Quarry has an eccentric upgrade

In the construction materials industry, you?d be hard-pressed to find a piece of equipment that works as hard as a vibrating screen. Every day tonne after tonne of material is dumped on them, shafts turn and bearings drive. Although they endure tough working conditions, they?re still held to the highest standards and expected to produce a clean, properly sized product every time.

It?s with good reason this equipment is held to such a high standard. Responsible for the primary material production tasks, one could argue vibrating screens are the heart of every processing operation. They can be the difference between uptime ? productive and profitable ? or downtime ? detrimental and costly. One quarry in the township of Presque Isle, Michigan, US, adjacent to Lake Huron, would certainly attest to the impact equipment, particularly vibrating screens, can have on an operation. 

Acquired in 2000, the Presque Isle (PI) quarry is one of nearly 1600 worldwide production sites under the Lafarge Corporation umbrella. As a leading materials producer in the US, offering everything from standard construction and asphalt stone to unique high calcium limestone (HCL), the PI quarry relies on the heart of its operation to ensure uptime remains high. For PI, this heartbeat comes from a fleet of 14 F-Class vibrating screens from WS Tyler. 

To say the equipment has been crucial to the company?s success is a strong statement that is backed by solid numbers. In just over a decade, PI has become the largest producer in Lafarge North America?s family, consistently averaging more than 2400 tonnes per hour and an average annual production of nearly eight million tonnes per year. 

{{image2-a:r-w:200}}The PI quarry isn?t an ordinary operation and didn?t get to where it is by accident. Rather, the entire team, from the maintenance staff to management, goes beyond ordinary and gives large doses of extraordinary. This is evident in both day to day operations, as well as long-term strategy. A prime example of PI?s tactical planning approach took place in 2002, when it was going through a scheduled upgrade of its vibrating screen equipment. 

It was at this time the new fleet of F-Class machines was purchased ? but it wasn?t a simple decision based solely on the typical criteria of tonnage output, high production or speed. While Dave Nelson, PI?s manager, did have specific and high expectations for the machines? performance, the real deciding factor was based on something that wouldn?t be measured by standard production benchmarks.

Multi-storey screen houses are quite common in the aggregates industry. Combine their size, capacity and force with the inherent function of a vibrating screen ? to vibrate ? and it?s easy to see that a resulting side effect can be transmitting vibration back into the structure, resulting in major structural concerns for a multi-storey building. The intrinsic vibration concerns often result in limitations on the placement of screens within the structure, with a limited footprint per floor. The combination of logistical factors can make screen selection quite complex, exactly the case for the PI quarry. Because his operation utilises a six-storey screen house, with machines on the second, third and fourth floors, both vibration concerns and a limited footprint were paramount considerations in Nelson?s decision.

Beyond logistics, Nelson had specific goals he wanted to achieve with the new machines. The key goal was to improve efficiency in load rates, particularly conveyor load rates. Because of its proximity to Lake Huron, the PI quarry transports material by water and utilises conveyors to load the shipping vessels. Especially with rising fuel costs in recent years, Nelson points out the distinct advantage PI?s location offers over its competitors. 

?Land-based producers typically only serve customers within a 100 mile radius or less,? Nelson explained. ?Our location grants us the advantage of waterway transport and allows us to expand our market reach hundreds, even thousands of miles and into multiple states.?

While the expanded reach has given PI a distinct competitive edge, what wasn?t helping it was less than adequate loading rates. Nelson says the conveyors are capable of carrying 4500 tonnes per hour but during the upgrade planning and operation assessment, it was found they were nowhere near close to maximum efficiency. In fact, he estimated they were sometimes loading as little as 800 tonnes per hour or less, depending on material. 

{{image3-a:l-w:200}}?To say we were underutilising our load-out facility is an understatement,? he said. This made it crucial for the company?s vibrating screen selection to offer enhanced performance. It required more than trading up to a newer model. Additionally, increased efficiency didn?t equal greater tonnage output or even faster operation. It wasn?t going to be as simple as machines bigger in size and production capabilities either. And the final choice couldn?t leave out the most important factor ? structural concerns.

PI?s plant engineer Dennis Skaluba explored options with vibrating screen manufacturer WS Tyler. Known for offering several types of vibrating screen equipment, WS Tyler was the company Skaluba sought for PI?s screening equipment needs. 

First, a precise analysis of the screen house?s structural integrity was needed. A local engineering company conducted a professional structural analysis and Skaluba was left with two options: choose a fleet that would transmit minimal or no vibration into the structure, or require significant infrastructure changes to the existing screen house to accommodate added vibration. The latter wasn?t an option in Skaluba?s mind, so after sharing the structural analysis results with Nelson, the two began looking at options. 

