Bolted joints are a type of detachable fastening. They offer simple structure, reliable connection, and easy assembly/disassembly, so they are widely used in mechanical manufacturing. Mastering and applying bolt-tightening technology is very important for improving product quality. There are two main tightening methods: automatic tightening and manual tightening (human operator with an electric tightening tool).

Automatic tightening

After the work-piece reaches the tightening station, equipment moves a high-precision electric tightening tool to the bolt that must be tightened. The machine’s PLC sends a signal to the tool, and the tool drives the bolt according to the pre-programmed sequence. When one bolt is finished the tool withdraws and, following the programmed path, moves to the next bolt. The equipment that carries the electric tool is usually either an industrial robot or a dedicated tightening machine. The difficult point of automatic tightening is the high demand for work-piece positioning accuracy and fixture consistency; poor positioning or inconsistent fixtures lowers the success rate of bit-to-screw engagement and therefore lowers the overall tightening success rate. Advantages of automatic tightening are: labour saving and lower labour cost, elimination of quality variation caused by human factors, high accuracy, good repeatability, short cycle time, modular control system, strong anti-interference ability, simple and flexible programming software. When an error or fault occurs the system alarms and logs self-diagnostic results, making failure analysis and maintenance easy.

Manual tightening

In manual tightening an operator brings an electric tightening tool to the bolt, engages the bit, and starts the spindle to tighten the bolt to the specified torque. Because of the reaction torque, a suspension balancer is usually fitted to the tool to offset the weight of the spindle and the reaction force, reducing operator fatigue.

Differences between automatic and manual tightening

1. Foot-print: automatic cells are generally large; many production lines would have to be re-designed to accept them. Manual tools are compact, occupy little space, and fit easily into existing lines.

2. Cost: automatic equipment is expensive—tens of thousands of USD per station is common—whereas manual tools are much cheaper.

3. Productivity: automatic tightening is faster. One manual tool roughly equals three skilled operators; one automatic station equals about five operators.

In summary, choose the appropriate tightening technology by considering product geometry, station layout, and required tightening rate, so that product quality is guaranteed and product competitiveness is improved.