Otto and diesel engine technology has evolved dramatically over the last years. Today’s engines produce far less particulate matter, less carbon dioxide, and provide more reliable detection of operational faults. Where a 2.5 litre combustion chamber was once required to produce 180 horsepower, a 1.4 litre chamber is now sufficient. In order to make these improvements possible nearly all of the engine components have had to meet more rigorous technical performance requirements. One of the parts critical to the engine’s improved performance is the connecting rod, a component necessary for power transmission in the combustion process. In order to ensure efficient quality control, passenger car connecting rods produced by IMR Gesellschaft für Prozessleit- und Automatisierungstechnik mbH  go to a testing station at Schmiedetechnik Plettenberg GmbH & Co. KG in Lennestadt-Meggen, Germany where they are inspected with Cognex vision technology for characteristics such as thickness, length, and width dimensions, symmetry deviations, concentricities, and displacement.

Component improvement creates production challenges

In order for engines to produce the same power with a lower cubic capacity, they require more compression, which results in improved engine lubrication. The reduced cubic capacity in the engine results in reduced clearance in the bearing shells of the connecting rod eyes. This presents new production challenges because it is very difficult for the connecting rod to handle the stronger forces that are exerted while operating with minimal tolerances. During connecting rod production, material temperature can reach approximately 1280°C (2,336°F) and this causes the C70 steel to react extremely sensitively to the mechanical shaping processes. Despite improved production methods, it is possible that during the forging of the connecting rod the components of it may turn out too thick or too thin. During automated forging, forging displacement occurs under certain conditions if the upper and lower die do not correspond absolutely exactly within a tolerance of tenths or hundredths of a millimeter. Even during the subsequent manual calibration process, there are occasional temperature fluctuations in the connecting rod itself, due to different machining times, resulting in minimal size and weight deviations.

A new testing paradigm

In order to guarantee IMR the highest quality connecting rods, Schmiedetechnik Plettenberg had to break new technical ground in how they to measure and inspect them. This was the first project in which the company used a vision system for automated quality testing. The core challenge for Schmiedetechnik Plettenberg was achieving sufficient testing repeatability on the forged connecting rods, which sometimes contained rough surface. Using the Cognex vision system, the company was able to perform the testing with great success. For example, the company was able to achieve height measurement repeatability within 0.03 millimeters (0.001 inches). As a result of this success and because of their many years of experience in the field of the design and construction of turnkey handling and testing units, IMR commissioned the Schmiedetechnik Plettenberg testing station after only a short time. Today the station inspects an average of 4.5 million connecting rods annually.

Full testing control throughout the process

Even before they reach the optical testing station the connecting rods undergo initial examinations. After sandblasting and a first visual inspection, a magnetic powder test is carried out to detect ultra-fine hairline cracks. Before component parts are directed to the connecting rod, the rods are weighed with tolerances being plus or minus six grams (0.21 ounces). The connecting rods are then fed on a conveyor belt into the testing machine. They are detected by a light barrier and their location and position are then transmitted to the tracking system. At the first image processing station, the Cognex In-Sight? vision systems, equipped with telecentric lenses, each measure the lengths and widths of the connecting rods, the concentricity of the crank eye, and the engraving. The length and width are determined according to the product specification on the basis of the ratio of the center of the crank eye to an established reference outer edge. In cases where measurements deviate outside the acceptable range, the vision systems transmit a signal to reject the connecting rod on the station I-PC.

In the image processing station, a Cognex In-Sight vision system is used to detect additional characteristic numbers on the upper side of the components, based on forged three-dimensional imprints with good OCR legibility. The different characteristics are recorded by a combination of different lighting concepts in opposition to the two In-Sight cameras. LED background lighting serves to record the component contours. Engraving and characteristic numbers are detected by means of red LED incident light.

Robust system

In the connecting rod testing station two parallel approaches are adopted to achieve the highest possible degree of testing accuracy. After a detailed evaluation of the records the information from both images is also compared in the computer. The intelligent Cognex In-Sight system provides one hundred per cent error detection in the first test step. Thickness and height queries are made in the third testing station by the laser triangulation method. Two further Cognex matrix cameras with VGA resolution, detect the H-profiles by means of laser lines. The height of the crank eye and shaft and the height of the piston eye are measured at several points. In addition to this, the system checks the symmetry and the deflection of the connecting rod shank.

In their raw environment the Cognex image processing systems excel with their standard industry-ready functionalities. Their robust injection-molded aluminium and stainless steel housing also renders them impervious to high vibration loads. Tight M12 connections protect them from dust, which for the quality control requirements of Schmiedetechnik Plettenberg provides an extremely high degree of reliability at low maintenance costs. The result of the cooperation between forging experts and vision specialists is superbly engineered connecting rods capable of meeting the requirements of powerful, state-of-the-art engine technologies.