Please understand that I maybe would fail mechanical engineering 101 (if I had taken it), and physics was never my forte.
I have a torque wrench. You attach a 3/8" socket to the end, set the torque to a given amount, and it won’t torque beyond that. But what if I want to torque a screw? I have a screwdriver set with removable tips, not supplied with the wrench, and the tips are about 1/4" hexagonal base. Can I just put a tip (Torx) in a 1/4" socket and expect it to apply the right amount of torque? Or have I screwed something up mechanically so that I won’t get an accurate conversion and I might strip some threads or else under tighten?
There is a relationship between the diameter of the thing being turned and the appropriate range of torque; probably the easiest way to understand that is to compare, say, the tiny bolts holding your computer together vs the massive ones securing a suspension bridge - setting aside operational loads, it’s obvious that the force required to properly tighten the huge bolts would shear the little ones off like they were made of butter.
So if you want to torque a screw, you probably need to take into account the (possibly smaller) diameter of the screw shaft and adjust accordingly.
You can buy a bicycle torque wrench designed to take the 1/4"bit and supply the proper torque, experience has shown too big a wrench will shear bolts, your torque setting needs to be in the middle of the wrench range.
Yep.
Torque is torque - as long as the attachment is axially aligned with the wrench it doesn’t know the difference. Torx heads are used in the highest-torqued fasteners.
If you installed a Phillips tip on an automatic torque wrench the axial force required to hold the tip into the screw MIGHT affect the wrench mechanism, depending on the design of the wrench.
I once worked on a fighter aircraft (F-111) that had dozens of panels held on by multiple screws in front of the engine intake. If even one screw popped loose and went down the engine, it was possible for that engine to be destroyed. We were supplied with torque wrenches that fit on the end of our speed handles to ensure these screws were properly tightened.
Finally, a to-the-point (and correct) answer. In-line adaptors and extensions do not affect the torque reading.
But let’s go back to “…set the torque to a given amount, and it won’t torque beyond that.” That is probably not the case. Typically a torque wrench that takes a setting (as opposed to simply giving a reading) will notify the user when the set torque level is reached, but will not prevent tightening beyond that point. You have to pay attention to the signal (click/give in the handle, light, buzzer, etc.) and stop turning then. The torque wrench does not make it impossible to overtighten, it just makes the user aware of when to stop.
As mentioned in previous answers it is important to use a torque wrench with a suitable range, but that’s a different question.
As mentioned above, yes, a torque wrench will indicate the torque upon a fastener, provided that you use it correctly (axial centerline of fastener matches axial centerline of shank).
However, I’ll throw in a wrinkle to complicate things.
The purpose of a bolt is to clamp two or more members together. Twisting the bolt (or nut) advances thread engagement and creates a tension (ie. pretension or bolt preload) within the bolt, which creates this clamping force.
However, the effort required to twist the bolt (ie. torque) has many variables, namely:
Bolt geometry (thread pitch, diameter of threads and shank)
Friction (are the fasteners clean or dirty/rusty/corroded? has lubrication been applied? are there imperfections or burrs on the bearing surface?)
Pretension
As you can see, pretension is only part of the equation. We know this through observation, for example - loosening a bolt that had anti-seize applied is “easy”, since anti-seize inhibits corrosion and provides lubrication, both things that are favorable for easy torquing. We also know this conceptually - imagine a bolt that has comically deep threads that extend very far from the core of the bolt. This bolt would be very hard to turn.
Side note: You did not make any reference to a particular type of screw. The torquing characteristics of driving a lag screw into an 8x8 post is much different than a machine screw that attaches the arm of a chair.
The conclusion is that knowing the torque applied to a bolt provides only an approximation of bolt pretension, which is what we’re after. There are less accurate methods (ie. hand tight + an extra 1/2 turn) and more accurate methods (ie. measuring pre-tightened and post-tightened bolt lengths). It is up to the user to judge whether ideal conditions exist (torque specs typically apply to clean hardware in good condition), the hardware being used (is it the proper grade material?) and the criticality of the application, AND THEN to match it to the proper method.
I work in a lab that is ISO17025 Accredited for torque and have seen quite a few tools.
The normal workshop torque wrench (twist handle to set, stop when it clicks) aka click wrench does have a risk of continuing torque after the click sounds. However, the easy adjustment makes it great for many applications.
This is also true of dial and beam tools.
Things are different in a production environment, though.
Liability and mistakes leading to premature warranty claims are huge, costly factors which means that different tools are used for different torque values.
These cannot be used in an overtorque situation except by the most clever of slacker operators.
There are two major types of hand tools, each relying on springs and cams.
The cam over type has a ratchet head and can accommodate different sockets. There are screwdriver-type tools that are similar.
A break over tool is more like a box or open ended wrench and usually needs to be calibrated with the particular head installed as different sizes change the breaking characteristics.
These tools need regular calibration as springs wear and preset screws give and take. The calibration does drift over time, usually toward the low side, al though some go higher like when jammed with metal shavings.
Power tools are another matter.
Some use a clutch that stops the motor and others use a cam over mechanism.
