What does it mean to torque a screw to a specific torque?

Submitted by alucemet t3_10anzxi in askscience

I know how to calculate a torque (force times distance) but I don’t understand what it means to torque a screw to a specific torque. For example, if a screw is torqued to 7 in-lbs, does that mean the screw will not come loose until I apply a load of 1 pound 7 inches away from the screw/7 pounds of force 1 inch away from the screw? In order to have a screw torqued to 7 in-lbs, do I apply a load of 1 pound 7 inches away until it no longer rotates?

Edit: Thanks for all of the replies! I don’t think I can respond to everyone, but I have a better understanding now. Really appreciate the help!

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die_kuestenwache t1_j45t9md wrote

The more you tighten a screw, the more "force" it takes to tighten it further. Since tightening a screw is a rotational movement, what you're interested in is torque, not force. If you tighten it to a specific torque, this means you stop tightening once the torque necessary to tighten it further would exceed your limit. Reasons to only tighten to a certain torque might be that further tightening could deform whatever you are fixing with the screw or it might snap the head if you go too far beyond.

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kilotesla t1_j46lu62 wrote

The third hazard of over tightening is stripping the threads, either the threads of the screw or the threads of the nut or threaded hole it is going into.

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[deleted] t1_j46nwig wrote

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lucidrage t1_j46rdko wrote

>You want to make sure the bolt doesn't encounter a resonance frequency, from the machine it's attached to, causing it to come loose.

Does this happen often with cars? Why do we have to retorque the tires 100km after changing it? Do they not tighten it after they swap it?

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davidm2232 t1_j46ryuj wrote

>Why do we have to retorque the tires 100km after changing it

I've found that is usually due to rust on the wheel/hub that crushes while driving. That being said, if the mating surfaces are clean and the lugs are tightened to the proper torque, they should be fine for thousands of miles/KMs

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hatsune_aru t1_j4a8m4q wrote

I've seen the paint on the mating surface of the lug nuts sometimes degrades and causes the nut to loosen.

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Different-Truth7668 t1_j46zobq wrote

Steel rims deform slightly, giving the connection some "springiness". But alloy wheels don't deform if you stay within the given torque range (if you don't, they crack) so any dirt particles, rust, sand etc. that grinds to dust after initial tightening will leave a gap between the hub and rim.

So forgetting retorque with steel rims is somewhat forgivable, but outright dangerous with alloy rims.

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davidm2232 t1_j470673 wrote

>but outright dangerous with alloy rims.

Not true at all. Mechanic/tire shops don't have every customer that had their wheels pulled off come back to have them retorqued. If they are torqued properly initially, no need to retorque.

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Different-Truth7668 t1_j474b8s wrote

For example, i forgot to retorque my alloys in the autumn when the mandatory winter tire months came on. When i finally did it, some of the bolts were already "finger tight".

Almost every garage i have worked at has reminded customers about retorque when they get their car back.

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[deleted] t1_j49ktq2 wrote

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spideywat t1_j49m0xk wrote

It has nothing to do with the torque value and everything to do with the material. I have had dozens of vehicles, driven hundreds. Aluminum rims often come loose. The nature of the material. I have torque wrenches and a big shop.

On one set of rims it takes about 15 minutes to torque each wheel the first time then a short drive and some of the nuts loosen. 2015 f350. The Ford diesel mechanic noted the same thing and he hates those rims too.

I was at a garage where the mechanic told the customers that retorquing was bull and he had never done it once in his life. He was a fool.

Wheels come off less often now then 30 years ago, after which they introduced retorquing tires.

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spideywat t1_j49mzcy wrote

We have put millions of kms on our vehicles. We know about them loosening, however, there will always be people saying it is nonsense or an error in the initial torque. Follow your pattern, set the proper torque, every once in a while some will loosen on their own. That’s why we have pre trip inspections and a ton of regulations on our vehicles.

And if you over tighten then you are weakening the studs and can ruin your threads. Ounce of prevention as they say.

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Coomb t1_j49xo5s wrote

Everything deforms when pressure is applied. Alloy wheels are actually considerably less stiff than steel so they will deform more under the same load.

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uwauwa t1_j46zwey wrote

its due to aluminum allow wheels, something about the softness of the metal "could" loosen after stress cycles.

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NiteShdw t1_j470fqu wrote

I’ve lost screws off my motorcycle because they vibrate out (small ones that hold on the fairings).

