Brinell Hardness of the steel according to the Brinell Scale.
  3. Date of conversion 19 June 2009.

Contact Information 

Frédéric Guerne
Digger Foundation
Rte de Pierre-Pertuis 26
P.O. Box 59
CH-2710 Tavannes / Switzerland
Tel: +41 32 481 27 73
Fax: +41 32 482 27 74
E-mail: info@digger.ch
Web site: www.digger.ch