Torque control of impact wrench

[Nick55]

Want to use an air impact wrench to torque bolts to an approx torque of 1200N-m (about 900 ft-lb). Is there any proven method available of doing this? Desire to use impact wrench because of speed of operation. Bolts are 1 inch diam.

 

[CWIC]

The fastener material and type/grade are important factors when selecting a method of torque or tension verification. Don't forget the number of plys joined for each type of connection and the use of hardened washers.

The use of a calibrated torque wrench can provide this data. This method provides a torque value used to tighten the fastener under specific conditions. A multiplier may be needed if there are numerous fasteners to torque or if a wrench with that capacity is not available, 900ft-lbs is quite a bit.

A Wilhelm-Skidmore fastener-tension tester can also be used to correlate the required torque with a tensile load applied to the fastener.

The turn of the nut method can also be used to achieve a target tensile value based on the number of turns after the fastener has reached a "snug-tight" condition.

 

[Nick55]

Thanks for your reply, however the whole point of my question was to avoid the need for the devices you are suggesting (torque wrench, etc). The turn of the nut method is, unfortunately, probably not practical because the elastic behaviour of the unit being bolted up is not highly predictable.

 

[jetmaker]

Nick55,

You may want to check with an automotive repair shop the specializes in tire installation... like Discount Tire, etc. I remember having some tires changed, and they use impact wrenches to put the wheels on. Their wrenches had a series of special bars that limited the torque for installation. The selection of the bar was based upon the vehicle type. They may be able to tell you where they got the tooling, and you may be able to find one for your torque range.

Let us know what you find.

jetmaker.

 

[TheBlacksmith]

I know Snap-On offers these torque limiters for the auto shops.

Blacksmith

 

[Nick55]

Thanks Blacksmith, I will check again. I had only found these in small sizes so far (3/8 drive). I need 3/4 drive or more.
Nick

 

[Koz]

Hi Guys,
I think CWIC's point is that unless you calibrate your equipment, you don't know what you have as far as "torque" values go. The numbers are meaningless unless you bounce the equipment, torque-limiters, et.al. against a known standard such as the pressure gauge on the calibrated Skidmore. I thinke even turn-of-the-nut needs qualification because of the variation in material such as the plies mentioned above. At those torque values, I'd want calibration of my torque wrench, unless it was for in-plant use. Are there TC bolts available for this torque value? You could use the manufacturer's certs.
Koz

 

[diamondjim]

if: because the elastic behaviour of the unit being bolted up is not highly predictable.

Where did the 900 ft-lb value come from if the system
is highly unpredictable.

 

[dwclark]

As long as your talking cars, and torque specs...
The torque applied to most passenger car wheels is in the range of 70 to 90 ft-lb. Just for a little perspective I note that the Ferrari F50 with a 286 cubic inch V-12 develops 347 ft-lb of torque at 6500 rpm. This allows it to achieve 60 mph in 3.7 seconds! But 900 ft-lb of torque... we'd have to talk Cummins and Kenworth for a good comparison.

 

[unclesyd]

Considering all of the above, which is correct, we need to reach out to other methods.

A torque wrench can be somewhat calibrated with a DTI. As you have a flexible connection this should be adequate.

Being a joint with flexible components we could use crush tubes.

One could also go to DTI Bolts or use it as calibration method.

A good approximation can be had if you can gauge the bolts with a micrometer. A lot depends on the grip length.

 

[evelrod]

I don't quite know what you are after as torquing one inch , indeed 1 1/4" to 1 1/2" A-325 and A-490 bolts in construction is done normally with a hand held pneumatic impact wrench. 900 ft/lbs is not all that big of a deal! Without a calibration schedule ( using a Skidmore and a "time to torque", eg.) you have no way of knowing what you have. If your needs are more "industrial", you might consider a hydraulic torque device (wrench) as it is considerably quieter and gentler on the equiptment. Also easier to maintain consistant torque readings and to calibrate for several applications in succession.
Several units from hand operated (porta-power) to power driven pump units are available at most of the larger construction/industrial rental companies.

As to wheel torque---my Dodge D-350 wheels torque to 350 ft/lbs OEM specs.(I don't go quite this high and I use anti seize, both deals definately NOT approved by the OEM!)

Rod

 

[griffengm]

Nick,
We have worked on this issue for a number of years. Pneumatic torque wrenches are not repeatable to any degree. That is why you occasionally hear of wheel studs snapping off. The adjustment is far too gross. There are some electric drivers that will work somewhat better while electric torque wrenches are nearly as bad as air.
Do yourself a favor and buy an tool designed for consistency. We use an Atlas Copco air torque ratchet that has served us well. Desoutter also has these.

 

[kstaylor]

Normal pneumatic impact wrenches are too variable to use for controlled torque bolting. The torque output is dependent on a number of things such as air pressure, number of other users on the air system, worn tools (air wrenches are some of the most abused tools I have seen in industrial settings), etc. The output torque can vary widely from tool to tool on the same air system at the same pressure.

We have used the RAD pneumatic torque wrenches (non-impact wrenches) from Global Bolting Technologies (

http://www.radseries.com/)

with good success on high pressure equipment. Accuracy (to the desired bolt torque) is +/- 5%, repeatability from bolt to bolt is +/- 1.5%.

KST

 

[mikedcm]

Assuming that a specific tension or preload in the bolt is the objective rather than a specific torque then an air impact can be calibrated - with a Skidmore or similiar device to apply a torque value consistent with the desired tension by using pressure relief valves in the air line or receiver tank. Results are variable depending upon condition of the guns, length of hose, temperature etc. High torque values may mean nothing more than that the bolts are rusty. Torque control bolts may be an option to explore.

