Adapting to Terrain: How Deflection Analysis Shaped a New Zealand Cable Logging Strategy

Cable logging operations require meticulous planning, to ensure cost effective and efficient outcomes. In this article, we explore how varying tower heights, skid placement and deflection influence payload capacity and engineering decisions.

Understanding RoadEng Cable Analysis

This analysis was conducted using the RoadEng Cable Analysis Tool, specifically employing the "Maximum deflection given load" method. RoadEng offers several other analysis options tailored to different operational needs, including:

Maximum flying load given clearance:

Deflection for fixed skyline given load:

Simple third point deflection:

Comparing Cable Analysis: 70ft vs 90ft Towers

Using advanced cable analysis, we investigated the potential of increasing payload capacity through greater deflection. Here’s what we found:

• 70ft Tower Configuration: This site had been planned for a 70ft yarder using a motorised grapple. However, as the modelled payload increased to 4.5 tonnes, the system experienced significant deflection loss. This loss of deflection was particularly problematic because it would reduce the system’s ability to safely and effectively extract stems over the terrain and would mean “high leading” logs and creating a significant “red zone”, which the breaker outs would have to manage.

• 90ft Tower Configuration: Increasing tower height to 90ft significantly improved deflection, raising the payload capacity to 8 tonnes. Although this would have been an ideal solution, access constraints in the area and the unavailability of a larger tower made this option impractical

  
  

Adapting Skid Placement to Optimise Operations

Rather than opting for a larger tower, alternatives were explored that balanced cost, accessibility, and operational efficiency. The original plan included a "hot deck", where logs are processed and loaded onto log trucks. To address the deflection issue caused by the terrain, the logging company had to rethink the approach.

The solution involved moving the skid site and creating a mini pad specifically for the yarder. This adjustment allowed the 70ft grapple yarder to sit lower on the slope, resolving the deflection issue. Although this two-stage logging process added harvesting expense

(approx-$5-$8 per ton), it was ultimately the preferred option. The benefits included:

Cost-Benefit Balance:

Accessibility:

Deflection Management:

Why Deflection Matters

Deflection in cable logging impacts how much weight a system can safely transport. Better deflection means less strain on the cables and equipment, translating into higher payload capacities and more efficient logging operations. Without adequate planning, there is a risk of designing skid sites and operations that may not support intended deflection and/or productivity levels.  Study the table below to see the dramatic effect deflection has on tension

Financial Foresight: Failing Safely $$$$

At Schroder Hill, we emphasise the importance of “failing safely” within a financial context. Using advanced GIS and CAD tools like RoadEng, we can test the limits of designs and strategies in a digital environment, where the cost of error is minimal compared to mistakes made in the field.

Let us work alongside you to leverage these tools, providing the expertise needed to optimise projects, reduce risk, and maximise efficiencies for your clients!!