Brushless motors are the hot new topic in cordless tools, but unlike, say, lithium-ion batteries, they are not being adapted to entire product lines all at once. The current state of the technology — coupled with the added cost and application limitations — means these motors are only going into certain types of tools, impact drivers chief among them. With 18-volt brushless impact drivers now available from nearly every major brand, the time seemed right to test them. (For more information on the technology, see “ Brushing Up on Brushless.”)
For this article I tested nine different tools but charted 10 sets of results. Since the latest Panasonic impact driver runs on either 14.4- or 18-volt batteries, I tested this tool both ways.
What sets these tools apart as a class (besides brushless motors) is the addition of control electronics that make them more versatile by allowing users to step down driving speed and power. For most fastening jobs, users will bang away at high speed like they usually do, but it’s still nice to have that extra element of control at your fingertips. The medium setting can help tame fast coarse-pitch screws in soft drywall and brittle plaster, and the low setting is good for snugging screws up against hinge plates — or for any driving task where overdoing it could strip or break the fastener.
Most of these tools have three speed ranges, though one has four and another has only one. Some of the tools have internal electronics that allow motor- and battery-protecting functions and diagnostic displays.
While some of these tools have unique design or performance features, for the most part they are all pretty evenly matched. Every model has an LED headlight, a sturdy reversible (and removable) steel belt hook, and rubberized grip surfaces. Only five have an onboard battery fuel gauge — even though every user I’ve surveyed appreciates this feature. Five have push-in bit holders, which are convenient but not that important in my experience, since all the bit holders can be operated easily enough with one hand.
In keeping with Tools of the Trade’s last test of 18-volt impact drivers (Fall 2011), I tested runtimes by driving 1/4-inch by 31/2-inch coated Simpson SDS lags into the sides of thick Weyerhaeuser Parallam beams until each tool’s batteries were depleted. I drove the lags in groups of 10 to avoid overheating the motor or batteries. Since I used the same test procedures, you can compare the performance of these latest brushless tools with the tools tested in 2011.
The runtime would be greater in dimensional framing lumber because it’s not as dense as Parallam. Our testing routinely shows an increase of about 25%.
Regardless of the no-load speed provided by the manufacturers, the most relevant measure of speed is the amount of time each tool needs to complete a uniform fastening task. To measure speed under a respectable load, I timed how long it took to drive a dozen and a half lags early in each tool’s runtime test while the battery was fresh. After discarding the fastest and slowest times, I averaged the remaining times to arrive at the listed speed value. Most of the drivers’ times were within seven-tenths of a second of each other for this task, but a few lagged behind. After 55 lags were driven, subsequent timing showed that the top tools retained more of their original speed. After more than 100 lags, the top two models still posted respectable times — good for carpenters who run a lot of fasteners per day. (See runtime and speed results on page 18).
I performed a similar test with 3-inch drywall screws, but they went in so quickly and the times were so close that it told me nothing about the tools’ relative power.
High-Tech Torque Testing
In the past, I rated impact drivers’ ultimate power by measuring how far they could drive giant lags into thick blocks of wood. The order of finish was telling, but the test failed to produce values that could be objectively compared, because there was no way to gauge how much more power it took to drive the lags deeper.
For this latest test, I employed two secret weapons courtesy of test-equipment manufacturer Skidmore-Wilhelm. I used the company’s T-2000 dynamic torque tester and Model J hydraulic bolt tension calibrator, the same device some manufacturers use to test impact tools.
These pieces of equipment were designed for use with impact wrenches and therefore test the hard-joint capabilities of the tools (like tightening a nut onto a bolt that’s already finger tight). Even though impact drivers are usually used for soft-joint driving (like burying screws into wood), the same force-delivery mechanisms are at work in both tools so they can be tested in the same manner. The major difference with impact drivers is that they rely on 1/4-inch hex shank bits — which are far less rigid than the socket drives on impact wrenches. Though this limitation isn’t a big deal for soft-joint driving, it can be a problem for hard-joint applications.
Dynamic torque testing. The T-2000 electronic measuring device consists of a metal beam with a 1/2-inch square socket at the end. When driven by an impact wrench — or in our case an impact driver with a 1/2-inch socket drive adapter attached — the finely calibrated beam senses the impulses delivered by the tool and transmits a signal to the digital meter, which converts the average peak impulse output into torque (measured in foot-pounds). This dynamic test method is necessary because impact tools don’t deliver consistent torque all at once; instead, they are impulse tools that do work with a series of high-energy blows of short duration.
Torque output is notoriously difficult to measure, so the numbers I recorded with the electronic test unit are best thought of as a relative performance measure and are not intended to supplant the maximum torque values published by the tools’ manufacturers.
Force testing. The Model J hydraulic bolt tension calibrator contains a hydraulic load cell that is squeezed between a backing plate and a special test bolt that gets tightened by the tool being tested — in this case, a cordless impact driver with a 1/2-inch socket adapter and socket attached. When the bolt is driven as tight as the impact tool can manage, the amount of pressure created by the test bolt is displayed on a gauge (measured in pounds of force).
After recording and averaging the force readings for each tool, I used a freshly calibrated Snap-On Techwrench to crank the gauge up to that same number and recorded the torque readout displayed on the torque wrench. This allowed me to convert the force values to inch-pounds of torque and compare them to the manufacturers’ specs.
In my testing, the tools produced roughly half as much torque as the manufacturers’ maximum torque ratings. I suspect this is because I used the 5/8-inch test bolt recommended by Skidmore-Wilhelm, and the toolmakers used a larger test bolt and socket. I tried it myself with a 7/8-inch test bolt and was able generate torque numbers approaching those of the manufacturers, but the socket adapters broke so frequently I went back to the recommended setup for tools of this strength range. (See tension and torque results at left.) As with the values recorded using the electronic tester, these numbers should be considered a measure of relative performance.