The best power tool for a particular application depends on the requirements for productivity, ergonomics, reliability and quality.

Today, assemblers have more options for power tools than ever. The array of tool types and brands can be dizzying. Even when you narrow your scope to one type, perhaps a dozen models fit a particular torque range. How do you choose? If you focus on several core issues, the choices become clear.

Clutch Selection

Many articles have described the various types of clutches for power tools. But, these articles rarely cover how differences between clutches influence tool selection.

There are two clutch classifications—continuous-drive and discontinuous-drive.

A continuous-drive clutch delivers power continuously without interruptions. Common continuous-drive clutches are automatic shut-off, direct drive or stall, and cushion. Continuous-drive tools are typically gear-driven, and they may or may not have torque control or an automatic shut-off function.

Discontinuous-drive tools deliver power in short bursts with no torque between each burst. Common discontinuous-drive tools are pulse tools and impact wrenches. Pulse tools generate torque hydraulically, while impact tools generate torque mechanically.

Application Criteria

So, which drive type is better? It depends. One factor to consider is whether the joint is "hard" or "soft." A joint is considered hard if it takes less than 30 degrees of rotation to go from snug to final torque. A soft joint requires more than 720 degrees, or two full revolutions.

If cycle time is important, discontinuous-drive tools can increase productivity with their high free speeds on hard to medium-soft joints with little prevailing torque. Most discontinuous-drive tools have free speeds exceeding 3,000 to 8,000 rpm—that’s 1,000 percent faster than most continuous-drive tools. (Under load, these tools will slow down as they rotate the fastener.) As an ergonomic benefit, discontinuous-drive tools produce virtually no torque reaction. Because each burst of power is measured in milliseconds, the operator barely feels it.

Continuous-drive tools may be more appropriate on softer joints and joints with high prevailing torque, because the speed of the tools does not change as much during run-down and under load. However, as a result of this continuous supply of power, the operator is exposed to the torque reaction for a longer period of time. This trade-off in ergonomics is especially critical when the torque exceeds 40 newton-meters.

The two most commonly used tools for assembling critical joints are continuous-drive tools with an automatic shut-off clutch and discontinuous-drive pulse tools. These tools primarily overlap in the broad range of 5 to 500 newton-meters. Tools with other clutch types are primarily for applications where cost alone is the major issue or where little to no torque control is required.

Power Sources

Pneumatic tools have been around since Charles King’s 1890 invention of the air hammer. All clutch styles are available in pneumatic tools. Electric tools were developed in the early 1980s. Most of the common clutch styles are also available with electric motors. With the advancements in battery technology, many clutch styles are available with cordless tools, too.

With the exception of a few operational issues, the power source has little effect on clutch selection. Tools with automatic shut-off clutches are available with pneumatic and electric motors from many vendors. Pulse tools are available in pneumatic, electric and cordless models, as well. Remember, the application’s parameters determine the class of clutch—continuous or discontinuous—not the power source.

So, why choose electric tools? Some say that DC electric tools are more efficient and use less energy than pneumatic tools. This is correct. However, DC electric tools are more expensive than pneumatic tools. In the typical application, the difference in energy costs requires a 20-year payback to justify the additional expense of DC electric tools.

Manufacturers choose electric tools primarily for their ability to control certain aspects of the fastening process, such as speed and power during run-down. Pneumatic motors, following a horsepower curve, slow down as they approach the stall torque. Electric motors maintain a constant speed under load, and they can run at multiple preset speeds.

Many engineers associate electric tools with the ability to control several fastening parameters with a single tool, to measure angle of rotation, and to measure dynamic installed torque. While electric tools can perform all these functions, pneumatic tools—both pulse and automatic shut-off clutch styles—are available with these identical capabilities.

Controlled Tools

With an increasing awareness of quality, many manufacturers are concerned about documenting process information to ensure that the operation was performed correctly. Controlled tools use transducers to measure dynamic torque. A controller uses the signal from the transducer to shut off the tool at a preset torque.

Many people assume that this can only be done with DC electric nutrunners. But there are many styles of controlled tools. Earlier generations were simply pneumatic angle nutrunners with built-in transducers, and those tools are still available today. The separate controller would stop the tool by closing a solenoid valve.

Today, many users are turning to pulse tools because of their speed and ergonomic benefits. Available in pneumatic and electric models, controlled pulse tools are a great alternative to DC electric angle nutrunners in the right applications.

Another less expensive alternative to fully controlled tools is a system that provides process verification without transducers. This is known as tightening monitoring or reporting. These systems confirm that the tool fully completed its cycle and shut off. Using a simplified controller, you can monitor the cycle time of each run-down, verify cycle completion, and count the number of bolts that have been installed. The best part is that many existing tools can be retrofit to perform these functions.

Individual Priorities

Even after you’ve narrowed your tool choice to a specific clutch and power source, you may still have many tools to pick from. This is where your priorities come into play. Do you want a tool with an automatic shut-off or without? Do you want a tool on the high end or low end of the torque range? Do you want a lighter or beefier tool? By identifying your priorities, you should be able to make the right decision quickly.

