The impact driver is another specialised power tool that has recently started to make its way into the tool kits of DIYers and tradesmen. Impact drivers were originally a manually-operated hand tool that worked by creating a sudden large turning force or torque at the head of the tool when struck by a hammer. This turning force was used to tighten or loosen stubborn screws while the simultaneous linear forward force produced from being struck ensured that the screwdriver bit was kept fully inserted in the screw head. In more recent times, as with many hand tools before it, the impact driver has become electrically-powered. In electric impact drivers the high levels of torque are now produced by the electric motor driving an internal hammering mechanism striking against the drive shaft of the impact driver.
In this review of electric impact drivers, we first look at the functionality of this power tool type, what it is used for, and how it compares to the related impact wrench. After that, we study the general characteristics of impact drivers and the features to look out for when buying one, before finishing with an examination of an array of popular impact wrenches that are sold in the UK today.
The Electric Impact Driver
The electric impact driver is not dissimilar from any other electric screwdriver or drill driver, but stands out from these other machines by the excessive amount of torque it can generate through its internal hammering mechanism. At the same time, the impact driver remains safe to use by anyone since the large amount of torque is applied so briefly at each strike of the ‘hammer’ that very little consistent reactionary force is produced. This means that there is little chance of the impact driver twisting in the opposite direction to the motor rotation, potentially injuring the operator, if the fastener binds or refuses to budge.
As a consequence of this exceptional torque, the impact driver can usually drive stubborn fasteners faster and more efficiently than any other equivalent electric screwdriver or drill driver. This makes the electric impact driver ideal for jobs with large and difficult-to-insert screws going into harder materials or in projects where a large number of long screws need to be driven home quickly.
Given how good impact drivers are at screw driving, one might be tempted to ask why bother with non-impact machines at all. The main reason is that although impact drivers work better than other screwdrivers on stubborn fasteners, the equivalent regular electric screwdriver is usually more efficient at normal unchallenging screw driving. In addition, impact drivers are louder than similar regular electric screwdrivers or drill drivers, often necessitating the use of ear protection if used frequently. And the high amounts of torque produced by an impact driver can more easily strip or cam out more sensitive screws. Finally, like impact wrenches which require special impact resistant socket sets, impact drivers also require bespoke impact resistant screw bits to withstand the high forces created by the impact mechanism. Impact resistant bit sets are generally harder to come by and are usually more expensive than regular screwdriver bits.
Impact Driver vs Impact Wrench
Impact drivers are very similar to impact wrenches in that they are electrically powered tools that use the repeated striking of an internal hammering mechanism to increase the torque needed to drive stubborn or stuck fasteners. However, impact drivers differ from impact wrenches in certain subtle ways. Firstly, impact drivers are less heavy duty and usually have lower maximum torque levels than impact wrenches. This is because the torque needed to release a stuck screw is usually significantly less than that which can be needed to release a frozen nut or bolt which impact wrenches have to contend with. Indeed, applying too much force to a screw can lead to a damaging cam out of the screw head. Impact drivers also differ from impact wrenches in their driving shanks, with impact drivers usually possessing a 1/4 inch (6.35mm) female hexagonal receptacle for different screwdriver bits while impact wrenches usually have square male anvils on which sockets for different sized nuts and bolts can be fitted.
What to look for when buying an Electric Impact Driver
The vast majority of electric impact drivers are battery-driven and, as a consequence, power is measured in voltage which partially correlates with the maximum torque that the impact driver can produce. However, most electric impact drivers in the consumer space are powered by an 18V or 20V battery so trying to differentiate between impact drivers based on this metric is usually not very useful.
Probably the most important technical specification to pay attention to when purchasing an electric impact driver is the maximum torque that it can produce. In the UK, this is usually measured in Newton-metres (Nm) and although one generally wants to go for an electric impact driver with the highest maximum torque to ensure it is able to tackle even the most stubborn of screws, it is very rare that the very high levels of torque that some impact drivers can achieve will ever be needed for screw driving.
No-Load Speed or Revolutions Per Minute (rpm)
With impact tools, the higher the speed of revolution of the motor spindle, the greater the force of impact of the hammering mechanism. This means that higher rpm on impact drivers correlates quite closely with the higher maximum torques that they can produce. However, the highest levels of torque are not always needed in every job involving screw driving, and sometimes lower levels of torque are needed to prevent damaging the screw itself or the application in which the impact driver is being used. Consequently, some manufacturers of impact drivers have designed their devices with the ability to set different maximum rotational speeds which translates into different levels of maximal torque. Impact drivers also often have variable speed triggers which can provide yet further control, albeit more approximate, over the amount of torque that is applied when tightening and loosing fasteners. Variable torque control can be particularly useful at the very start of unscrewing a fastener when the risk of damage to sensitive screw heads is at its greatest. Likewise, it is also useful during the last millimetre or so of driving home a screw to prevent over-tightening.
Impacts Per Minute (ipm)
As the name suggests, the impacts per minute metric (also sometimes referred to as ‘blows per minute’) of impact drivers refers to how quickly the internal hammering mechanism of the tool operates. Intuitively one can deduce that the higher the strike rate, the more often the peak torque is applied and the faster the fastener is driven home or loosened. In reality however, most impact drivers have such high impacts per minute speeds that one will be hard-pressed to notice a difference in the efficiency of different impact drivers with different ipm values, making this metric another relatively unhelpful one when choosing one’s next machine.
As with most power tools, especially those that are used frequently or repeatedly, one generally wants to aim for the lightest possible machine that one can afford in order to reduce the likelihood of hand or arm fatigue when using it. Lighter machines are also more easily manipulated especially when working up high or in confined spaces. One other point to note here is that impact drivers manufactured by the bigger name power tool companies are often compatible with a range of their batteries that have different electrical storage capacities. In general, increasing the amounts of stored electrical energy in a battery increases its weight, so the overall weight of some impact drivers can also vary depending on the type of battery that is installed.
Impact drivers like most power tools are weighty bulky affairs and owners usually benefit from being able to place them in a carry case during storage and transportation. As a consequence one should generally aim to buy an impact driver that comes with its own bespoke bag or case.
Like impact wrenches, impact drivers are noisy machines often breaking the 100 dB mark. This means that the operation of impact drivers should be used with hearing protection especially if the impact driver is to be used repeatedly or on a regular basis.
Popular Electric Impact Drivers in the UK-- Information not currently available
* Weight includes battery pack
** The use of different battery pack sizes changes the overall weight of the impact driver