Two-bearing configurations, a common design in the vibrating screen world, do not produce a consistent stroke, which is one of the hallmarks of four-bearing, eccentric shaft designs. This means excessive vibration is transmitted back into the structure. With that option ruled out, the two were hoping to find a four-bearing machine large enough to meet efficiency expectations that would also, and most importantly, fit logistical needs. They were pleased to learn that WS Tyler had the answer to meet both demands: the four-bearing F-Class vibrating screen.

Featuring advanced, double eccentric shaft technology, the F-Class screens produce a constant, positive stroke that handles material volume spikes without losing momentum. As the eccentric shaft turns, the screen body is forced to follow the shaft movement. While it travels upward, the counterbalance weights move in the opposite direction and create an equal force to that generated by the body. As a result, the forces cancel each other, allowing a dynamically balanced system that transmits minimal to no vibrations into the structure and enables optimum production and reduced CAPEX.

{{image5-a:r-w:500}}Nelson and Skaluba purchased a new fleet of 14 machines. The units were of varying sizes, based on the location within the screen house, and broke out as shown in Table 1.

Though no additional vibration would be transmitted into the structure, the upgrade did require minimal structural reinforcement to accommodate the larger equipment. The impact of the new F-Class machines though? Anything but minimal.

PI increased screening area by 60 per cent while staying within the designated footprint and vibration limitations. The biggest impact, however, was realised in Nelson?s primary goal ? efficiency. Nelson said he and Skaluba were able to measure a difference within the first year. ?Probably the most dramatic example of this increase was what we saw in our one-inch minus material,? Nelson said. ?Our loading rate increased by 50 per cent.?

A primary contributor to this increase, he explained, is the cleaner product. In addition to the minimal vibration offered by the screens? consistent stroke and G-force, the F-Class? eccentric design also optimises screening performance, resulting in virtually no blinding and pegging issues. The self-cleaning design of the machines is complemented by the chosen screen media to produce a cleaner product, which makes the conveyor and loading process much more efficient.

While the one-inch minus material was the clear leader in terms of increased efficiency, Nelson said the company forecast a 15 per cent increase in overall load rates within the first year. 

The PI quarry was upgraded 10 years ago and those same machines are still in operation today. Throughout the years, Nelson and Skaluba have kept a close eye on the plant?s efficiency, load rates, maintenance requirements and a host of other key performance indicators to assess the equipment. This extraordinary monitoring and preventative maintenance schedule is based on one of Lafarge?s unique initiatives: the Lafarge Maintenance Module. 

Essentially a benchmark to ensure better accountability and documentation for preventative maintenance needs, the maintenance module is a clearly defined program of everything from periodic screen inspections to regular temperature readings on the equipment?s bearings. Skaluba feels preventative maintenance is the key to high uptime and performance. 

?I take it very seriously,? Skaluba said, referring to the program Lafarge implemented in 2004. ?It?s a good, solid program and we have seen first hand the value preventative maintenance brings to our equipment life and uptime.?

One of the module?s parameters includes planned maintenance and scheduled downtime, to ensure as little interference with the operation?s uptime as possible. Combined with the natural result of good preventative maintenance, uptime at the PI quarry has been exceptional. The proof is in the percentage, which, according to Nelson, is an impressive 94 per cent.

Though Nelson is quick to credit Skaluba?s extraordinary commitment to the maintenance module as the reason for such impressive uptime, he also recognises WS Tyler?s role beyond the equipment itself. 

?The screening machines are exceptional but a primary reason we wanted to work with Tyler was the service aspect,? Nelson said. ?For example, Tyler representatives were here on-site, day one, to train our maintenance team on the equipment.? 

{{image4-a:l-w:200}}Additionally, the Tyler team provides ongoing support. One particular service Skaluba appreciates is dynamic vibration analysis. This performance test evaluates the product, screen media and vibrating screen, and tests speed, stroke and overall performance. A report is provided, with professional recommendations to improve vibrating screen performance and reduce maintenance costs. Follow-up assessments were conducted in 2004, 2006 and most recently 2011. 

?The analysis reports have helped us troubleshoot a few times, so in those cases they?ve been invaluable,? Skaluba said. ?Equipment failure and downtime is probably the biggest issue we deal with in our industry, so on the rare occasions we experience it, it?s imperative we get back up and running quickly. Every minute a piece of equipment isn?t working, we miss out on production and profit.?

The F-Class machines have been PI?s workhorses for the past 10 years and Nelson expects them to last at least another decade. The longevity is especially remarkable to him because, as he puts it: ?They?ve gone through 10 years of nearly eight million tonnes per year in production. Do the maths ? now that?s impressive.?

Often, all that separates ordinary from extraordinary is that little ?extra?. Lafarge PI has taken every key component of success, from world class maintenance to strategic planning, and sprinkled it with enough ?extra? to go far beyond the ordinary and become the leading producer it is today. ?