November 21, 2011

Impact Power Tools

An Overview of Impact Power Tools

Introduction

Impact power tools, or impact wrenches, are designed to deliver high torque output with minimal exertion by the operator, by storing energy in a rotating mass and delivering it suddenly to the output shaft.

Compressed air is the most common power source to drive impact power tools due to the low-cost design and good power-to-weight ratio. Electric and hydraulic power sources are also used and with the increasing popularity of cordless operation in recent times, lithium ion batteries have become another source of power to drive these tools.

Many designs are used, but all of them accomplish the same goal of allowing the hammer to spin freely of the anvil, allowing it to be accelerated and store energy, then delivering that energy suddenly to the anvil, before allowing the process to repeat.

Impact wrenches are available across all standard socket wrench drive sizes and are popular in industries such as automotive repairs and product assembly.

How Do They Work?

The basic principle behind impact power tools is that a rotating mass (the hammer) is accelerated by a motor and then suddenly locking to the output shaft (the anvil) to create a high-torque impact. After delivering the impact, the hammer is released to spin freely again. There are many different designs to accomplish this task, some more effective than others. Depending on the design, the hammer may drive the anvil either once or twice per revolution, with some designs delivering faster, weaker impacts twice per revolution while others deliver a slower, more powerful, impact only once per revolution.

Different Designs

This design allows the hammer to slide and rotate on a shaft, with a spring holding it in the downward position. Between the hammer and the shaft is a steel ball on a ramp allowing the input shaft to rotate ahead of the hammer with extensive torque. The spring is compressed and the hammer is slid backwards. At the bottom of the hammer, and the top of the anvil, are teeth designed to withstand high impacts. The hammer rotates until its teeth engage the teeth on the anvil, stopping the hammer from rotating. The input shaft continues to turn, causing the ramp to lift the steel ball, lifting the hammer assembly until the teeth no longer engage the anvil, and the hammer is free to spin again. The hammer then springs forward to the bottom of the ball ramp and is accelerated by the input shaft, until the teeth contact the anvil again, delivering the impact. The process then repeats itself, delivering blows every time the teeth meet, generally twice per revolution.
This design uses a hammer fixed directly onto the input shaft, with a pair of pins acting as clutches. When the hammer rotates past the anvil, a ball ramp pushes the pins outwards against a spring, extending them to where they hit the anvil and deliver the impact, then release and spring back into the hammer, by having the balls drop down the other side of the ramp at the instant the hammer hits. Since the ramp only needs one peak around the shaft, the engagement of the hammer with the anvil is not based on the number of teeth between them, this design allows the hammer to accelerate for a full revolution before contacting the anvil, giving it more time to accelerate and delivering a stronger impact.
This design uses a rocking weight inside the hammer and a single, long protrusion on the side of the anvil’s shaft. When the hammer spins, the rocking weight first contacts the anvil on the opposite side than used to drive the anvil, nudging the weight into position for the impact. As the hammer spins further, the weight hits the side of the anvil, transferring the hammer’s energy to the output shaft and then rocks back to the other side. This design has the advantage of hammering only once per revolution, as well as its simplicity, but has the disadvantage of making the tool vibrate as the rocking weight acts as an eccentric, and can be less tolerant of running the tool with low input power. To help combat the vibration and uneven drive, sometimes two of these hammers are placed in line with each other, at a 180 degrees offset, both striking at the same time.

All hammer designs require a certain minimum torque before it is allowed to spin separately from the anvil, allowing the tool to stop hammering and instead smoothly drive the fastener when only low torque is required, rapidly installing/removing the fastener.

Impact Driver

Impact power tools, or impact wrenches, shouldn’t be confused with impact drivers.

An impact driver is a tool that delivers a sudden rotational and downward force. In conjunction with toughened screwdriver bits and socket sets, they are often used to loosen larger screws (bolts) and nuts that are corrosively “frozen” or over-torqued. The direction can also be reversed for situations where screws need to be tightened with torque greater than what a screwdriver can provide.

Manual impact drivers consist of a heavy outer sleeve that surrounds an inner core that is splined to it. The spline is curved so that when the user strikes the outer sleeve with a hammer, the downward force exerts pressure on the spline to produce a rotational force on the core and any work bit attached to it. The tool converts the rotational inertia of the sleeve to the lighter core to generate large amounts of torque. At the same time, the impact from the hammer forces the impact driver down into the screw, reducing or eliminating cam out. This attribute is most beneficial for Philips screws which normally cam out as part of their design. It is less beneficial for flat head screws and is not beneficial for most other types of screws.

An impact driver can use a motor to automatically deliver the downward and rotational forces. These have the advantage of greatly increased speed. They are often used in construction and manufacturing to replace screwdrivers where speed and operator fatigue are at stake. In many instances they fail since they cannot deliver the heavy downward impact of a manual unit. This can be especially true when dealing with very stubborn fasteners.

Buyers must be aware that tools can be incorrectly advertised as impact drivers while they are in fact just impact wrenches. The only way to verify that a motorized impact driver is truly what it claims to be is to try it before purchasing.