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Lift Lets Down Simplot

The high-speed canning line at Simplot Australia’s Edgell Division located at Manly was plagued by frequent failure of a scissor lift. The lift was used to lower canned food from one conveyer to another. Pins linking the scissor arms were breaking every few weeks, halting production while new pins were made at the plant’s workshop and installed. Technical staff tried several grades of steel, to no avail. Gilmore Engineers were engaged to identify the problem and recommend a solution.

Visual inspection. There was a suggestion that a part of the lift might periodically catch on the surrounding structure, causing a catastrophic overload. However, close visual inspection of the pins showed a velvety texture on the broken surface and fine, curved marks characteristic of the slow advancement of a crack in the pins. This phenomenon is called fatigue and is associated with fluctuating loads, such as the loading and unloading of a lift.

Mathematical analysis. Knowing the lift geometry and the weight of the load, we were able to calculate the forces in every part of the lift This complicated calculation was made quickly and error-free by our use of Mathcad® software.

The results showed that the linkages in the lift were multiplying the load, particularly when the lift was near the bottom of its travel. In fact, some pins were taking ten times the weight of the cans on the lift! Other pins the same size were taking one-twentieth of this load, indicating unequal load sharing.

Computer modeling. Finite element analysis of the pins (illustrated) using Lusas® showed that the stresses in parts of the pins were more than sufficient for cracks to start and propagate through the pins. Manual calculations backed up the computational results.

Finite Element Model of Pin Showing Stress Contours

The solution. The lift was redesigned to accommodate larger pins. The pin shape was altered to eliminate stress raisers such as changes of shape, and the material chosen was stronger. All of these changes were made to resist fatigue. Two lifts were made to our design and each has been working problem-free on the high-speed canning line for seven months.