High-Speed Video INSIDE Impact Wrenches: How They Work

Welcome back to the Torque Test Channel. Today we have another one of our four science episodes, which might just be the most popular series we have. Appreciate that guys. We'll continue making them. When we dive into stuff for science, that usually includes us stumbling into some tool related questions by just sort of having fun with it, providing some info and data in the process that can sometimes accidentally teach us a few things and perhaps you too along the way. And today's subject is how impact drivers and impact wrenches work in both air and cordless. This is something probably long overdue for us, but with the help of our new high speed camera, we felt it was about time. So by cutting into or removing the hammer covers on these tools altogether or using a clear plastic housing, we can take a peek into these tools and see how they work at 1,000 to 1,500 frames per second. Let's start in order of size now with cordless impact drivers. Inside an impact driver, you will find a few things. First, as with all cordless tools today, is a brushed or brushless motor driving the first half of what's called the drivetrain. In most cases, brushless is preferred for space savings, power and heat reduction, but they both work and do the same sort of thing. And that thing is generating a ton of RPMs, often around 20,000 RPMs or above. Inside of these tools, including larger impact wrenches, you'll find an impact mechanism with a planetary gear set on the back. This works to take that high RPM of the motor and turn it into lower but higher torque capable RPM, like in the case of the new Rigid High Torque and the Milwaukee 2767 here, that both have a 11 to 1 reduction or 5 teeth on the sun gear and 55 teeth on the ring gear. The purpose of that lower RPM, higher torque is to overcome the spring tension and hammer mass on the ball and cam sprung impact mechanism, which is what pretty much all of them use in cordless impacts today. And that mechanism inside works like this as Toolstop demonstrates. This business end has two components, a hammer and an anvil. When the torque required is low, the tool just works sort of like a drill. The tool's RPM free spinning that fastener on or off. But when torque is required, the hammer binds up from spring tension and is forced up in the process, then over and leaps forward to strike into the next wing of the anvil, which is called a dog. The faster you can repeat this process, the faster the hardware can be driven. But on the other hand, more gear reduction or aka lower RPM would allow you to overcome a larger spring and a larger hammer and make for bigger hammer blows, which is called higher dynamic torque. The first impact driver we've had a part on this channel shows this process and that's the Metabo HPT triple hammer, which works in much the same way, but has three anvil dogs and the hammer body has three hammers to match. This usually impacts three times per revolution instead of two. Here's what that looks like at full speed. And here's what that looks like at a thousand frames per second. It's a pretty good demonstration of how more impacts per minute can drive something quicker, but with less degrees of rotation afforded to the tool here before hitting that next dog, it also has less angular momentum and speed. Force does equal mass times acceleration. So without increasing mass, a loss in acceleration into that next dog can mean less power, which is what we saw when this tool is on our dyno flat lining versus some of its contemporaries. While according to Hitachi slash Shikoki in Japan, these tools in power mode are supposed to result in hammering one and a half times per rotation, skipping over every other dog and therefore increasing that acceleration, which makes a lot of sense to us. Though in practice, our footage seemed to show the hammer hitting or scraping the top of the next dog rather than just skipping it altogether, perhaps slowing it down. Not really sure. Either way, the dyno's bolt was less tightened by this tool than others, which means we noticed this before even opening the tool up. So we maintain it's perhaps best intended for screws and other stuff like that, rather than large bolts and lags, which realistically is fine for impact driver work. Looking forward to testing their new 36D model. One misconception about impact drivers that some people have is that they're different somehow than impact wrenches in design. That an impact driver has some forward hammering going on, like a rotary hammer drill or something. We can dispel that for you. For one, any forward blows you're encountering are simply an accidental byproduct of the ball and cam sprung hammer mechanism cordless tools need to use. The mechanism allows the electric motor to operate at its peak power and efficiency of say 19, 20, 22,000 RPM or whatever without being bogged down. DC motors don't take kindly to being repeatedly stopped and they don't operate very well that way. The spring allows the motor to continue spinning while the hammer catches up. One downside of this is that the spring is launching the hammer forward, not just rotationally, meaning there is some forward force felt each blow. On the triple hammer we shot it's even more so because of the hammer landing sort of on top of the next anvil dog sometimes, resulting in the collet nearly letting go in some cases. But on most drivers it's more negligible and on wrenches they can even often use a Delrin bushing like the RIDGID in Milwaukee here to help mitigate that vibration. We'll get into that felt vibration in a bit. But that does mean impact wrenches like this bastardized Makita clone have pretty much the same thing going on inside. No window cutout needed for this one as they designed a clear cover over the hammer cage housing. Let's take a look at this first with 1000 frames per second. What you're looking at here is exactly how impact drivers work too. This