Dewalt and other Stanley Black & Decker cordless power tool brands use UWO, or unit watts out, as a way to compare the performance potential of cordless drills and drivers.
In a nutshell, the takeaway is supposed to be that a Dewalt cordless drill with a higher UWO rating will perform work, such as a drilling or driving task, faster than another Dewalt cordless drill with a lower UWO rating.
Let’s say you have a Dewalt cordless drill with 340 UWO rating, and a Craftsman cordless drill with a 280 UWO rating. The Dewalt is going to be more powerful.
UWO is intended to give a fuller picture of a cordless drill’s power or performance potential than torque specs alone.
In North America, Dewalt, Craftsman, Porter Cable, and other Stanley Black & Decker cordless power tool brands generally do not publish the maximum torque specs of their cordless drills and drivers.
Can you convert UWO, or unit watts out, to a maximum torque rating, to allow for easier comparison across multiple brands?
The short answer is that you can’t. What follows is a detailed explanation about how and why UWO simply can’t be converted to a torque rating.
If you’re not in the mood for math, stop here!
UWO, or Unit Watts Out: What Does it Mean?
Unit watts out is a measure of the power output that can be achieved by a cordless power tool’s motor and drive components. It can be considered as a measure of a drill’s ability to perform work, and is a function of torque and speed.
Torque is a measure of a tool’s ability to produce turning force, and is often used as a way to compare different cordless drills and other power tools with respect to power. However, maximum torque ratings alone can lack context.
Dewalt adapted UWO as a way to combat the misconception that the higher the torque rating, the faster a drill can perform or complete an application.
The issue with this, or at least consumers’ frustrations, is that no other brands use UWO outside of Dewalt and other Stanley Black & Decker power tool brands.
The Math: Power, Torque, and Speed
Let’s talk about the relationship between a cordless power tool’s motor’s power, torque output, and speed.
For the sake of simplicity, we should consider the entire power transmission system as one component. The motor, gearbox, and chuck together are what deliver cordless drill/driver or power tool performance. For ease of explanation and analysis, let’s assume this is all combined together as “the motor.”
For a typical power tool motor:
P is power, in Watts, is torque, in Nm, and is the angular speed.
If you want to use in-lbs of torque, you’ll need to add in another step. 1 Nm is equivalent to 8.85 in-lbs. I’ll spare you the details, here’s the Wolfram Alpha conversion if you want to double check. So, if you have torque specs of 885 in-lbs, you’ll need to convert it to 100 Nm.
Since we typically work with motor speeds of RPMs, rotations per minute, we need to do some substituting. The angular speed is in units of radians per second. 1 rotation per second is equivalent to 2π radians per second, so you just plug in 2π for proper conversion.
N is the rotations per minute (RPM). Since we actually need that speed to be in rotations per second, we just divide by 60 (1 min = 60 seconds).
That gives us:
Simplifying for the constants, we get:
If you want to use torque values in inch-pounds, N that equation becomes:
To simplify things:
K is just a constant that depends on the units you’re using. If you want to use foot-lbs of torque, for example, that constant K is what changes. For torque in Nm, K = 0.1047, and for torque in inch-pounds, it’s 0.0118.
Convert UWO to Torque
Okay, so how do you use that equation, ?
Let’s say you want to buy a new cordless drill, and its specs say it’s rated at providing a maximum power output of 500 UWO. Another cordless drill that you’re looking at from another brand is rated at 640 in-lbs of max torque. Which cordless drill is more powerful?
Here’s the hard part, and I know some of you are going to get frustrated now. The short answer is that you cannot convert from UWO to torque. It simply doesn’t work that way.
UWO is a measured specification, derived from measured torque and measured speeds.
When you look up a Dewalt or Craftsman cordless drill, its box or product page will usually have a power specification, such as 500 UWO, and speed range specifications, such as 0-450 RPM and 0-2000 RPM.
The maximum torque will be available at lower speeds. But at 450 RPM? That’s hard to say, because those speed ratings are no-load settings, when the drill is allowed to free-spin. Cordless power tools sometimes operate at slightly different speeds under load.
Let’s say you have a Dewalt cordless drill with the following specs:
- 820 UWO
- 95 Nm max torque
- 0-450 RPM lowest speed range
- P = 820 W
- = 95 Nm
- N = unknown
- K = 0.1047
So, solving for N, you have:
N = 82.4 RPM
If you are given the max power of a drill in UWO, and a speed range, such as 0-550/0-2000 RPM, you simply don’t have enough information to determine the maximum torque of that drill.
