The avoidance or reduction of chainsaw kickback is a key safety feature for manufacturers. The challenges for engineers are to obtain more accurate analyses of the problem over the tool’s lifecycle, improve post processing analyses and then to validate the analyses experimentally.
In this technical article, EnginSoft and chainsaw manufacturer Emak explain how they used the advanced system modeling capabilities of the MapleSoft products (Maple and MapleSim) to determine maximum chainsaw system performance, detect performance issues early, manage multiple engineering domains and improve pre- and post-processing analyses, before conducting structural analysis and experimental validation with ANSYS Mechanical Workbench 18.
The article describes the various phases of the workflow process adopted by the engineers in the modeling process, from the creation of the initial model, the mathematical and physics analyses that they conducted, to the analyses of the results obtained, the DOE verification process and finally the experimental validation of their findings.
The use of the MapleSoft and ANSYS software in combination allowed the engineers to drastically reduce trial time and fine-tuning, while also saving time on data interpretation and mathematical analysis in order to improve chainsaw performance, minimising damage and fatigue over the product life to increase chainsaw safety for operators.
The Emak Group operates worldwide, handling distribution in Italy and in another ten foreign markets - USA, France, Germany, UK, Spain, Poland, Ukraine, China, South Africa, Chile and Brazil - through subsidiary companies, offering a vast assortment of products with recognized brands and addressing a highly diversified customer target group.
Emak products address gardening and forestry duties and agriculture such as brushcutters, lawnmowers, garden tractors, chainsaws, rotary tillers and motor hoes. Emak also produce diaphragm pumps for the agricultural sector (irrigation and spraying), piston pumps for the industrial sector, professional high-pressure washers, hydrodynamic units, and urban cleaning machines.
The Emak Group concentrates its efforts on achieving customer satisfaction with an offering of advanced technological solutions and services characterized by impeccable standards of reliability and innovation and the creation of value to benefit all its stakeholders. In order to achieve its goals the Group bases its activity on specific critical success factors.
Chainsaws is one of the flagship products from Emak. It incorporates numerous safety features common to many engine-driven power tools. Manufacturers have introduced numerous design features to improve safety. Some features have become de factor standards, and others are legal requirements in particular jurisdictions. Best practice dictates that an operator should inspect the saw before starting work and only operate the saw if all the safety features are properly functional. Additional safety features provide a significant commercial advantage to chainsaw producers. Most chainsaw safety features are focused on the kickback problem, and seek to either avoid it (chain and bar design), or to reduce the risk of injury should it occur (chain brakes). Especially, the chain brake ensures maximum safety in using the chainsaw. It protects the operator from dangerous kickbacks which can occur during working phases. It is actuated, with consequent instant locking of the chain, when the operator presses the lever or automatically by inertia when the protection is pushed forward in the event of sudden kickback.
The main challenges can be summarized into three main questions that Emak want to answer:
To answer questions 1 and 2, the Emak team needed a software solution that could provide an advanced system level modeling to maximize system performance, detect performance issues at an early stage, handle multiple engineering domains and improve pre and post processing analyses. Emak discovered in MapleSoft products, Maple and MapleSim, the perfect pairing to perform these tasks.
Maple is a math software that combines the world’s most powerful math engine with an interface that makes it extremely easy to analyze, explore, visualize, and solve mathematical problems. Maple can:
MapleSim, instead, is a Modelica®-based system-level modeling and simulation tool that applies modern techniques to dramatically reduce model development time, provide greater insight into system behavior, and produce fast, high-fidelity simulations.
To answer to question 3, ANSYS Mechanical Workbench 18 has been used for structural analysis, including linear, nonlinear and dynamic studies.
A single cylinder gasoline engine model has been created to predict engine performance in MapleSim, providing the first step of the workflow. The model describes the piston, cylinder, connecting rod and crankshaft behaviour. The chain brake connected to the crankshaft has also been modeled in MapleSim in order to optimize the stop time and calculate forces on the quadrilateral. Chain brakes prevent movement of the saw’s cutting chain by applying a steel brake band around the driven clutch drum. Clamping force for the brake band is provided by a powerful spring. The chain brake has two purposes. First, it can be used to secure the chain when changing position, moving between cuts or starting a cold saw, which requires a partly open throttle. This would otherwise lead to uncontrolled chain movement, a major hazard in older saws. Secondly, the chain brake can activate under kickback conditions to prevent the operator from being struck by a running chain.
Every physical modeling or physics-based modeling in MapleSim, incorporates mathematics and physics laws to describe the behavior of an engineering component or a system of interconnected components. In addition, for every block imported in MapleSim its properties can be specified: for example, the revolution joint used in the previous schema has some properties like angular velocity, initial angle, spring constant and so on. Beside these, also related system units can be selected. Once connected, all MapleSim blocks, useful to design the model, generate a system of equations which is automatically resolved to get all physical quantities required.
After the single cylinder engine model in MapleSim, the structure forces excitings obtained from bench experiments are extracted and imported into Maple. This task is supported by customizable and animated plots available in Maple.
As a third step, a custom template for signal processing has been implemented in Maple: the exciting forces were collected and then analyzed to reduce the frequency content through a Discrete Fourier Transform study. In particular, time histories were analyzed in order to extract the coefficients of the Fourier series that represent them, so to identify the dominant frequency inputs of the input signals, and use them for subsequent analyses. This allows for the frequency content to be decreased for post processing fatigue analyses that was executed later in ANSYS WorkBench 18.
Furthermore, a DOE with a graphical interface has been prepared in Maple: graphical Maple components were imported and connected to a Maple code to carry out a time reconstruction and to verify that the new signal was consistent with the original one, exporting it to the properformat. After this step, all results are exported into text files.
A model of the machine was implemented in ANSYS WorkBench and a related fatigue analysis is executed to validate experimentally all of its aspects (modal shapes, rigidity, damping). The reconstructed signal is imported through text files into WorkBench using the Imported Load block. The procedure requires, as a starting point, the resolution of the harmonic analysis taking into account all the frequency components for the reconstruction of the input load cycle. Frequency contributions were reduced and the results are used to perform harmonic simulations (one for each frequency contribution), crucial for time reconstruction of system regime response. Even for these harmonic simulations, the harmonics are pre-stressed by a previous static condition that represents the tightening of the bolts.
Following the harmonic simulations, the time history of the stress history in points of interest for the duration of the cycle progresses, in particular to get the temporal history of signed Von Mises. This time history is decomposed into a history of peaks and valleys, useful for counting load cycles through the Rainflow Algorithm. Once the current load cycles are known in the temporal history of signed Von Mises in terms of average and alternating voltage and their number, the alternating voltage is corrected taking into account the average value, according to Soderberg’s theory. With these values the S-N curves are plotted to calculate the number of breaking cycles of each contribution and thus the total damage and fatigue life.
By coupling Maplesoft products with ANSYS WorkBench 18, EMAK achieved high computational accuracy. In particular, by using Maple, the Emak engineers were able to drastically reduce the time typically spent on multiple trials and fine-tuning. Moreover, Maple’s graphical interface makes reading the data far easier. On the other side, MapleSim, allows to create models quickly, and the mathematical analysis tools enable to really understand what is going on the chainsaw’s single cylinder engine and brake, and how to optimize them, exploiting the best in class capabilities of ANSYS WorkBe
Simulation is nowadays strongly connected to new product development in most high-technology industries.