same sort of two hammers, two dogs configuration that's just slightly bigger on this compact impact wrench, but similar RPM even to those drivers. Notice how not only do the hammers land squarely onto the next anvil dog, they aren't really sliding off of those dogs either. They impact onto and then rebound off of that dog and rotate onto the next. Pretty efficient, no wasted friction, and the shorter amount of time you can convert potential spinning energy into kinetic impacting energy, the more you're going to optimize the tool's capability for higher dynamic torque or force per blow. Not that this tool made a lot of beans for us, but it does show at least how it works. We noticed looking at the footage here that the cover wasn't really doing much despite being see-through, so we decided to just take it off and try to film it that way. Here's the exposed version of this tool at 1500 frames per second now. So this is just starting to pull the trigger. And here it is at speed. You can now briefly see the notches in the center of this mechanism that represent the cam part of this design. The hammer rides on this, and when confronted with spring pressure while seated against a dog, is forced to climb upwards before smacking back down onto the next dog and then repeating. It may not look like this tool is making much progress, but this all has been about 1 second of footage in real life. Here's a clip of the tool sped up to show it working. Now it's hard to talk about cordless impacts and not look at the Milwaukee 2767, our old faithful. We bought a spare hammer nose cover for this tool for just about 17 bucks and hacked up the old one here like a blind surgeon with an angle grinder. Not very pretty. The difficulty with this tool and ones like it is that it maximizes its hammer mass by extending past and around the anvil dog, so there's not a lot of peaking potential into this one, which we'll try to overcome shortly for you. Here's a clip of what you'll be seeing in slow motion looks like in real life. And here's the high speed. So you'll notice right away that you only see the line on the hammer once every two hits. That's a result of that hammering once every 180 degrees design that's carried over from the last impact. Except unlike that one, Milwaukee and most high torques use a larger diameter hammer cover to allow for a larger hammer itself inside, and that larger footprint even goes past and over the anvil dogs. Since high torques are obviously slower, that mass being maxed out in that force equals mass times acceleration formula is pretty important. And the larger in diameter that mass is situated, it puts that mass to the power of two exponentially in the rotational inertia formula, which I think is why while cordless impacts are getting shorter nowadays, they're also getting usually chunkier towards the front, capitalizing on this sort of physics. To demonstrate this, we took the just two ounces heavier hammer mechanism out of the Milwaukee high torque and swapped it into the rigid high torque, which was then after still operating at the same RPM as before. That rigid was just going from a two pound two ounce to two pound four ounce hammer, and it made this much of an increase in peak torque. Now, just as we did with the compact impact, we felt it might be possible to just lose the hammer cover altogether and take a peek at the insides in action without obstruction. This requires some forward pressure to keep it seated, but seemed to work pretty well to our surprise. Notice how when transitioning from early hammering to full power, the hammers go from sliding up on the dogs to get past them to when at full beans the hammer just recoils and makes a jump off the dog, then in mid air floats past it and onto the next dog and then onto the next, creating more angular travel or basically more degrees for it to accelerate. And here we can finally see that spring at work to winding up. Then the hammer is stationary for a second and releasing that tension as for rotation after each blow, the hammer travels backwards quite a bit more than I would have assumed. But here's another angle at 1500 frames per second. Notice that the square drive wiggles after each hit. It really is like a hammer each blow hitting then recoiling. No real constant linear torque applied seen here, like you would get from a drill. This tool is currently operating in clockwise, but that's loosening for this dyno on its reverse threaded bolt. And at the edge of your screen, you can see the progress of each hit that it's making from the travel of this pinhole on the socket, which is pretty cool. And you can also see faintly in the background, the famous Milwaukee M18 battery shake going on, delayed in its reaction from the vibration traveling down the tool. That battery is definitely living a hard life. But this brings up a good topic, cordless impact wrenches. Why do they shake you around so dang much? Well, you may remember me talking about that spring and how more powerful impacts have a bigger, harder to overcome spring, which results in the ability to push a heavier hammer forward and with more acceleration. Well, with Newton's third law equal and opposite forces and all, a spring applies equally the amount of force it takes to accelerate that hammer as it does versus what it's attached to. And in an impacts case, that's attached to you, your grip holding onto the tool. It wants to turn your arm very suddenly and very briefly, several times a second. We measured this Milwaukee at about 2,200 impacts per minute, or just over the 2,100 specification it carries. You experience this as vibration and just sort of teeth chattering good times when a bolt is stuck. Some designs can help mitigate that impact transmission internally and with other tricks like Flex's shock shield shown here, but it's unavoidable in the sprung ball and cam mechanism cordless tools use. And the more powerful the impact, the more it's going to be using you to