With this equation, you can calculate the max power output of a cordless drill motor, or any other cordless power tool or DC motor, using measured values of torque and rotational speed. The constant K is only a unit-conversion factor that allows for deviation from Nm and radians/second.
In the European market, Dewalt and other Stanley Black & Decker brands provide added specifications for their cordless drills:
- Power output in UWO (unit watts out)
- Max soft torque in Nm
- Max hard torque in Nm
Hard and Soft Joint Torque
A soft joint is a physical connection where the material between a nut and bolt have low stiffness when compress when tightened. Additional tightening is needed to tighten things up snug.
A hard joint is a physical connection where there is high stiffness in the material between the nut and bolt surfaces. With such a connection, the fastener is tightened snug and then can only be turned a little more before it reaches final torque.
So, let’s say that you’re fastening two steel plates together. The nut and bolt are tightened to snug, and then you reach full torque in less than 1/8 turn. That’s a hard joint.
Now let’s say that you are instead fastening a wood board to a wood stud. Once snug, the fastener needs another full turn or more before it’s at final tightness or torque. That’s a soft joint.
There are different ways of defining hard and soft joint parameters, but we won’t get into that here. A cordless drill or driver will have a higher hard torque rating than a soft torque rating.
Generally, a “hard torque” rating will be its “maximum torque.”
|Dewalt Cordless Drill Specs|
You can get the torque specs in inch-pounds by multiplying by 8.85 if you’d like. For instance, 60 Nm hard torque is 531 inch-pounds.
|Dewalt Cordless Drill Specs|
Regardless of the units, there’s no patterned correlation between these numbers.
Imagine that the torque isn’t given.
K is 0.1047 when Nm is used, 0.0118 when in-lbs is used. But that’s not important here.
You can use this equation to determine any value, if the other two values are known. Let’s say you know that the DCD791 cordless drill can deliver 460 UWO, and that its max torque is 620 in-lbs. Well, it can give you this max power output at ~63 RPM. But you’re not given that information!
A Broken Equation
A valiant attempt was made by another tool review website to find a way to ballpark-estimate max torque from UWO.
Torque (in-lbs) = MWO x K / Speed (RPM), where K is 560.
Unfortunately, this just doesn’t work.
Using a set of known values for max torque, MWO (maximum watts out), and speed, they saw the ratio to be ~560. However, that ratio isn’t a constant. When looking at multiple data points, such as those shown above, things don’t match up.
Unit watts out is instead calculated using torque and speed measurements. You’re given zero-load speeds in on-paper specs, and there’s no way to use that to calculate torque. You need UWO and speed under load to determine torque.
So for UWO, torque, and speed, you need 2 corresponding values to get the third. In testing, speed and torque can give you UWO. On paper, UWO and speed can give you torque, or UWO and torque can give you speed. You’re never given that speed at which maximum power is achieved, and so you can never calculate max torque.
Can We Find a Pattern?
DCD777 is rated at having 340 UWO, 65 Nm max torque, and 500 RPM for the lower setting.
DCD791 is rated at having 460 UWO, 70 Nm max torque, and 550 RPM for the lower setting.
Both are compact cordless drills. Comparing the two, DCD791 has 35.3% greater UWO, 7.69% more torque, and 10% faster speed at the low speed/high torque setting.
Looking at the different examples in the table above, we can use the UWO and max hard torque specs to calculate the approximate speed at which the torque was measured for the calculation of UWO.
|Looking for a Pattern|
Take that speed, N, and divide it by the max speed of the high torque setting, such as 450 RPM, to get a ratio.
Potentially, we *could* take an average and come up with a category estimate, say 10% for compact drills, and 18% for heavy duty.
The first three model numbers are compact cordless drills with 2 speed ranges, and the other two are heavy-duty 3-speed models. All of these models are brushless drills, by the way. The more different drills have in common, the more reliable, or rather less unreliable, any conclusions can be.
So, let’s say we now have an unknown, such as the Dewalt Atomic series DCD708 cordless drill. The Dewalt Atomic DCD708 is rated at 340 UWO, with a lower speed range of 0-450 RPM. With 340 UWO and 450 RPM, and guessing that its max torque is at 10% of its speed, we can calculate a ballpark torque of 72Nm. However, I dug up a Czech product catalog, where they say that the DCD708 has a max torque of 65 Nm, just like the DCD777.
