Air Clamp method and design study｜Thorough explanation from calculation to safety circuit

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Many machine designers are concerned about whether the clamping force they have designed will be able to withstand the actual machining load and workpiece retention, or what if the clamping arm breaks because the calculation of the moment of inertia was not accurate enough.

In addition, many failures and regrets occur as a result of selecting equipment based on catalog specifications alone, such as difficulties in piping routing at the site or near-misses during maintenance due to inadequate safety circuits.

This article systematically notes specific design processes to solve these issues faced in practice.　　The "specific load calculation using inertia force coefficient G" and "procedures for constructing safety circuits compliant with JIS standards," which are discussed only in fragments on other information sites, are also discussed in depth based on field experience and the latest manufacturer's technical data.

We hope that by reading this article, you will be able to confidently choose the "air clamping method".

Air clamping methods and types from the basics

Grasping characteristics as affected by compressibility of air

The first physical challenge faced in designing air clamps is the compressibility of the air.　　While hydraulic oil used in hydraulic clamps is an incompressible fluid, air has the property of being compressed like a spring.　　While this characteristic has the advantage of softly gripping the workpiece, it also has the disadvantage of low rigidity against external forces such as cutting, for example.

Specifically, when intermittent cutting resistance is applied during milling or other operations, the air inside the cylinder is compressed and the piston moves slightly, causing "chatter" on the machined surface or, in the worst case, the workpiece may be displaced.　　In addition, at the moment the clamping operation is completed, the rebound of the compressed air can cause the head to bounce, a "bounce phenomenon.

To prevent this, it is important not only to increase the pressure, but also to use the mechanical locking mechanism described below, and to determine whether the process (assembly, transfer, light cutting) is suitable for pneumatic pressure in the first place.

The table below summarizes a comparison of pneumatic and hydraulic characteristics. Use this as a reference to determine if a pneumatic clamp is appropriate for your target process.

Thrust generation based on Pascal's principle

The force generated by air clamps is the basis of physics, "Pascal's Principle."This is determined by the following law.　　This is the law that pressure applied to a fluid in a closed container is transmitted equally in all directions. In design practice, the following equation is used to calculate the theoretical thrust F

F = P × A × η

where P is the supply pressure (MPa), A is the pressure-receiving area of the cylinder (mm²), and η is the mechanical efficiency (typically around 0.8 to 0.9).　　As can be seen from this equation,To obtain the required clamping force, the two choices are "increase the pressure" or "increase the cylinder diameter (pressure-receiving area).　　However, since factory air supply pressure is generally fixed at around 0.5 MPa, designers need to ensure thrust by selecting an appropriate cylinder diameter.

What needs attention,The point where the pressure-receiving area differs between the "push side" and the "pull side" of the cylinder　It is.　　On the side where the rod is present (pull side), the pressure-receiving area is reduced by the cross-sectional integral of the rod, so the thrust force generated is also smaller.　　When using a retracting or swinging clamp, be sure to check the area in the direction of motion used when referring to the thrust table in the catalog.

Lead groove structure of swing clamp

Swing clamps," in which an arm comes down from above the workpiece to secure it, are one of the most frequently employed devices in jig design because they ensure workability during attachment and removal.　　This device uses a single cylinder to perform the complex movement of the rod as it swivels, descends, and finally clamps vertically.　　This movement is achieved by "lead grooves (spiral grooves)" engraved on the rod and internal cam.

In the general construction, the guide pin traces the lead groove as the piston rod raises and lowers, creating a rotational motion.　　However, long-term use or high-speed operation causes uneven wear at the contact area between this lead groove and the pin, resulting in rattling of the arm (play in the direction of rod rotation).

To solve this problem,Kosmek Corporation　Some manufacturers, such as TOYO, use a lead groove with a special cross-sectional shape called a "Gothic arch shape".　　This is a technology that optimizes the contact area and reduces surface pressure by making the contact between the guide ball and groove "two-point contact" instead of "point contact.