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hatsune_aru t1_j4a8rpg wrote

Motorcycles are subject to significantly more vibration, and some motorcycles are even designed such that the fasteners aren't guaranteed to not back out (in cars, they are guaranteed to not back out, at least if designed right)

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dockriefer69 t1_j46ozle wrote

Torque requirements can also be used to make sure a fastener is tight enough. A torque wrench or screw driver gives positive feed back when you've reached you're torque setting. So whether you want to make sure it's tight enough or not too tight it works.

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Bromm18 t1_j49hefi wrote

Another example is, as a machinist, we tighten the collet chuck nut to different torque specifications depending on the collet holder (tool holder) size. As different torque settings can have different effects on the tool as it spins up to 10,000 rpm (and many even higher). Can cause the tool to be held to tightly and deform the holder, or to loosely and the tool could come out. And if the torque is too high or low it can cause deflection as well (deviation in the center of the tool over a distance).

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danieljackheck t1_j479oor wrote

Fastener engineer here!

Torque is a means to an end. The end goal is actually to stretch the screw. The screw behaves like a rubber band. As you stretch it out, it wants to return to its original length. As it does so, it squeezes the components you are fastening together. This is typically called clamp load. This clamp load is difficult to measure directly, requiring modifications to include an expensive load cell or ultrasonic measurement. Neither of these are practical in any production environment, and basically impossible at home. Torque on the other hand is really easy to cheaply measure.

What dictates how much clamp load you can get for a given torque is the friction you have to overcome as you tighten and how much clamp load the bolt can sustain. The total load be the proof load of the part and is based on the grade. Friction primarily comes from the contact between the threads and the contact between the underside of the screw's head and your joint components. This friction is also what prevents the screw from coming loose on its own. The amount of friction must be controlled so that you require a consistent amount of torque to reach a consistent amount of clamp load. This is typically done by careful control of the screws finish and application of lubricants.

​

There is a pretty simple formula for figuring out approximate torque if you know a few things about the components.

T = KDP

T= torque

K= dimensionless friction value for the entire joint. Takes into consideration the finishes and geometry of all of the components. Can be approximated in non-critical joints, for safety critical should always be experimentally derived.

D= nominal thread diameter

P= clamp load, often 75% of the proof load of the bolt

​

If for example I wanted to figure out what torque I should tighten a 1/4-20 grade 5 hex cap screw into a joint that has a matching nut or a tapped hole at least 3/8 deep:

K= .22 (typical for zinc plated parts, would change if using something with significantly different geometry like a flange bolt, different finish, or with lubricant added.)

D= .250 in

P= 2025 lbf (75% proof load)

.22 * .250 * 2025 = 111 in-lb

​

One consideration for the DIY at home is that lubrication will often reduce the amount of friction, meaning LESS torque is required to reach the required clamp load. The consequence here is that the torque value your service manual says you should use may actually be enough to damage the threads if you use lubricants that were not originally used during manufacturing.

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ScootysDad t1_j487zdc wrote

Damn interesting from a professional perspective. Thanks for the thorough explanation.

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hatsune_aru t1_j4a8vws wrote

Do you determine K with a bolt stretch gauge?

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danieljackheck t1_j4abbmu wrote

K is determined by using a torque transducer, load cell, and the other hardware like washers and nuts that you are going to use in your joint. You want to make sure your setup is as close to actual production as you can get. You then tighten to 75% of the bolts proof load as measured by the load cell while also measuring torque. That gives you enough information to use the formula to determine K.

K = T/(DP)

T = Torque achieved at 75% of bolts proof load

D = Nominal thread diameter

P = Proof load of bolt

​

Using our example above:

111/(.250*2025 = ~.22

​

This would be an example setup from work:

https://pieng.com/testing-overview/torque-tension-testing/

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Scuka1 t1_j45thhk wrote

If a screw is torqued to 7 in-lbs torque, that means that, if you used a wrench 7 inches long, and pushed with 1 lbs of force at the end of that wrench, you'd achieve that torque.

By tightening with different torque, you're achieving a larger preload on the bolt.

If you look at that picture, the bolt acts like a spring. Bolt head and nut are in fixed position (to simplify), but as you tighten the bolt, the bolt threads "travel" down the nut so the bold stretches just like a spring would. The elastic force of that stretch keeps the two plates together. The tighter you turn the bolt, the more you stretch it, the more preload you get, and ultimately the more clamping force there is.

