Choosing the Optimum Fastening System
Selecting the best tightening system for a bolted joint requires close attention to the factors that contribute to the overall quality and durability of the final assembly.
Many factors contribute to the overall quality and durability of a bolted joint.
Fastener quality is often considered a given in these days of ISO certification and statistical process control. However, in today’s world of cost pressures, short lead times and global sourcing, fastener quality may be less than expected.
Another factor that affects overall joint quality is an accurate preload, or clamp load, requirement. Engineers seldom calculate this accurately. Fortunately, software based on mathematical models developed in the 1970s has unraveled some of the mysteries surrounding preload.
Similarly, a thorough knowledge of the application will prevent quality problems. Engineers must understand the working environment and service conditions to which the bolted joint assembly will be subjected, including external loading, operating temperatures and corrosive media.
But perhaps the most important influence on joint quality is the strategy used to tighten the joint and the equipment used to achieve that strategy.
How Critical Is It?
One factor that influences the choice of fastening system is the criticality of the joint.
Obviously, the most critical bolted joints are safety-related. A failure of one of these joints, often referred to as Class A joints, may result in catastrophe or bodily injury. Examples are wheels, brakes and steering gear.
Intermediate, or Class B, joints are reliability-related. Failure of these joints may result in disability of the equipment. Examples are bolted joints in engines and transmissions.
Class C and D joints are related to customer satisfaction. A failure in one of these joints might cause an annoying squeak, leak or rattle.
Preload and Accuracy
Preload is the initial, tensile clamp load generated by the fastening tool. The preload requirement for the joint must be determined during the design stage. This can be complicated, particularly if the joint will be exposed to eccentric and dynamic loads or different rates of thermal expansion.
Some joints require all fasteners to have similar preloads. For example, the preload on bolts used to assemble connecting rods and cylinder heads must be held within a tight tolerance to avoid bore distortion.
Conversely, some joints do not warrant such in-depth analysis, because preload level and accuracy are not important. In these cases, simple assembly tools can be used.
Several strategies can be used to tighten threaded fasteners, but not all tools are capable of performing the necessary measurements.
Tightening threaded fasteners is an energy-transfer process. Most of the energy transferred from the tool to the fastener is consumed in overcoming the frictional resistance between the parts. As much as 90 percent of the energy is absorbed in overcoming friction, and only 10 percent generates clamp load. As a result, the most common tightening strategy, torque control, has severe limitations in providing accurate preload.
Torque control is easy to apply and check. It can be measured directly with a transducer, or indirectly with a clutch or stall tool. The relationship between torque and preload is shown in the formula:
T = Fp x d x k.
In this formula, T is torque, Fp is the preload, d is the fastener diameter, and k is the friction factor.
The major drawback of this method is that the resulting clamp load is greatly influenced by friction. Friction is difficult to predict and control. It depends on many factors, including the surface finish of the parts, platings and coatings on the fastener, and lubrication. Even fastening speed can indirectly contribute to frictional variation.
With torque control, preload variation, or scatter, is typically 25 percent to 30 percent.
An improvement over the torque control method is torque control with angle monitoring. To do this, a tool with both torque and angle transducers is required.
This method has no effect on preload variation, but it does provide a check that the fastening process was completed as expected.
By monitoring the angle turned from a snug, or threshold, torque—usually about 30 percent to 50 percent of the target torque—defects such as crossed threads, bottomed-out bolts or distorted components can be detected.
Again, the preload scatter with this tightening strategy is typically 25 percent to 30 percent.
To reduce this variation, a tightening strategy based on the relationship between the thread pitch and the angle turned during fastening is required. This strategy is called angle-controlled or "turn-of-the-nut" tightening. The strategy starts with tightening the fastener to an initial snug torque and then turning the fastener to a predetermined angle of rotation. Once the joint is consolidated, the angle turned is proportional to the amount of fastener elongation, which in turn is proportional to the preload. Friction does not influence the angle portion of this tightening strategy, except that it determines the final torque, which is merely monitored for quality purposes.
The relationship between the angle turned and fastener elongation is:
dl = (a x P) / 360.
In this equation, dl is fastener elongation, a is the angle turned, and P is the thread pitch.
Preload scatter with this strategy is typically 15 percent.