This dramatically improves durability and enables high-precision positioning compared to conventional products.　　In design selection, not only thrust but also durability due to such differences in internal structure should be taken into consideration.

Link clamp features

Whereas the swing clamp "swivels",link clamp　The arm moves back and forth to clamp a workpiece by means of a link mechanism utilizing the handpiece principle.　　The greatest feature of this mechanism is that the thrust of the cylinder is amplified by the link ratio to provide a powerful clamping force in a compact body.　　In addition, since the arm does not swivel, it has the advantage of being easy to place even in spaces where there are walls on the sides of the workpiece or where there is severe interference with adjacent fixtures.

However,The structure of the link mechanism is more complex than that of the swing clamp because multiple parts are pin-coupled.　　Therefore, if cutting coolant or chips accumulate in the linkage, it tends to cause malfunctions.　　When designing the machine, it is necessary to take measures such as covering the linkage or incorporating a circuit that allows periodic air blow cleaning.

The following table compares the major pneumatic clamping mechanisms.

Retention function of mechanical lock

It was developed to prevent "loss of holding force when air supply is stopped," which is a drawback of pneumatic clamps,mechanical lock(or toggle lock) function.　　It has a mechanism that geometrically locks the internal wedge or toggle link in the position where the clamping operation is completed.

With this function, even in the unlikely event of a power failure or hose disconnection accident that results in zero air pressure, the clamp will maintain its clamped state due to its powerful spring force and frictional holding force.　　From the perspective of Japanese occupational safety and health regulations and JIS standards, the use of clamps with this type of safety mechanism is strongly recommended for processes in which heavy objects are gripped and transported, welding lines, and other high-risk facilities.

Note, however, that when unlocking (unclamping), a strong force is required to release the lock, so the lower limit of supply pressure may be set higher than that of a normal cylinder (e.g., 0.4 MPa or higher required).

Selection to determine the best air clamping method

Theoretical thrust calculation formula

As mentioned above, the theoretical thrust is obtained by the product of the pressure-receiving area and the pressure, but more detailed study is required for practical selection.　　Specifically,It is necessary to estimate the "effective thrust" taking into account the pressure loss due to piping resistance and the sliding resistance of the packing inside the cylinder.　　Generally, the theoretical thrust listed in the catalog is multiplied by the load factor (inverse of the safety factor).

F_req = F_theoretical × η_load

where F_req is the practical clamping force, F_theoretical is the theoretical thrust, η_load is the load factor.

One mistake that many novice designers make is to select a size that is just at the theoretical thrust in the catalog, resulting in insufficient gripping force when the pressure fluctuates (decreases) in the actual field.Even if the original pressure at the factory is 0.5 MPa, selecting a cylinder diameter with a sufficient margin in anticipation of a momentary drop to about 0.4 MPa at the end actuator section is the basis for trouble-free design.

Set appropriate load factor

So, exactly what level of load factor (safety factor) should be set?　　The technical documents of major Japanese manufacturers such as SMC and CKD provide clear recommendations according to the application.

• Static clamping (simply holds the workpiece in place): Load factor 70% or less (safety factor of approx. 1.4 times or more)
• Dynamic clamping (with transport, with impact): Load factor 50% or less (safety factor of approx. 2.0 times or more)

For example, assume that the required gripping force calculated from the cutting resistance and the dead weight of the workpiece is 500 N.　　For static fixation, the required theoretical thrust is 500 ÷ 0.7 ≈ 714 N. On the other hand, for a dynamic condition such as grasping and swinging by a robot hand, a cylinder with a theoretical thrust of 500 ÷ 0.5 = 1000 N or more must be selected.

When pushing a workpiece with a simple cylinder without a guide, it is also important to design the cylinder so that no lateral load (radial load) is applied to the rod.　　If a lateral load is applied, a separate linear guide must be used together or a guided cylinder must be selected to keep the load within the allowable lateral load range of the cylinder.

