Learning about Power Drivers and Drills

From DIY projects to large contracts, power drivers and drills always make an appearance.

Traditionally, the drill is seen as a multipurpose tool, perfect for both drilling and driving. However, using the battery-powered impact driver for other projects like hanging drywall shows off the efficiency and accuracy the tool can offer. Here’s a quick guide to learning more about the difference between drills and impact drivers.

Different Internal Mechanisms

Both drills and impact drivers utilize trigger switches that activate an electric motor to turn electrical power into rotational power. But their similarities end with the drive shaft of the motor. Though drills and impact drivers both rely on torque, they apply the rotational force in different ways. Torque measures (in inch-pounds) the quantity of force on an object that causes the object to rotate.  

In a power drill, the drive shaft connects to a slip clutch that controls the amount of torque exerted by the drill. The clutch fits between the motor and the chuck, allowing the motor to turn the drive shaft when engaged. Disengaging the clutch disconnects the motor from the chuck. Using a slip clutch in an electric drill allows the operator to control the amount of torque applied to a screw.  

Numbers on the adjustable collar of the clutch indicate the amount of torque produced by the drill. Higher numbers correlate to greater amounts of torque and are utilized when drilling bigger holes, driving large screws, using hole saws, or drilling through harder materials. Lower numbers apply lesser amounts of torque and prevent overdriving the screw. If the torque applied to the bit exceeds a preset value, the clutch disengages.

In contrast to power drills, impact drivers do not rely on a clutch. Instead, an impact driver places a rotation impact mechanism between the motor and the drive shaft. The rotation impact mechanism works as a hammer, while the side of the drive shaft functions as an anvil. As the motor torque is transferred to the drive shaft, the rotation impact mechanism places percussive force in the same direction as the rotating force on the shaft. Within the drill housing, a spindle rotated by the motor connects to a hammer and the shaft. When the load on the drive shaft exceeds a preset value, the hammer moves backwards and disengages from the anvil. At that point, a coil spring based at the front reengages with the anvil.

The combination of rotating and percussive force produces significantly more torque than seen with a conventional drill. While a power drill delivers constant torque, the impact driver delivers bursts of high intensity torque. The spring-loaded hammer strikes the side of the drive shaft at a rate of thousands of blows per minute. This additional force allows the impact driver to deliver a higher amount of torque.  

Different Purposes

The distinction between the power trains for these two instruments – the power drill and the impact driver ­– points to the different purposes for each tool. Although a power drill delivers constant torque, it can stall when driving long or large fasteners. The application of constant torque can also strip a screw head. Nevertheless, the benefit of using a power drill becomes apparent when drilling large holes with spade bits, auger bits, or hole saws; when using the drill and paddle bit to mix various types of wet materials; and when drilling with accessories, like wire wheel brushes. Having a range of speeds and clutch settings allows a power drill to perform more precise work than that of an impact driver, such as drilling smaller holes or driving smaller screws.

On the flipside, impact drivers deliver the power needed to drive screws through dense materials and to drive long screws with ease. The combination of downward, rotational, and side-facing percussive forces holds a screw and prevents it from being stripped. Because the impact driver relies on percussive force, it does not work well for applications that require drilling or driving precision. As opposed to power drills that can hold different diameter sizes of drill bits or other tools, impact drivers cannot accept anything that does not have a ¼” hex shank.

Different Weight, Size, and Ease-of-Use

Impact drills offer smaller size and less bulk than power drivers. Considering the combination of rotational and side-facing percussive forces, the efficient delivery of power by the impact driver does not require a tight grip when driving a screw. The hammering action holds the screw and prevents slipping. With power drills, the application of constant torque requires an operator to use both hands to prevent the drill from slipping.