Another strategy—tightening to the yield point of the fastener—is based on the maximum preload capacity of the fastener. This strategy calculates the yield point of the fastener under the combined action of tension and torsion. This is done by monitoring the rate of change of torque over fixed angle increments. The value obtained is the torque gradient. As the yield point is reached, the torque gradient declines rapidly and the tightening process is halted. In a typical joint, the amount of permanent fastener elongation is 0.025 to 0.05 millimeter.
Preload depends on the tensile yield strength of the fastener material and the shear stresses developed from friction in the threads during tightening. The relationship is:
Y2 = s2 + 3t2.
In this equation, Y is the tensile yield strength of the fastener, s is tensile stress, t is shear stress.
This method is much less influenced by friction and joint variations. At the end of the tightening process, the final torque and angle can be inspected to ensure that they fall within predetermined limits. In this way, every tightening cycle is 100 percent checked.
Preload scatter with this tightening strategy is typically 8 percent.
Other, less common methods for controlling preload include ultrasonics, load-indicating fasteners, load-indicating washers and hydraulic tensioning.
The torque level of the application will sometimes be the determining factor in choosing a tool. Assemblers can choose between manual, pneumatic, electric and hydraulic tools. These tools may be handheld, or they can be partially or fully fixtured in single or multispindle configurations.
Manual tools are used for low torque levels, particularly in mass production. While the average human can apply up to 500 ft-lb with an extended-length torque wrench, this could not be done continuously. For this reason, manual operations are normally limited to 20 ft-lb.
Pneumatic and electric tools are used for applications ranging from 0.5 in.-lb to 1,000 ft-lb.
Hydraulic tools are sometimes used in applications above 1,100 ft-lb.
Fastener and Drive Style
Some power tools are better suited to certain fasteners than others. When evaluating fasteners, engineers should ask the following questions:
- Do the fastener threads have features, such as locking patches, that might cause a high prevailing torque?
- Will the drive recess allow easy access to the tool?
- Can the drive recess withstand the torque?
- If fastening a nut and bolt, how will the part that is not tightened be retained during assembly?
- Will the tool need special fixturing?
- Will special equipment, such as screw feeders, interface with the tool?
- Are the fasteners started by hand, or will one tool start the fasteners and another complete the process?
- Is a torque-reaction device necessary? How will it be applied to the tool?
- Is a magnetic socket or bit required?
Size and Space Constraints
Space limitations often affect the choice of tool. Some applications require power tools with special offset attachments. For example, a crowfoot head is needed to fasten a brake banjo bolt.
Special attachments also are available for tube nut applications, such as attaching a hydraulic brake pipe to a master cylinder, and for hold-and-drive assemblies, such as a shock absorber, where an internal fastener is held in place while an external fastener is tightened.
Other applications may require vertically or horizontally applied tools or power heads. Sometimes these tools have to be applied at an inclined angle. Fasteners that are difficult to access often require spindle extensions, universal joints or wobble sockets. These devices can affect the final torque.
The power source of the tool is rarely a major consideration for most applications, but it deserves some review.
Most pneumatic tools require a lubricated air supply. As a result, pneumatic tools may be inappropriate for applications that require a clean environment, such as a vehicle interior during final assembly. In this case, a cordless tool or DC electric tool should be used.
Large DC electric tools need enough current to achieve full torque, and special power drops may be required. Multispindle power heads usually require a three-phase power supply.
Microprocessor-controlled fastening equipment should be located away from sources of electrical noise.
Cycle Rate and Joint Type
The cycle rate affects the choice of tool. If the application has many fasteners and the production rate is high, a multispindle power head is required.
In addition, some tools are better for certain joint types than others. If prevailing-torque or thread-forming fasteners are being assembled and a DC electric tool is used, it should be oversized slightly to reduce the risk of overheating the motor. In general, the tool should operate at no more than 85 percent of its capacity.
Similarly, pulse tools are not the optimum choice for very soft joints with large tightening angles. The tool’s energy is absorbed by the joint rather than transmitted to the fastener.
Workers are often injured by the misapplication of power tools. Common injuries include cumulative trauma disorders (CTDs) due to vibration, repetitive motion injuries from repetitive work tasks, and hearing loss from loud tools. The estimated cost of one CTD is $27,000, so selecting an ergonomic tool is critical.
