Why can’t a cordless impact wrench unscrew the wheel?

The evolution of cordless tools is moving fast. Twenty years ago, even a screwdriver seemed to many as a little-known device, needed only by those who earn their living by repairing or installing. And today, almost everyone has it in their home and garage tools, including the very lazy ones, who take it out of the drawer once a year in order to fix a stool in the kitchen, and with a sigh cleaned back, realizing that it is easier to buy a new one… They buy wireless Bulgarians, jigsaws, polishing machines, and much more.

Very much contributes to the expansion of the amateur tool arsenal and modern approach of manufacturers, allowing you to save and have a single battery and charger for several tools of the same brand.

One of the popular tools, which are bought both purposefully and consciously, and on the principle of “just in case”, has recently become a percussion battery impact wrench … And the first thing that comes to mind for car owners – easy, beautiful and elegant wheel loosening, as in a car repair shop or tire service. However, the expectation and reality do not always coincide here!

Battery wrench in the arsenal of a car enthusiast

The impact wrench, powered by a screwdriver battery, is a very useful tool for the owner of a young car who prefers to repair it with his own hands. The standard half inch “square” on the output shaft allows you to install on it the whole range of end heads from any available universal set – from at least 8 to 24, which covers almost all the fasteners of most cars. The torque of middle class models is from 200 to 300 newton meters, which is quite a lot. At the same time, the tool is compact, does not require a compressor, hose, water separator and other pneumatics, and in general can work in a garage that does not have a socket, and even on the street at the entrance, which also sometimes happens …

The nutcracker facilitates the work and reduces the time spent in a twisted form under the open hood or under the arch of the wheel. Especially when it’s cold. The price of such a tool is relatively affordable – from 5-6 thousand rubles for a good brand. Many buy this gadget for a wide range of tasks, including repair and seasonal change of wheels, others – only for the latter. However, both the first and the second sometimes turn out to be deeply disappointed: the wheel bolts can’t be handled by the impact wrench, although, if you believe the characteristics, you just have to! Why does this happen? Find out all the reasons!

“Knock longer, and you will be opened!

Many people are disappointed in the battery impact wrench too quickly – before they have had time to “crack” it. And also without reading the instruction – on the fig, and so everything seems to be clear… In part, this is the reason for the popular belief that “impact wrenches have a slightly different torque. It is said that the same amount of torque from the aggregation of blows is “weaker” than the torque applied in the form of continuous traction, developed by the hand with a cylinder wrench.

In fact, the torque is, of course, the same. The same bolt, which is twisted with the same torque by the impact wrench and the wrench in the hand, will stop rotating at the same point. But a lack of understanding of how the impact wrench works is the first reason why wheel bolts cannot be unscrewed.

Any wrench has a so-called torque increase graph. According to this schedule, the maximum torque is never reached from the first revolutions – a wrench needs to knock on its torque clutch to develop full power!

This graph represents the torque dependence on time, taking into account the different thread diameters, and can be found in the instructions for the tool. However, if the impact wrench is not very branded, the manufacturers take it easier and replace the graph with the phrase “maximum torque is reached after the impact mechanism has been running for 8-10 seconds”. Accordingly, only after 10 seconds of “knocking” (or even more!) the impact wrench comes to full power. After that, it needs, in a good way, at least the same amount of time to move the nut or bolt out of place. Without knowing it, some people “push” the wrench several times for a few seconds and are disappointed to conclude that the tool “doesn’t pull”… Although it’s actually quite possible that it does!

Tightening moment of wheel bolts

A huge number of motorists will learn about the norm of torque for tightening the wheel bolts of their “swallow” just after buying a wrench. Before that, they somehow did not think about this norm, tightening the bolts as hard as possible, which is facilitated by popular scarecrows about the self-spinning wheels on the go. Some of them are still using the grandfather’s method of rolling the bolts on the ground before screwing – they say it will be harder with sand!

However, the instruction for any car clearly indicates the torque of the wheels, expressed in newton meters. Each car has its own, but roughly can be designated as follows: for sedans and small class crossovers, it is 80-120 newton meters, the average – 120-160, for large SUVs – 170-200. These are approximate figures, but they give a general understanding of the values of the moment. That not thirty, and not three hundred, figuratively speaking.

In fact, the torque wrench, which can show real torque on the arrow/digital scale or by clearly audible warning clicks, has and regularly is used only by rare perfectionists. Everyone else delays as they have to, mostly on an imaginary scale from “tight” to “dead”. As a result, the majority of car owners who twist the wheels with their own hands and lean on the sensations, rather than on a torque wrench, BELIEVES them in 1.5-2 MINIMUM DESIGN!

