Maximum Torque Possible with a Screwdriver

I have no idea. How about testing a screw; tighten it as much as possible by hand, then use a torque wrench with a screwdriver tip to see what the torque is when it first starts to move?

Thanks Mikerho - I'd love that to happen out in CR4 land somewhere... here I have no torque wrench, so can't do the necessary experiment!

Thanks - Robert

I think your reply has merit Mikerho and a spring balance pulling on a lever 12" from the centre, whilst getting someone to push on the screw driver, negates the need for a torque wrench. Also mounting this under a bench drill for support would work. A cheap Torque wrench is not going to break you.

But... the quality of contact and screw head type needs to be considered, is the screw head soft or hard, are the axes of the screw and driver in line, is the screwdriver worn or the wrong size, is the screw accessible. How many turns will the operator do at this torque? Will screwdriver and screw lose contact before the full torque is exerted or will the hand slip on the handle.

The last consideration, to overcome start-up friction requires considerably more torque.

Tony

It looks like you're going to have to rely on mechanic's feel. In Robert Pirsig's book Zen and the Art of Motorcycle Maintenance, he describes mechanics feel as an intrinsic feel for the elasticity of materials. I'm paraphraseing, but the essence of the matter is that experience trains the muscles and the mind to know: without really even thinking about it, when a nut, bolt , or screw is tight enough without reaching its mechanical limits.

In the absence of a torqueing device, that's all you have.

I have mild arthritis in my hands and I can do 9 ft·lb with an ordinary screwdriver.

I'm only good for 9 inch pounds, and that's with a T-handle

I may not be strong, but I sure am stupid!

I had a quick attempt at an answer after the guys had gone = empty workshop, just me.

Took a screwdriver with a 6mm hex shaft, put a ring spanner on it, found a plastic bucket & played with lumps of lead in the bucket, to see what I could lift by turning the screwdriver with the bucket handle hooked on the spanner.

Came out about 6kg at 75mm, which works out somewhere near 4.5Nm (3.3 lbft).

Don't suppose this is anywhere near accurate as an "average" - the experimental method was crap & I was afraid of breaking either the bucket or the spanner, but it gives an idea.

I'm a skinny old bloke, but have reasonably strong wrists for my physique.

Would've been so much easier if I could've found the torque wrench!

If I get a chance, I'll get some of the guys involved tomorrow - they're always good for a laugh - we can use a longer lever arm & smaller weights (for better accuracy) & have the screwdriver blade supported, so we're not trying to both lift & turn.

That's very kind of you JohnDG - and a very good idea I should have thought of myself (I have no torque wrench, but plenty of screwdrivers!).

I'll try it too - but let me know if you get any consistent results out with the rest of the crew involved.

Thanks

Robert

A couple of suggestions:

Look up in a catalog such as McMaster Carr for "Torque Screwdrivers". They sell them and look at the range of the units that they sell. This should give you a ball-park range.

If you don't own a torque wrench borrow one or go to a store that sells them. Take an appropriate size deep socket wrench (size based on the diameter of the simulated screwdriver handle). Put the socket on the wrench and twist the socket with your hand. Read the torque on the wrench. Since the outside of the socket might be polished and hard to grasp you could use an "Allen Driver" (like a screw driver but with a hex end instead of slotted or phillips point on it) with a same size socket on the torque wrench. When you twist this you will probabley be able to recreate very closely what you can do with a screwdriver without all the "assumed" variables.

Both good ideas - I did have a look at the torque screwdriver ranges on the web, but I was concerned I didn't know the size of the lever arm (some of them seemed to have almost L shaped handles which would considerably increase the torque). They were claiming up around 90Nm, or 66 ft.lb ... I'm just about willing to bet I couldn't do that with a screwdriver.

However I can get hold of a torque wrench tomorrow if I play nice with some workshop guys I have to visit ... I'll take a trusty socket with me - I have one just about the right size.

Thanks for the advice.

Robert

A 1" diameter handle is ussually found on cheap screwdrivers and you can't hold them well enough for most applications.

Here is a list of several torque measuring screwdrivers, you take an average from their capabilities

or a quality tool

Klein 57035

Not sure what you mean but, I have an idea; Most battery drills have torque settings, how about inserting the screwdriver end of a screwdriver into the chuck of a battery drill ,then hold the screwdriver with one hand and run the drill, if it pulls away from you hand to much torque, now start reducing the torque setting to find where it slips, this would have to be the max torque that could be applied by a person.

You can then look at the manual that was supplied with the drill to calculate the torque setting you require.

Disclaimer: The above test may not be relavent to all people, especally Truck Drivers, Pen Pushers or Lawyers, they all have different strenghts. All 3 of the above have strenght in their hands, but in different ways. The Truckie has Brute Strenght, The Pen Pusher is Nimble with his hands (and probally has a limp handshake) The Lawyer on the other hand is very quick with his fingers, and before you know it all you cash is gone.

Regards

Joe

Good idea, but may be better to start at a low torque setting & work up. If you hang on too long, you could end up with a broken wrist.

Thanks John, have not forgotten you mate, just had other projects that need more attention.

Cheers

Joe

This remind me that there was a thread on lengths of screwdriver and the effect on the torque. As I remember, the available torque was not just a function of the grip diameter, which optimally is what you can grip comfortably, but also the amount of extra moment length that can be applied by rotating the lower arm from the elbow to the grip.