When evaluating power tools, most engineers are concerned with four key factors: productivity, ergonomics, reliability and quality. The value of each factor relative to the other varies from one engineer to the next. If your company is trying to double its production rate, high speed may be the most important criteria for choosing a tool. If your product has a high potential for liability, quality or repeatability may be the deciding factor.

Before discussing these four factors in detail, it might be helpful to define them, since these common terms may have different meanings to different people.

Productivity. This is really a measure of the tool’s speed. A high-speed tool can run down more fasteners in less time than a slow tool. However, the trade-off may be accuracy.

Ergonomics. This indicates how safe and easy the tool is to use during a shift. An ergonomic tool is lightweight, balanced and less fatiguing. It produces little torque reaction and has a comfortable grip.

Reliability. This is a measure of the overall longevity of the tool. The key factor here is the cost and frequency of maintenance. Pulse tools require less expensive, but more frequent maintenance. The process involves changing fluid and replacing soft parts, such as gaskets. Angle nutrunners require more expensive, but less frequent maintenance. The parts that fail on these tools are hard parts, such as clutches and gears. Over the life of the tool, pulse tools and nutrunners should have comparable maintenance costs.

Another consideration is that pulse tools are self-diagnostic. The tool indicates when maintenance is needed well before the tool stops performing. Most nutrunners run for long periods but have instant failures with no warning.

Quality. This is a measure of how well the tool should meet your engineering specifications. This might mean that the tool should drive a bolt tight enough so that it won’t loosen up, but not so tight that the bolt breaks. Or, it could mean the tool should tighten the bolt to 30 newton-meters within 8 percent repeatability and a Cpk of 1.67, measured dynamically.

A common misperception is that pulse tools without an automatic shut-off are less accurate than pulse tools with such a clutch. If used correctly, both tools should perform almost identically. A pulse tool without an automatic shut-off may not completely tighten a bolt if the operator prematurely lets go of the trigger before the cycle is complete. A shut-off pulse tool reduces this possibility because the operator must wait for the tool to shut off on its own.

Making Trade-Offs

To make choosing a tool easier, engineers should rank two of these four factors as most important to their application. From those two choices, you can narrow the field to a specific type and size of tool. This will also help you recognize the trade-offs associated with choosing those priorities.

The following examples show how these priorities play out in applications where both pulse tools and angle nutrunners could be considered.

Productivity and ergonomics. In this case, a small pulse tool without an automatic shut-off has the edge over an angle nutrunner.

Pulse tools boost productivity because they are faster on hard and medium joints. They can also be operated one-handed, allowing the worker to feed and start fasteners with the opposite hand. Ergonomics will benefit because the pulse tool will be smaller and lighter than the nutrunner. In addition, a tool without an automatic shut-off will be lighter than a tool with such a clutch.

The trade-offs involve reliability and quality. Pulse tools require regular, simple fluid changes. And, because the tool lacks an automatic shut-off, engineers must be willing to live with some potential variability in the fastening process.

Productivity and quality. Here, a midsized pulse tool with an automatic shut-off gets the nod over the nutrunner. Again, the tool is fast and can be operated one-handed. The automatic shut-off ensures consistent quality. If process verification or documentation is necessary, a controlled pulse tool should be considered.

The trade-offs involve reliability and ergonomics. Maintenance may be an issue, and the automatic shut-off will increase the tool’s weight. Torque reaction will still be lower with the pulse tool than with the nutrunner.

Productivity and reliability. On hard or medium joints, a large pulse tool without a shut-off is a better choice than a small pulse tool or an angle nutrunner. On soft joints, the nutrunner is better than a pulse tool. Large pulse tools and angle nutrunners require less maintenance. The trade-offs involve quality and ergonomics. Without an automatic shut-off, there may be some operator influence over the fastening process. And, the beefier pulse tool will weigh slightly more.

Ergonomics and reliability. Here, the choices are a large pulse tool without a shut-off or an angle nutrunner, depending on the joint and torque requirement. Large pulse tools are still ergonomic compared with many nutrunners, because they produce little or no torque reaction.

The trade-offs are quality and productivity. The nutrunner will be slower than a pulse tool. In this case, an automatic shut-off on a nutrunner is strictly a side benefit.

Ergonomics and quality. Here, a pulse tool with an automatic shut-off is better than a nutrunner, even on softer joints. Look for the lowest weight model possible for the given torque. The automatic shut-off will eliminate operator variability.

The trade-offs are productivity and reliability. Pulse tools will slow down on softer joints and may require more frequent fluid changes.

Reliability and quality. For these priorities, the angle nutrunner is the best option. These tools need less frequent daily maintenance and do not require periodic fluid changes. They also have a very reliable automatic shut-off. If process verification is needed, consider a controlled pneumatic or electric model.

The trade-offs are ergonomics and productivity. Nutrunners are slower and heavier than pulse tools. They will also produce torque reaction.

Finally, it’s important to note that priorities can change. A manufacturer that has been using small, lightweight tools for ergonomic reasons may find that the tools aren’t durable enough to withstand the rigors of the assembly line. For this company, reliability becomes the most important issue, and larger, more rugged tools are needed.

For more information about power tools, call 503-254-6600 or visit