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Quality control testing of engine hydro-mounts

Engine hydro-mounts offer the highest possible driving comfort for motorists. They combine the acoustic isolation function of a conventional rubber mount with balanced damping performance. In a hydro-mount, the main rubber spring has an upper and lower chamber. The chambers are connected by a series of canals and separated by a rubber valve. The main spring is filled with hydraulic fluid which acts as a secondary damper. On excitation or shock input, the rubber valve opens and closes the bypass channel redirecting glycol flow between the two chambers and changing the stiffness of the mount.

A global market leader in the field of automotive vibration control technology needed to implement a company-wide testing procedure at all their engine mount production facilities, to control the quality of products destined for the automotive OEM industry. Historically, their quality control testing was performed at their R&D facility in Germany using a conventional servo-hydraulic testing machine. Having recognised the high operational and maintenance costs, let alone the associated infrastructure costs, it was decided that a hydraulic-based testing machine would not provide the optimum solution for the next generation of product testing equipment.

According to the quality compliance requirements by their OEM customer, the engine mount supplier was required to verify the viscoelastic properties of their engine mounts using a dynamic-mechanic alanalysis (DMA) testing protocol. The protocol comprised of a preliminary static compression test, to define an allowable static stiffness, followed by a three-phase dynamic test up to 100Hz. Acceptable tolerances of the viscoelastic performance of the engine mounts were defined within dynamic stiffness and loss angle limits.

The engine-mount supplier acknowledged the difficulties associated with ensuring controlled and stable testing at higher frequencies. Viscoelastic components, in particular, provide challenges to materials testing since the displacement and force are out-of-phase with respect to each other. Testing at increasing higher frequencies for a given displacement causes the force amplitude to increase, when operating in displacement control. This therefore requires quick attainment and steady control of the required peak-values.

The company turned to ZwickRoell for an oil-free testing solution. It was concluded after preliminary testing that the installation of an electro-dynamic, linear-drive (LTM) testing machine would provide best value for the customer’s needs and demanding testing requirements. The complete ZwickRoell testing package included a 5kN LTM with customized component grips, testControl ll and testXpert R Sequencer software. The LTM incorporates a centralised internal incremental encoder, positioned directly on the force axis, to ensure that very precise piston measurements within +/-2µm are achieved. This guarantees very accurate measurement results, when used as a control variable in combination with the 10kHz control and data acquisition rates of the testControl II controller.

The LTM only requires an electrical supply foroperation. Unlike a servo-hydraulic solution, the LTM requires no water, no oil and no hydraulic hoses. The electric motor only extracts enough amperage to perform a test and nothing more, saving a considerable amount of money, particular for testing at high frequencies. In the long term, the LTM requires negligible maintenance, as the system incorporates a wear-free, piston and braking system.