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A battery powered impact driver is capable of driving a 6? screw into a solid piece of wood, without the need of predrilling, in less than 10 seconds. The impact unit consists of a gear drive, spindle, spring, hammer and anvil, to which a tool is connected to drive the screw or bolt. The periodic torsional impacting action of the hammer is achieved by a windup and release mechanism.
The dynamic interaction between these parts is simulated using ABAQUS/Explicit. With the model, it is possible to predict the kinematics of the impact mechanism, including torque spike characteristics and driving speed. Key characteristics of the model have been validated by tests.
Thus, analysis leads the design towards finding the most efficient combination of cam lead angle, hammer release clearance, inertias, and other design variables.
High-speed camera test video clips compare well with simulation animations.
1. Introductory Remarks
The current presentation is part of consulting work performed at Manta Corporation. The reader is requested to understand that due to confidentiality considerations, the extent and level of detail of disclosed material must remain limited.
2. A battery powered impact driver
The battery operated impact driver is a new type of hand tool for the construction industry and light mechanical industry (Fig. 1). It has become a highly desirable tool due to its portability and ability for continuous usage when alternating between two sets of batteries. Thanks to the impacting mechanism, it can produce torque impulses of sufficient magnitude and duration to drive a typical wood screw without the need of pre-drilling. Contrary to a conventional driver, it does not require a high pushing force on the part of the operator to keep the bit engaged while driving the screw.
The key elements of the impact driver lay in the impact unit (Fig. 2). This unit is driven by a motor through a planetary gear and consists of a spindle with a V-grove, a steel ball, a spring compressed between a hammer and a disk at the top of the spindle, an anvil and a stopper (Fig. 3). The anvil has a chuck for attaching different drive bits.
As the planetary gear output spindle rotates at a fairly constant speed, it winds up the spring when the hammer is impeded from rotation by the anvil, which feels the resistance of the screw. This windup happens because the steel ball is trapped in the spindle V-groove and simultaneously presses against an inverted V-shaped cavity on the inside of the hammer. The hammer rises and gathers momentum, so it eventually clears the top of the anvil. At this point, the spring has accumulated a large amount of elastic energy and wants to unwind. The only way for this to occur is by causing the hammer to rotate, as it starts moving down, guided by the ball in the groove. It will accelerate forward to maintain position with respect to the spindle, and eventually strike the anvil again as it lowers towards the next impact position. Because of the high velocity and kinetic energy of the hammer upon impact, the anvil will undergo a finite amount of rotation before it stops. At this point, the process is repeated and the next impact cycle is initiated.
In the design of an impact driver, several design variable combinations may have to be considered to increase operation efficiency. For example, the spring stiffness and preload, hammer/anvil clearance, inertia and mass, V-groove cam lead angle, etc., all affect performance, i.e. driving speed.
The purpose of simulation is to capture the effects of these design parameters and to be able to help predict tool efficiency. A simulation model (Fig. 4) is described in detail below. Selected stages of the operation described in this section can be observed in a series of high-speed camera frames in Fig. 5, together with equivalent simulation animation frames which will be discussed in later sections.
3. Model description
The anvil and hammer are modeled with solid C3D8R bricks (Figs. 4 and 5). The spindle is modeled with rigid elements and kept rotating at constant speed. The hammer spring is modeled with a series of preloaded springs whose upper end rotates with the hammer but is kept restrained vertically.
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