Reaction torque of angled nutrunner I have a question regarding the behavior of an right-angled, air/electrical powered nutrunner. More specifically, the reaction torque that the operator is subject to during operation. When the tightening procedure is near completion, the tool acts on us with a sharp, counter-clockwise torque (assuming that the screw/nut is tightened clockwise, of course!).
I assume that one may argue that the joint responds to the tool with a corresponding, opposite torque. This would make sense if the tool would be considered a rigid lever, as "conventional" non-driven tightening tools are. However, I am not sure if this applies to the driven tool, which transfers torque by the drive line. The fastener torque is acting on the output axle of the tool, which is rigidly connected to the drive line, which is made up of bevel gears, planetary gear sets and an electric motor. How can this result in a counter-clockwise torque at the operator?
Could someone elaborate on this?
 A: Just like we can ignore purely internal forces when analyzing the translational motion of a body, we can ignore internal torques when analyzing the rotational motion.
The gears and other mechanisms within the tool mean that some parts may be experiencing different torques within.  But assuming the tool doesn't yank itself out of your hand, then we can look at the lack of rotational acceleration as:
$$I\alpha = \Sigma \tau$$
$$0 = \tau_{\text{operator}} + \tau_{\text{nut}}$$
$$\tau_{\text{operator}} = -\tau_{\text{nut}}$$
The internal mechanics don't matter, even if there is a clutch and some portion is slipping (We assume the spinning mass inside the tool is small and not significant).  Anything about the tool that modifies the torque does so on both sides.
So for sustained torque, all the above holds.  But in the comments you mention impact devices.  These apply short bursts of high torque.  Because these last for very short periods of time, the assumption above about no angular acceleration no longer holds.  Instead masses inside the device (and probably the body of the tool itself) spins... (just a bit).
The angular momentum impulse is then absorbed by the operator during the time between impacts.  One way to think of it is as a "lever" that allows the sustained, operator-provided, lower torque to be converted to short periods of higher torque on the output shaft.
This is similar to a centering punch tool.  You provide a certain force over a distance, but then the tool releases a hold and the center punch delivers a very high force for an instant.
A: The difference between the manual and powered wrenches is an illusion. In order to turn the nut relative to the thread, the tool must have something to push against that is stationary relative to the thread. In both scenarios the push/pull is in the same direction.
The illusion arises because, with the powered wrench, you don’t have to move your hand through an arc like with the manual wrench. This lack of motion makes it the action feel different to your senses.