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PLZ_STOP_PMING_TITS t1_j46ygbl wrote

If you used a 1 inch long wrench and put 7lbs on it's end would that be equivalent?

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ThetaReactor t1_j47b77d wrote

Yes, you're just multiplying the length of the arm by the force applied.

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W0tzup t1_j45yiny wrote

Torque = force x radius.

If the force applied is constant but say the wrench is longer, then the torque generated will be more, and vice versa.

When you use a wrench with a preset amount of torque, it tightens the screw to that torque value and in order to loosen it up you will need to generate a minimum amount of that torque in the opposite direction.

The purpose of tightening screws/fittings to a specific torque value is so the thread does not get damaged; especially when you need to undo/redo them often.

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5J7XM33IXN4XCQI6B2BB t1_j46oexa wrote

The purpose of torquing is to apply a specific(ish) amount of preload to the screw. It has nothing to do with protecting the threads.

In many applications, a bolt can only be used once anyways because it stretches during torquing the first time it is used.

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Chasethemac t1_j471380 wrote

Protecting the threads is a huge factor in torque spec.

Material and thread type would determine what you can torque something, too. It's basically the starting point in determining a spec. Valve cover onto aluminum head, for example.

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5J7XM33IXN4XCQI6B2BB t1_j480mmf wrote

Torqued bolts/screws exist to provide a certain amount of clamping force. You do not choose a torque spec based on the threads, you choose the bolt size, material, and thread profile based on the clamping force requirements.

Anyways, I prefer angle control over torque control since it's much more repeatable, assuming your bolts are to spec.

I used to work for a fastener company in QA.

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minkey-on-the-loose t1_j46fvem wrote

Not only the thread, but the compression on a gasket, no?

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PsychoEngineer t1_j47bwob wrote

In a way; applying the torque translates into a force the screw/bolt head is applying axially down the screw/bolt into whatever it’s being driven into; this force is what holds the parts together, or in the sense of a gasket crushes the gasket a certain amount. (Think of the gasket like a spring not a solid).

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Bwyanfwanigan t1_j4617yr wrote

What is missing here is the practical part. You use a torque wrench to accomplish this. They have a part that let's you set the required torque, and then you turn the bolt with the torque wrench until the wrench clicks. Google torque wrench for an image. Older torque wrenches had just a pointer and scale and depended on the elasticity of the wrench handle to get the right torque. They were fairly inaccurate.

Theoretically undoing a torque bolt requires the same torque. Real world that depends on a lot of things...

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julie78787 t1_j469nx0 wrote

It actually doesn’t because the coefficients of static and sliding friction are different.

There are practical implications of that, but unless you assume frictionless parts, the torque immediate prior to motion will be greater than the torque immediately after.

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fighter_pil0t t1_j46hn81 wrote

I feel like this would be a factor for automatic torque wrenches but not manual ones. As you try to hit the torque correctly you usually slow to a complete stop.

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julie78787 t1_j46nf6e wrote

So long as you don't stop, then start, then stop, hoping to hit the correct torque.

Once you stop, the coefficient of static friction will increase the apparently torque, potentially leaving the fastener under-torqued.

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minkey-on-the-loose t1_j46fqa0 wrote

Those old pointer torque wrenches were good enough for grandpa, they are good enough for me. But those setting torque wrenches sound awesome.

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davidm2232 t1_j46seqy wrote

Both have their purpose. The click ones are great to quickly get through a torque sequence. The beam or pointer style give a nice visual representation of the torque. Snap On makes a digital one that will give an instant torque readout and beep when you reach the setpoint. Best of both worlds

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aspheric_cow t1_j4735jb wrote

I'd argue the old beam type torque wrenches were more accurate and reliable. They are so simple there's not much that could go wrong with it. They just aren't very convenient to use.

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abat6294 t1_j45txun wrote

Yes, you described it nearly perfectly. I'd make two corrections:

  1. In reality, the torque required to loosen a fastener doesn't always equal the torque applied when it was tightened especially if a lot of time has passed. But if you immediately loosen a fastener after tightening, it should be about the same.

>In order to have a screw torqued to 7 in-lbs, do I apply a load of 1 pound 7 inches away until it no longer rotates?

  1. I think this definition works, but when applied in reality it's more like: Rotate the screw until 7 in-lbs of torque is reached. Because as you know, more and more torque is required to spin a fastener the tighter it gets. When it's loose, you can only apply torque up until it spins.