As mentioned by iceiso, there are all sorts of ways to trick the tool into giving a false indication. There is the whole science of ‘characterizing the joint’ in calibration techniques. You can use what are called rundown adapters which are something like nut and bolt combinations that get tighter in a controlled manner.
The gold standard here, though, is an inline rotary transducer with a position encoder. This way, you can use the actual power tool, the actual bit, the actual fastener and the actual materials being joined and correlate the torque with the angle of rotation. This means that data can be graphed and analyzed six ways from Sunday.
It can be in furlong jovian atmospere square cubits ( area * pressure = force, and then that force is acting at one furlong from (closest approach to ) the axis of rotation ) if you like … the ISO standard says to use Newton Metres.
Of course he is stuck with whats on the graph, but yeah Torque is the same, as shown on the guage/dial , no matter what sort of attachment is at the end of the wrench.
One thing no one has mentioned is the dirty little secret of torque wrenches.
At less than 20% of full scale torque wrenches aren’t very accurate.
So if you have say a 5-100 ft/lb torque wrench and you want to torque a small screw to 5 ft/lb you might as well do it by hand, as the torque wrench won’t be accurate.
Because of this I own several torque wrenches ranging from small in/lb units to a great big honking 300 ft/lb unit for the mother of all bolts.
This is true up to a point. I also have many torque wrenches. However, as an aircraft mechanic, all of my torque wrenches are required to be certified to be accurate throughout their entire range.
The FAA does not allow me to use a non-certified torque wrench. When I bought my brand new Snap-On torque wrench I could not use it on aircraft until it had been tested & certified to be accurate throughout its entire range. It was brand new & was not able to meet these specifications. My Snap-On dealer & I went through six wrenches before we found one that would pass the FAA specs. He sold the other ones to auto mechanics. Try not to think about that.
All of my wrenches must be certified at least once a year. Since the certification process takes from two days to two weeks, & I use mine at least three times a day, I have two sets of wrenches & send them in six months apart.
All of my torque wrenches are either Mac, or Snap-On. Most other brands will not pass the FAA required certification year after year. I had one Craftsman large (300ft-lb) torque wrench pass certs for 12 years before it failed the certs. Not bad.
For home use, most brand name torque wrenches are fine. The ones that do not have a known good brand are very suspect as to their accuracy. If it says “made in China”, it is most likely scrap metal.
As has been said, torque is torque. As long as the torque wrench & the driven fastener are aligned concentrically, the wrench is accurate. If you need to use a crows foot or an offset attachment with your torque wrench, some math will need to be done.
Lastly, store your torque wrench with the adjustment at the bottom setting. It will stay accurate longer.
When you use a torque wrench, the effort required to twist the bolt or nut comes from three places:
1)Axial tension on the bolt itself. This is also known as thread extension. The bolt can be modeled as a very stiff spring, and when you apply torque you are pulling this spring apart. Length and diameter of bolt, as well as the bolt material, will affect this.
**2) **Thread friction. When applying torque, there will be dynamic friction at the thread surfaces. Factors that affect this include thread surface area, thread geometry & pitch, material, surface finish, and whether or not a lubricant is used. As for material… cadmium, for example, has a lower coefficient of friction versus zinc, which means (all else being equal) a cadmium-plated bolt should be torqued to a lower value versus a zinc-plated bolt. Some people call this “lubricity.”
3)Nutface friction. This is similar to #2, except this refers to the flat contact area of the nutface (or the flat contact area of the boltface if the bolt is rotated instead of the nut).
I’ve never used it, but there’s a software program called BOLTCALC which does all the math for you. And MIL-HDBK-60 is a handbook that exhaustively covers the subject of bolt torque.
Finally, it should be mentioned that a correction factor needs to be applied to a torque wrench when using a crowfoot wrench/socket. The correction factor can be calculated using Physics 101 laws & equations, but it is better to determine the correction factor empirically during calibration.
Thanks for the program; the last time I ever had to do the analysis by hand was back in school. I went through my old textbook, and here’s the formula for anyone who’s interested:
T = K * Fi * d T = Bolt torque
Where:
Fi = Preload
d = Major diameter
K = torque coefficient = [dm/(2d)][(tan(lamba)+fsec(alpha))/(1-ftan(lamba)sec(alpha))]+[0.625fc]
Where:
dm = average of major and minor diameter
lamba = lead angle (think, “sharpness” of the thread, sharper = small lamba)
alpha = 1/2 thread angle (think, how “flared out” each thread is from the adjacent)
f = coefficient of thread friction
fc = coefficient of collar friction
Grab you Snap-on catalog and read the fine print on the torque wrench page carefully. While they advertise 1% accuracy, that is only for 20%-100%. There will be a disclaimer that says at <20% the accuracy is worse, usually + - 20%
Interesting. So if a 5-100 ftlb wrench is only nicely accurate from 20-100 ftlbs, why do they even bother starting at 5? Who wants a wrench that implies it can be used in that 5-20 range but is not accurate therein? Why not just call it a 20-100 wrench with high accuracy through out that entire range?
Setting aside whether it would make sense from a cost of sales standpoint, if accuracy is +/- 20% @ say 10 lbs (reading), you know real torque is between 8 and 12 lbs. With no reading, you could be anywhere between 0-20 lbs.
