Screw fastening processes for industrial assembly: torque control technologies. (Part 3)
November 24, 2021
Part 3: Torque control technologies.
In this post (which we could call part 3 of our series on industrial tightening processes) we will talk about the different techniques and industrial tightening control tools that we usually find available on the market. For a reminder of the basic concepts in bolts, joints and dynamics we invite you to review our previous entries in this link for part 1 (bolts and their variants) and this other link for part 2 (torque and dynamics in different types of joints and fastening of materials). In addition, I clarify that I will use the term "torque" to refer to the tightening torque interchangeably.
Tightening screws correctly is one of the great concerns of the assembly industry. Screws that are not tightened with the proper force and technique will eventually cause early problems in the quality, functionality, and useful life of the assembled product. A first impression that differentiates a high-end product from a low-end product (derived in addition to cost) is the quality of the assembly; What will the customer think if the product becomes loose or disassembled after a short time of use? Would the customer seek to purchase this product again? Obvious answer, don't you think?
No control (direct clutch or direct connection): The first control principle that we will mention is, contradictorily, the lack of control. In this type of solution, the motor force is transmitted to the shaft, tip or key, either directly or through a series of reductions depending on the motor force and the rotational speed implemented in the tool. These are the least expensive solutions of their kind, making them very versatile to be used in multiple applications at the same time. Its main disadvantage, the total lack of control in the final tightening force, so that in addition to not obtaining consistent tightening, there is a risk of damaging the screw or the part to be assembled if the tool has more force than the maximum torque than support these parts. It is usually used for assemblies where the clamping force is not a determining factor in the final quality of the product, such as the assembly of domestic furniture in general.
Impact tools (impact wrench, pulse wrench): Impact wrenches or drivers have a system that allows the arrow to keep moving by providing torque force through blows or "pulses" with a well defined average force. They are usually used for very high torque, greater than 20 Nm and that can reach an average of 2000 Nm. The most common are mechanical, where a rotating head (usually called a “hammer”) is connected to the motor and axially pushes the arrow with the tip or key (usually called an “anvil”, and when it encounters resistance, it slides, rotates and hits again the arrow thus transmitting the nominal force of the motor to the clamping element, and the others are those of hydraulic pulse, which replace the mechanical mechanism with a pair of oil chambers that are pushed by the rotation of the motor. Both technologies allow good repeatability of tightening (max. +/- 10 ~ 15% for hammer, 5 ~ 10% for hydraulic), so it can be ensured that the result in tightening torque will be consistent each time the tool is used. They are used primarily for the assembly of heavy machinery and engines, thus they are found in the automotive, aerospace, railroad, petrochemical pipeline, heavy construction and shipyards industries.
Embragues deslizantes (slip clutch): Cuando se alcanza el torque establecido se activa el deslizamiento de dos trinquetes que separan la rotación del motor de la flecha con la punta o llave. La parte importante a notar es que el motor no se detiene, por lo que depende del operador cuando terminar el proceso. Este tipo de embragues es adecuado para el apriete de uniones suaves, o autoperforantes donde el aumento en la curva de apriete no es constante y requiere la supervisión del operario para detener o continuar el apriete. El problema con los embragues deslizantes es que generan mucho ruido y vibración, además que tienen un desgaste más rápido comparado con otras tecnologías.
Automatic shut-off tools: Automatic shut-off tools are characterized by their ability to stop the operation of the tool at the moment the pre-established tightening torque (torque) is reached. In general, tools are stopped by cutting off the power supply, either the air flow or the electrical current, by mechanical or electronic means. Automatic stop tools are the most used in the assembly and manufacturing industry, as their precision is greater than 5% in pulse and pneumatic tools and up to less than 1% in electronic transducer tools.
The proper selection of each type of tool will depend on the type of result you want to obtain and the mechanical limitations of the joint and the type of adjuster that is being used. Broadly, it can be summarized as follows according to the type of joint:
- Rigid or elastic assembly less than 20Nm: Ideally, automatic stop should be used, but also clutch tools if noise and precision are not objective. - Rigid assembly greater than 20Nm: The best option are pulse tools if high precision is sought, otherwise, impact wrenches. - Elastic assembly greater than 20Nm: Impact wrenches. - Assemblies with self-drilling, self-tapping screws: No clutch or slip clutch.
This is only a quick suggestion, to ensure the best option for your process, contact your Yamazen sales representative.