 

[Nick55]

Dear KSTAYLOR, thanks for the tip. This looks like a good tool and I am pursuing it. Strange, since I searched extensively on the web without noticing this company. Unfortunately it requires a reaction arm (one of the advantages of an an impact wrench is the lack of need of a reaction arm), however it looks like it will run-up the nuts at a reasonable rate. I agree that the impact wrench has so many variables that it is probably not practical to have it determine the torque. The other possible solution that came up is DTIs, and in particular "squirt nuts" which squirt out some orange silicon when they reach a predetermined load.
Nick55

 

[unclesyd]

Here is good website for the DTI washers. Make sure you get the right one for you bolting. The standard ones are normally set for a specific fastenere, A325, A490, etc.
Using a DTI you might want to match mark the bolt and nut on a couple of joints and see where the nut side ends up. You can get a pretty good Idea of the torque. It might be good enough for your application.

http://www.appliedbolting.com/

 

[FWijaya]

Nick,
I had faced the same problem with you. Let me describe myself first. I work with an offshore-contractor company. As usual, the project specification sometimes call for a bolt to be torqued (sometimes to be tensioned) to a certain value.
If this is above water surface work, I am willing to use a torque wrench. But, once we are requested to use torque wrench under water (diving job), our diver are reluctantly to use it(in-practical). So, we always come out with an idea to calibrate the (air/hydraulic)impact wrench.
How? I believe this is the question you raised before.
We can use impact wrench to replace tensioner or torque wrench. The way is as follows:
1. Provide the tensioner or torque wrench (yes, you have to have this tool to perform the calibration. The idea of this is we want to perform quicker and practical way to tighten the bolt. I am not trying to save money by avoid in providing the torque or tensioner. Eventhough, you might end up saving money since your job will be completed quicker than suppose to be)
2. Provide a sacrificial bolt (with all appurtunances).
3. Tighten the bolt to the required tension or torque tool by the torque/tension tool.
4. Try to UNBOLT/untighten the bolt with impact wrench by adjusting the released pressure (gradually increase the pressure).
5. Once you have the "correct" released pressure, re-tighten the bolt with the correct released pressure.
6. For final confirmation, try to un-bolt AGAIN it with an increasing torque or tension value until achieve the required value. If the bolt is loose on the required tension/torque, than you have calibrated your impact tool.

The reason why some client refuse to perform this way is that we are hardly to ensure that the impact wrench will not exceed the bolt's breaking load, thus we can not guarantee that we will not damage the bolt. However, by a slow increment on the pressure, we might be able to get the "most correct" pressure value.

Hope this answer your query.

 

[unclesyd]

How do you account for breakaway torque? The best method is to use a DTI and feeler gauge or "squirter".
The reuse of a fastener for more than one test is ususally prohibited. It is normal to use 3 bolts of each size to set a tightening device. Then 3 new ones to verify.

How does the torque values change at 100' with an air impact wrench? Do you have a special type wrench?

 

[Nick55]

Thanks to all who have replied. We are buying a non-impact air wrench (Atlas Copco - horribly expensive). It is called a nut runner and appears to have an internal trigger mechanism which shuts it off at a preset torque. Of course it needs a reaction arm. I personally liked the idea of squirt washers, but the longterm corrosion issue was a consideration. I will try to follow up with feedback on how it works out.
Nick55

 

[zepp]

Nick 55

Having gone through this post (late I know, but Im new here)first of all I would like to say, your final choice is slightly better than your original, yes the tool that you have selected is expensive, but is it acurate?
I have faced this argument many times in the field, basicaly the impulse gun is a modified impact wrench, and the torque control specified by all the manufacturers is some what dubious to say the least. I have tested many brands on torque transducers fitted with rundowns to recreate a bolt being tightened, and each time, I have been left nothing short of dissapointed.
The only time I would advocate the use of such tools is when being employed to break out bolts, even then I am not happy with the use of them.
the problems arise with these tools from the very nature of their running, basicaly, most manufacturers use a kind of relief valve for the impulse wrenches, and is usualy colour coded, the problem with this is, the changing properties of the oil used in the impulse chamber, on start up it is thicker due to it being cold, through use it thins down, as you may know, with a direct operating relief valve on any hydraulic system, cracking pressures vary from 50% to 25% of the set load, and this is so with the impulse wrench and this is the very reason why they are not reliable as true torque tools, and I wont even get into repeatability.
I see mentioned above that some one says that no pneumatic tool can give good repeatability, this is wrong, there is a manufacturer in the UK who Rad have based their tool on that have repeatability to +/- 3% on their pneumatic torque guns, and they have very smooth action, dont cause white finger vibration problems, and they are not noisy, but above all, after many tests, I find that although slower than the impact/impulse tools, it does give a finished torque figure that can be trusted, and this unit does not keep going, when it has reached the set torque, it stalls, Impact/impulse wrenches do not, the motor keeps banging away, and the final outcome of the bolt is very uncertain, the problem that I find with the rad tool is again the motor driving it, i dont know if they have changed this in recent years, but it used to be a motor from an impact gun, and the problems occured because the motor is not designed to stall.
As for costs, the tool i am talking about comes in slightly cheaper than the one you have selected, and like all torque wrenches, the one I would choose has to be calibrated every year.
I hope this throws some more light on the subject for you, and I wish you luck with your adventures into torque and the many methods to apply it.