The fact is that most people have a subjective sense of “norm” very far from the manufacturer’s recommended numbers. And there is also a subconscious, genetically embedded fear of losing wheels on the move. If a dozen car owners give in their hands a torque wrench, ask to tighten the wheel bolt blindly, without looking at it, and stop them when the norm is reached, eight or nine people will exclaim “How, already all?! That is, the passport effort will seem to them insufficient. At automobile forums, revelations of such enlightened people are regularly met. Here typical: “… in the book on service Renault Duster it is told, that it is necessary to twist wheel bolts with effort 105 Nm. I took a high quality and expensive torque wrench from my garage neighbor, set the torque and tightened the bolts as it should be in the book. So it turns out that after the dynamo wrench can still be quietly screwed on at least half a turn! The casting is original factory, as well as wheel bolts …”.

So the wheels on Duster are tightened with the effort of newtons under two hundred, like for Cruzak … And this is also if the neighbor, a lover to help, with a pipe will not fit … This is the second reason why a sufficiently powerful battery nut wrench can disappoint you by refusing to unscrew the wheel bolts or nuts.

Tightening the fasteners by hand uncontrollably and on a hunch, you’ll almost certainly be able to tighten them! Let’s be frank: in most cases, nothing wrong with it does not happen, if the key spins with his hands from the whole fool adult not a small man, but at least does not jump on it and does not use an extension pipe. However, purposefully bought for the seasonal change of wheels wrench with a passport torque of, say, 200-250 newton meters, will be powerless, although at first it was fair to pretend that its power with a reserve should have been enough for the wheels that require 100-120 newton meters.

“It should be, but not enough…”

If, however, you want to unscrew the wheels with a fashionable gun a la “squeaky, and ready”, then we will consider that we have paid for a powerful battery impact wrench with a passport moment with a decent stock (250-300 newton meters) and understand the principles of its work. We also took a torque wrench from our spare neighbor in order to tighten the wheel bolts as prescribed by the manufacturer, and not to know the grief of turning away in the future.

Oops … And still a bummer … Bolt, screwed with a torque of 120 newton-meters, according to the manual, does not unscrew with the impact wrench with the declared power of 250-300 newton-meters. Why not?!

Here’s the thing…

First, the bolted connection always has different torques for tightening and unscrewing. The torque to unscrew a nut or bolt is one and a half times greater than that which was applied at the time of screwing. Therefore, by twisting a bolt with a force of, say, 120 newton meters, we need about 180 newton meters for the reverse process.

And second. The fact is that any bolt, and wheel bolt in particular, has an effect called “tacking”. And we are not talking about corrosion! It can certainly take place in a threaded joint and significantly complicate its disassembly. But the “tacking” effect, in our case, means an increase in friction force in the bolt head area over time.

That’s why cordless nutrunners are good at loosening wheel bolts that are tightened at nominal value and immediately after they are screwed in! Even the difference between tightening and unscrewing torque does not prevent them from doing so. But when this difference is added to the tightening force with manual tightening without a torque wrench, plus six months between the removal of the wheels, for which the friction force in the head has increased, and here even a 300-newton wrench will be powerless! And you still have to move the “120-newton” bolts with a cylinder wrench! The same applies to bolts in the suspension of a car with heads from 17 mm and above – wrenches from the above category of power most often can not cope with them.

Buy or not buy a cordless impact wrench?

The impact cordless impact wrench is definitely a great all-round helper in car repair. However, if you limit yourself to the seasonal change of wheels of your personal car twice a year, you probably do not need this tool.

In the most common segment of budget battery powered impact wrenches with a developing force of 200-300 newton meters can not be fully confident in the effectiveness of a particular model for 100% reliable wheel loosening in cars of small and medium classes, not to mention large and heavy. There is no guarantee that the bolts, even tightened strictly at face value, can be unscrewed without a manual “cylinder” after six months.

Plus, manufacturers often measure the maximum power of the impact wrench in very specific conditions, which are unrealizable in practice (as it, for example, happens with the power of a car stereo), and the figure drawn on the box and in the passport in normal practice may be unattainable. Accordingly, it is possible to try the actual, rather than advertising opportunities of a particular wrench model only by purchasing it. It is impossible to make a confident conclusion that a particular model will cope with your wheels, only by the torque characteristics in the description of the tool. Unfortunately, customer feedback is also not a guarantee.

Yes, more powerful impact wrenches than the 200-300 newton-meter class, which we have indicated, with the declared torque of 500-600 newton-meter and higher, will be guaranteed to cope with any wheels. But the price of them abruptly goes away, and the average car owner will hardly dare to make such a purchase.

To use a budget impact wrench, which has not overpowered the wheels, just to speed up the unscrewing / unscrewing of manually loosened bolts of the wheels in an independent seasonal “reworking” of a personal car is an excessive luxury. For this task, if you really want, it is enough to use the already existing screwdriver with adapter “HEX hexagon ¼-square ½”.


Leave a Reply

Your email address will not be published. Required fields are marked *