Looks like the highest available settable torque for a conventional screwdriver is 70 inch pounds.

DeWalt do a (drill shaped) model which can sustain 52 Newton Metres that's 38.35 foot pounds, and I doubt if many people could hold that even with the drill handle.

Just one note: (Don't know if it's relevant in your application) I think I could manage a lot more torque on a 1 inch diameter handle if I didn't have to worry about pushing the screwdriver at the same time to avoid damaging the screw head.

It depends too much on the health & strength of the person concerned, therefore its totally unpredictable, surely you can work that out for yourself???

As already posted, buy or make a device for proper setting of torque.

Thanks guest - I'm not looking for a correct answer - just a basic range for an adult male... a bit like house bricks are sized to be the right size and weight for an adult male - obviously some people can't hold them, some could hold bigger, but the house brick is the size it is for a good reason, and it works, even though the "right size" is totally unpredictable.

So I am looking for a comfortable range of torques that I don't want to exceed for a hand tightening application - if I knew the maximum I would probably design for 1/3 of that because I am not looking to make it difficult. Then I would cut the thread at a certain angle etc to get the required tension at a comfortable torque. You're right, I could buy or make something to work it out for myself... alternatively I could ask the people here if they have a sense of what is about right. I've had some really interesting responses.

Not sure why you're wasting your time on it if you don't think it's a worthy question.

Robert

You wrote:-

Not sure why you're wasting your time on it if you don't think it's a worthy question.

Answer:-

We try and help everyone, even the dummies

My comments at least added something to the discussion and I identified myself, your reply was just rude, maybe time to return to Twitter, your not welcome here!

thanks guest.. it's nice that even the dummies try and help.

Hi,

I knew a blacksmith who had no problem handling 30 Kg with one hand, and I visited once a shop where beautiful Thai women were busy at assembling tiny parts and wires that nearly no European nor American men or women would be able to do.

So I would take a suitable screwdriver and if not enough torque then drill a hole through the handle and put a suitable big nail through to amplify the torque.

RHABE

No more experimental results yet, but I found this on the 'net.

It has summary results of a study using 30 male and 25 female subjects, and includes the statement "Mean torque values for the long and short screwdrivers were only 2.7 and 3.1 Nm".

Looks like my 4.5Nm wasn't too far of the mark.

Hello RobertOz,

I spoke with a skilled tool maker some time a go, and I asked him the same question.

He said you tighten the screw until it makes a cracking noise and you back it off counterclock wise a 1/4 turn.

Ha Ha pretty funny guy!

I believe JohnG Reply 23 is as close as your going to get to your answer.

Regards,

Tim

I am 6' 4", 250 pounds. I just used a screwdriver to turn my torque tester. I was able to turn the screwdriver to 100 inlb. I would guess most people would fall just short of that value.

my tendon snapped @12lbsinch - need help

1/ off topic, but relevant, Can anyone explain why it is much easier to get more torque with a long screwdriver than an identical handle and blade but shorter shaft. I know it works but it seems totally illogical.

2/ While accepting that you are only looking for a range, you also need to take into account HOW someone arrives at "hand tight". Use of both hands is still hand tight! T bar handles or 1/4" square drives multiply many-fold of course. so if you specify "hand tight" you ought to spec how to gauge it as well.

3/ Is this a once only adjustment? if so how about putting a shear pin in the system or a weak point in the shaft that breaks at the correct torque.

If something that has to be done and undone multiple times then fit some sort of simple clutch,

Just my €0.02 worth

Chas

Re: your point (1).

The link I gave at #23 (repeated here) gives evidence of the opposite phenomenon - the longer drivers averaged 2.7 Nm against 3.1 Nm for the shorter.

Other comments I came across (without reference to controlled experiments) supported your contention, giving 3 suggested explanations:

1. A longer driver makes it easier to attain a comfortable stance, where the full strength of the arm and shoulder can be applied.
2. A longer driver often has a longer (and possibly larger diameter) handle^[1], making it easier to get a good grip.
3. (IMHO this one's specious to say the least) - the longer shaft may act as a kind of torsion spring, allowing a higher torque to "build up".

Of these, only the first seems to hold water as reasonable.

[1] This as reported; I realize that you specified "an identical handle and blade".

Length of the screwdriver shaft will not affect the amount of torque you will apply. Torque is simply a function of length and force about an axis.

1/ see 15 above and add in a factor for the blade not being exactly normal to the screw head thus allowing extra moment.

By the way, I forgot to say how I did my 9 ft·lb (actually +/- 2 ft·lb since I don't have a peak reading torque wrench). The trick is to get your hands dry and let the skin sorta bind up like an oilfilter band and then rotate from the shoulder. You can get right much torque that way.

I have worked for a number of years in the Hydraulic Field. Vickers had some slotted screws on their "air gap" solenoids housing asking for 35 in-lbs of torque. I torqued a couple of them up with an inch-pound wrench and found it very difficult to remove the screws with a screw driver (not impossible but..). I tell my guys "torque em up tighter than tight" and they'll be OK. Use a 1/4- 3/8 " shaft full blade and about 1 1/4" dia. handle. Handle shape can make a difference.