Many users have applications for an affordable and easy-to-use dc electric system for tightening threaded fasteners. The goal is a robust tightening system with precise torque control comparable to today’s state-of-the-art dc assembly systems, but at a lower cost.
The most accurate dc assembly systems use a torque transducer to continuously monitor the dynamic torque delivered to the fastener during the tightening process. The controller electronics monitor this dynamic torque signal to control the precise shut-off point. These systems usually include a method of data output to document joint quality. Documentation may not be required to error-proof the tightening process. Some lower cost options that do not require a torque transducer include:
Clutch Control: A mechanical clutch can be directly connected to a dc electric tool to control torque without the need for a torque transducer. This provides torque control comparable to pneumatic tools and can be calibrated to a reference torque control standard. However, a mechanical clutch adds weight and size to the tool that can decrease ergonomics. The wear of components requires periodic manual torque adjustment.
Basic Current Control: Another torque control method is to measure the power consumption of the motor. A controller monitors the electric current used by the motor, and when this current reaches a predetermined level, the motor is shut off to complete the tightening process. However, the flow of current into the motor is not directly related to the torque delivered to the fastener. Many other factors can affect this current measurement. As the temperature of the motor increases, less torque is produced for a given amount of current. Shutting off the tool at a preset current level will produce less torque when the tool increases in temperature with use.
Secondly, the inertia of the rotating parts within a tool, combined with the response time of the control system, will cause torque overshoot that is not at all related to the amount of current used by the motor. The amount of torque overshoot can be negligible when the tool is operated at a slow speed or on a soft joint, but it will be higher when the tool is at a high speed or on a hard joint. Simply measuring the current into the motor does not differentiate between these conditions.
A third factor is the efficiency of the entire gear system. While a torque-sensing transducer will automatically compensate for any change in efficiency of the planetary gear system, a system based on current control would need recalibration to a reference torque control standard as the efficiency of the gear system changes throughout the life of the tool.
Advanced Current Control: A comprehensive software algorithm can integrate several other factors to arrive at a more accurate estimation of torque using current control. Sensing the motor temperature allows compensation for the difference in torque output with changing temperature. Operating the system in a “learn mode” on a specific test joint while sensing the joint rate, and then reducing the tool’s speed to a level where the torque overshoot produced by the rotating inertia becomes negligible, can help reduce the effects of inertia and can reduce torque overshoot. But this is only effective on production joints that are consistently similar to the sample joint. When these systems are used on production joints of varying torque rates, they fail to meet expectations. And when used on multiple joints of different torque rates, they require a joint type selector, which adds cost and complexity to the fastening system.
One of the basic requirements of the ISO 5393 test method for torque capability over a wide range of joints is that tools be tested on both hard and soft joints without any adjustment to the control system. Systems that must “re-learn” or have control parameters changed for either type of joint will violate this basic requirement of this industry-standard performance test.
Data to document process control capability from current control systems usually is not accepted. Current control data is merely a target, or a calculation based on a number of measured properties, not the actual dynamic torque delivered to the fastener. When compared to a reference master torque transducer, the indicated torque can differ substantially from the actual torque.
Transducer Control: The simplest, most direct, and most accurate torque control method measures dynamic torque using a torque transducer. Not including networked data collection can simplify the system: a simplified system confirms tightening to within the specified torque limits and enables bolt counting for Poka Yoke process control.