YOKOTA Impulse Wrenches

Produce faster and with higher accuracy and safety

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YOKOTA Impulse wrenches are used for threaded fasteners when torque values with small tolerances are requested, like in the assembly industry. The machines are light in weight, have low noise and vibration levels, very comfortable power-to-weight ratio and have a patented cooling system for the impulse mechanism. Impulse wrenches do not give any kick-back to the operator.

Produce faster and with higher accuracy and safety

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Why impulse wrenches?

The speed of production by means of impulse wrenches is very high. The speed (up to 10.000 rpm during pre-tightening) of the tool is of big importance for the assembly industry. Research in the car manufacturing industry has shown that tightening of threaded fasteners can be as high as 10% of the total assembly time. If this process can be sped up, there are good chances for major increases in production efficiency. Faster production also induces lowering the cost of production. The reduction in time that can be achieved by using YOKOTA impulse wrenches depends of different factors. The length of the thread on the fastener, the hardness of the joint and other influences. Recent measurements have shown time savings of up to 35% (compared to the use of angled nut runners) that can be achieved in reality.

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Better clamp forces. Tightening of bolts with a specified torque level is only a means to achieve the target of the designer of the construction: Clamp force that makes sure that assembled parts will stick together. During the tightening process with an impulse wrench, the relaxing forces in the joint are reduced by the impulses. And it is just for this reason, that tightening with impulse wrenches achieves better clampforces, compared to continuous driving tools.

YOKOTA Impulse wrenches have a light weight, give no reaction forces and have thus better ergonomics for the operator.

Why impulse wrenches?

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Working principle of the YOKOTA impulse wrench.

Stated simply, the YOKOTA impulse wrench consists of two parts: The highly efficient air motor and the impulse unit. The air motor is in most cases equipped with a so-called double chamber. The rotor (with 9 vanes) revolves in a cylinder that have two chambers, enabling to feed and exhaust air flow on two sides of the rotor simultaneously. This principle causes a high torque of the air motor, which allows very short acceleration times. The driven shaft of the rotor is connected with the impulse mechanism.

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Working principle of the YOKOTA impulse wrench.

The impulse mechanism has an oil filled cylinder, that is revolving around this driven shaft of the air motor. It is the cylinder, that gets driven by the air motor. The main shaft of the impulse wrench has two blades that are driven by the oil in the cylinder and the specially shaped chambers. By opening or closing a release valve, the oil pressure can be reduced or increased. Higher oil pressure achieves a higher torque of the main shaft. By using oil, the set torque value has a very high repeatability. Every revolution of the air motor, a new impulse is generated on the main shaft.

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Classification of bolted tightenings.

In the European vehicle manufacturing industry, often a guideline is used for the classification of bolted tightenings. It is the German VDI guideline Nr. 2862. In this guideline, bolted joints are judged as a function of the results when the tightening was NOT made OK..

Category A: Direct or indirect life danger. Example: Bolted joint in the brake pedal, safety belt or seat.

Category B: Functional error. Example: Waterpump to engine fixture. Category C: Customer annoyance. Example: Rumbling noise of the hat shelf.

For a category A bolted joint, it is obligatory to use tools in which two different variables are measured (Eg: Torque and Angle).

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In the international vehicle manufacturing industry, quality and safety are very important. All functional and safety critical tightenings must be counted, documented and of course tightened to the correct clamp force. To achieve this, one needs a YOKOTA impulse wrench with integrated torque-transducer and integrated angle encoder on the main shaft, connected to a controller. If the result of the tightening does not comply with the pre-set demands, the controller stops the process, making it possible to check manually the joints. This system for zero-failure assembly fits perfectly for the Poka Yoke strategy.

Classification of bolted tightenings.

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Apart from the vehicle manufacturing industry, more and more manufacturers of machinery and appliances in other sectors of industry like to check these variables during the tightening process. Intrinsically the quality of the end product improves. And at the same time the risk for need of service or repair costs minimizes. This is especially interesting in case of export of the end product. Examples of these industries are manufacturers of industrial robots, heavy earth moving equipment, hydraulic valve blocks and pumps.

Classification of bolted tightenings.

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Functioning of the YOKOTA impulse wrench with integrated torque transducer and angle encoder.

In the main shaft of the impulse wrench are mounted (within the case of the tool) four restrictors (in a bridge of Wheatstone) that measure the torsion of this shaft during the tightening process. The measured values are transmitted by induction, so contactless, to the controller. Next to that, a rotary encoder is built onto the main shaft, also transmitting contactless the angular displacement in 1 degree accuracy to the controller. The fact that the transmitting of these values happens contactless is of importance to the users that make hundreds of thousands of bolt tightenings. In the controller, the limit values and the framework for correct tightenings are set on beforehand. (Minimum torque, maximum torque, target torque, angle during free running, angle during pulsed tightening, times, number of impulses).

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Explanation Poka Yoke strategy.

Poka yoke (ポカヨケ ) is a Japanese term used in Lean Manufacturing and translates best as foolproof (A poka is an error by accident and yoke translates as prevent). It describes a method to design a production process in a way that it is almost impossible to make mistakes. Any operation is executed in such a way that the correct method is forced. This concept is launched by Shigeo Shingo as part of the Toyota Production System.

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How does the YOKOTA System Wrench fit so well within the Poka Yoke strategy?

Due to small irregularities in the supply of semi finished products or parts, there is always a small risk for a manufacturer that for example the tightening thread is damaged. Or that remains of paint or glue are attached to the thread. This could possibly not be noticed by the production employee / operator, but could at the same time have fatal consequences for the correct tightening of the threaded fastener. By monitoring torque levels and angle during the process, these types of errors are noticed by the YOKOTA system wrenches. And the problem can be mended directly, during the production process. Direct corrections in the process prevent expensive repair jobs at the end of the production line. And more important: It prevents expensive repair jobs after the end product has been delivered to the end customer (although the latter is not part of the Poka Yoke strategy).

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CONCLUSIONS

YOKOTA impulse wrenches with integrated torque transducer and rotary encoder offer:

• Speed• Accuracy• Poka Yoke• Ergonomics • Positive worker attitude • motivation • Top quality of the end product

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