Reference source: SMC Corporation (https://www.smcworld.com/catalog/BEST-technical-data/pdf/6-2-1-m21-43-tech.pdf）

Moment of inertia and inertia force coefficient G

In selecting a swing clamp, the calculation of "moment of inertia" is more important than the calculation of thrust and is often overlooked.　　When an arm swings, a large amount of inertia energy is generated by the mass of the arm itself, the attachment at the end, and the rotation speed.　　The moment the clamp stops at the swivel end, this energy strikes the internal cam mechanism and guide pin as impact torque.

To quantitatively evaluate this impact force, a parameter called "inertia force coefficient G" is used in manufacturer's materials such as CKD.

M = m × G × L

• M: Allowable bending moment or torque (N-m)
• m: Mass of arm etc. (kg)
• L: Distance from center of rotation to center of gravity (m)
• G: Inertia coefficient (depends on operating speed)

It is important to note that the coefficient G increases exponentially with increasing operating speed. For example, if the speed doubles, G may jump several times.　　If the calculation results show that the allowable moment is exceeded, consider the following measures before increasing the cylinder size.

1. Lightweight arm: Change from steel (S45C) to aluminum (A7075, etc.) and cut out unnecessary parts.
2. Speed reduction: Reduce the turning speed with a speed controller. This is most effective.

Many breakage troubles are caused by insufficient calculation of this moment of inertia. Please be sure to verify the weight based on the calculation formula, not on a sensory value such as "this much weight should be OK.

Reference source: CKD Corporation (https://www.ckd.co.jp/kiki/jp/file/2778）

Circuits for safe air clamping methods

Residual pressure drain valve for safety

The installation of a "residual pressure discharge valve" is a legal and practical requirement to ensure the safety of the air clamping system.　　Even if the air supply source is shut off during maintenance or emergency shutdown, compressed air remains trapped (residual pressure) in the piping between the cylinder and the switching valve.　　If the piping is disconnected in this condition, the compressed air will be released with an explosive noise, the hose will go wild and hit the operator, or the cylinder will unexpectedly move due to residual air, resulting in a serious accident such as pinching a finger.

JIS B 8370 "General Rules for Pneumatic Systems" also requires that the structure be capable of safely releasing residual energy when energy is shut off.　　Specifically, it should be placed near the entrance to the circuit, such as just after the FRL unit (filter/regulator/lubricator).3-port residual pressure drain valve(VHS series, etc.)" will be established.

By operating this valve closed, the supply can be shut off and at the same time the downstream air can be released to the atmosphere.　　The designer should be sure to incorporate this valve in the circuit drawing stage and place it in a position where anyone can see how to operate it.

Pilot check valve to prevent falling

When a clamp cylinder is used in the vertical direction (Z-axis), there is a risk that the cylinder will drop under its own weight or the weight of the workpiece if the air supply is cut off.　　This is used to prevent this from happening.Pilot check valve".　　This valve allows air to flow freely while supply pressure (pilot pressure) is applied, but when the supply stops, the internal check valve closes and blocks the exhaust side of the cylinder.　　This allows air to be contained in the cylinder and held in that position.

SMC and Koganei's technical documents also recommend pilot check valves for building fall protection circuits.　　However, since it is only "air containment," it is not suitable for long-term retention.　　This is because minute leaks may occur over time, resulting in a gradual decrease in pressure.

Therefore,For retention over several days or when absolute security is required in areas with human access, use the aforementioned cylinder with mechanical lock in combination with multiple safety measures, or add a physical anti-drop pin (shot pin).

