David Krauter, a mechanical engineer for Herrenknecht, has spent three decades in the tunneling industry, most of them directly involved in cutters and excavation tools. Throughout that time, mechanized tunneling has improved in its efficiency and reliability across the board, including the cutters themselves. We sat down with Krauter to discuss his background as well as the past, present and future of cutter technology.
How did you get involved with the tunneling industry?
I grew up on a farm south of Bakersfield, California, and I actually had planned to be a farmer. It was on the farm that I got practical experience with machinery, which led me to pursue a degree in mechanical engineering at UC-Berkeley. From there I went to work for a shipyard in Seattle, and in 1988, I went to work for The Robbins Company.
What was your first experience in tunneling?
I first started working in the conveyor department, but quickly began working with roller cutters. Almost immediately after that I was sent to a project site in Norway to learn more about the business and what crews were experiencing. At the time, V-Block cutters were predominant, but that design led to lots of broken fixation bolts which caused excessive downtime and delays. Cutterhead maintenance costs were very high, and it could take up to 4 man-hours just to fish a broken bolt out of the cutterhead. It was a bad situation.
So when I got back to Seattle, I was assigned as part of a team of young engineers to come up with a better way for the TBMs planned for the Svartisen Hydroelectric Project in Norway . Together we developed the first front-loading Wedge-Lock cutter mounting system for 19” cutters. That has evolved into the back-loading design that is the predominant cutter housing today. It is still amazing to me that my boss at the time, John Gibson, entrusted a group of young engineers with the task of devising a new way.
How was the new cutter mounting system better?
The wedge was a beautiful thing. The key was getting the bolt or fastener out of the load path between the rock and the cutter. With the V-Block the bolts were in the load path, whereas on the Wedge-Lock the bolt was off to the side in the wedge and that greatly improved the situation for the bolt not to see fluctuating loads and break. This concept eventually would become the industry standard for all diameters of roller cutters.
Where else did you work?
I worked all over the world – in the United States, Africa, Asia, India and Europe; I even got to visit the Channel Tunnel while it was being constructed. Eventually I joined CTS in 1997, and then in 2006, I joined Herrenknecht after it purchased CTS. Joining Herrenknecht was a real eye-opener for me because up until that time I had only been involved with hard-rock cutting tools. At Herrenknecht, we work on all different types of cutting tools from static soft ground tools to roller cutters for shaft machines and small diameter machines. At our shop in Sumner we manufacture more than 60 different types of roller cutters from 6.5 to 20 in. in diameter.
What are some of the more recent changes to cutters?
The biggest right now is the availability of different cutting tips. There is the standard tool steel ring, but now we have a hard-faced tool steel and tungsten carbide insert with hard facing (TCI-HF). Tool steel is the most commonly used cutting tip and the best choice for most hard rock boring applications where high impact loads are expected. Hard faced tool steel is effective in soft, extremely abrasive rock like sandstone and completely decomposed granite. TCI-HF is often effective in boulder laden ground types like glacial till. The inserts “grip” soft ground and cause cutter to roll instead of slide.
Another development has been in the area of pressure compensators. I was skeptical that pressure compensators would be effective in the underground environment when they are exposed to pressurized mud, dirt and water. But we supplied roller cutters that worked under 27 bar pressure for a shaft drilling job in Nevada in which we used two types of pressure compensators – one piston-type compensator and one bladder-type compensator. At the end of the project we took apart the roller cutters and the cutters that had bladder compensators were still full of oil – not dirt and water like the cutters with the piston compensators so that proved to me that they work. Initially they were developed for deep mining shafts, but we have adapted them for TBMs, including the Robbins TBM that will mine the Rondout tunnel in New York and the Herrenknecht Lake Mead TBM in Nevada.
How does one select the right size cutter to use?
On hard rock, typically you go as large as you can – 19-in. cutters if you can fit them. But as the diameter of the cutterhead gets smaller, below 5 m or so, it becomes difficult to fit 19-in. back-loading cutters so you have to go to a smaller cutter – 17 in. or smaller. For soft ground machines it is more an art of compromise. If you want a maximum amount of openings for driving in sand or clay, the available space for cutters is reduced. So, you need to weigh what you will expect as the predominant ground along with the likelihood of encountering boulders or lenses of hard rock or silt – it is a compromise. Ultimately the contractor will choose the set up he is most comfortable with, and that varies from contractor to contractor.
Now that you know what size cutter, what other factors need to be considered?
Once you have determined the cutter size, then you have to determine what kind of cutting structure to use – steel, hard face or tungsten carbide insert with hard facing. That comes down to the ground conditions. Beyond that you have to determine whether you want monoblock or replaceable ring cutters. Replaceable ring is the economical choice and start at 12 in. and go up to 20 in. Below 12 in. it has to be a monoblock as the rings get too weak, which is made of the same material that the replaceable disc rings are. Finally, you have to determine if other features are needed – whether to use pressure compensators or excluder seals, for example. There are all kinds of designs in order to keep the cutter rolling. That is the process we go through with customers.
What’s next in cutter technology?
One new thing we are working on is a radio frequency ID tag (RFID) for cutters. The hardest thing about getting the RFID tag to work was finding one that would stand up to the underground environment. With the RFID tag and software we are developing, you can just point a reader at the cutter and it will tell you its history, like how many times it has been through the cutter shop and what parts have been used on it. It also helps with inventory management in the cutter shop. The first iteration will just be used for the shop but eventually we want to integrate that into the cutter changing process in the tunnel as well. The RFID technology will make it easier for the cutter shop personnel to make informed decisions about how the roller cutters get rebuilt.
What steps can a contractor take to get the most life out of its cutters?
The key is to be proactive and not reactive with maintenance, and it is important to change cutters before they block. On soft-ground machines sometimes that is difficult as contractors may want to push longer distances to avoid the time and expense of performing a hyperbaric intervention. Careful steering in hard rock conditions can also help ensure that individual cutters are not being point loaded, which could lead to failure.