From L. R. H., North Argyle, N.Y. — Will you please inform me what philosophical principle is involved in the fact that a long screw-driver will turn a screw with less power than a short one? Can you explain the phenomenon to an unphilosophical subscriber?
Answer.—Our correspondent, in asking his question, virtually asserts that, with a long screw-driver, more power can be applied to the head of a screw than with a short one. This fact has often been denied, yet we believe it to be well established. The difference in the lengths of the screw-drivers admits of a difference in the manner of using them, and this difference in the way of using accounts for the difference in the power exerted.
In Fig. 1 of the accompanying illustrations we represent a short screw-driver at work. It will be noticed that the center line of the screw coincides with the center line of the screw-driver. If a long screwdriver were placed in exactly the same position, and were to be held, while being used, as straight as it is necessary to hold the short screw-driver, the results obtained would be substantially the same in the two.
The usual manner of using a long screwdriver, however, is like that shown in Fig. 2. Instead of the screw-driver being held exactly in line with the center of the screw, it is inclined to one side, which, in effect, makes the handle very much larger than it really is. The space A in Fig. 2 shows the inclination of the handle from a vertical line. The space B, which is double that of A, represents the diameter which the handle of the screw-driver in effect has by virtue of being worked in the manner indicated.
Fig. 3 shows an enlarged section of the screw head, showing the point of a long screw-driver in the slot, and illustrates the leverage obtained by the long screw-driver when inclined. A short screw-driver inclined so as to move in a circle having a diameter equal to B of Fig. 2, would be thrown out of the slot in the screw head.
Figs. 4 and 5 represent an experiment made to demonstrate the truth of the principles we have just described. The point of a small screw-driver is ground, beveling so as to give its handle, when revolving, motion in a circle of diameter equal to that of the long screw-driver. In other words, the space C in Fig. 4 is made to correspond with the space A in Fig. 2. With the short screw-driver thus managed, no more power is required to operate it than is required to operate the long screw-driver. Hence it would seem that the reason for a long handled screw driver 
requiring less power than a short one lies in the facts we have described—partly in its having a larger handle, whereby power is more readily communicated, and partly because a larger handle, in effect, is given to the screw-driver by its handle being moved in a circle.
Carpentry and Building – August, 1880
– Jeff Burks
I’d suggest that because the longer handle allows you some distance from the work you can apply more apparent force to the turning, rather than in keeping the screwdriver head in the screw. Essentially that the geometry of how the power is applied is different due to length, and that the force used to control the screwdriver can be applied with more control, thereby requiring less concentration, so that more concentration can be applied to the turning. Conversely, if the handle is too long it becomes more difficult to control and so concentration shifts back to keeping screwdriver and screw connected. This may also be why phillps, torx, and square head screws are easier to apply more apparent power to. There’s an analogy in weight lifting where you can lift more weight by grasping the bar more firmly and even more by concentrating a bit on that grasp–more control of the connection between weight and lifter allows the mind to focus on the lift.
I may be misunderstanding this explanation. I think this is a weird comparison and questionable explanation. Not sure I would use the screwdriver at an angle and risk slipping out then marring the surface of the wood.
The comparison should be between screwdrivers with handles of the same diameter and different lengths of shafts. Certainly increasing the handle diameter in creases the torque
I thought the same thing about the handle Lew60; then I reread and noticed this “inclined to one side, which, in effect, makes the handle very much larger than it really is.”
The answer is simple physics, longer shaft equals more torque equals more force on the screw head. It is the same as the lever principle in lifting. This is true even when the shaft is directly inline with the screw head and not canted as in strange Burks demo. Agree with lew60 too.
I don’t think this is right. According to wikipedia, torque is “a force applied at a right angle to a lever multiplied by its distance from the lever’s fulcrum.” In this case, I think the “lever” is the axis of rotation. A longer handle doesn’t get you any further from the axis of rotation. If you put a really thick handle on it, though, you’d get more torque since that allows your hand to exert force further from the axis of rotation. I found this that describes the same thing: http://van.physics.illinois.edu/qa/listing.php?id=294.
I think the canted screwdriver thing above is just saying that you have a little more room for error with a long screwdriver. You wouldn’t purposely angle the screwdriver, but we aren’t perfect and can’t keep the screwdriver absolutely perfectly vertical – especially if we are really cranking down on something. If you get the handle a quarter inch off center with a long shafted screwdriver, the angle of the screwdriver head with the screw isn’t as bad as if you did the same with a short shafted screwdriver.
Comparing a lever that uses a fulcrum point to a screwdriver that turns about an axis isn’t the same principles in my way of thinking. Placement of the fulcrum can change the advantage without changing the length of the lever. So same lever different fulcrum point equals more lifting force. What’s all that have to do with turning a screwdriver?
But I think they are the same principles…that being physics. The first poster made the mistake of thinking that the fulcrum point is at the tip. The screwdriver has a fulcrum point, that being the central axis that stretches from the tip through the handle. It also has a ‘lever’, that being the handle. Perhaps this is easier to think of in the sense of a lock’s key, which is just a screwdriver with a semi-unique pattern.
In fact, the Wikipedia entry for lever explains how torque is the unifying force in mechanical advantage. I’m a bit incredulous of the explanation above, but it would be a wonderful experiment to test this. Maybe Chris can build a device to measure the torque possible with a given screwdriver, then one could try a little experiment and test this hypothesis…
“longer shaft equals more torque equals more force on the screw head”
This isn’t true. For a screwdriver held directly in line with a screw,
Torque = (radius of screwdriver handle) x (force applied by hand). The length of the screwdriver shaft simply doesn’t enter into the equation (simple physics).