Looking at those two models,
DCD777: 340 UWO, 65 Nm max torque, and 500 RPM for the lower setting.
DCD708: 340 UWO, 65 Nm max torque, and 450 RPM for the lower setting.
There’s no high-confidence pattern, but I’m open to being wrong. If you play with these or other drills, and can find a pattern which can be used for a simplified UWO to torque conversion equation, please let us know!
However, there’s also the fact that UWO is a measure of a cordless drill system’s performance, including the motor, gearing, chuck – the power transmission from motor to accessory mount. Different motors and gearboxes will challenge any patterns that might appear.
Maybe this pattern can still be used for rough calculations? I reviewed a Craftsman compact cordless drill a few months ago. It’s a brushed motor drill not brushless. It’s rated at 280 UWO and 0-350/0-1500 RPM. Using the pattern from above, let’s assume that the maximum torque is achieved at 10% of the low-speed max, or 35 RPM.
So, 280 UWO divided by (35 * 0.0118) = 678 in-lbs max torque? That’s definitely not right. Based on performance, I’d guestimate that the maximum torque is under 300 in-lbs, since this drill seemed to lack power compared to the Skil 12V brushless model. If true, this would mean that the max torque is achievable at around 80 RPM, which would be nearly 23% of the max speed at the high torque mode.
Additionally, there is no guarantee that UWO is calculated at the time of max torque. UWO could be at an intermediate speed, corresponding to an intermediate torque.
Dewalt now describes UWO on their website as:
The point where the drill’s speed and torque output are the highest.
That point might not be at the maximum torque point in a speed vs. torque chart. We can make assumptions, simplifications, and generalizations based on trends and perceived patterns, but there’s no way to validate any of them, unless you either work for Dewalt or Stanley Black & Decker, or have a dynamometer, a very expensive piece of equipment used to measure motor torque, speed, and power.
So What’s the Point of UWO?
A cordless drill or driver’s max torque tells you about its maximum torque at 0 RPM.
More than 10 years ago, when UWO came out, Dewalt explained that UWO was a more accurate portrayal of a cordless drill’s performance. Indeed, looking at UWO is a great way to compare different cordless drills in Dewalt’s product family. It’s a measured value that is dependent on speed and torque combined.
One drill is rated at 400 in-lbs max and with 0-400/0-1500 RPM speed settings, and another at 380 in-lbs max torque, with 0-550/0-2000 RPM speed settings. Which is the more powerful tool?
With UWO, you can look at different values and see how different tools compare against each other. It’s intended as a measure of performance and application speeds, rather than simply being a zero-RPM torque measurement.
Unfortunately, as helpful as it might be, there’s no quick, easy, or confident way to convert a UWO value into max torque specs.
If you want to know the max torque of a specific cordless drill, your best bet is to look at Dewalt’s UK website, or overseas retailers’ product pages, where soft and hard torque values are often published alongside UWO.
If you want to know the max torque of other Stanley Black & Decker brands’ cordless drills, such as Craftsman or Porter Cable, you might simply be out of luck.
You can’t draw parallels between different models either. Two drills might both be rated at similar UWO ratings, but one could potentially deliver higher torque and lower speeds, or lower torque at higher speeds.
Dewalt intended for UWO to be a new standard by which cordless drills could be better compared against each other. However, no other power tool brands outside of Stanley Black & Decker are using it yet. Given the amount of time that has passed, other brands might never adopt UWO as a measure of cordless drill or power tool performance, at least not in marketing materials.
To summarize everything, UWO is a marketing convention that can be used for comparing cordless drills and drivers by Dewalt, Craftsman, Porter Cable, and other Stanley Black & Decker brands. Unfortunately, there’s no easy way to use that UWO when comparing SBD brands’ drills with other brands’, such as by Milwaukee, Makita, Bosch, Metabo, Ridgid, Ryobi, or any of the power tool brands that typically publish max torque specs.
But, hopefully at least this discussion helps you better understand what UWO is and what it means.
I am definitely open to different opinions. If you know of an accurate and consistent way to determine maximum torque from UWO and no-load speed specs, please let us know!
Also, I took all reasonable efforts to check, double-check, and triple-check all my math, assumptions, simplifications, and conclusions. Please let me know if you find anything that doesn’t add up, make sense, or hold true.