Reference: Koganei Corporation (https://product.koganei.co.jp/common/pdf/tech/SP237_Pilot_check_Valve_J_Ver1.pdf）

Reference: Kosmek, Inc.https://www.kosmek.co.jp/data/pdf/jp/BWS_R00_2022FA.pdf）

Speed control is meter-out control

In adjusting the speed of air clamps,Adopt "meter-out control　The main principle is to　　This method adjusts the speed by throttling the air (exhaust) coming out of the cylinder, rather than the air entering the cylinder.　　When the exhaust air is throttled, back pressure builds up inside the cylinder, and the piston moves stably with a cushion of air between it and the cylinder.

If "meter-in control" is used to squeeze the incoming air, the piston will not move until pressure builds up in the cylinder, and "stick-slip," which occurs when the piston accelerates at once as soon as it starts moving, is likely to occur.

This is fatal to clamping equipment, resulting in impact to the workpiece, misalignment, and even damage to the equipment.With the exception of single-acting cylinders and other exceptions, clamp circuits using double-acting cylinders should always select a meter-out type speed controller and mount it directly on the cylinder port.　　This provides stable operating speeds that are resistant to load fluctuations.

Drainage measures to be required

One of the top causes of pneumatic equipment failure is problems caused by moisture (condensate) in compressed air.　　Hot air compressed by the compressor is cooled in the process of passing through the piping, where it condenses and forms water droplets.　　When this moisture enters the clamp cylinder, it washes away the internal lubricating grease, causing accelerated wear of the packing and rusting of the cylinder tube.

In particular, recent high-performance air clamps have precise internal tolerances, so the slightest rust or debris can directly lead to malfunction.　　Designers should incorporate the following drain measures into the circuit as a flow

1. source control: Install an air dryer (e.g., refrigeration type) immediately after the compressor to lower the dew point.
2. removal measure: Install an air filter and mist separator in front of the equipment to remove moisture and oil.
3. Piping Installation: Provide a drain valve at the end of the piping, or start up the piping and then branch off to the equipment (because water flows to the lower end).

Also,If lubrication is inadvertently performed with a lubricator on a lubrication-free cylinder, the initial grease will flow out and shorten the life of the cylinder.　　It is also an important role of the designer to clearly state "no refueling" to maintenance personnel.

Reference source: Fukuhara Corporation (https://www.fukuhara-net.co.jp/necessity）

Summary of air clamping methods useful in the field

• Air clamp design must take into account the lack of rigidity and bounce phenomenon due to the compressibility of air and determine the application.
• Thrust calculations are selected by applying Pascal's principle (pressure x area) as well as load factor (static 70%, dynamic 50% or less)
• Swing clamps are easy to attach and detach, but the risk of damage due to the moment of inertia during turning is high, so calculation is essential.
• Lead groove shape (Gothic arch, etc.) is directly related to durability and repeatability, and should be used as an indicator for selecting a manufacturer.
• Link clamps provide a strong clamping force due to a doubling mechanism, which saves space and avoids interference.
• Mechanical locking feature is safe because it physically maintains gripping force even when air supply is interrupted
• Since the inertia force coefficient G increases in proportion to the square of the speed, arm weight reduction and speed reduction are the most effective measures
• As a safety measure, always install a residual pressure drain valve at the circuit inlet to prevent runaway accidents during maintenance
• For vertical installation, use a pilot check valve and set up a circuit to prevent the cylinder from falling when the air is shut off.
• The meter-out method is used for speed control, and back pressure is used to prevent pop-out and stabilize operation.
• Since condensate (moisture) is a major cause of equipment failure, multiple measures should be taken with air dryers, filters, and mist separators.
• Since the pressure receiving area of the cylinder differs between the "push side" and the "pull side," calculate the area in the direction of motion used.
• Lubrication of unlubricated cylinders is prohibited because it causes grease leakage, and we will make sure that everyone at the site is aware of this.
• Pipe diameter and fitting size also affect flow rate (speed), so select the appropriate size for the cylinder.
• The shortcut to zero trouble is to consider all these elements as a flow and design it with logical support.

That's it.