:: Easy Stroke Adjustment for GP300 Cone Crushers

Metso Minerals now offers externally adjustable strokes for its Nordberg GP300 and GP300S cone crushers. It allows easy changing of eccentric bushing positions without dismantling the crusher.

Traditionally, stroke adjustment in a cone crusher has not been used to optimize production in crushing plants. This is mainly due to the difficulty in accessing the eccentric bushing, as the top frame and main shaft need to be dismantled. However, the stroke influences cone crusher performance in several significant ways. Not least among these is that in varying operational conditions, choosing the right stroke (larger/smaller) provides an opportunity to maximize the end-product capacity and product shape with the GP300 and GP300S cone crushers.

The idea of externally adjustable stroke came from a Metso Minerals customer, Reijo Savolainen, who is chief executive of Rudus Murskaus. A pioneer in crushing and screening operations, Reijo recalls how he first thought of the adjustable stroke in the mid 1980s when he was faced with bearing problems in a crusher. "We had an old G14 cone crusher and were running it at too tight a setting to make fine fractions or achieve a good shape for the product. As a result of overloading the crusher, we had problems with the bearings. While I was dismantling it, I thought that the dirty work and the crusher downtime could be avoided if the crusher had not been loaded by too high a stroke compared to the closed side setting. Decreasing the stroke without dismantling the crusher would have been the solution to the problem."

Reijo Savolainen, chief Executive at Rudus Murskaus was the father of the idea. "We did the stroke change a couple of times, but dismantling the crusher at the quarry is a demanding job.” Reijo continues. “There is always the risk of a mounting fault and of dust entering the bearing faces. Because of the potential risks, we preferred not to perform the stroke change in the quarry. This means that the stroke often isn't changed at all during the crusher's lifetime," he explains.

Reijo resurrected the idea a decade later in the mid-1990s when he was working as a technical director at Lohja Rudus Oy. "It was a very lean time in the crushing business, and we were looking for ways to improve our company's profitability,” he says. “On realizing that our plants were running with similar layouts and the same stroke in G cones – even if the crushing requirements and the rock harness changed every day – we began to study which stroke and setting combination would yield the best production in each job. Since we are aware of the risks of stroke adjustment involved in dismantling the crusher, we were forced to find another way," he reveals.

The idea of externally adjustable stroke took several years to develop, and finally Reijo demonstrated his idea in scale model form to Metso Minerals in early 2000. "The people at Metso were excited about this innovation,” he says. Further developing the concept took several years and now, after more than 10,000 hours of testing, Metso is offering it as an option on its GP300 and GP300S series crushers.

Rudus Murskaus has two of Metso’s Lokotracks and a Nordberg NW300GP wheel-mounted crushing plant equipped with the externally adjustable stroke. The crushers are running in two, three, and four-stage crushing plants, and their work takes them all over Finland. The Lokotrack plant consists of LT125, LT300GPS, and LT300GPB units. The plant typically operates in two ways; either with the LT125 + LT300GPS producing 0/32 or with the full train attached with LT300GPB producing 0/5 and 5/16. "When we are producing 0/32 with a two-stage crushing plant, we typically adjust the secondary GP300S stroke to 25 mm, because then we are able to use a small enough setting," explains Reijo. "Thus, we are able to maximize the amount of quality product we produce," he continues.

The optimal stroke and setting combination changes when the plant takes on the third stage. "We are making two products in our three-stage Lokotrack plant, 0/5 and 5/16. Depending on the market need and stockpiles, we will need to make more of either 0/5 or 5/16," Reijo calculates. "For example, if we want to maximize the 5/16 fraction in our present quarry in Helsinki, we change the GP300S stroke to 18 mm and the GP300 stroke to 25 mm. However, if we want to maximize the 0/5 fraction, we use a 32-mm stroke in the GP300S and a 40-mm stroke in the GP300. By changing the stroke in different jobs and with different rock types, we are able to get the best performance and product shape from the LT train at all times," he elaborates.

Reijo explains how the stroke increase improves the quality of the end product. "In one of our quarries, the end product's flakiness index was 11–12%. This was produced mainly by the GP300, at a 25-mm stroke. The stroke was increased to 32 mm, and the shape improved so much that we met the requirement for under 10% flaky particles in the end product. We didn’t, therefore need to use the VSI to shape the end product, which was a huge benefit for us."

The financial benefit of stroke adjustment has been shown in the figure below. If there is a possibility to upgrade production by increasing the stroke from the normal 25 mm to 32 mm and the change is not made, the blue curve illustrates the annual lost production in relation to the average contract size. It has been assumed that in every other contract the optimal stroke is 25 mm and in the rest 32 mm.

If the stroke is changed the traditional way, by dismantling the crusher, the red curve demonstrates the annual production lost because of the extra time needed to change the stroke. It is assumed here that changing the stroke in the traditional way takes eight hours, and the externally adjustable stroke takes 10 minutes. Apart from the lost production, the additional costs incurred for labor, crane work, etc. come to about 2,500 euros a year (an example cost from Finland).