How Does A Pulse Tool Work?

Reaction-less Assembly Tools That Offer The Utmost in Precision…

Pulse nutrunners (pulse tools) are discontinuous-drive tools. They apply torque in small increments rather than in one continuous blow. When the fastener is running free, the tool doesn’t pulse and the driveshaft spins rapidly. Free speeds of 4,000 to 8,000 RPM are typical for pulse tools, with some smaller pistol-grip models topping out at 10,500 rpm. Once the fastener is snug, the tool begins pulsing, that is, applying short bursts of torque that only last a few milliseconds. Depending on the degree of hardness of the joint, 10 to 15 pulses is usually enough to tighten a fastener. Note: Due to their nature, pulse tools are not recommended for soft joints.

The source of the pulses is a unique hydraulic mechanism that consists of two rotating, cylindrical parts. The first part, called the casing, has an oblong inner chamber filled with hydraulic fluid. It’s connected to the tool’s air motor. The second part, called the anvil, fits inside the casing. The anvil is connected to the tool’s driveshaft and is bisected by two blades that are pushed outward by springs. These blades separate the chamber into two halves. As the casing rotates, the anvil’s blades contact the inner wall of the chamber, thus generating pressure.

When the tool is running at free speed, the casing and anvil rotate in unison. At this point in the cycle, the blades are not touching the inner wall of the casing, and there is no pressure on the hydraulic fluid. As the fastener provides resistance to rotation, the anvil begins to slows, but the casing continues rotating at its original speed. The blades are pushed inward toward the anvil’s center, compressing the springs. The oblong shape of the chamber combined with the compression of the blade springs reduces the volume within the chamber and increases pressure on the hydraulic fluid.

As the casing continues to rotate, fluid pressure accumulates and the anvil rotation begins to hesitate slightly. Hydraulic pressure increases, pushing against the blades and forcing the anvil to rotate. When the anvil is displaced 90 degrees from the casing, the blades are fully compressed at the seal point, which is the narrowest section of the oblong chamber. The hydraulic fluid reaches maximum pressure, and the anvil comes to a virtual stop. At this point rotational forces on the anvil are also maximized.

When the anvil pushes past the seal point and begins to move forward, the blades are again forced outward. Fluid pressure drops, and the anvil accelerates, starting another pulse cycle.

The key advantage of the pulse tool design is that the hydraulic fluid absorbs most of the vibration from the fastening process. Because torque is applied in short pulses, torque reaction is almost non-existent. Thus, allowing a pulse tool to be used without a torque reaction arm and still ensuring proper ergonomics.

Interested in learning more about pulse tools? Feel free to browse through our website, send us an e-mail at, or give us a call at 800-608-5210 for more information.