DC electric tools are often justified for ergonomic, as well as control, reasons. The tools are clean, quiet and produce less torque reaction compared with similar sized air clutch or stall tools.
For applications involving pistol-grip tools, a device to minimize torque reaction is necessary to prevent wrist injuries if the torque exceeds 18 to 20 in.-lb. Pulse tools eliminate this need, because they produce little or no torque reaction. For in-line or angle wrenches, a torque reaction device is recommended for applications over 15 ft-lb.
Many tools and small power heads are suspended from torque-reaction balancers. These devices suspend the tool, enabling it be moved in and out of position as required. At the same time, they absorb all of the torque reaction generated at the end of the tightening cycle.
The need to gather data on the assembly process has led to the widespread use of power tools equipped with torque and angle transducers. Torque and angle data is used to monitor and document the quality of the bolted joints. The peak torque applied to the fastener, and the angle of rotation after tightening, are used to stop the torque at a target limit or to indicate out-of-limit conditions.
This data is often carried from the shop floor to a remote computer for analysis and storage. DC electric tools are used almost exclusively in these situations.
Installation and Maintenance
The cost of installation, service, spare parts and maintenance should be considered when choosing a power tool.
Percussive tools are robust and require little maintenance. Clutch tools require little maintenance, as long as the air lines remain lubricated. Failure to do this will reduce the life of the motor vanes and clutch components.
Pulse tools require more frequent maintenance. The hydraulic fluid and seals should be changed regularly.
DC electric tools are prone to cable failure if they are not supported correctly. The tools are less durable than air tools and should be handled more carefully.
All transducerized tools should be recalibrated regularly. Depending on the cycle rate, the tools can be recalibrated as often as once per shift or as little as once per year.
Torque auditing procedures are recommended on both controlled and noncontrolled fastening systems to ensure that the equipment has not been maladjusted or misapplied. Microprocessor-based fastening systems contain built-in methods for checking the integrity of the transducers. This check occurs at the start or end of each tightening cycle.
Now that all the critical factors have been considered, it’s time to select a tool.
Pneumatic clutch screwdrivers are available with a number of clutch mechanisms for different applications. An adjustable shutoff clutch is used for critical applications involving composites, plastics or metals that require precise torque control. The automatic shutoff reduces air consumption and torque reaction. These tools are good for machine screws in hard and soft joints. They are also used for self-tapping or sheet metal screws, as long as the tapping torque is below the final torque.
With an adjustable cushion clutch, steel balls roll between indented plates to provide smooth disengaging at a preset torque, while minimizing vibration to the operator. This is a good, general purpose torque-limiting clutch. These tools can drive machine screws on hard and soft joints, but they should not be used for thread-tapping screws if the tapping torque exceeds the final torque.
The positive jaw clutch is for applications where the driving torque may exceed the final seating torque, as in thread-forming operations. The torque is controlled by the operator and is limited by regulating the air pressure.
Direct-drive tools are for soft joints that do not require critical torque control.
The torque is controlled by the operator and can be limited by regulating the air pressure.
Impact tools work like this: Repeated blows from small, rotating hammers transfer energy to an anvil that generates pulses of torque to the output shaft of the tool. How much torque is applied to the fastener depends on how much energy is transferred to the hammers and how long the tool is held on the joint. The torque is also influenced by the joint’s stiffness.
A major benefit of these tools is that they deliver high torques, at high speeds, in a relatively small package. Available in pneumatic and electric models, these tools have a high power-to-weight ratio and produce little torque reaction. They are generally inexpensive with low maintenance costs. They are ideal when preload accuracy is not critical, because the torque may vary by as much as 50 percent.
However, they produce a lot of vibration and noise. Most conventional impact tools have no torque output setting, but they rely on air pressure, flow rate and operator judgment to determine when a fastener is sufficiently tightened.
A torque-control impact tool has a built-in or detachable torsion bar that provides some control upon reaching a target torque. Detachable torsion bars have reduced diameters for decreasing torque outputs. They are usually referred to as "torque sticks." The built-in types have a mechanism that incorporates an internal torsion bar and shut-off valve. Torque control is achieved by preloading the torsion bar to the desired torque. The torsion bar flexes at the set torque and trips a shut-off valve that instantly stops the tool. This tool is a good solution when the positive features of an impact tool are required, a wide torque range is acceptable, and a maximum torque value should not be exceeded.