So if a screw spins with 5 in-lbs of torque, then you simply can't apply 7 in-lbs - it moves before 7 is reached.

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kilotesla t1_j46lmjd wrote

>So if a screw spins with 5 in-lbs of torque, then you simply can't apply 7 in-lbs - it moves before 7 is reached.

Or if you did apply 7-in pounds, you would accelerate it rapidly and it would spend very fast, which isn't really possible with a manual tool although it could be possible with an electric tool, but spinning very fast would make it hard to avoid overshooting the final level of tightening that you want to achieve, and the whole thing becomes simpler when you go slow enough that inertial forces are negligible.

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iboxagox t1_j46pzo9 wrote

The torque requirement equates to tension on the screw. When you tighten a screw or bolt, the bolt stretches slightly(gets longer). Think of it as a spring. A bolt might have a torque requirement that will create a tension on the bolt called the proof load. Any more torque, and the bolt will permanently stretch and will essentially not be usable. (There are applications where this is actually desired. "Stretch Bolts". ). Think of bending a metal spoon. You can bend it slightly, and it will return to its initial position. Bend it more and it will be permanently bent. (You exceeded the "yield" point of the metal.) Typically, the torque and hence tension will be in the elastic region( the screw will not be permanently stretch ed and can be reused). Anyway, the tension on the bolt clamps the items together and the frictional force created between the two parts is what prevents the parts from moving. More torque, equates to more load which equates to more clamping strength.

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TrappedOnARock t1_j47tldq wrote

One thing about torque specifications I don't understand is when a manufacturer calls for a certain lb-ft of torque, why do they sometimes add an additional degrees of rotation afterwards?

That's seems counter-intuituve and less accurate of a fastener safety measurement.

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danieljackheck t1_j482m9u wrote

If you look at my explanation in this thread, (almost certainly the longest one) the primary factor dictating how much torque you need to apply is friction. The better you control that friction the more accurately the torque you call out gets you to the clamp load required. Unfortunately friction is sometimes hard to control.

One of the other ways you can get an accurate tightening is by turning the fastener the same amount every time. Stretch in the fastener, and therefore clamp load, increases linearly with rotation of the fastener. If the distance between my threads is 1mm and I turn the bolt 360 degrees, I will have stretched my fastener by 1mm.

In practice though the challenge is determining where to start measuring that rotation. Everybody has their on definition and perception of where a thread starts to engage. You need to establish a starting point that is relatively consistent for everybody. That is where the torque comes in.

At lower torque values, the variability in friction has less of an impact on the clamp load you ultimately achieve during tightening. So I can pick a lower torque value, and then once I hit that value I can start counting degrees of rotation. This would get me to roughly the same location every time. This is much more accurate than purely relying on a maximum torque value for joints that have a lot of variability in friction.

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feyzquib7 t1_j474pqg wrote

Put simply, it means you don’t want to over or undertighten it. Whatever you’re using it for has been identified as mission critical to the object. Undertightening would mean it’ll come loose during normal operation. Overtightening means you risk sheering the bolt or attached surface during normal operation.

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darrellbear t1_j478h5t wrote

Get a torque wrench of the appropriate range (in/lbs, ft/lbs, etc), set it to the desired amount and tighten the screw/s. The wrench will let you know when you've reached the setting--it will click, release, illuminate or such. Specific tightening techniques can be required for certain applications such as engine head bolts, wheels, etc.--bolts may be required to be tightened in a certain order/pattern/amount, multiple passes, etc.

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biff64gc t1_j47fn29 wrote

It has to do with with the turning force being applied to the head when tightening the screw/bolt/nut into position during assembly. Generally the higher the torque used to put the screw in, the better it will stay in position due to increased pressure on the threading. Putting too much pressure into turning the screw and you could damage the screw threading, the shaft threading, the screw head could snap off, you could strip the head, or damage the material you're fastening together.

So torque settings are recommended to both ensure the screw/bolt/lug will lock into place as tightly as possible, but not so tight that you will break something completely. The torque setting is determined by a variety of factors ranging from the materials being fastened, material of the screw, threading size, and physical size of both.

You set your tool to the torque setting. As you apply turning pressure with the tool it will hit that setting limit and click as the tool shifts/gives under the pressure. This is essentially the tool giving out before the screw does to ensure you don't over-torque and break something.

How well a screw stays in place is determined by threading size, original torque pressure, the material it's connected to, the environment, and other fastening additives like glue or locktite.

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