The article is also correct about a canted screwdriver, though I don’t think I have ever knowingly used a screwdriver in that manner, long or short. I am curious to try it out though.
THanks for Patrick Lund and Glad my gut knowledge and high school physics served me well in understanding the issue. I still feel canting the screwdriver will lead to marring the surface of the work, damage to the screw and possible puncture injuries.
But how does the longer shaft equal more torque?
Eric Erb
eric.erb@bioclinica.com
mobile: 240-328-3373
You don’t get more torque from a longer shaft.
If the handle is thicker, that would give you more torque. If the handle is longer, it might allow you to apply more force because of a better grip, which would increase torque, (but not by virtue of the length of the shaft as such.) Torque is the force applied times the distance from the axis you are rotating around, not the distance along the axis. If the longer handle allowed you to get your whole forearm into the mix, you could apply the force out by your elbow (thereby increasing the distance from the axis) but you’d need a damn good grip on the handle…
Eric Erb
eric.erb@bioclinica.com
mobile: 240-328-3373
I would like to help clarify something or confuse everyone. The torque is equal to the force times the lever arm, which is the radius of the handle
The article’s argument:
If both a short screwdriver and long screwdriver are held at the same angle the apparent handle of the long screw driver would be larger, hence less force required for the longer screwdriver because the lever arm appears to be larger.
But this is incorrect because you are still turning the screw driver about it’s axis which produces the torque and the radius is still the radius of the screw driver.
Physics:
If the handles were the same diameter then both screw drivers would produce the same torque. . A larger handle would give you more torque. Length of the shaft is irrelevant.
Please note that a longer shaft on a screw driver does give you more torque if you are opening a can of paint.
“this is incorrect because you are still turning the screw driver about it’s axis ”
The problem set up is wrong, or as a physics professor of mine was fond of saying, your conception is wrong. I’m sure the double entendre was intentional.
Anyway, you are turning the SCREW about it’s axis, not the screw driver.
A perfect illustration of the concept is using a flex tip breaker bar and a socket to loosen a bolt. With the bolt (screw), socket (screwdriver tip) and breaker bar (screwdriver shaft) all aligned straight up and down, the force applied is your arm strength alone, some base level X. When you move the breaker bar off of dead center, the force applied _to the bolt (screw)_ is X + some Y mechanical advantage. When you flex the breaker bar fully perpindicular to the screw forming a perfect 90 degree angle, you have the most force you can apply to the bolt.
The longer screwdriver can be offset from dead center a lot further than a short screwdriver and _still engage the screw head_ in a meaningful manner. Therefore, the longer screwdriver is capable of giving the flesh a mechanical advantage.
I hate to reply to myself but I ran out or room to type. Never did that before on this blog.
I wanted to add two things. First, the above is of course a simplification to illustrate the force on the screw, not the screwdriver. After all, of course the screwdriver is being turned.
Second, I have seen spoon tipped screwdrivers and drivers with the flat ground at other than 90 degrees purposefully to engage a slot at an offset. The biggest I’ve seen was on a train caboose for manually tripping switches–the shaft was at least six feet long.
The different length of screwdriver really doesn’t affect the torque directly. Torque is the force applied times the distance from the axis you are rotating about. So the torque of the screwdriver only really depends on the thickness of the handle. Longer screwdrivers tend to have larger diameter handles (not always) and hence the application of more torque.
Patrick
There is supposed to be a relation between the length of a screwdriver and the force you can produce at the business end – a product of the torsion (not torque) developed in the driver shaft. Or is this advertising hype?
Mallettowardnone, rusty and Patrick have it correct, torque is the product of the force applied to a lever arm of a certain length, perpendicular to the axis of rotation of the object the force is applied to, in this case a screw. Perpendicular is the key word.
Yucky technical stuff alert: Physicists and engineers define Torque as Force x Lever arm length x Sine of angle between lever arm and axis of rotation. The definition allows for non-perpendicular angles. So in the case of a screw driver handle which is assumed perpendicular to the axis of rotation, the sin(90) = 1, leaving Force x Lever Arm Length. A larger diameter handle will create a longer lever arm. Longer screw drivers often have larger handles, so you can get more torque that way.
Longer handles may give more apparent force in those cases where people were working in tight spaces, but a longer handle moves them away from the tight space allowing their arm to put more force on the screw too.
I should be specific here, a point out on the edge of the screw driver handle is perpendicular to the axis of rotation. Another example would be the T-handle we use on our Tap and Die sets. You can put a lot of torque on the axis because the T-handle, perpendicular to the axis of rotation, on the tap is so long.
Here is an idea (didn’t say it was good or not; but it is an idea). Take a machine thread screw double nut it. Get a torque wrench (and something to clamp it to) a socket and two different length screwdrivers. Set the wrench at the proper torque setting for the screw. Trust me the torque wrench won’t know it’s being held still and the screw is being turned. See if you can hit the limit with each screw driver in line. Test two; tilt the screw driver at an angle and repeat. Then keep raising the setting until you realize the length doesn’t matter and you would rather be woodworking.
By the beard of Zeus! I’m struggling to fathom how much mental energy has been exerted above in the name of turning a screw with a couple of screwdrivers around 1880! I’m with ‘markdorman,’ wouldn’t you rather be woodworking??!
I don’t know anything about the physics just what I know from personal experience. I always use a six inch long driver in my cordless drill. It is much easier to control the bit and I am guessing much more likely to drive the screw easier.