# Mechanics of using a torque wrench to drive screws **URL:** \ **Category:** Factual Questions\ **Created:** [August 18, 2015, 5:20am UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151 "2015-08-18T05:20:35Z")\ **Posts on this page:** 10\ **Page:** 2

**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [August 19, 2015, 5:50pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/21 "2015-08-19T17:50:50Z")

**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [August 19, 2015, 10:31pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/22 "2015-08-19T22:31:37Z")

> [@Isilder](#): > > 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. If you are torqueing screws then the amount of torque is required is going to be small, and being off a little can make a snapping difference. Carefully setting a torque wrench rated in foot pounds your setting is going to be set point ± 1 or 2 foot pounds. That would be ± 12 to 24 inch pounds. And a small screw that that is a lot of difference. A inch pound torque wrench is going to be =- 1 or 2 inch pounds a much smaller amount. You are right torque is torque but wrenches errors differ. So it does matter. ---

**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [August 19, 2015, 11:40pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/23 "2015-08-19T23:40:03Z")

Perhaps \*\*Max Torque \*\*will drop by and straighten this all out. ---

**Author:** ![48Willys](https://avatars.discourse-cdn.com/v4/letter/4/db5fbb/32.png) [@48Willys](https://boards.straightdope.com/u/48Willys)\ **Post date:** [August 20, 2015, 3:55am UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/24 "2015-08-20T03:55:58Z")

> [@Rick](#): > > 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% Rick, Yes, my Snap-On dealer & I read that. I can not have that kind of inaccuracy in the work I do. I cannot take Snap-Ons word for it that their torque wrenches are accurate, I must prove, by testing, that they are accurate. As I told the dealer, the wrench is of no use to me until it passes a certification test. Guessing is not allowed. Neither the FAA, nor I, care what the catalog says, we care what the certification lab says. The Snap-On dealer & I set about finding an accurate torque wrench for me. I had a total of six brand new torque wrenches tested & one certified to be accurate to within, IIRC, +/- 3 %. So, IME, 1 out of 6 torque wrenches are accurate at the lower settings. Not all of the tested wrenches were inaccurate at the lower settings. Hence my statement “This is true up to a point”. How else should I have said that? ---

**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [August 20, 2015, 6:03pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/25 "2015-08-20T18:03:33Z")

Well if 5 out of 6 are inaccurate and you don’t send it in for certification then as a blanket statement I would say they are not accurate. As 83% aren’t. ---

**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [August 21, 2015, 12:18am UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/26 "2015-08-21T00:18:58Z")

> [@MikeG](#): > > Hey jnglmassiv, can you calibrate torque wrenches? I’ve got three snap on clickers I’d like to have checked. Yep! It’ll cost you lunch, though. Do you still live in the same place here in the neighborhood? Send me a message at my user name at hotmail or gmail. ---

**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [August 22, 2015, 11:36am UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/27 "2015-08-22T11:36:14Z")

> [@Gary\_T](#): > > 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? Errors typically have a fixed offset plus a percentage. You can get 7% at 100, then 52% at 10, then 502% at 1. For situations where this matters, you can specify a standard “accuracy class” instead of an accuracy. For all other situations, you can use the right size tool/meter/guage. Regarding the substantive question “why don’t they just leave the bottom of the scale blank”, it’s because for some applications you don’t need to have ths same percentage accuracy at the low end as you require at the high end, and for other applications the percentage accuracy at the high end is just something that is thrown in free as a side effect of achieving the required accuracy at the low end. (In my workplace we don’t require certified torque wrenches. We use the torque setting on the Li-ion screwdriver.) ---

**Author:** ![Kedikat](https://avatars.discourse-cdn.com/v4/letter/k/ed655f/32.png) [@Kedikat](https://boards.straightdope.com/u/Kedikat)\ **Post date:** [August 23, 2015, 6:15pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/28 "2015-08-23T18:15:09Z")

Properly setting a screw can be far more complex than a bolt. A screw is usually cutting it’s way into the materials to be fastened together. A long screw in hard material may exceed the screws torque limit and break the screw. So use a torque setting that will not exceed the screw strength. A broken screw is hell to remove. But, that can still allow you to strip out the threads that the screw has cut. I usually choose a power driver with clutch style torque system. If you go over torque, it lets it spin free. If I am spinning free before setting the screw. I increase the torque setting in steps. If it seems to be getting too heavy, I reverse out and then back in. If you are just ahead of the torque required to drive the screw. Then you will probably set it correctly when the head embeds more or less flush. Too much and the head may keep right on sinking in. Or the screw may spin at the surface and rip out the threads. It is a much fussier operation than nuts and bolts. Sheet metal screws will very quickly and easily strip out their threads if over torqued. ---

**Author:** ![casdave](https://avatars.discourse-cdn.com/v4/letter/c/c6cbf5/32.png) [@casdave](https://boards.straightdope.com/u/casdave)\ **Post date:** [August 24, 2015, 3:07pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/29 "2015-08-24T15:07:03Z")

When you are torqueing screws and slotted head bolts there is another issue. A torque wrench measures turning force, well duh! however if you press down hard on the wrench, especially a screwdriver torque wrench, you will get a false reading - it will be too low. In other words, you have to be certain you are only turning your screw, and not pressing or pushing it into the material - it seems obvious when its mentioned, trust me, plenty of mechanics don’t realise it. Not all cycle mechanics are trained fully in the engineering principles of their trade - you’ll find that out when your brakes suddenly fall off. ---

**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [August 24, 2015, 4:51pm UTC](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151/30 "2015-08-24T16:51:36Z")

> [@Kedikat](#): > > … A screw is usually cutting it’s way into the materials to be fastened together. … In construction this is usually true. In an auto mechanic’s shop it’s almost never true. In a factory it could be either depending on what they’re making, but I’d bet that self-tapping is by far the exception. And is certainly by far the rare case to have both self-tapping fasteners and a formal torqueing standard that requires a specific torque value. [Previous page](https://boards.straightdope.com/t/mechanics-of-using-a-torque-wrench-to-drive-screws/728151.md?page=1)