provide that foundation for its power. Not all mechanisms work that way. However, when it comes to air tools, far and away the most common impact mechanism in those when space allows is a twin hammer. In that design, there's a hammer cage with two separate hammers in it. Then the anvil extends down in the middle and has two opposing dogs that engage with each of those hammers simultaneously. Then once they impact, it acts as a clutch on the empty side of that anvil and skips the hammer on its next pass, which means on these tools, almost all air tools, they impact once per 360 degrees rather than every 180, like on cordless mechanisms. Now showing the tool stripped down like we did with the cordless tools and like we're showing you here isn't possible as the hammer nose cover keeps that air cylinder snug down and from blowing out the front of this thing. But here's how an air impact wrench works with a window cut out of it showing its relevant bits. We're having to shoot this in the shop where there is an airline supply. So it's quite a bit darker despite about 3000 extra lumens being pointed right at this thing. But right away, you can already see how the line we painted on the hammer cage is the side that's sort of being skipped over before it sort of hits on the opposing side each time. Only once per full rotation or 1100 IPM as we measured it here versus the 2200 of the Milwaukee. This sort of design has its benefits. For one, it allows the tool to speed up more before the next blow. Air tool mechanisms are also often called clutches because they engage and then disengage. Here's another good angle showing that the design of the hammers allow them to impact then sort of get the heck out of the way quickly so the air motor can spin up again. We estimate the hammer speed to be around 1.7 times that of the cordless tool in this just before it strikes the hammer. This speed is a cheap way to add rotational inertia without adding size or weight to the gun. The second benefit is that the impacting itself is quieter. The gun, yeah, definitely louder than a cordless tool, obviously. But at least on a stuck bolt, cordless can be a bit more ear splitting in my experience due to that double time IPM. There is certainly a laundry list of benefits to a cordless tool, of course. Not being tethered to an airline being chief among them, but there are also three benefits to this sort of mechanism versus cordless that don't have to do with IPM. One, the entire air motor and vanes are attached with usually a spline output shaft to the whole hammer mechanism. This means the combined weight of the motor, hammers, and hammer cage account for the total mass being used for impacting. So basically, this much of the tool's mass is impacting on an air tool rather than this much of a tool on a cordless tool, which is the main reason it's hard to make high torque shorter nowadays than what they already are and why air tools are typically smaller. Then number two, the entire mass comes to a complete stop when it impacts before spinning up again, which is the most efficient conversion of that potential energy stored in the mass rotational inertia into kinetic energy that moves your socket. A cordless tool needs to continue spinning, so a portion of that energy spills over into the spring used to keep things going. And third, finally, air motors of this size range spin pretty fast, 8,000 to 11,000 rpm. And while that's pretty slow compared to, say, a 22,000 rpm brushless motor and a cordless tool, those all have planetary gear reduction to increase torque and combat that spring tension. Air impacts are typically more direct driven, which means a 10,000 rpm air impact will drive a loosened nut off of a wheel stud pretty darn close to 10,000 rpm, while a high torque can do this task at usually around 1,700 to 2,000 rpm. So with all that said, does that mean air tools are just better? Well, I certainly like working on them, obviously, so I'm biased. But even so, the answer to that would be definitely not. Besides reliability, one reason to go with a top brand historically associated with making an impact wrench is that they have the resources, experience, and reputation of being able to tune all of these things together. A brushless motor with its peak operating rpm and power has to be tuned to the hammer spring size, which has to then be tuned to the hammer design and hammer mass, which has to be tuned to the anvil design and shape. These tools are basically multiple independent systems working all in harmony, that if one is out of balance with another, for example, on an air tool, you could have wasted time and power squandered at the hammers with these things sort of jumbling around, which they could do. And on a cordless tool, it can result in the tool being needlessly too big and or underperforming despite its larger than average size. As always, just choose the tool that works best for you. As we've shown on this channel, there's more than one road to take nowadays if you want a lot of beans. Some of the hammer mechanisms, which I'll leave cards for at the top of your screen that I personally think are cool, but we couldn't film in slow-mo for you today because they either don't look like anything when they're working on the insides or would throw necessary bits out into outer space through a hole we would cut in the side. That includes the rocking dog mechanism used on old school NASCAR five lug pit stops, the three jaw hammer mechanism used on Paoli pit guns for modern F1 pit stops because we just didn't have one to show you yet. And that three jaw is very different to the Metabo HPT triple hammer and a twin pin clutch style found in like an Aircat 1250K and others that operate inside of a oil bath. I appreciate you coming along with us on these sorts of episodes. Hopefully you saw or heard something for the first time that you found interesting. Click subscribe to catch us on the next one. And thanks as always for watching.