Pulse tools use a standard air motor to drive a fully contained hydraulic mechanism. The torque is transmitted by compressing the hydraulic fluid in the chamber, creating a pressure differential. This differential then creates a torque impulse that is transmitted to the fastener.
The tools can be supplied with a torque transducer and shutoff solenoid, linked to electronic controls that can be programmed for gang counting and multiple parameter sets.
Pulse tools are best applied to medium and hard joints. They are fast, powerful and quiet. Like impact wrenches, they do not produce a torque reaction. However, maintenance costs with the tools can be high. In addition, these tools are usually heavier than equivalent impact wrenches. And, they have a lower power-to-weight ratio due to the inefficiency of the hydraulic fluid.
Pneumatic clutch angle wrenches contain a mechanism that is driven by the air motor to a preset torque. Then, in a fraction of a second, the mechanism sets off an instant drive disengagement process and an air-charged shutoff valve. Torque transmission is immediately cut to zero, resulting in low inertia effects on the operator. This combination of instant response and total disengagement means more precise control and less operator fatigue.
These tools are relatively inexpensive and provide good torque repeatability with low torque overshoot. They produce a low torque reaction. However, they do not provide a torque readout, unless equipped with an expensive transducer. They are noisy, and they emit an oily film with the exhaust air.
DC electric clutch screwdrivers are available as straight, push-to-start, pistol-grip or angle tools. They contain an adjustable clutch and dynamic motor brake for accurate control. The power source is usually a standard brushed electric motor.
These lightweight, adjustable tools can provide variable speeds and a soft start to facilitate fastener location. With a controller, they can perform gang counting.
On the downside, they have a fairly limited torque range and they do not have a torque readout. Additionally, the brushes require periodic replacement.
Cordless screwdrivers are powered by rechargeable batteries. They typically provide a maximum torque of 11 ft-lb. They are widely used in automotive interior applications, which require clean, low-torque, go-anywhere convenience. The tools have a precision shutoff clutch that provides torque accuracy of 5 percent to 10 percent, with smooth, quiet operation.
The main disadvantages of cordless tools are their low power-to-weight ratio and the relatively low number of fasteners that can be tightened between recharges or battery changes. However, with the rapid recharging times of the latest batteries, this is usually not a limiting factor. A low battery charge is indicated by a visual indicator or the failure of the clutch to activate.
Although DC electric tools cost more than other tools, they provide numerous benefits, including an operator interface and data acquisition. The tool usually contains a brushless DC motor with torque and angle transducers. The controller houses the servo drive and microprocessor-based control electronics. The controller acquires, processes and reports tightening information. This data can include individual end-of-cycle torque and angle results, as well as parameter sets, date and time, and statistics based on predetermined sample sizes.
In addition, the controller can receive external commands from intelligent devices, such as programmable logic controllers or bar code readers, and provide external outputs to line controllers or data acquisition networks.
DC electric tools are accurate, clean, quiet and ergonomic. They have a high power-to-weight ratio and are economical to run. However, they are expensive, somewhat application-sensitive, and less robust than equivalent air tools. And, the cables of these tools should be managed carefully.
There can be distinct advantages in considering the fastening system for an assembly during the design stage, rather than waiting until it reaches the line.
Let’s say, for example, that an impact wrench is used to fasten an M12 bolt. This fastener size was chosen so that in a worst-case scenario, a minimum clamp load will be assured. Such conditions include minimum air pressure, maximum joint friction and an operator who stops the tool prematurely.
If the same joint is designed for yield control with a DC electric system, an M8 bolt could be used. This is because the preload variation is much less with a DC electric tool than with an impact tool, even under adverse conditions. In a mass production environment, the cost difference between the two bolt sizes would soon return the extra investment in the more expensive fastening system.
Fastener accessibility is another example of how money can be saved when the fastening equipment is considered during product design. Many engineers and tool suppliers have been left with the frustrating task of developing an attachment that can deliver high torque in inaccessible spaces, with infinite durability.
Often, positioning a fastener 1 or 2 millimeters to the right or left, or providing a little extra clearance for a power tool, will not adversely affect the performance or cost of an assembly. But, it will allow the use of standard tools without the need for costly attachments.