Poka Yoke approach to fastener torque control

Crane Electronics has supplied a TMAC Opta torque monitoring and control system and an IS WrenchMaster torque wrench to a major UK vehicle assembly plant, where a new application required the accurate tightening of the exhaust manifold to petrol engines. The essential requirement of the system was to recognise those engines requiring additional tightening before fitting to the vehicle. In contrast to a diesel engine, the exhaust manifold of a petrol engine is only tightened to 50 per cent of final torque before leaving the engine facility. It was decided not to use normal power tool technology to complete the tightening process; instead, a simple Poka Yoke error-proofing system was used to inform the line supervisor of any problems. The preferred supplier was Crane Electronics and the TMAC Opta is central to the system.

There were also other issues to consider. Firstly the system, and not the operator, had to be able to distinguish between different types of engine. Secondly, the operator needed to receive a visual indication that the status of the assembly was good (OK). Finally, all recorded torque values had to be documented electronically.

Crane Electronics' proposal was to interface a TMAC Opta with the engine conveyor belt. This involved liaising with an appointed subcontractor at the plant, who installed the facility and then integrated the TMAC Opta's input/output capability to the production line control. This proved to be a straightforward process.

How it works

Every time a petrol engine is fed to the conveyor belt, the control system stops the engine on station. The operator knows then that the fasteners on the engine manifold have to be tightened using the IS WrenchMaster tool. The TMAC Opta gives the operator a visual indication once the final torque has been achieved.

The sequence of the fastener tightening is always the same and if any are not within tolerance, the system will not release the engine until it is either corrected or the line supervisor has tagged the engine for rectification.

All torque results are saved and processed by the TMAC Opta, which means that all documentation for product reliability and production analysis purposes is automatically archived.

Crane's system is now fully implemented and ensuring that the safety-critical tightening of exhaust manifolds to petrol engines is an extremely accurate process. The system has also minimised the risk of production line stoppages.