How do I achieve required torque when using an impact driver?

Question

There's such tool called impact driver (like this one for example). The manual says it uses a special mechanism to produce a series of torque impulses to fasten/loosen nuts and screws.

The tool has only two adjustments - the forward/reverse switch and the On/Off switch. The deeper the On/Off switch is pressed the faster the tool motor rotates.

There's no clutch to adjust the torque value after which the clutch would disconnect the motor. How do I control the torque without a clutch?

Suppose I need to assemble a section of fence that will require say one thousand screw/nut connections with nuts of the same size (that's not an imaginary task - I did such job without an impact driver). Obviosly I need to tighten all nuts to more or less the same torque. If I overtighten a nut it can just break or cut the screw or bend the parts it connects - that's bad. If I undertighten a nut it will soon loosen and fall out - that;s bad as well.

So let's pretend I use nuts that need to be tightened to approximately 70 Newton-meters and the driver I use is said to produce torque up to 100 Newton-meters.

And of course I need to drive those nuts fast - otherwise why would I use a power tool instead of a screwdriver and a usual wrench? So obviously I can't time the periods of applying torque or count impacts or whatever else - I need some reliable way to achieve more or less the same right torque for all nuts.

Please note: I don't care of achieving exactly the right torque. I only care of achieving more or less the required torque - so that the nuts don't loosen on themselves and parts are not damaged while the nuts are being tightened. The primary concern is that the driver works very fast, so if I hold the switch pressed for just too long I can easily overtighten the nut and cause unintended damage.

How do I achieve roughly the same torque repeatedly for multiple nuts when using an impact driver?

Answer 1

Usually, around the home anyway, there are other signs to let you know when you've applied enough torque. MarkD noted a common one -- when you see the screw head just go below the surface of the wood. In your fencing example, if you're tightening lag screws or nuts with washers, you'll see the washer start to sink into the wood long before you've applied enough torque to shear off or strip the screw or bolt.

It's a great tool for this kind of work, and it's super fast. Don't slow yourself down unnecessarily by underestimating your own ability to judge how much torque you've applied, and how much is enough for your application.

Answer 2

"How do I achieve the same torque repeatedly for multiple nuts when using an impact driver?"

Short of manually using a torque wrench like woodchips recommends, you don't. The point of an impact driver is not to apply a consistent torque. It is to apply more torque than can be obtained from a traditional driver. One of the more common uses is to loosen seized up, or over-tightened bolts/screws. I also use mine quite a bit when driving screws into wood when the final torque doesn't matter (e.g.- I just need the screw head to be slightly below the surface of the wood.. rough work).

In other words, if your need is consistent torque, an impact driver is not the tool for the job. A traditional clutched driver is going to be more appropriate.

Answer 3

I would get all the bolts driven, quickly, but not tightly using the torque driver. Don't cinch them down. Then walk along and use a hand held torque wrench on each.

Answer 4

The torque applied to a faster by an impact wrench can be controlled by a torque limiting extension bar. The torque limiting extension bar looks like a normal socket extension, except that they are designed to flex at a predefined torque. This flex absorbs the impacts from the wrench and stops the transmission of energy to the fastener at the predefined torque. The extension bars come in kits and are usually color coded for different torque values. For example a black bar for 30 ft/lb, green for 65 ft/lb and so on.

Answer 5

Assuming the torque can in fact be adjusted you can use a regular hand-help torque wrench to get a pretty close calibration. It's a matter of clamping the later, which has been set to the desired reading, in some way. Start with the impact driver set somewhat lower than the desired setting and connect the two using whatever socket extension joiner you have available. Now gradually adjust the impact driver till it makes the torque wrench clicks or does whatever it does when it's up to its setting.

Answer 6

I know this is 6 years old, but the answer to OPs question is simple and not answered fully to this point in time. The simplest way to adjust the torque on a torque wrench that is hpa (high-pressure air) powered as is the case in this example is to simply turn down the pressure on the air regulator (there is almost always a turn knob which limits the working pressure in an 'air-hog' setup (as they are known colloquially throughout the midwest and southern states).....Here are the steps to set the torque gun to 'torque' a fastener to a desired level.

1.) get a comparable scrap of lumber to adjust the torque settings to a desired level 2.) grab one of the fasteners that you intend to use 3.) Screw it in to the desired depth/penetration/(torque?? ;)) that you want... 4.) If you need less torque (i.e. if the screw or bolt or whatnot is twisting off or splitting the wood), then just TURN DOWN THE PRESSURE....so if it was at one hundred I might use what I call the 'bracketing method' to get to the correct working pressure by cutting the pressure back WAY TOO far....say to 50 or 60 psi (as a concrete example). 5.) 50 or 60 psi should only screw it in part way....so to you will need to increase the torque by increasing the working air pressure....so just keep turning the knob on the air pressure regulator until it seats the screw like you desire. 6.) get out to the field with your adjusted tank and torque/fasteners and drive a few thousand screws at the new customized 'torque level' you figured out in steps 1-5 :) .